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Status of Pollinators in North America (2007)

Chapter: 6 Strategies for Maintaining Pollinators and Pollination Services

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Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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6
Strategies for Maintaining Pollinators and Pollination Services

Although information on the status of most pollinators is incomplete, much can be done to maintain commercial and wild pollinator populations and to prevent future shortages of pollination services. The sustainability of the European or western honey bee (Apis mellifera), the principal managed pollinator in North America, could be buttressed through the development and adoption of parasite- and pathogen-resistant stocks of bees. Several developments could help the bee industry reach this goal: use of modern molecular techniques for identifying superior Apis stocks, effective methods for the preservation of honey bee germplasm, a suitable infrastructure for maintenance and use of resistant stocks, and adoption of practices by commercial queen producers and beekeepers that are consistent with these goals.

The development of mite- and pathogen-resistant stocks, however, is a long-term solution, one that will require extensive collaboration among researchers, extension personnel, and the queen-and-package industry. In the meantime, beekeepers require immediate relief. Other pest management strategies include programs that mitigate the effects of pesticide resistance in mite populations and cultural and other nonchemical techniques for disease management in commercial hives. Management techniques also must be implemented to reduce the impact of Africanized honey bees, which have begun to colonize areas of the United States critical to the beekeeping industry. The development of methods that support the commercialization of non-Apis pollinator species is also a high priority.

For wild, unmanaged pollinators, the most important goals involve conservation and restoration of habitat. Many pollinators can survive in small habitat patches and use the resources in natural areas, wildlands,

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

and even human-dominated areas including appropriately managed farms, urban parks, and golf courses. Small changes could produce substantial benefits, but basic information on the resource requirements of a wider variety of pollinator species is needed to improve habitat management. Also, economic and policy incentives would encourage the stewards of a wide range of urban and rural areas to adopt pollinator-friendly practices and also to encourage information exchange and outreach. The most effective and sustainable route to stability in pollination services is to identify and implement practices that promote the availability of diverse commercial and wild pollinators.

MAINTAINING COMMERCIAL POLLINATORS

Apis mellifera: Problems and Solutions

The beekeeping industry is at a critical juncture as it faces a suite of challenges that defy easy solution. The parasitic honey bee mite Varroa destructor, now ubiquitous in North America, is the single greatest threat to a sustainable supply of healthy and affordable honey bee colonies worldwide (DeJong, 1990; DeJong et al., 1982a, 1984). Major wintertime losses of honey bees in the United States every few years since 1993 (Burgett, 1994; Caron and Hubner, 2001; Finly et al., 1996; Lumkin, 2005) are almost certainly attributable to varroa mite infestation, which was exacerbated by the evolution of resistance to standard miticides. The tracheal mite Acarapis woodi also contributes to the periodic catastrophic winter losses, but reliable data on its prevalence in North America are not available. There are effective treatments for management of tracheal mites, including trachealmite-resistant stocks of bees (Chapter 3). Problems with tracheal mites, to the extent that they exist, can most likely be ameliorated by improved detection and control among beekeepers.

Another serious challenge to the beekeeping industry is the Africanized honey bee, which has colonized several regions of the United States that are important to the commercial queen-and-package bee industry (northern California and the southeastern United States). The bees also migrate with beekeepers to hospitable wintering grounds. Because the Africanized bees have several traits that are undesirable for beekeeping (Chapter 3), it is imperative that the genotype be prevented from coming to predominance in the United States and Canada. The bees’ presence in the southeast—an important area of queen-and-package production for the rest of the United States and a primary wintering ground for beekeepers (Chapter 3)—makes this objective paramount.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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Resistant Honey Bee Breeding

A long-term solution to the problems of parasitic mites and honey bee pathogens is the development of resistant stocks of bees. Several traits associated with varroa mite resistance are heritable (that is, available for selection) (Camazine, 1986; Camazine and Morse, 1988; DeJong, 1996; Harbo, 1992, 1993; Harbo and Harris, 1999a,b; Harbo and Hoopingarner, 1995; Harbo et al., 1997; Moritz, 1985; Moritz and Hanel, 1984; Rinderer et al., 2003). Similarly, tracheal mite resistance is a heritable trait (Gary et al., 1990; Page and Gary, 1990). A varroa-resistant stock of honey bees was developed at the U.S. Department of Agriculture (USDA) Agricultural Research Service (ARS) honey bee research laboratory in Baton Rouge, Louisiana (Harbo and Harris, 1999a), and is available commercially as SMR (suppressing mite reproduction) or SMART stock.

Related efforts also have focused on identifying honey bee populations with a long history of exposure to V. destructor as a potential source of resistant stock (Rinderer et al., 1999, 2001, 2003). ARS began to import bees from the Primorsky region in far-eastern Russia beginning in the early 1990s (Rinderer et al., 2005). The Russian bees were quarantined on an island off the coast of Louisiana, and they have been subject to further selection. The Russian bees exhibit a high degree of varroa mite resistance (Rinderer et al., 2003, and references therein), and they are now available commercially.

Resistance to American foulbrood and other bee pathogens was shown to be heritable in the 1930s (Park, 1936). Although other traits contribute to foulbrood resistance (Spivak and Gilliam, 1998a,b), the principal mechanism is hygienic behavior (Rothenbuhler, 1964). Stocks that exhibit hygienic behavior have been developed at least three times since the 1930s (Park et al., 1937, 1939; Rothenbuhler, 1964; Spivak and Reuter, 2001). Hygienic behavior also could operate in mite resistance (Boecking et al., 2000; Harbo and Harris, 2005; Spivak and Rueter, 2001), and the University of Minnesota has developed hygienic stocks that are available commercially.

Another challenge to the bee industry is the synthesis of results from federal and academic research into sustainable commercial queen-and-package operations. There are well-developed methods for quantifying resistance to mites and pathogens (Harbo and Harris, 1999a; Harbo et al., 1997; Spivak and Downey, 1998; Spivak and Gilliam, 1998a,b) and for breeding and maintaining resistant stocks (Harris et al., 2002; Page and Laidlaw, 1982a,b; Page et al., 1983, 1985). Perusal of trade journals reveals beekeepers’ interest in mite-resistant stocks of bees and the low availability of such stock: Several suppliers advertise Russian, SMR, or hygienic bee stocks, but there are no data on the number or quality of queens available. It is not clear why resistant stocks have not yet been widely adopted (Sheppard, 2006), but it is possible that the impediments include the difficulty of maintaining inbred

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

lines, the negative consequences of selecting one trait over others that are commercially important (Page and Laidlaw, 1992), and the time and effort involved in replacing queens (Laidlaw, 1992).

Of particular importance is the lack of locally adapted stocks. Typically, although not universally, southern queen producers use stocks that perform well in the warmer south but that might not do well in the north, where winters are more severe. This is especially problematic for stocks that are affected by tracheal mites or diseases such as chalkbrood, both of which affect bees more in the cooler, damper regions of the north (Flores et al., 1996). Establishing locally adapted populations of bees is difficult because more than 500,000 queens are shipped each year throughout the country from southern production sites (Schiff and Sheppard, 1995, 1996).

Instrumental insemination (Laidlaw, 1992) is ideal for bee-breeding programs (development and maintenance), although it is more costly than is natural mating. Moreover, the honey bee mating behavior presents a challenge to the development and maintenance of selected lines of honey bees. Honey bee queens are naturally polyandrous (Winston 1987), mating with 7–17 drones on 1–5 mating flights, usually within the second week of life. Queens and drones fly to discrete spaces in drone congregation areas, located some distance above the ground and away from their nests. Mating takes place as the queen flies through one or more drone congregation areas, where the sources of drones are uncontrolled. It is not clear whether some percentage of mating with a specific desired stock is necessary to ensure a mite- or pathogen-resistant colony (Box 6-1) and likely depends on the genetic mechanisms involved (dominance, additive, epistasis).

Most commercial queen producers probably do not use resistant stocks, and most queens shipped throughout the United States apparently still come from susceptible stocks of bees. Susceptible queens also produce drones that flood local mating areas, so it is difficult to establish a sustainable resistant population.

Genetic Solutions to Problems with Mites and Pathogens

Genomics and germplasm preservation could be used to facilitate the development and maintenance of selected honey bee stocks. The traditional breeding process could be augmented through the use of genetic markers (expressed sequence tags and quantitative trait loci) for desirable traits. Markers already have been identified for defensive behavior (Hunt et al., 1998) and for hygienic behavior (Lapidge et al., 2002), and more research could facilitate development of commercially viable selected stocks of honey bees. The recent sequencing of the honey bee genome by the Baylor College of Medicine (The Honeybee Genome Sequencing Consortium, 2006) and

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

BOX 6-1

Development and Maintenance of Selected Stocks: Controlled Mating

The development and maintenance of selected stocks and breeder queens require controlled mating, generally through instrumental insemination (Laidlaw, 1992). Breeder queens (selected queens inseminated with semen from selected drones) are transferred or sold to commercial queen producers who use them to produce large numbers of production queens for sale to beekeepers. The parentage of the production queens is controlled through the use of breeder queens. Before a production queen is sold to a beekeeper, it is first mated to several drones, and the mating of production queens is invariably natural. Because commercial queen producers cannot completely control the sources of the drones that mate with their production queens (Laidlaw and Page, 1998), the queens often mate with drones from unselected stocks of local wild bees or from colonies belonging to other beekeepers. Thus, production queens will often produce hybrid workers that do not exhibit the desired traits or that do not exhibit those traits to the desired extent, depending on the genetic basis of the variation under selection (for example, dominance, additive, epistasis).

The percentage of matings that must occur with a specific desired stock to ensure a mite or pathogen resistance in a colony is not known and could depend on the trait. Some work suggests that open-mated queens from selected stocks can produce colonies with useful—but incomplete—mite resistance (Harbo and Harris, 2001; see also Spivak and Reuter, 1998, and Spivak et al., 1995, for response to American foulbrood), but another report suggests that both male and female parents should be from selected stocks (Harris and Rinderer, 2004).

Although instrumental insemination is currently complicated for use in commercial queen production, there are other options for controlling commercial mating—drone saturation and isolation (Laidlaw and Page, 1998). The former achieves varying degrees of controlled natural mating by stocking mating areas with large numbers of drone source colonies from the desired selected source (Hellmich, 1986, 1991; Hellmich et al., 1988). The latter uses isolated mating yards to control mating. The opportunity to employ isolation is limited because a separation of several kilometers from other sources of drones is required.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

related developments in honey bee genomics (Robinson et al., 2005) provide outstanding resources for these efforts.

Maintaining selected stocks of honey bees is difficult because of the generally uncontrolled mating behavior of queens and because queens have relatively short and unpredictable lives of 1–3 years (Seeley, 1985). Given the ephemeral nature of honey bee stocks, honey bee germplasm (sperm, eggs, embryos) is an ideal candidate for preservation, which would allow stakeholders an economical way to maintain large quantities of desirable germplasm from a nearly unlimited number of sources. The benefit seen in the increased access to resources would well justify the investment required to identify or develop the germplasm. This work would fit within the mission of the USDA National Animal Germplasm Program (http://www.ars-grin.gov/animal/), which coordinates and supports the cryopreservation of U.S. animal genetic resources (Blackburn, 2002). Preservation of honey bee germplasm has been attempted, so far with limited success (Collins, 2000, 2004).

