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Genetically Modified Pest-Protected Plants: Science and Regulation (2000)

Chapter: 3. Crossroads of Science and Oversight

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Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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3

Crossroads of Science and Oversight

This chapter focuses on the scientific basis of the oversight of transgenic pest-protected plants. The committee recognizes that there is an urgency to solidify the regulatory framework for transgenic pest-protected plant products because of the potential diversity of novel traits that could be introduced by transgenic methods and because of the rapid rate of adoption of and public controv1ersy regarding transgenic products.

For comparison with transgenic pest-protected plants, a case study concerning conventional pest-protected plants and a discussion of scientific issues surrounding them are presented. Then case studies of transgenic pest-protected plants are discussed; these case studies provide examples of scientific review of transgenic pest-protected plants by federal agencies.

After the case studies, EPA's proposed rule for the regulation of genetically modified pest-protected (GMPP) plant gene products as plant-pesticides is then evaluated in light of the discussion and other scientific criteria. Finally, the committee suggests guiding scientific principles for oversight of transgenic pest-protected plants and sets forth specific research needs.

3.1 CASE STUDIES OF PEST-PROTECTED CROPS AND THEIR OVERSIGHT

3.1.1 Conventional Breeding for Rust Resistance in Wheat

Three main rust diseases affect common wheat (Triticum aestivum L.) and durum wheat (T. durum Desf.): stem rust (or black rust), caused by

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

Puccinia graminis Pers. f. sp. tritici Eriks and Henn.; leaf rust (or brown rust), caused by P. recondita Rob. Ex Desm. f. sp. Tritici; and stripe rust (or yellow rust), caused by P. striiformis West. All three rust diseases are fungi which are obligate parasites in nature (that is, they require a living host to survive). They all need free moisture for infection, but they have different optimal environmental conditions for disease development, so they often do not damage wheat production concurrently in the same region (Knott 1989). For example, stem rust tends to require higher temperatures than leaf rust, which requires higher temperatures than stripe rust; hence, stem rust is usually more damaging in the northern Great Plains, leaf rust in the southern Great Plains and the East, and stripe rust in the West. Of the three diseases, stem rust can cause the more devastating epidemics; for example, in 1916, a stem rust epidemic was estimated to have reduced total US wheat production by 38% (Loegering 1967). But leaf rust is more common (Schafer 1987; table 3.1). Because wheat is used primarily as a food grain, losses in total production underestimate the true economic loss when wheat is damaged so severely that it must be sold as a feed grain.

Research to Reduce Losses Caused by Wheat Rusts

Losses due to rusts can be reduced by cultural practices, removal of an alternative host, chemical control, and genetic protection from the pathogens (also called host-plant resistance) (Knott 1989; Schafer 1987). The goal is to break the life cycle of the pathogen.

After the 1916 stem rust epidemic, a major barberry-eradication program was started in North America (Roelfs 1982). Roelfs suggested four benefits from the success of the program: disease onset was delayed by 10 days, initial inoculum was reduced, the number of pathogen races was reduced, and the pathogenic races of stem rust were stabilized. The reduction in the number of races and their stabilization were due to eliminating the sexual cycle of P. graminis. P. recondita also has alternative hosts, but none is known for P. striiformis (Schafer 1987). Chemical control of rust diseases with fungicides has been successful, but the cost of the fungicides, the economics of US wheat production, and concerns about chemicals in food grains have limited their use in the United States (Rowell 1985). Fungicides are widely used in Europe and the Pacific Northwest to control other wheat diseases.

By far the most common approach to the control of rust diseases in wheat is the use of conventionally bred resistant plants because it costs less than fungicide applications and there are numerous sources of genes for pest-protection (for example, McVey 1990; Cox et al. 1994). The most important aspect of breeding for rust-protection is that not only the genet-

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

TABLE 3.1 Wheat Yield Losses Due to Stem, Leaf, and Stripe Rust in United States, 1995-1998

   

Yield loss, % of harvested bushels

   

Common

Year

Disease

Winter

Spring

Durum

1995

Stem Rust

0.01

0.01

0.0

 

Leaf Rust

2.36

0.10

0.0

 

Stripe Rust

0.11

0.03

0.0

1996

Stem Rust

0.23

0.00

0.0

 

Leaf Rust

0.78

0.03

0.0

 

Stripe Rust

0.26

0.05

0.0

1997

Stem Rust

0.00

0.02

0.0

 

Leaf Rust

2.85

1.10

0.0

 

Stripe Rust

0.07

0.04

0.0

1998

Stem Rust

0.09

0.03

0.0

 

Leaf Rust

1.60

0.83

0.0

 

Stripe Rust

0.27

0.17

0.0

Average, 1995-1998

Stem Rust

0.08

0.01

0.0

 

Leaf Rust

1.90

0.52

0.0

 

Stripe Rust

0.18

0.07

0.0

Source: USDA (1999g).

ics of the host but also the genetics of the pathogen must be considered. Both are subject to change—the pathogen by mutation and sexual hybridization, the host by plant breeding. Because of changes in the pathogen, protective genes in the host are overcome by new virulence genes in the pathogen.