Transition to Resistant Stocks

Converting the current U.S. honey bee population to one that is resistant to parasites and pathogens is an enormous challenge that would require unprecedented cooperation among queen producers and consumers, federal and university research facilities and extension programs, and, most important, beekeepers. A successful transition would require improved identification methods, including the use of genetic markers in mass screening for desirable traits; new stocks that are viable in several regions; an industry infrastructure that maintains superior stocks; and a mechanism for third-party certification of new product lines. Certification of breeder stock, mating technology, production methods and facilities, and commercially produced bees and queens would be necessary.

Managing Miticide Resistance

Pesticide resistance has become the major problem for the management of parasitic mites. Populations of V. destructor that exhibit resistance to fluvalinate (Baxter et al., 1998; Elzen et al., 1998, 1999a,b,c,d; Hillesheim et al., 1996; Macedo et al., 2002), coumaphos (Elzen and Westervelt, 2002; Milani and Della Vedova, 1996; Pettis et al., 2004), or amitraz (Elzen et al., 1999c, 2000) are widespread.

Resistance management programs would provide beekeepers with a significant tool for mite management. Such programs could be built around results from several areas of research, including projects on the mechanisms and management of resistance to various pesticides (Gerson et al. 1991; Ting et al. 2003; Wang et al. 2002; Wu et al. 2003), the identification of genetic

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

correlations among resistance mechanisms, determination of the fitness consequences of pesticide resistance, and determination of optimal intervals for pesticide rotation (Hall et al. 2004).

The industry also could benefit from the development of synergists to inhibit enzyme-based resistance in mite populations, thereby restoring the effectiveness of existing miticides. And the identification of new, less toxic pesticide compounds derived from natural products would provide beekeepers with still more options. In particular, the work should focus on improving the efficiency and reliability of such commercial products as Mite-Away II and other soft chemicals (Apiguard and Api-Life VAR).

The design of resistance management programs could follow up results from research projects outlined above. Although there are no comprehensive programs for beekeepers, there is considerable literature that could be used as a starting point for research on pests of bees (see Batabyal and Nijkamp, 2005; Benting et al., 2004; Comins, 1986; Elzen et al., 1999b; Georghiou, 1980; Green et al., 1990; Hall et al., 2004; MacDonald et al., 2003; Phillips et al., 1989; Thompson, 2003; Walker-Simmons, 2003; Williamson et al., 2003).

The tracheal mite has dropped into relative obscurity over the past decade, overshadowed by problems with varroa mites. The current effects of tracheal mites on honey bee populations are not known. Fortunately, several remedies are available for control of tracheal mites, including “grease patties” (made from vegetable shortening and granulated or powdered sugar) (Baxter et al., 2000; Calderone and Shimanuki, 1995; Liu and Nasr, 1993; Wilson et al., 1989), formic acid (Baxter et al., 2000; Feldlaufer et al., 1997; Hoppe et al., 1989), and menthol (Baxter et al., 2000; Duff and Furgala, 1993; Wilson et al., 1989, 1990). Amitraz, although not currently marketed, also can be effective against tracheal mites under some circumstances (Duff and Furgala, 1993; Wilson and Collins, 1993). Treatment results have been mixed (Duff and Furgala, 1993; Scott-Dupree and Otis, 1992), and honey bee populations have evolved resistance to tracheal mites (Gary et al., 1990; Page and Gary, 1990), an attribute that likely has contributed to a reduction in concern about this pest.

There is an additional economic benefit to deploying mite-resistant bees and reducing pesticide use—over and above the savings realized from eliminating the need to purchase chemical pesticides. The use of resistant stocks allows beekeepers to eliminate pesticide use, and some beekeepers could potentially sell their products at a premium (NRC, 2000).

Other Methods of Managing Parasites and Pathogens

Nonchemical control methods—such as cultural methods or biological control—offer many advantages for beekeepers. Combined with third-party

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

certification of honey (as pesticide-free or organic, for instance), those methods expand the beekeepers’ options in the marketplace, enabling them to take advantage of the more lucrative trade in natural foods. Among the cultural methods for mite control, drone brood removal, which exploits mite preference for drone brood, can be effective albeit labor-intensive (Calderone, 2005). The Beltsville screen insert, a piece of wire mesh inserted 3–5 cm between a hive’s bottom and its bottom board, traps the mites that typically fall to the bottom of the hive as bees groom to rid themselves of mites. The insert prevents the mites from climbing back up to reinfest the bees. The screen insert has yielded mixed and generally disappointing results (Ellis et al., 2001; Harbo and Harris, 2004; Pettis and Shimanuki, 1999; Rinderer et al., 2003), but it could become an effective management tool if it were combined with pesticides that have a rapid knockdown effect for application during honey-producing months.

The fungal pathogens Hirsutella thompsonii and Metarhizium anisopliae have shown promise as potential biological control agents for varroa mites (Kanga et al., 2003a,b), but problems with the pathogens’ sensitivity to temperature and spore distribution within hives remain unsolved. If these could be overcome, biological control could become a viable option for managing parasitic mites.

Perhaps even more important than developing new treatments for bee diseases and parasites is reinforcement of regulations aimed at prevention. Protection of North America against invasive pests and diseases from abroad is the cornerstone of pollinator protection on the continent, but existing regulations should be strictly enforced and strengthened to remain effective. The Federal Honey Bee Act of 1922 “prohibits the entry of honey bees from countries where diseases and parasites harmful to honey bees are known to exist” (USDA-APHIS, 2002). The act authorizes the Animal and Plant Health Inspection Service (APHIS) to regulate importation of honey bees in the United States. In 2004, APHIS changed the regulation to allow honey bee packages from Australia and New Zealand to be imported to pollinate California almond groves (USDA-APHIS, 2004).

Although honey bee colonies from Australia and New Zealand can offer a short-term benefit in the pollination marketplace, great care must be exercised to ensure that they do not carry new pests, parasites, pathogens, and predators. APHIS and corresponding agencies in Canada and Mexico should conduct periodic, coordinated monitoring of honey bee populations to determine whether specific pests are present. Target species for monitoring should include Tropilaelaps clareae (parasitic mite), Hyplostoma fuligineus (large hive beetle), Varroa spp. and V. destructor haplotypes that are not present in North America, Apis mellifera scutellata (African honey bee), Apis mellifera capensis (another potentially invasive subspecies of honey bee from South Africa), and other Apis species. APHIS could coordinate the

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

efforts with cognate agencies in Mexico and Canada. State departments of agriculture should be included in the development of monitoring programs and could provide valuable personnel. Shipments of bees from countries or territories that have pests that are not already present throughout North America should not be permitted if long-term safeguarding of North American pollination capacity is a priority.

APHIS should carefully assess the integrity of inspection in countries interested in supplying bees to North America, and it should collect and analyze samples of adult and immature honey bees from producers who wish to ship to North America. Sampling in the countries of origin is necessary because the bees could have pests that are currently unidentified and therefore not on the list of target species. Also, North American countries should proceed with research on honey bee pests in the potential source countries that have not yet arrived in North America to prepare the countries’ beekeeping industries for possible or eventual introductions.

Africanized Honey Bees

The consequences of the Africanized honey bee (AHB) infiltration of U.S. and Canadian honey bee populations are difficult to predict. However, uncertainty and precedent in other nations suggest that it is prudent to prepare for the worst. There are three general methods for managing AHBs: eradication, genetic isolation, and breeding (http://www.ces.ncsu.edu/depts/ent/notes/Bees/ahbactionplan2001.pdf). Several states, including North Carolina, have developed action plans that include recommendations for best management practices for beekeepers, and procedures for abatement, quarantine, outreach, and first-responder training. Other states should develop similar plans, and much of the information they need is available from existing resources.

Eradication is most effective against confirmed or suspected founder colonies that are inadvertently imported by truck or ship, but before the Africanized bees can become established. Genetic isolation is achieved through various controlled-mating techniques—such as geographic isolation, instrumental insemination, and drone saturation (Laidlaw and Page, 1998). Geographic isolation requires European honey bee production apiaries to be established at a distance from AHB colonies that is sufficient to prevent mating of the European queens with the Africanized drones. Queen-and-package producers might be able to use this method to a limited degree by placing operations in places that are so far free of Africanized bees: the northern United States, Hawaii, Canada, Australia, and New Zealand. Northern U.S. and Canadian operations could be of limited use, however, because the colder weather prevents production of queens and packages until late in the season. The United States began to import honey

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

bee packages from Australia and New Zealand for early-season pollination after winter losses in 2004–2005 (Sumner and Boriss, 2006). Australia, New Zealand, and Hawaii could be important sources of uncontaminated germplasm in the future, but extreme vigilance would be needed to ensure that additional invasive diseases or pests not be introduced.

A second way to control mating is through instrumental insemination (Laidlaw, 1977), which allows for control of male and female sources of germplasm and for maintenance of a secure, closed breeding population. Instrumental insemination is a highly effective tool in the hands of qualified practitioners and it is effective for the maintenance of domestic supplies of germplasm that is free of AHB traits. However, it is impractical for the production of commercial queens for sale to beekeepers: it is too time-consuming and labor-intensive to be profitable (Laidlaw and Page, 1998).

The final method for controlling mating is drone saturation: flooding an area with enough drones from a desired source to enhance the probability that young queens will mate with them. More research is required, however, to determine the degree of mating control required to produce behaviorally acceptable colonies (Guzmán-Novoa and Page, 1994a).

Beekeepers are aware of liability issues that could result from stinging incidents that involve Africanized bees. Guzmán-Novoa and Page (1994b, 1999) have reported that selective breeding within Africanized populations can result in a reduction in defensive behavior. However, continuous breeding selection could be necessary to suppress defensive behavior, especially where AHB stocks are prevalent.

Integrated Pest Management

Integrated pest management (IPM; Kogan, 1998) provides a unifying framework for the management of many agricultural pests, including those of honey bees. IPM coordinates the use of several pest control methods for sustainable, economically feasible management. Whenever possible, IPM uses reliable pest-sampling methods and economic injury thresholds to guide treatment decisions. IPM is desirable because it allows beekeepers to use pest information to avoid economically unnecessary applications of pesticides and antibiotics, thereby extending the long-term utility of those products by reducing the rate at which resistance evolves. It also allows beekeepers to reduce or eliminate pesticide residues in hive products.

Each sector of the beekeeping industry will require an IPM program to fit its size (the number of colonies) and its marketing goals (commercial or natural foods). American foulbrood is one disease that is effectively treated with IPM approaches. The combination of cultural methods with inspection programs and the proper use of antibiotics provides good results for control (Goodwin and Van Eaton, 1999). Continued extension efforts should be en-

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

couraged to widen acceptance of IPM of parasitic mites, especially by large commercial operators. The future of IPM’s application in the industry will depend on the development of additional treatments and on the creation of economic incentives to compensate for the additional costs involved.

Extension activities provide a primary mechanism for informing bee-keepers about pest management options and the best ways to implement them. Extension apiculturists should encourage the use of IPM whenever possible, and extension personnel should encourage beekeepers to demand third-party certification of resistant stock from commercial queen producers. Extension efforts directed toward queen breeders and commercial queen producers should emphasize methods for stock development and maintenance and the use of controlled mating, primarily through geographic isolation and drone saturation.

ARS Honey Bee Research

Much of the applied research on honey bees in the United States is conducted in ARS honey bee laboratories. Research funding has increased from $5.6 million in 1996 to $9.2 million in 2006, although the number of fulltime scientists has declined since 2003 (Table 6-1). Some of the approaches to preventing or reversing pollinator decline outlined in this chapter depend on strong ARS involvement in honey bee research. Maintaining current research support and restoring lost scientist positions—with a special focus on honey bee pollination—at ARS is critical to pollinator conservation and restoration.

TABLE 6-1 Funding and Staffing ARS Bee Research

Fiscal Year

Funding ($ U.S.)

Full-Time Permanent Staff Scientists

1996

5,574,000

23

1997

5,913,000

23

1998

6,380,000

23

1999

6,599,000

26

2000

7,009,000

26

2001

7,629,000

27

2002

8,037,000

25

2003

8,450,000

28

2004

8,844,000

27

2005

8,861,000

27

2006

9,227,000

24

SOURCE: USDA-ARS.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×
Government Intervention

Although beekeepers have faced recent substantial increases in the cost of mite control and hive transportation (Chapter 3, Chapter 4), those expenses have been offset somewhat by higher pollinator rental rates and, in some years, by higher honey prices. The higher prices signal shorter supplies of honey bees. Honey bee populations recovered after the winter kills of 1995–1996 and 2001–2002 (USDA-NASS, 1999, 2004a) and pollinator rental rates have increased as have competitive honey prices. Continuing indirect federal price supports through the loan deficiency payment and marketing assistance loan programs (USDA-FSA, 2006) also strengthen the market. Beekeepers can be expected to re-establish the honey bee colonies lost during the 2005–2006 winter. In general, although honey bee colony numbers are much more volatile from year to year since the arrival of the varroa mite (Chapter 2), the market for honey bee pollination services appears appropriate, and that signals help in stabilizing the number of pollinator colonies available (Sumner and Boriss, 2006). However, government intervention could reduce volatility by encouraging research, extension, and certification efforts; by creating stricter controls for importation of honey bees from other countries; and by better monitoring of honey bee colonies and pollination services (Chapter 5).

Faced with managing the varroa mite threat to the North American honey bee population, the beekeeping industry might find that its funds alone are insufficient to cover immediate research needs. Special, limited-term federal support should be made available through a competitive research program targeted at honey bee genetics and management to protect populations. The program could be administered by the USDA National Research Initiative Competitive Grants Program or by the National Science Foundation. Given the targeted agricultural nature of the problem, however, a USDA program would be more suitable. Long-term, programmatic research support should continue through ARS.

The effects of increased research for improved varroa mite management will be emasculated in the absence of effective communication with the honey bee industry. The recent reductions in federal funding for state extension programs leave two avenues for improving communication. First, state land grant universities should seek ways to cooperatively finance positions for honey bee extension specialists, who could then increase the benefits of research through education and outreach. Second, the honey bee industry, represented by the American Beekeepers Federation and the American Honey Producers Association, should continue and intensify its efforts to communicate advances in honey bee hygiene and management information. The industry also could collaborate with researchers to help

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

identify obstacles to the transition to IPM-based beekeeping, with resistant stock as a foundation.

Continuous protection against invasive pests and diseases from abroad is crucial to pollinator protection on the continent. The federal Honeybee Act of 1922 authorizes APHIS to regulate imports. The 2004 revision (USDA-APHIS, 2004), prompted, in part, by honey bee shortages in California almond groves, led to extensive APHIS collaboration with the Australian Quarantine and Inspection Service and New Zealand’s Ministry of Agriculture and Forestry to establish rigorous inspection and quarantine procedures (USDA-APHIS, 2004). Rigorous enforcement of sanitary rules on honey bee colony and queen importation must continue, along with protection against interstate transmission of pests. The revision (USDA-APHIS, 2004) permits importation of other pollinator bee species (Bombus impatiens, B. occidentalis, Megachile rotundata, Osmia lignaria, and O. cornifrons). Importation of these bees is regulated to prevent the introduction of new of diseases, parasites, and pest species.

Industry Initiatives

Beekeepers and the crop producers who require pollination have a special interest in the health of honey bees. The main fundraising mechanism available to U.S. agricultural producers for research and promotion is the Commodity, Promotion, Research and Information Act of 1996, which authorizes “check-off” programs administered by the USDA Agricultural Marketing Service (AMS) but managed by an industry board (USDA-AMS, 2005). Since its creation in 1987 through a USDA-administered referendum of honey producers, the National Honey Board has administered a check-off program that pools revenues from the fee of a penny per pound of honey that is collected from producers who sell at least 3 tons of honey in a year. The Honey Research, Promotion, and Consumer Information Order collects the funds for marketing and research to reduce production costs and to enhance demand for honey (USDA-AMS, 2004). The specific focuses are honey research and promotion, not work on pollination or pollinators.

Given the increasing importance to beekeepers of revenues from pollination fees and the importance of honey bee pollination to producers of almonds and many other fruit and vegetable crops, the two groups might consider joint fundraising to focus support on pollination-related research and education. Research on methods to mitigate damage caused by parasitic mites and to ensure strong colonies each spring are among the relevant topics that joint support could fund. Another is research on honey bee stock development and maintenance.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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Non-Apis Commercially Managed Bees

The potential for using wild bees as managed crop pollinators has long been known (Bohart, 1972a) and several approaches have been explored (Bosch and Kemp, 2002; Macfarlane et al., 1994; Stephen, 2003; Torchio, 2003). There is an extensive body of published work on methods of rearing wild bees (Griffin, 1993; Hughes, 1996; Maeta and Kitamura, 1981; Strickler and Cane, 2003; van Heemert et al., 1990) that provides a strong foundation for efforts to identify and cultivate commercial pollinators among the large number of wild bee species (Strickler and Cane, 2003).

Several native and nonnative species currently are being used commercially or have potential for use as agricultural pollinators in North America (Chapter 1). Among the native North American bees, Osmia lignaria is an efficient and cost-effective pollinator of sweet cherry, plum, and prune (Bosch and Kemp, 1999) that has demonstrated potential as an almond pollinator (Bosch et al., 2000; Torchio, 1981a,b, 1982). O. ribifloris is an effective pollinator of blueberry (Sampson and Cane, 2000; Sampson et al., 2004; Stubbs et al., 1994; Torchio, 1990). O. aglaia can be an effective pollinator of cultivated blackberry and raspberry (Cane, 2005), and O. excavata and O. jacoti have potential as commercial pollinators (Wei et al., 2002). Bumble bees and Andrena spp. are better pollinators of lowbush blueberry than are honey bees (Javorek et al., 2002). Bumble bees are also highly efficient greenhouse crop pollinators (Box 3-1; Free, 1993).

O. cornifrons, the hornfaced bee, is an Asian species used extensively for apple pollination in Japan (Batra, 1982; Maeta, 1990; Sekita, 2001) that has good potential for North American pear pollination (Maeta et al., 1993). It was imported into the United States in 1977 (Batra, 1979), but it has not become established as a commercial pollinator. The alfalfa leafcutter bee, Megachile rotundata, introduced from Eurasia, is superior to honey bees for alfalfa pollination (Cane, 2002; Tepedino, 1997). M. rotundata also can be an effective pollinator of blueberry (MacKenzie, 1997; Stubbs and Drummond, 1997a) and oilseed rape (Soroka et al., 2001) but not cranberry (MacKenzie, 1997).

Although USDA no longer introduces exotic bees to North America for development as commercial pollinators, prospecting among native fauna for new agricultural pollinators is an important way to encourage redundancy that will promote pollination services and food security and stability. Although prospecting programs have been in operation for 50 years at ARS and several university laboratories, the search for, biological evaluation of, and development of propagation and rearing methods for alternative bee pollinators have resulted in the wide-scale propagation of only a single species. The alfalfa leafcutter bee was propagated successfully as the result of development of trap nest technology at the ARS Bee Biology and Systematics

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

Laboratory in Logan, Utah (Bohart, 1962, 1972 a,b; Bohart and Pedersen, 1963). The entire contemporary U.S. alfalfa leafcutter bee industry results largely from this pioneering work by a few USDA and Canadian agricultural scientists who, because of its behavior in Eurasia, recognized the leafcutter bee as a legume crop pollinator. Strickler and Cane (2003) discussed lessons learned from past experience on developing alternative pollinators and offered suggestions to guide future effort.

In view of the effort required to develop new commercially managed pollinators, to complement efforts at the Logan laboratory, ARS could well benefit from creating positions in research entomology for the ARS fruit and vegetable laboratories across the United States. Their work might identify candidate pollinators for the major crops requiring pollinators in different regions, study the life history of promising species, identify potential pest problems, and develop viable management and rearing methods for commercial use of those species.

In addition to increasing the effort to identify new commercial pollinators, research investments are needed to prevent declines in existing commercially important species. The alfalfa leafcutter bee was devastated by chalkbrood—a fungal disease—and the absence of any successful management strategy (Chapter 3) should spur research to develop tools for an effective response.

U.S. bombiculture, the rearing of bumble bees, faces many more serious problems than does megachileculture—the rearing of leafcutter bees. Bumble bees are susceptible to some of the same diseases and parasites that plague honey bees, and disease limits their utility as commercial pollinators. Infections of the bees can complicate long-term maintenance of captive colonies. Two native species (B. occidentalis and B. vosnesenskii) have been evaluated, mass reared, and used as pollinators in the United States, but infections of Nosema and other pathogens in commercial insectaries led to discontinuation of these efforts (Thorp, 2003; Winter et al., 2006). Today, the only bumble bee raised for commercial greenhouse tomato pollination in the United States is B. impatiens, which is native to the eastern United States. Multinational agribusinesses have developed large-scale insectaries for year-round production of bumble bee colonies in Europe, Israel, Canada, Mexico, and the United States (primarily B. terrestris in Europe, B. impatiens in the eastern United States). In 2005, about 90,000 hives—from all suppliers of bumble bees—supplied bumble bees for pollination throughout Mexico, the United States, and Canada (René Ruiter, Koppert Biological Systems, personal communication, February 2006). U.S. bombiculture presents risks to native bees that could be greater than the risks posed by U.S. apiculture to honey bees (because there are no native North American Apis species). If managed nonnative bumble bees escape, hybridization and competition with native Bombus species could occur (Thorp, 2003; Winter

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

et al., 2006). The potential for such hybridization between European and Japanese species has been demonstrated in the laboratory (http://www003.upp.so-net.ne.jp/consecol/english/goka_report/goka_report.html).

Large-scale transportation of native and exotic bumble bee colonies among regions of North America and internationally is already thought to be a source of introduced pathogens in native North American Bombus species (Chapter 3; Thorp, 2003). During the initial stages of bombiculture development in the United States, native bumble bee queens of B. occidentalis captured in the United States were transported to Holland and used to start colonies that were later returned to the United States (René Ruiter, Koppert Biological Systems, personal communication, February 2006). The concern has been raised that the bees’ reintroduction carried new parasites or diseases (Chapter 3) into the United States. According to Colla and colleagues (2006), there is evidence that “commercially-reared bumble bees have higher prevalence of various pathogens than their wild counterparts. Several studies have found that the intestinal protozoa Crithidia bombi Lipa and Triggiani (Kinetoplastida: Trypanosomatidae) and Nosema bombi Fantham and Porter (Microsporidia: Nosematidae), and the tracheal mite Locustacarus buchneri Stammer (Acari: Podapolipidae) are far more abundant in commercial than wild bumble bees.” Because bumble bees often escape from and forage outside greenhouses where their colonies are used for tomato pollination, they could transmit diseases to wild colonies of the same and other Bombus species. Colla and colleagues (2006) reported a significantly higher incidence of infection with parasites and pathogens in various bumble bee species collected near greenhouses, than in individuals collected farther away. In Japan, Niwa and colleagues (2004) documented the transfer of pathogens from European to Japanese bumble bees, and comparable “pathogen spillover” might have caused or contributed to the recent decline and extirpation of several bumble bee species in the subgenus Bombus and to the likely extinction of B. franklini (Chapter 3; Colla et al., 2006; Thorp, 2005; Thorp and Shepherd, 2005).