Kilpatrick (1975) used an international testing program to estimate that the average lifetime of a gene for protection from leaf, stem, or stripe rust was 5-6 years. The rapid loss of genetic pest-protection due to new virulence genes led researchers to look for new protective genes and for durable resistance (for example, Line 1995). R. Johnson (1984) has defined durable resistance as the “resistance that remains effective during prolonged and widespread use in an environment favorable to the disease.” Considering the definition, genes for durable resistance are identified only after they have been deployed in widely grown cultivars.

New genes for pest-protection are constantly being searched for in wheat and its wild relatives, and plant breeders try to create new combi-

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

nations of protective genes. Increasingly, new genes are being identified and transferred from wild relatives of wheat (for example, Cox et al. 1993 and 1994; Sharma and Gill 1983). Wild relatives of cultivated plants have coevolved with the crop pathogens and so are often extremely useful sources of protective genes (Leppik 1970; Wahl et al. 1984). Although many of the wild relatives of wheat are Triticum spp., many are more distant (McIntosh et al. 1995). For example, the protective genes Lr24 and Sr24 came from tall wheat grass, Thinopyrum ponticum (Podp.) (Barkw. & Dewey); and Lr26, Sr31, and Yr9 came from rye, Secale cereale L. Little is known about the biochemistry of genetically based rust-protection, so most breeding programs use phenotypic selection (the presence or absence of the disease) and some use molecular markers to track protective genes.

The main phases of any wheat-breeding program are introduction of genetic variation, inbreeding and selection of useful variants, and extensive field testing of selected variants to determine their agronomic or commercial worth (Baenziger and Peterson 1992). All the standard plant breeding methods are well documented (for example, Fehr 1987; Stoskopf et al. 1993), as are the methods specifically applied to breeding for rust-protection (Knott 1989; MacIntosh and Brown 1997). The most common breeding method for moving one or a few genes into an elite line or cultivar, especially when the genes are being transferred from a wild relative or an unadapted line, is backcrossing. It has been widely used to introduce protective genes into cultivated wheat whether those genes are derived from Triticum spp. or from more distant but sexually compatible relatives.

The Agricultural Result

As mentioned previously, the effect of rusts can be devastating when susceptible wheat cultivars are grown. However, estimating the value of crop resistance to rust accurately is difficult because the widespread growth of resistant cultivars affect the yield-loss estimates. A well-documented estimate (based on potential yield losses due to the disease-infecting susceptible cultivars) of the annual value of having stem rust resistance in wheat grown in western Canada was Can$217,000,000 (Green and Campbell 1979). The annual yield losses due to stem rust have averaged between 15% in Saskatchewan to 25% in Manitoba. in the United States, epidemics are localized, but the yield losses due to plant susceptibility to stem rust were as high as 56.5% in North Dakota and 51.6% in Minnesota in 1935 (Roelfs 1979). For leaf rust, the yield losses due to susceptibility were estimated at 50% in Georgia in 1972. For stripe rust, yield losses due to susceptibility were estimated at 30% in 1961 in Washington. The losses

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

should be viewed as high estimates because the weather conditions, pathogen, and host susceptibility were optimal. The average annual US losses are probably more similar to those estimated by Green and Campbell for Canada (1979). The three rusts combined reduce the annual US wheat crop by about 2% (table 3.1); most of the losses are caused by leaf rust. The low level of rust losses is attributable mainly to the use of resistant cultivars.

Health and Environmental Impacts

No formal assessment of the health or environmental impact of conventionally breeding wheat for resistance to rust has been undertaken by regulatory agencies, inasmuch as the products of conventional plant breeding have generally not required their oversight. Rust-resistant wheat cultivars, regardless of the source of their resistance genes, have been widely grown and consumed in food products with no history of causing health problems.