Recently, bumble bee rearing in the United States has been accomplished without international bees in facilities certified monthly by APHIS to be free of known bee diseases (René Ruiter, Koppert Biological Systems, personal communication, February 2006). The United States and Canada also have blocked imports of nonnative bumble bees (such as B. terrestris). Industry groups are lobbying the Mexican government to allow introductions of B. terrestris (from Europe) into Mexico for tomato pollination in greenhouses (Winter et al., 2006). Restriction of bumble bee transfers both within the United States and from other countries is advisable because of the potential for disease transmission to managed and native wild bumble bee species and the invasiveness of some species (such as B. terrestris; Dafni and Schmida, 1996; Hingston and McQuillan, 1997, 1998).

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

MAINTAINING WILD POLLINATORS

Wild pollinators are mobile organisms that often use many resources in noncontiguous habitats. Some butterflies visit suburban gardens for nectar but oviposit on the foliage of tree species in forest habitats. Many species of hummingbirds that breed in the United States and Canada overwinter in Mexico (Nabhan et al., 2004; Shepherd et al., 2003). Maintaining diverse communities of wild pollinators therefore requires an understanding of various habitat needs and of managing habitats and landscapes to provide necessary resources (Table 6-2). Populations of pollinators can be supported if habitats are managed to provide food, and areas for nesting, overwintering, and breeding (Dover, 1991; Erickson and West, 2003; Evelyn et al., 2004; Fenton, 1997; Schultz and Dlugosch, 1999; Scott, 1986). Because pollinators are mobile, the area over which they forage, disperse, and migrate must be considered in strategies to maintain populations. Adequate resources must be available within foraging and dispersal areas (Westrich, 1996) and along migratory routes (Nabhan et al., 2004).

Managing pollinator populations and communities requires planning and action locally, regionally, and across continents. Because of their ecological and economic significance and because they are in some respects better known than are many other wild pollinators, bees can serve as a paradigm group to illustrate how multiscale approaches can be implemented in habitat restoration, conservation, and management.

Resource Requirements for Bee Species

All native and introduced bee species, whether solitary or social, require the correct balance of water, floral hosts that offer sufficient pollen and nectar of the correct types (Roulston and Cane, 2000; Roulston et al., 2000), nest-building materials (leaves, resins, sap, gums, floral oils, essential oils, bark, plant trichomes, old mouse nests, snail shells, mud, sand, pebbles), and nesting substrates (O’Toole and Raw, 1991; Roubik, 1989; Shepherd et al., 2003) to survive as adults and rear their larval broods (Michener, 2000).

Michener (2000) provided a comprehensive review of floral resource requirements for bees. Bees obtain pollen and nectar from cultivated and wild plants. Pollen (usually moistened with nectar or floral oil) is used to feed larval bees, and nectar is used to fuel the flight of adults. Many solitary bees are active above ground as adults for only a few weeks or months. Oligolectic bees specialize on one or a few closely related species within a genus of flowering plants; polylectic bees collect pollen from an array of unrelated plants. Species with long flight seasons are usually polylectic and include the long-lived carpenter bees and euglossine orchid bees, those that produce multiple generations within a season, and highly social bees with annual or permanent colonies (honey bees, bumble bees, stingless bees).

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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TABLE 6-2 Pollinators and Resource Requirements

Pollinator Group

Resource Function

Resource

Honey bees, bumble bees, batsa

Nesting, roosting sites or substrates

Cavities (underground, hollow trees), large caves, mines

Hummingbirdsb

Nesting, roosting sites

Trees (horizontal branches with shelter from night sky)

Nonsocial bees, waspsc

Nesting sites or substrates

Bare ground, vertical cliffs or ditch banks, adobe walls

Large and small carpenter bees, leafcutter bees, mason beesd

Nesting sites or substrates

Soft woods, pithy twigs, beetle burrows

Bumble beesa,e

Nesting sites

Rodent, mouse nests

Flies

Adult food

Pollen, nectar

Flies

Larval food

Insects, organic matter, water

Leafcutter and mason bees(European)f

Nesting sites

Plant galls, snail shells

Nonsocial bees, wasps

Nesting sites

Sand dunes, sand or burrow pits, gravel pits, quarries

Highly eusocial bees, honey bees, bumble bees, stingless bees

Building materials

Glandular secretions (beeswax, exocrine secretions, Dufour’s)

Nonsocial bees, some wasps; mason bees, leafcutter bees, masarid wasps, potter wasps

Building materials

Mud, clay, sand

Leafcutter bees, mason bees, masarid wasps

Building materials

Debris (bark, floral parts, seeds, dead insect parts)

Nonsocial bees, some wasps

Building materials

Water (mixed with soil to make mud)

Leafcutter bees, mason bees

Building materials

Leaves cut into pieces or masticated

Leafcutter bees, especially anthidiines (carder bees)

Building

Plant hairs (trichomes)

Leafcutter bees, Apis, Melipona, Trigona, orchid bees

Building materials

Floral, plant resins

Orchid bees

Pheromones

Essential oils, such as monoterpenoids collected by males

Bees

Food, building materials

Floral oils (Clusia, Dalechampia, Krameria, Malpighiaceae)

Birds, some bats, bees, masarid wasps, butterflies, flies

Food

Pollen, nectar

Centris, Epicharis, Paratetrapedia bees

Food, building materials

Floral oils (Clusis, Dalechampia, Krameria to mix with pollen)

Wasps, Pompilidae (spider wasps)

Larval food

Paralyzed spiders

Parasitic, nonparasitic wasps

Larval food

Insect prey

Butterflies, moths

Larval food

Leaves, other plant parts (often taxonomically restricted)

Ants

Adult, larval food

Nectar, honeydew, insect prey

Beetles

Adult, larval food

Pollen, nectar, food bodies, organic matter

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

Pollinator Group

Resource Function

Resource

Thrips

Adult, immature food

Floral tissue, leaves, pollen, nectar

Hummingbirds

Food

Nectar, small insects

Hummingbirds

Building materials

Spider webs (silk), lichens, fibers

aMichener, 1974.

bEhrlich et al., 1988.

cMichener, 2000.

dKrombein, 1967.

eHeinrich, 2004.

fO’Toole and Raw, 1991.

The bee species might require floral resources for weeks or months, so a diversity of floral sources must be available; at least some of the flowering plants should have overlapping blooming periods that encompass the bees’ long flight periods. For social bees that overwinter as adults (bumble bees, honey bees), the temperate bloom of fall-blooming asteraceous species (goldenrods) are nectar and pollen sources that provide protein and carbohydrate resources essential for winter survival (Shepherd et al., 2003; Vaughan et al., 2004).

Bee species vary in floral resource requirements, and there is a wide variation in nesting habits. Many dig nests in the ground (Halictidae, Andrenidae), others occupy abandoned rodent nests (Bombus spp.), some use preexisting tunnels or cavities in dead tree trunks and limbs (Megachilidae, some Apinae), and others actively excavate cavities in wood (Xylocopinae). The diversity of a bee community is tied to the availability of different nesting substrates (Potts et al., 2005).

Most North American bees are ground-nesting. Like their more familiar sand wasp relative, they vary by species in nest site selection criteria (Cane, 1991; Michener, 2000; O’Toole and Raw, 1991). Some ground-nesting bees prefer to nest in open, horizontal areas of soil devoid of vegetation or debris; others seek small areas of bare soil within lawns or nest in vertical banks, either naturally occurring ones or those formed by adobe structures, wood frame houses, and other buildings. Patches of bare earth warmed by the sun and protected from flooding are especially preferred as nesting sites. Many species prefer to nest in well-drained sandy soils or silty loams (Cane, 1991). Some nest in dense aggregations that persist for decades (Michener, 2000); others construct highly scattered ephemeral nests.

About 10 percent of North American bees nest in wood (Michener, 2000). Carpenter bees (Xylocopa spp.) have strong jaws to excavate extensive galleries in soft, dry, dead wood. Small carpenter bees (Ceratina), mason

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

bees (Osmia spp.), and some leafcutter bees (Megachile spp.) use cavities made by wood-boring cerambycid or buprestid beetles. Nesting female bees match the diameter of their bodies to evacuated natal tunnels of the beetles (Krombein, 1967). This nesting biology makes it possible to “trap nest” and collect numerous females of diverse species in the twig-nesting guild. Those species will readily build nests in blocks of wood that have been drilled with appropriate-sized holes (Gathmann et al., 1994; Krombein, 1967; Tscharntke et al., 1998).

About 45 species of North American bumble bee nest principally in cavities within the ground. Upon emerging from diapause in the early spring, bumble bee queens seek rodent burrows, abandoned mouse nests, and other cavities in which to found their colonies and rear their first broods (Goulson, 2003c; Heinrich, 1979, 2004; Michener, 1974). In the southwestern United States, bumble bees (such as B. sonorus) often nest in or near human structures—under boards, in sheds, in walls, or even in abandoned mattresses or automobiles (S. Buchmann, unpublished data).

Once a nest is built, it can be modified by the addition of construction materials or glandular secretions. Ground-nesting bees typically use nothing more than exocrine gland secretions (Michener, 2000; O’Toole and Raw, 1991; Stephen et al., 1969). Bees that nest in pithy twigs, stems, or dead wood often forage at some distance from their nests for additional construction materials (Roubik, 1989). Pieces of foliage often are used by leafcutter bees (Megachile spp.) to form cell walls and end plugs. Osmia spp. often construct cell walls and end plugs from mud gathered nearby (Bosch and Kemp, 2001). Other Megachile species use resins, pebbles, and plant materials to form divisions between larval cells or to prevent entry to their nests by ants, parasitic wasps, or birds (Krombein, 1967; Michener, 2000; Stephen et al., 1969).

In agricultural plantings, where leafcutter and mason bees are used to pollinate crops, it can be necessary to provide patches of fresh mud (for Osmia spp. mason bees) or appropriate plants from which bees can cut leaves to form their cells. In the case of the introduced alfalfa leafcutter bee, alfalfa plants themselves provide pollen, nectar, and the leaves the bees use to create their nests. Other twig- or wood-nesting bees (such as Xylocopa) line their nests with layers of glandular exocrine secretions (Cane, 1991; Michener, 2000; Roubik, 1989). Social bees, including bumble bees, stingless bees (meliponines), honey bees, and some orchid bees (euglossines), use collected materials and beeswax secreted from their abdominal wax glands to build nests (Michener, 1974). Even for commercially managed bee pollinators, it can be necessary to provide supplemental sources of nesting materials.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

Restoring, Managing, and Conserving Wild Bee Habitat

The plant resources required by bees for food and nesting dictate that strategies for maintaining healthy and diverse communities of pollinator populations must focus on conserving and restoring diverse plant communities (Forup and Memmott, 2005; Kremen et al., 2002a; Matheson et al., 1996; O’Toole, 1993, 1994). Providing sequences of blooming plants that encompass the entire flight period of the pollinator is one important component of maintaining pollinator populations, whether these series result from small-scale modifications on farm sites or in gardens or from large-scale regional restoration (Vaughan et al., 2004). Similarly, plans to provide habitat for bees or other pollinators (Table 6-2) must consider not only food resources, but also the specialized resources used for breeding, nesting, or overwintering.

Several factors should be considered in determining appropriate planting mixes. First, native plants are generally preferable to introduced species because they help maintain North American biodiversity of plants and pollinators (Shepherd et. al., 2003). Ideally, plants are not just native to the continent but they are native to and genetically adapted to the region and to the site conditions (McKay et al., 2005). Second, plants must provide a complete phenological suite of resources for key pollinator species (Kremen et al., 2002a). Developing an optimal plant list requires research on the network of interactions between plants and pollinators and on which critical “bridging” plants might be needed to provide resources during periods of dearth (see, for example, Forup and Memmott, 2005; Kremen et al., 2002a). Finally, conserving existing original habitats generally should take priority over restoration, because restored habitats might not replicate every component that is functionally significant to pollinator species (Zedler and Callaway, 1999), and goals for restoration can be difficult to establish (Ehrenfeld and Toth, 1997).

Nesting Sites and Substrates

The alkali bee (Box 6-2), which has been successfully managed for pollination of alfalfa in the Pacific Northwest (Chapter 2), provides an example of how creation of supplemental or artificial nesting sites can enhance bee populations. Remarkably, few restoration ecologists have tested the efficacy of supplemental nesting sites for ground-nesting bees to enhance wild populations or to provide stable long-term habitat (but see Forup and Memmott, 2005). Investigating the effects of supplementation on nest occupancy, abundance, and persistence of wild bees is therefore a high priority for research.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

BOX 6-2

Alkali Bee: Case Study in Managing a Ground-Nesting Commercial Pollinator

Nomia melanderi, the alkali bee, is native to arid areas west of the Rocky Mountains. It nests obligately in alkaline areas in California, Colorado, Idaho, Nevada, Oregon, Washington, and Utah. Before human colonization, alkali bees nested in dry lake beds and similar habitats, requiring soils with an alkaline surface crust of salt. The bees nest at depths of 8–16 cm in aggregations of as many as 240 bees per square meter (Bohart, 1958, 1967, 1972b; Cane, 2003). Alkali bees visit native legumes for pollen and nectar and are extremely efficient pollinators of alfalfa, for which they are managed as commercial pollinators. Today, specially prepared alkali bee nesting beds have been created in four states. In Washington’s Touchet Valley the bee nesting beds average 6200 m2—the largest site had 1.7 million bee nests. In 1992, the cost per acre to pollinate alfalfa with alkali bees was estimated at $30 (Willet and Gary, 1992).

Methods for creating appropriate nesting conditions for N. melanderi were developed and tested at the ARS Bee Biology and Systematics Laboratory in Logan, Utah (Cane, 2003). To create artificial nesting beds, prepared soil is moved into basins with underlying gravel or plastic and standpipes to create an upwelling of moisture to the surface. Salt is applied heavily to the soil surface to form a crust that mimics the salt pans and playas where bees nest naturally. Backhoes fitted with hydraulic punches remove block soil cores from existing nesting aggregations, and the cores are planted in new alkali bee beds to establish nesting sites adjacent to commercial alfalfa fields. Although moving nests and underground cells of ground-nesting bees is notoriously difficult and rarely attempted, it has worked well for establishing nesting aggregations of alkali bees in artificial nest beds. The bee beds are expensive initially, but once established they are sustained by the bees themselves. Maintenance costs are low and the beds last for decades.

In the 1960s and 1970s, alfalfa growers began to rely less on the native alkali bee for pollinating their crops (Mayer and Johansen, 2003) and shifted to using the alfalfa leafcutter (Megachile rotundata), an introduced species. The decline in the alkali bee industry probably was the result of pesticide use for controlling rangeland grasshoppers, competition from honey bees, and several rainy years, when the alkali bees’ underground cells suffered unusually high mortality (Mayer and Johansen, 2003; Chapter 3). Although few managed alkali bee beds remain, new research and educational efforts are beginning to attract new practitioners to the field (Cane, ARS, personal communication, January 2006).

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×
Ground Nesters

Suitable habitats, including open ground or vertical walls, ditches, or banks, can be created or modified within natural or anthropogenic habitats as bee nesting sites. For example, piles of sand have been used in Europe to create nesting substrates for bees (O’Toole and Raw, 1991). Some habitat manipulations are simple and compatible with human habitation in Mexico and the southwestern United States; solitary bees often nest within adobe walls, which are relatively inexpensive, available, and durable (Buchmann, unpublished data; Stephen, 2003).

Twig Nesters

Because wood-loving bees vary in size, nesting females choose tunnels of appropriate diameter and depth that have been vacated by the emerging adult wood-boring beetle (Buprestidae, Cerambycidae) (Linsley, 1958). Given that beetles and bees are ecological partners, actions to increase larval substrates for wood-boring beetle taxa can increase the availability of nest sites for pollinating bees and some wasps (Jones and Munn, 1998; Shepherd et al., 2003). Not all woods or plant species provide suitable nesting substrates. Generally, soft woods that are not colonized by fungi are preferred by guilds of wood-nesting bees (Krombein, 1967). Thus, retaining dead branches or trees is an essential part of habitat management for healthy bee populations and communities. Removing large amounts of dead wood for fire wood (mesquite, palo verde, and ironwood in the southwestern United States) results in the rapid elimination of many native bees (Buchmann and Nabhan, 1996; Buchmann, unpublished).

Bee nesting habitats also can be created by attaching drilled-board trap nests to fence posts, dead trees, or buildings (Griffin, 1999; Krombein, 1967; Shepherd et al., 2003). A balanced strategy of conserving beetle-infested dead trees and branches, setting out trap nests, and drilling holes into dead trees should increase local bee populations (Buchmann, unpublished). Detailed instructions for drilled-board trap nests are in the literature (Bosch and Kemp, 2001; Griffin, 1999; Krombein, 1967; Shepherd et al., 2003) and online (http://snohomish.wsu.edu/mg/ombblock/ombblock.htm; http://www.nwf.org/backyardwildlifehabitat/beehouse.cfm).

Cavity Nesters

Nest boxes made of wood or Styrofoam with plastic or rubber hose entrance tunnels can be provided for bumble bee species that nest underground. The boxes can be buried or nestled into bank or ditch sides to attract founding bumble bee queens in the spring. Adding upholsterer’s cotton, abandoned mouse nests, or other nesting materials can improve the nests’

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

attractiveness (Heinrich, 2004; Intenthron and Gerrard, 1999). Nest boxes yield variable success that depends largely on the skill and knowledge of the builder and person placing the boxes in the field (Heinrich, 2004; Kearns and Thomson, 2001; Prŷs-Jones and Corbet, 1987). Extensive work on the chemistry of bumble bee pheromones has been conducted for some species (Bergström et al., 1996; O’Neill et al., 1991); the use of pheromone lures in the early spring to attract females to nest boxes could be useful although it has not yet been evaluated.

If colonies are started in the laboratory from wild-caught queens, their diet must be supplemented with pollen (often collected from Apis colonies using pollen traps) and sugar water or diluted honey. Colonies replaced to the wild should be kept away from areas where insecticides are sprayed, or spraying should occur at night when bees are inside their nests. Although established Bombus colonies can be purchased from commercial insectaries, they should not be used in reintroduction programs or as crop pollinators outside of greenhouses because of the possibility of transmitting pests and pathogens to local conspecifics or congeners (Colla et al., 2006).

Seeding Areas with Established Nests

Nests of wood or ground-nesting bees can be collected in natural habitats and introduced elsewhere to reestablish populations, although this approach is still experimental. Occupied branches or inhabited dead trees can be moved from one area to another to seed a new habitat with bees (Yurlina, 1998). Trees and nesting blocks were used in New Jersey at the Fresh Kills landfill to reintroduce native bee species to the active restoration site (Handel, 1997; Handel et al., 1994; Yurlina, 1998). Introductions of occupied nests, however, are more common in commercial agricultural pollination. Twig-, wood-, and cavity-nesting bees are generally more manageable than are ground-nesting bees (Bosch and Kemp, 2001). Several leafcutter and mason bee species are routinely moved in artificial domiciles to orchards and alfalfa fields for agricultural pollination (Chapter 1). Other than alkali bees (Box 6-2) (Bohart, 1958, 1962, 1972a; Cane, 2003), ground-nesting bees have been difficult to manage commercially as pollinators. There is not enough information available to determine whether reintroducing native twig-nesting bees into restored habitats would allow establishment or whether ground-nesting bees can be similarly reintroduced and established.

Agricultural Landscape Management
Floral Resources

Many of the options for increasing the diversity and abundance of floral resources on farms to accommodate the needs of a diverse pollinator

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

community (Batra, 2001; Bugg et al., 1998; Matheson, 1994; Pywell et al., 2006; Vaughan et al., 2004) do not necessarily reduce farming productivity, and they can even improve productivity by providing additional benefits beyond pollination services, such as nectar for natural enemies of crop pests (Pickett and Bugg, 1998). The options are listed here in order from the least expensive, most easily implemented to larger scale, more costly, or more complex changes.

  • Growing polycultures rather than monocultures in a field results in a more diverse set of floral resources. Including flowers that bloom at different times of the year provides for and attracts a greater number of pollinator species, including those with long flight seasons.

  • Tolerating weeds along crop borders can provide a diverse and abundant set of floral resources, at no cost to the farmer.

  • Insectary strips planted within crop fields or in field margins and in buffer strips provide abundant pollen and nectar sources and attract bees to the fields (Altieri and Nicholls, 2004; Carvell et al., 2004; Pywell et al., 2005).

  • Planting cover crops on resting fields or as orchard understory and allowing cover crops (such as clovers) to bloom before plowing them under provides “green manure” that benefits both pollinators and soil fertility.

  • Planting wildflower mixes in fallow or old fields or allowing weeds to colonize creates meadows that support pollinators as the fields rest.

  • Planting permanent hedgerows of native perennial forbs and shrubs provides nest sites and preferred pollen and nectar sources for a diverse community of pollinators in the spaces between fields. Such hedgerows may also serve as wind-breaks or provide erosion control.

  • Restoring natural habitat patches on farms in permanent set-asides—focusing on areas that are more difficult to farm, such as edges of ditches, ponds and riparian areas, on hills, or around utility poles—can provide undisturbed habitat for pollinators.

Some governments, particularly in Europe (Box 6-3), have developed extensive monetary incentives to encourage environmental stewardship by farmers and ranchers. They include promotion of fallow and set-aside programs, as well as the planting of annual or perennial wildflowers for forage for pollinators along field margins and between fields. Among the specialty seed mixes of local wildflower species that have been developed in Europe is the “Tübingen mix,” which is in wide use in Europe (Engels et al., 1994; Matheson, 1994). Monitoring programs have demonstrated that integrating low-cost pollen and nectar sources into field borders provides measurable improvements in abundance and richness of several pollinator groups (Carvell et al., 2004; Pywell et al., 2004, 2006). The increase in land-

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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BOX 6-3

EU Biodiversity Conservation in Agriculture

The European Union (EU) and most of its member states have set a target of halting the loss of biodiversity by 2010. Detailed EU-sponsored research programs provide the scientific rationale for policy development (including the ALARM program [Chapter 5] and the SAFFIE project). Several policy instruments are available (agri-environment schemes and biodiversity action plans) to provide incentives for implementation.


Science Programs

ALARM, or Assessing Large Scale Environmental Risks to Biodiversity with Tested Methods is funded by the European Commission under Framework 6. The overall research program has several aims:

  • Quantify distribution shifts in key pollinator groups across Europe.

  • Measure biodiversity and assess economic risks associated with the loss of pollination services in agricultural and natural systems through the development of standardized tools and protocols.