Little is known about the biochemical basis or gene products for plant protection against rust, but these genes are likely to be similar to other genes in the large class of race-specific resistance genes isolated from other plants. The presence of pest-resistance genes can affect end-use quality by affecting the grain protein content. For example, leaf rust detrimentally affects leaves reducing their potential for nitrogen remobilization to the grain and reducing grain protein content (Cox et al. 1997). Lower grain protein content is generally considered a detrimental effect in hard wheats but a beneficial effect in soft wheats (Finney et al. 1987). Stem rust tends to reduce the flow of photosynthate and nitrogen to the grain; but because nitrogen is mobilized early in grain development, the overall result of stem rust is generally an increase in protein content in the grain, possibly including shriveled kernels.

Environmentally, the use of rust-resistant wheat cultivars has reduced the use of fungicides, but the extent of this reduction is not well documented, because effective pest-protection has been widely deployed for many years and the economics of wheat production often preclude the widespread use of fungicides that are effective against rust.

3.1.2 Bt Crops

The most widely used transgenic pest-protected plants are cultivars that express insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt). Cotton and corn are protected from some of their lepidopteran pests by Bt proteins in the Cry1A and Cry9C groups. The po-

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

tato cultivars are protected against the Colorado potato beetle by a Cry3 Bt protein.

There is a tendency to consider Bt toxins as all biochemically similar, but the DNA sequence similarity among toxins can be less than 25% (Feitelson et al. 1992) and the biochemical properties of the more than 100 different Bt toxins vary widely.

Potato

Transgenic potato was the first Bt crop variety approved for commercial use (EPA 1995a). The target pest for transgenic Bt potatoes is the Colorado potato beetle. This pest is not a major problem in all areas of potato production, but the need for an alternative to conventional insecticides for controlling it by conventional farming techniques was apparent in the years before approval because the beetle had become resistant to all available classes of conventional insecticides. Just as Bt potatoes reached the market, a novel insecticide, imidicloprid, also reached the market. The new insecticide was so effective on a number of potato pests that it competed effectively with Bt potatoes that controlled only the beetle pest. In 1998, Bt potatoes in the United States were planted in 50,000 acres, which is 3.5% of the total US potato acreage (Idaho Statesman 1998). Strains of Colorado potato beetle resistant to imidicloprid are already evolving in a number of locations (for example, Suffolk County, NY), so Bt potatoes may soon be planted on a much larger scale. However, low rates of adoption of Bt potato may ultimately be due to the need for potato growers to use chemicals to control insect pests other than the Colorado potato beetle. In those cases, protection from the Colorado potato beetle may not offset the cost of the chemicals and the transgenic seed (Gianessi and Carpenter 1999).

Some of the Cry3 protein produced in Bt potatoes is coded from the full length bacterial gene. However, a significant fraction of the Cry3 toxin produced in Bt potato is a smaller, truncated form of molecule (Perlak et al. 1993). EPA documentation indicates that the potential for the smaller Cry3 molecule to induce a food allergy is similar to that of the larger Cry3 molecule. EPA (1995c) indicates that “despite decades of widespread use of Bacillus thuringiensis as a pesticide there have been no confirmed reports of immediate or delayed allergic reactions from exposure”. Bt toxin's history of use (microbial Bt sprays have been registered since 1961) and rapid digestion in simulated gastric fluids (in less than 30 seconds; EPA 1995c) are considered evidence of safety by the EPA. In addition, in acute toxicity studies, no adverse effects were exhibited (Lavrik et al. 1995) (see section 3.1.3). However, it must be recognized that the microbial Bt toxins that have been widely used for decades to

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

control lepidopteran pests have less than 60% molecular similarity to the toxin which is active against Colorado potato beetle (Feitelson et al. 1992). This beetle-active toxin has only been used in spray form since the mid-1980s on limited acreage of potatoes. Also, the sprayed beetle toxin is not applied to the tuber; whereas, in the transgenic potato varieties, it is present in the tuber (Rogan et al. 1993).

As indicated in section 2.6, field studies that compared the biodiversity of insects in fields with transgenic pest-protected potatoes and in fields with nontransgenic potatoes treated with synthetic insecticides found higher densities of above ground beneficial arthropods in the transgenic Bt fields. USDA referred to the findings in its positive response to Monsanto 's request for nonregulated status of transgenic Bt potato (USDA 1995a). EPA's pesticide fact sheet for Bt potatoes (EPA 1995a) did not refer to those field data but concluded that no negative ecological effects of Bt potatoes were expected on the basis of a series of laboratory tests conducted by Monsanto (for example, Sims 1993; Keck and Sims 1993).