  • Determine the relative individual and combined importance of drivers of pollinator loss (land use, climate change, fertilizer and pesticide contamination, invasive species, socioeconomic factors).

  • Develop predictive models for pollinator loss and consequent risks (Settele et al., 2005). ALARM has 54 EU partner institutions working in a 5-year, 22-million-euro project (2004–2009; Box 2-3, Box 5-1).

SAFFIE, Sustainable Arable Farming for an Improved Environment, is a United Kingdom research program designed to sustain the management of arable farms to support more wildlife. Its main objectives concern testing methods for enhancing farmland biodiversity. Farmers are encouraged to use alternative approaches to habitat management of crop and field margins as a way to develop more sustainable farming. The project has 20 partners and £3.5 million in funding over 5 years (2002–2007).


Policy Instruments

Agri-environmental schemes provide programs that encourage EU farmers to carry out environmentally beneficial activities on their land. The aim is to enhance biological diversity in a range of plant and animal groups, including pollinators. Farmers recover the cost of supplying environmental services through government payments. Examples of activities include the following:

  • Reversion of intensively used land to biologically diverse but less profitable extensively farmed land

  • Reductions in nutrient use

  • Reduction or cessation of pesticide use (such as through organic farming)

  • Creation of nature zones from lands removed from production

  • Continuation of traditional environmental land management in zones liable to neglect

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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  • Maintenance of landscape features that are no longer agriculturally viable

The programs are managed by regional or national authorities under a decentralized system, subject to approval by the European Commission. The costs are partly financed by the EU. Fifteen member states are operating agrienvironmental programs that cover 900,000 farms and 27 million hectares, about 20 percent of EU farmland (for information on EU, visit http://www.europa.eu.int/comm/agriculture/envir/index_en.htm; for information on its member states visit http://www.europa.eu.int/comm/agriculture/rur/countries/index_en.htm).

The agri-environment schemes in various European countries have yielded mixed, taxon-specific results. Although positive results have not been demonstrated for all taxa studied (Kleijn et al., 2001, 2004), some pollinator groups, notably bees, butterflies, and flower flies, appear to benefit in some cases (Carvell et al., 2004; Kleijn et al., 2001, 2004, 2006; Pywell et al., 2005, 2006; Weibull et al., 2003). Scientific monitoring of the schemes, particularly before-after control-impact monitoring (Potts et al., 2006), is critical to assessments of effectiveness, and much can be learned and applied from the work in Europe.

The United Kingdom operates four optional agri-environmental schemes (U.K. Department of Environment, Food and Rural Affairs, 2002) that pay farmers to practice environmentally friendly farming. The Countryside Stewardship Scheme aims to conserve, enhance, and restore target landscapes. The Organic Farming Scheme facilitates the shift from conventional to organic farming. The Environmentally Sensitive Areas Scheme covers 22 specific areas of national environmental significance.

The Entry Level Agri-Environment Scheme is a new program that is expected to include more than 70 percent of British farms. The intention is to promote simple, effective environmental management to enhance farmland biodiversity across a range of plant and animal groups, decrease diffuse pollution, maintain landscape structure, and conserve the historic environment. The program has several areas that promote pollinator biodiversity according to replicated field experiments (Carvell et al., 2004; Pywell et al., 2004, 2006):

  • Field margins sown with buffer strips provide forage (nectar and pollen) and nesting resources for pollinators and safeguard boundary habitats against chemical sprays.

  • Grasslands sown with mixes that include pollen-and-nectar flowers can increase the diversity, abundance, and availability of forage resources, and increases bumble bee diversity and abundance.

  • Careful management of hedgerows can create and protect habitats suitable for pollinators.

  • Permanent grasslands can be established with very low input to provide long-term pollinator habitat.

SOURCE: http://www.defra.gov.uk/erdp/schemes/default.htm#land.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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scape heterogeneity that accompanies these methods also can be beneficial (Tscharntke et al., 2005).

The U.S. federal government also offers incentives through the Farm Bill, which is administered by USDA’s Natural Resource Conservation Service (NRCS). This agency maintains and offers state lists of approved or suggested plants for revegetation (in buffer strips or for hillside erosion control) or for seeding rangelands for cattle production (http://www.nrcs.usda.gov/; http://plants.usda.gov/). Some recommended species are potentially invasive exotic plants or are grasses that provide little or no floral reward for bees and other pollinators. In some cases, most notably the new Conservation Security Programs, pollinator-friendly plants are specifically recommended (USDA-NRCS, 2004, 2006a,b). More work is needed to develop appropriate lists of plants that support pollinators and to customize those lists for different ecoregions within North America.

Nesting Substrates

Methods also are available for providing or protecting nest sites and substrates for bee species in the agricultural landscape (Matheson, 1994; Vaughan et al., 2004); many of them do not interfere with farming. They range from simple, low-cost measures to more complex and expensive methods:

  • Management of irrigation to preserve ground-nesting bees. By using drip or spray irrigation instead of flooding, farmers can avoid drowning ground-nesting bees and larvae. Interference with foraging and nest cell provisioning can be avoided by irrigating at night.

  • Management of tillage to protect existing bees’ nests. By shallower tilling or using no-till agriculture, disturbance of nest sites can be avoided. The density of squash bees (Peponapis pruinosa) on squash and pumpkin farms in the eastern United States that practice no-till agriculture is three times that of tilled farms (Shuler et al., 2005).

  • Active land management to provide nesting sites for bees. Examples include creating patches of bare ground for ground-nesting bees within perennial plantings, such as hedgerows, or mowing or weeding within pastures; leaving dead wood and standing snags, drilling holes in dead wood, and putting out trap nests for twig-nesting bees; providing a sand-loam mix for ground-nesting bees; putting out bumble bee nest boxes, buried or above ground; and creating specialized conditions for nesting aggregations (Box 6-2). More research is needed to determine which active management techniques are most effective for pollinator conservation and to adapt them for different bee faunas and site conditions.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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Alternatives to Chemical Pest Controls

According to the National Center for Food and Agricultural Policy, more than 68 active ingredients are used to control insect pests on North American farms (http://www.ncfap.org/whatwedo/index.php). Insecticides are differentially toxic to nontarget species, depending on the active ingredients, the strength and composition of the formulation (dust, powder, liquid), and the behavioral and physiological response of the target insect (Johansen, 1977; Johansen and Mayer, 1990). Some pollinator species might not be killed outright by pesticide applications, but they could suffer sublethal effects, including reduced foraging ability, that ultimately hamper their productivity (Morandin et al., 2005; Vaughan et al., 2004).

Short of eliminating insecticide use altogether, growers can reduce risks to pollinators from pesticides in several ways (Johansen and Mayer, 1990; NAPPC, 2006; Vaughan et al., 2004):

  • Choose appropriate pesticides. Some insecticides have active ingredients that are less likely to cause mortality or sublethal effects in bees, to have formulations that are less toxic to bees (for example, granular powders are less noxious than dust; Johansen and Mayer, 1990), and to break down more rapidly than others do. Microencapsulated formulations should be avoided because they mimic pollen.

  • Apply pesticides selectively. Growers can avoid using insecticides during a crop’s bloom period, apply them at night while bees are in nests, and apply them on the ground rather than in aerial spray.

  • Convert some or all fields to organic production. Growers thus provide areas that are refuges from pesticides (Vaughan et al., 2004).

Grassland and Grazed Land Management

Natural grasslands (prairies) are now considered the rarest North American biome; more than 90 percent of the continent’s grassland area is now in agricultural use, and 14 of the 16 temperate grassland, savanna, or scrub ecoregions in North America are classified as either critical or endangered (Ricketts et al., 1999). The loss involves more than grasses; annual wildflowers and perennial plants are important vegetative components of grassland biomes. Flowering plant, arthropod, and vertebrate biodiversity is often higher in grasslands than in other North American biomes (Butaye et. al., 2005; WallisdeVries et al., 2002).

Management of prairies and grazed lands includes mowing, grazing, or prescribed burns that can either harm or benefit pollinators (Carvell, 2002; Potts et al., 2003; Rathcke and Jules, 1993; Smallidge and Leopold,

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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1997). Plans for managing pollinator populations and communities also must consider the effects of invasive species on the composition of natural vegetation. Specific practices that provide nest sites for pollinator species might need to be considered for management protocols.

Mowing or Grazing

Many butterflies and other insects depend on habitats in early succession (Smallidge and Leopold, 1997), and mowing or grazing can be essential to maintaining the early successional habitat types as patches within the landscape, particularly if organisms that formerly grazed there (such as bison) are now missing. In habitats where fire is the natural agent of disturbance, mowing or grazing can be more beneficial to the maintenance of pollinator habitats, particularly if habitat patches are small and isolated. In particular, pollinator species of interest suffer some larval or adult mortality from fire (Smallidge and Leopold, 1997). Mowing at the appropriate time (August) is also a good method for maintaining early successional patches for the endangered Karner blue butterfly (Lycaeides melissa samuelis) and its lupine host plant (Lupinus perennis) within sandy pine barrens, pine-oak savannahs, and oak savannahs in the Great Lakes region. Mowing also allows new patches to be localized within the dispersal limits of the butterfly, permitting colonization from nearby occupied patches of lupine (Smallidge and Leopold, 1997). Recent grazing was linked to increased bumble bee richness and abundance in calcareous grasslands. In the United Kingdom, grazing probably contributed to bumble bee abundance by enhancing diversity and the abundance of forage plants preferred by bumble bees and by reducing vegetation height, canopy closure, and moss litter (Carvell, 2002).

Burning

Fire can cause mortality in pollinators that nest above ground (larval and pupal lepidopterans and twig-nesting bees) and lead to reductions in abundance (Smallidge and Leopold, 1997). Most ground-nesting bees, however, nest at a soil depth of more than 5 cm (Michener, 2000), so the soil could insulate the nests and reduce or eliminate mortality from wildfires or prescribed burns.

In fire-adapted communities, many plants require fire or heat to open fruits or scarify seeds (for example, Givnish, 1986). Such communities often respond to a fire with abundant new growth, including annual wildflowers, plants from bulbs, or regenerated sprouts. The flush of postfire vegetation often produces an equally dramatic spike in nectar and pollen for local pollinators; one example is fireweed, Epilobium spp., in the eastern United States (Heinrich, 2004).

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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From a site in Israel that had high pollinator biodiversity (Mt. Carmel), Potts and colleagues (2003) discovered that fire initially was catastrophic to plant and bee communities, but that recovery was rapid. Within 2 years of the fires there was a peak in plant and bee diversity that was followed by a long and steady decline over the next 50 years. They reported that bee pollinator communities closely matched the plant community in recovery and regeneration (Potts et al., 2001, 2003).

Like mowing and grazing, fire is an important management tool that can be used to reset the successional sequence and maintain the diverse and heterogeneous mosaic landscapes that include early successional stages (oldfields) and late primary stages (climax forests). Resetting the successional sequence provides resources for a wider array of species (Pickett and White, 1985; Smallidge and Leopold, 1997). More information is needed on the short- and long-term effects of fire—and its use as a management technique—on diverse North American plant and pollinator communities.

Nesting Habitat

Although solid expanses of grasses and forbs are not productive nesting habitats for bees, they do provide nest sites (larval host plants) for a variety of Lepidoptera. Thus, grassland management protocols that are well adapted for Lepidoptera also should consider provisions for bee-nesting sites. Nesting sites can be provided by creating patches of bare ground or sand-loam mixes for ground-nesting bees; by maintaining a landscape mosaic of wooded and grassy areas, protecting some dead wood and standing snags and drilling holes in some dead wood; putting out trap nests for twig-nesting bees; and putting out bumble bee nest boxes, buried or above ground (Box 6-4). Large-scale herbicide applications, such as are applied in the southwestern United States to remove undesirable scrub and brush (mesquite and Prosopis plants), should be discouraged because they remove not only nesting sites and refuges, but also pollen and nectar sources for native bees, honey bees, and other pollinators (Buchmann and Nabhan, 1996).

MAINTAINING POLLINATION SERVICES

Maintaining commercial pollinator stocks and the diversity of wild pollinator communities differs from maintaining pollination services provided by pollinators, because pollination services could be enhanced without an increase in pollinators. This section presents strategies for maintaining pollination services to crops by commercial pollinators and pollination services to crops and wild native plant populations by wild pollinators.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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BOX 6-4

Golf Courses and Other Urban and Suburban Green Spaces

Traditionally, golf courses have been inhospitable areas for pollinating birds, bats, and insects because of the large amounts of fertilizers, herbicides, and pesticides used and their close-cropped mowing. The U.S. Golf Association has adopted pollinator-friendly practices (Shepherd, 2002; Shepherd and Tepedino, 2000; Shepherd et al., 2001) for out-of-play areas (roughs), where wildflowers are planted, nesting domiciles (drilled bee boards) are provided, and few or no pesticides and herbicides are applied. Some golf courses have combined to form an association of organic golf courses (http://www.usga.org/turf/green_section_record/2005/jan_feb/Inorganic.html; http://www.epa.gov/oppbppd1/PESP/strategies/2005/ogmd05.htm).

Similar techniques could be applied in urban parks and greenbelts, on large corporate campuses, and at a smaller scale in home gardens, to improve habitat for pollinators in urban and suburban areas. The abundant floral resources in backyard gardens in some urban areas already support diverse communities of bees and nest sites for twig-nesters in wooden fences or houses (Cane et al., 2006; Frankie et al., 2005).

Commercial Pollinators

Crops that require or are improved by animal pollination benefit from the services of commercially managed honey bees or other commercially managed bees. The supply of commercial honey bee colonies can be stabilized by reducing bees’ vulnerability to pests, parasites, pathogens, and pesticides. If honey bee colonies are in short supply, a new and potentially useful compensation is to increase the available colonies’ efficiency. Honey bee brood pheromones have been identified that temporarily increase the proportion of a colony’s foragers that collect pollen (Pankiw, 2004). Hormone manipulations also can advance the age at which bees switch from working in the hive to foraging and increase the proportion of a colony’s foraging-worker force (Robinson and Ratnieks, 1987). These pheromones and hormones could be developed into slow-release stimulants to increase a colony’s pollinator force in a grower’s field, although possible negative effects on bee hives also should be explored. The supply of alternative commercial pollinators requires caution to reduce losses to pathogens and parasites, as happened to the alfalfa leafcutter bee, for example. Intensified research and technology transfer will be required for development of new species of alternative pollinators.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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Commercially managed pollinators can be brought to the crops that need their services, ensuring service delivery. Thus, growers of commodities that require pollination follow recommendations for pollinator stocking. For example, hybrid sunflower production requires two colonies of honey bees per hectare (Delaplane and Mayer, 2000). Although some improvements could be made to maximize benefits by altering the spacing of colonies in fields and the spacing of self-incompatible cultivars (Chapter 4), in general, the great advantage of using commercially managed pollinators is that service delivery can be controlled, or at least manipulated, by relative placement of pollinators and cultivars.

Wild Pollinators

It is far more difficult to ensure that services from wild pollinators are delivered to crops. Because the mechanisms are not still well understood, managing wild pollinators requires a better understanding of foraging ecology and population biology and how they are influenced by landscape properties (Kremen and Ostfeld, 2005). The few existing studies, however, suggest that healthy (diverse and abundant) pollinator communities could provide enhanced pollination services for a wider array of crops, and ensure stability of services within seasons and across years (Klein et al., 2003; Kremen and Chaplin, in press; Kremen et al., 2002a).

Because pollinators are mobile and they collect resources within the foraging range of a nest, roost, or territory (for example, hummingbirds), environmental qualities of the immediate site (local) and the surrounding area (landscape) affect their population sizes, densities, and persistence. Many pollinator species use a variety of floral and nesting resources that can be distributed across different habitat types at different times of the year (Westrich, 1996). Some pollinators use native plant resources that occur only in natural habitats in season, and weedy resources that occur in agricultural habitats in the summer (Kremen et al., 2002a). Mass-flowering resources provided by crops can also be important for selected species in a landscape (Westphal et al., 2003).

Evidence suggests that the character of a landscape is important in determining the richness, abundance, and composition of pollinator communities on farms. Pollinator species differ in their ability to provide services to different crops (for example, Free, 1993; Kremen et al., 2002b), and their effectiveness could vary with the community context in which they exist (Greenleaf and Kremen, 2006b; Thomson and Goodell, 2001; Thomson and Thomson, 1992). Therefore, alterations in the composition of pollinator communities due to landscape change influence both the quantity and quality of pollination services to crops—although local site characteristics also influence pollinator communities and services (reviewed in Kremen

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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and Chaplin, in press). In California’s Mediterranean climate environment, landscape factors (the proximity or proportional area of natural habitat within a site) are the dominant factors for pollinator richness, composition, abundance, and services (Greenleaf, 2005; Greenleaf and Kremen, 2006b; Kremen et al., 2002b, 2004), although site characteristics (conventional or organic management of farm sites) modulate these responses at the population level (Kim et al., 2006; Williams and Kremen, in press). In tropical rainforest biomes of Central and South America and Indonesia and in temperate grassland biomes in Germany and Canada, pollinator richness, abundance, and services also respond primarily to proximity to natural or seminatural habitat at the landscape level (Chacoff and Aizen, 2006; Klein et al., 2002, 2003a; Morandin and Winston, 2005; Ricketts, 2004; Ricketts et al., 2004; Steffan-Dewenter and Tscharntke, 1999; Steffan-Dewenter et al., 2001, 2002), but local factors, such as light (Klein et al., 2002, 2003b) and the abundance and richness of weedy floral resources (Morandin and Winston, 2005), also have statistically significant effects.

Pollination services for wild plants that depend on or benefit from animal pollination are generally provided exclusively by wild pollinator populations, although managed honey bees often forage on wild plants and, thus, provide some services (Kremen et al., 2002). Managing wild pollinator communities is needed to ensure pollination function for natural plant communities. Pollination services to wild plants in habitat fragments can be influenced by the size and isolation of the fragment, the characteristics of the surrounding human-modified matrix, and the resulting population responses of plants and pollinators (Bronstein, 1995; Ghazoul, 2005c). Small fragments tend to have small plant populations (MacArthur and Wilson, 1967), which can be less attractive to pollinators (Brody and Mitchell, 1997; also reviewed in Kunin, 1997), and thus become pollinator limited (Box 4-1; Groom, 2001). Smaller fragments often also contain smaller populations and fewer pollinator species (MacArthur and Wilson, 1967; Miller et al., 1995; Ricketts, 2001; Steffan-Dewenter, 2003) thus reducing pollinator visitation (Aizen and Feinsinger, 1994; Cresswell and Osborne, 2004). Empirical studies, however, have revealed positive, negative, and neutral effects of fragment size on pollinator abundance, richness, and services (Aizen and Feinsinger, 1994; Cane et al., 2006; Danielsen et al., 2005; Miller et al., 1995; Tonhasca et al., 2002; Winfree et al., 2006). The variability in response is probably attributable to differences in habitat specificity and dispersal ability among pollinator species (Law and Lean, 1999; Saville et al., 1997; Steffan-Dewenter, 2003).

Geographic isolation also can affect pollination services to wild plants (Ghazoul, 2005c). Plant populations in isolated fragments could be self-limited by the amount of compatible pollen available (Duncan et al., 2004). Isolated fragments contain smaller populations and fewer pollinator and

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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plant species (MacArthur and Wilson, 1967) thus reducing pollinator visitation and fruit set (Cunningham, 2000; Steffan-Dewenter and Tscharntke, 1999). Corridors that link habitat fragments have been shown to increase movement of selected pollinator species and enhance pollination of target plants (Tewksbury et al., 2002; Townsend and Levey, 2005). Isolation also can reduce pollinator visitation and seed set (Jennersten, 1988), but in some cases, even highly isolated plants are known to receive sufficient out-crossed pollen to reproduce (Nason and Hamrick, 1997; Schulke and Waser, 2001; White et al., 2002).

All of the fragment-specific factors are likely to be modulated by the type of human-dominated matrix that surrounds natural fragments (Ricketts, 2001). If the surrounding matrix is hospitable to wild plants (Mayfield and Daily, 2005) or contains nesting or floral resources for some pollinator species (Klein et al., 2002; Westphal et al., 2003), the effects of fragment size and isolation can be alleviated. Relatively few studies of pollinator communities and pollination function in fragmented landscapes consider matrix effects (Cane et al., 2006; Dauber et al., 2003; Hirsch et al., 2003; Steffan-Dewenter et al., 2006; Williams and Kremen, in press; Winfree et al., 2006).

Clearly, managing landscapes and sites will be important for restoring, preserving, or maintaining diverse pollinator communities and ecological service functions to crops and wild plants. How much natural habitat is sufficient in the landscape for pollinator maintenance is an open question. Kremen and colleagues (2004) observed a log-linear relationship between the amount of pollination services provided to a watermelon crop and the proportional area of natural habitat within several kilometers of a farm. Full pollination services could be provided by wild bee communities at 30 percent or more natural habitat cover. Morandin and Winston (2006) determined that removing 30 percent of land from canola seed production would actually increase profits to canola farmers, because of the resulting increased diversity, abundance, and services provided by wild bees. Ricketts and colleagues (2004) suggested that fragments of at least 20 hectares of tropical rainforest provide valuable services to coffee from wild bees that are comparable to other land use values. Equivalent studies of native plants in natural habitat fragments are lacking.

How patches of habitat should be configured to deliver pollination services into the surrounding agricultural matrix (in the case of crops) or to maintain gene flow and population persistence for isolated populations of wild plants that are confined to fragments also is poorly understood. If wild pollinators in an area indeed depend on natural habitat fragments for nesting sites and critical floral resources, then crop pollination can benefit from a “service halo” around the habitat fragment corresponding to the foraging ranges of individual pollinator species (Ricketts, 2001; Ricketts

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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et al., 2004). Dispersing small fragments extensively throughout an area seems logical but leaves open the question of how to configure large parcels to allow pollinator populations to persist. Both metapopulation theory (reviewed in Hanski and Ovaskainen, 2000; Harrison and Fahrig, 1995) and empirical data (Harrison et al., 1988) suggest that some larger patches are needed to support larger sized populations that are more resistant to extinction (see also Berger, 1990; Zayed and Packer, 2005). Larger areas also will, in theory, contain more diverse assemblages of pollinators (MacArthur and Wilson, 1967; Simberloff and Wilson, 1969) that might provide more services, more consistently, and contribute to pollination of a wider variety of crops (Kremen and Chaplin, in press) and other plants (Memmott, 1999; Memmott et al., 2004). More research is needed to determine the optimal configuration of landscape fragments and their connectedness to maintain pollinator populations, communities, and functions.

PUBLIC POLICY AND POLLINATOR POPULATIONS

U.S. Endangered Species Act

The Endangered Species Act (ESA) of 1973 is the broadest and most powerful U.S. law for the protection of endangered species and their habitats (NRC, 1995). The act lists species of plants and animals (vertebrate and invertebrate) as endangered or threatened according to assessments of their risk of extinction (Congressional Research Service [CRS], 2006). Once a species is listed, ESA’s strict substantive provisions become legal tools to assist in the species’ recovery and the protection of its habitat. Endangered species and their critical habitats are entitled to strong protections. It is illegal, for example, to take any endangered species in the United States or its territorial waters, and any federal action that will jeopardize the future of an endangered species is prohibited, including any action that threatens to destroy or damage critical habitat. At press time for this volume, in the fall of 2006, 1879 U.S. and foreign animals and plants were listed as endangered or threatened (U.S. Fish and Wildlife Service [USFWS], 2006).

ESA’s major goals include the recovery of endangered and threatened species to the point at which protection is no longer needed. As this volume went to press, USFWS (2006) had cataloged 17 U.S. and foreign species that had been recovered and removed from the list. The populations of other listed species have increased, and some appear to have stabilized even though they remain on the list.

A species is placed on the Endangered Species List on the initiative of the secretary of the interior or of the secretary of commerce. The decision is based on the best available scientific and commercial information and a

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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lengthy procedure that ensures public participation and the collection of relevant information. Because Congress directed that listing have a scientific foundation for the label of threatened or endangered, economic factors are not considered in the listing decision. In June 2006, there were 282 “candidate” species for which no decision had been made. The status of those species is to be monitored and, if any emergency poses a significant risk to their continued existence, they must be listed promptly.

Modifications of ESA and other recently proposed changes could make it more difficult to list pollinators than some other animals. A 1981 congressional revision specifically exempted any “species of the Class Insecta determined by the Secretary to constitute a pest whose protection under the provisions of this Act would present an overwhelming and overriding risk to man.” Any species that has caused economic damage or could do so is less likely to be protected. The larvae of some lepidopteran pollinators, for example, and the adults of some hymenopteran pollinators can under some circumstances cause economic damage. Securing endangered status for them could prove problematic.

Recent efforts to amend ESA also could add new barriers to listing pollinators. H.R. 3824, passed by the House of Representatives in 2005, “To amend and reauthorize the Endangered Species Act of 1973 to provide greater results conserving and recovering listed species, and for other purposes” replaces the criterion of “best scientific and commercial data available” with “best available scientific data.” More important, unlike ESA itself, H.R. 3824 for the first time defines “best available scientific data” as “scientific data, regardless of source, that are available to the Secretary at the time of a decision or action for which such data are required by this Act and that the Secretary determines are the most accurate, reliable, and relevant for use in that decision or action.” The secretary is directed to issue regulations that establish criteria for “best available scientific data” and must ensure that the information consists of empirical data or data found in sources that have been subjected to peer review by people recognized by the National Academy of Sciences [NAS] as qualified to independently review a covered action in a manner that is in compliance with the Data Quality Act (44 USC 3516) (Congressional Research Service, 2006). According to CRS, “Some contend that the specification of empirical data in H.R. 3824 would exclude estimates derived from models and limit the type of data available for use…. However, estimates derived from modeling could be allowed under H.R. 3824, if it meets the NAS peer-review conditions set forth in the bill.” Because of the paucity of data available for many pollinator species (Chapter 2), assessments of species status often are based on information derived from population models or from genetic studies, which could be excluded if ESA is amended as proposed.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

Incentives for Stewardship

The public benefits provided by wild pollinators justify public policy that encourages stewardship of wild pollinators. Given the importance of habitat, land stewardship policies constitute the majority of relevant mechanisms. However, consumer-oriented measures also have a role in pollinator conservation policies.

For agricultural lands, there are four voluntary programs that can be used or adapted to create or maintain pollinator habitat. The Farm Security and Rural Investment Act of 2002 (the Farm Bill) authorized them:

  • The Wildlife Habitat Incentives Program (WHIP) (NRCS, 2006a) provides cost sharing and incentive payments to eligible farmers for planting native and nonnative plants that could enhance wildlife habitat (including pollinators) through early successional habitat development, riparian herbaceous cover, tree and shrub establishment, and upland and wetland habitat management.

  • The Environmental Quality Incentives Program (EQIP) (NRCS, 2006b) also provides money to eligible farmers who focus on soil and water conservation. The program can be customized to include pollinator habitat through improvements in hedgerows, riparian buffer strips, tree and shrub planting, and wildlife habitat management.

  • The Conservation Reserve Program (CRP) (USDA-FSA, 2006) pays eligible farmers to convert agricultural land to conservation uses under a 10-year contract. Farmers make bids that describe their land management plans and the annual payments they would require. The Farm Service Agency (FSA) evaluates the bids in light of technical advice from NRCS. The evaluation is based on state priorities, and points are awarded for expected conservation benefits from plans that include native species, especially flowering shrubs and forbs. Currently, no points are assigned explicitly for pollinator habitat.

  • The Conservation Stewardship Program (CSP) (NRCS, 2006c) awards 10-year contracts to eligible farmers according to the farmers’ proposed intensity of stewardship and their proposed practices. CSP payments for pollinator habitat are available as “resource enhancements” under the rubric of “wildlife habitat management.” In 2005, North Dakota’s state NRCS program covered pollinator habitat under three CSP enhancements involving native herbaceous cover plots, unharvested tame hay land, and native woody cover plots (NRCS-North Dakota, 2005).

WHIP, EQIP, and CRP are available to farmers who have traditionally raised wheat and feed grains eligible for federal price supports. Because of the tightening federal budget, access to payments for conservation practices is rationed through priorities established by state technical committees and

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

according to the characteristics of individual conservation plans submitted by landowners. Many states do not assign points for enhancing pollinator habitat or provide guidance for doing so. In the states that do provide points, such as Michigan, very few landowners had enrolled as of December 2005. CSP eligibility is further restricted to farmers in a limited number of watersheds in each state on a list that rotates annually, with the goal of making each watershed eligible every 7–10 years (A. Herceg, NRCS, personal communication, December 2005).

Although the four U.S. farm environmental stewardship programs provide a sound vessel for encouraging landowners to enhance pollinator habitat, interest among farmers has been limited. The research base for NRCS to estimate the on-farm and external conservation benefits from pollinator habitat also is limited. Development of a national monitoring program for pollinator species would provide a remedy (Chapter 5).

For nonfarm, private landowners—homeowners, public utilities, or businesses—investments in pollinator habitat could be encouraged through income tax deductions. Public agencies involved in land management, such as the U.S. Forest Service, the U.S. Department of the Interior, and the U.S. Department of Transportation, could include provisions for pollinator protection or enhancement in their guidelines. The inclusion of pollinator protection in the criteria for federal land leases for grazing and timber harvest also could encompass large areas of land. Some interstate highways already have wildflower plantings, which could be enhanced by purposeful selection of appropriate native plant species favored by wild pollinators.

Volunteer networks also could encourage creation or restoration of pollinator habitats much as they have done for pollinator monitoring in the Audubon Society’s annual Christmas Bird Counts and the North American Butterfly Association’s Fourth of July counts (Chapters 2 and 5). Monarch Watch’s Monarch Waystation program has already resulted in the creation and registration of more than 600 butterfly-friendly habitats with nectar resources and host plants. A private, nongovernmental organization interested in pollinators might establish a “friends of pollinators” network that could be diffused through school programs and public service announcements. Interest could be sparked through activities such as a landscape architecture competition for designs that invite and support pollinator populations.

Even consumers can engage in pollinator protection. Following the successful ecolabeling campaigns for dolphin-safe tuna and shade-grown coffee, a label could be used to certify pollinator-safe fruits and vegetables. With the important exception of the USDA organic label, most food certification labelling is done by private organizations. Currently, however, there are no known organizations that are both interested in and capable of developing and providing certification for a pollinator-protector label.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
×

ADAPTIVE MANAGEMENT AND POLLINATOR MONITORING

Different management strategies can be used across landscapes, including public and private lands, working lands, and natural areas, to improve conditions for pollinators and to maintain pollination function in crops and wild plants. Strategies range from site-specific management that could be performed by private landowners, to landscape and regional actions that would require coordination by county, state, or regional authorities and nongovernmental organizations. Although management actions can be guided by a body of existing scientific knowledge, all are experimental; therefore, concurrent monitoring of pollinator status and of pollination function is needed (Chapter 5) to determine the efficacy of different strategies and to adapt measures to provide even better performance (Kremen et al., 1993; Margoluis and Salafsky, 1998; Walters and Holling, 1990).

CONCLUSIONS

This chapter presents various actions that could be taken to maintain commercial pollinators, wild pollinator species and communities, and pollination function. The committee suggests the following as priorities.


Apis

  • Develop and refine both traditional and molecular methods for identifying bees with economically desirable traits for inclusion in honey bee breeding programs.

  • Select model populations of honey bees with economically desirable traits for adoption by the beekeeping industry.

  • Develop educational materials and programs to enable private-sector queen producers to develop and maintain pest, parasite, and pathogen resistant stocks of honey bees and to serve as reliable sources of quality production queens that produce colonies expressing useful levels of economically important traits.

  • Develop sustainable methods for ensuring that Africanized bees do not degrade the commercial value of existing stocks of honey bees.

  • Develop resistance management programs to mitigate the adverse effects of pesticide and antibiotic resistance in honey bee pest, parasite, and pathogen populations.

  • Develop methods for the preservation of honey bee germplasm.


Other Commercial Species


  • Identify commercially viable solutions to the problem of chalkbrood in the alfalfa leafcutter bee, Megachile rotundata.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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  • Identify non-Apis bees with the potential to be developed into economically useful pollinators.

  • Develop commercially viable methods for culturing economically important species of bumble bees and solitary bees for use as crop pollinators.

Wild Bees
  • Inform the public—in particular, the agricultural community and managers of golf courses, urban parks, and other large urban-suburban areas such as industrial and academic campuses—about current knowledge of actions (such as creating pollinator habitat) that can be taken to manage pollinators.

  • Conduct field studies in different regions of North America to determine the suites of key floral resources for use in restoration protocols in each region.

  • Conduct additional studies that can be used to improve existing restoration protocols, including monitoring the influence of restoration activities on population and community dynamics of pollinators and understanding land managers’ willingness to adopt restoration practices.

  • Define land-management practices (by NRCS state offices) that encourage pollinator populations that are eligible for federal payments under existing Farm Bill conservation programs such as EQIP, WHIP, CRP, and CSP.

  • Integrate land management practices that encourage pollinator populations at the state level into existing Farm Bill conservation programs such as EQIP, WHIP, CRP, and CSP.

  • Conserve existing natural habitats in human-dominated landscapes.

Suggested Citation:"6 Strategies for Maintaining Pollinators and Pollination Services." National Research Council. 2007. Status of Pollinators in North America. Washington, DC: The National Academies Press. doi: 10.17226/11761.
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Pollinators--insects, birds, bats, and other animals that carry pollen from the male to the female parts of flowers for plant reproduction--are an essential part of natural and agricultural ecosystems throughout North America. For example, most fruit, vegetable, and seed crops and some crops that provide fiber, drugs, and fuel depend on animals for pollination.

This report provides evidence for the decline of some pollinator species in North America, including America's most important managed pollinator, the honey bee, as well as some butterflies, bats, and hummingbirds. For most managed and wild pollinator species, however, population trends have not been assessed because populations have not been monitored over time. In addition, for wild species with demonstrated declines, it is often difficult to determine the causes or consequences of their decline. This report outlines priorities for research and monitoring that are needed to improve information on the status of pollinators and establishes a framework for conservation and restoration of pollinator species and communities.

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