Details of methods and results of the laboratory tests were voluntarily provided to the committee by Monsanto. Examination of this information indicated that most of the procedures and conclusions were valid. However, in some cases, the approach to testing seemed inefficient. For example, tests for nontarget effects on honeybee larvae used a bioassay in which 5 uL of a Bt-toxin solution was pipetted into the bottom of larval cells and observations for potential mortality were made (Maggi 1993b); this approach would be better for a contact toxin than for toxins such as Bt toxin, which must be ingested. In the tests for adult honeybees (Maggi 1993a), the amount ingested was estimated by weighing the solution before and after presentation to honeybees and controlling for evaporative loss; however in the larval study (Maggi 1993b) the amount ingested was not estimated and it is unclear how much of the solution was consumed by the larvae. A positive control (that is, a group of larvae presented with a solution that will definitely kill them) was not mentioned in the study provided to the committee.

For some other tests, the absence of information made interpretation of results less clear. For example, tests with ladybird beetles used adults and provided the Bt toxin in a honey solution (Hoxter and Smith 1993). Consumption was measured by comparing the weight of the test diet before and after presentation to the beetles. However, the measurement of consumption was not useful, because there was no control for evaporative loss. Without knowledge of the amount consumed, it would be better to gather data on egg production which is more sensitive to stress. Tests of larval ladybird beetles would also offer a more sensitive toxicity test for Bt.

The ladybird beetle test and other tests for EPA used Bt toxin produced by bacteria instead of plants. In some of the nontarget testing, that

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

seems to have been done to increase the toxin concentration to more than 100 times the concentration in pollen1 or nectar. That is justified, but it would also be ecologically relevant to determine effects of the actual pollen and nectar produced by the plants under field conditions. In soil-degradation studies in which the bacterially produced toxin is used at concentrations that could be obtained from the plant itself, there seems to be less justification for not using the plant itself. It is surprising that in the Bt soil-degradation studies, either bacterially produced toxin or freezedried and highly pulverized Bt-potato plant material is used (Keck and Sims 1993). An ecologically more realistic approach was used in peer-reviewed studies by Donegan et al. (1995) and by Palm et al. (1996): where Bt and non-Bt plant material was placed in the field and monitored for decomposition, microbial diversity, and Bt-toxin titer. Donegan et al. (1996) also used an ecologically realistic system to test for differences in rhizosphere and leaf-dwelling microorganisms associated with field-grown transgenic Bt and non-Bt potatoes. No biologically significant differences were found. Similar tests would be valuable in regulatory assessments.

Overall, the data presented to EPA by Monsanto demonstrate that the transgenic Bt potatoes are likely to be environmentally much less disruptive than current chemical control practices against the Colorado potato beetle.

The concentration of toxin produced in the foliage (19.1 µg/g ) of Bt potatoes (EPA 1995a) far exceeds the concentration needed to kill young Colorado potato beetles (Perlak et al. 1990 and 1993). This level of toxin is expected to fit the EPA Scientific Advisory Panel's (SAP 1998) definition of a “high dose” that can be useful in delaying evolution of resistant pest strains (see section 2.9). The concentration of toxin in the tubers themselves is low (1.01 µg/gm), but potato beetles do not typically feed on the tubers.

Corn

Unlike the Colorado potato beetle, which can devastate potato production in some areas when not controlled with insecticides, the European corn borer, which is the major target of transgenic Bt field corn, has not commonly been controlled with insecticides. A survey of the literature (Gianessi and Carpenter 1999) indicates that across the corn belt only

1  

Some varieties of potatoes (for example, Russet Burbank) produce very little pollen, so the discussion of pollen refers to other potato cultivars that have been commericialized (for example, Superior).

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

5.2% of the acreage is sprayed annually for corn borers and in Iowa only 2.6%. Some of the reasons for the lack of chemical control are that the perceived yield loss has always been considered small (estimated at about 4%), the cost of pesticides is high relative to the crop 's value, and typical insecticides have not been very efficient at killing the pest after it bores into the plant.

In addition to the European corn borer, other insect pests are targeted by the Bt corn cultivars. For example, the southwestern corn borer, a major corn pest in some areas of the Midwest, can be controlled by Bt corn; and the corn earworm, a minor pest of field corn but a major pest of sweet corn, is also a potential target. A recent USDA study (1999d) indicates that in two of five regions mean yields were significantly higher with Bt corn than with non-Bt corn. How the increase in yield will affect farmers' profits is not evident, given increased seed cost and the increased potential for higher national production of corn which could lead to a decrease in prices.

At least three Bt toxins are produced by commercial transgenic pest-protected corn cultivars. The most common, Cry1Ab, is produced either as a full-length protoxin, as produced in B. thuringiensis (Monsanto variety), or as a truncated, preactivated toxin (Novartis variety). In nature, the 130 to 140 kilodalton protoxin is converted to a 60-65 kilodalton protein when the target insect ingests Bt (Federici 1998). Cry1Ac toxin is produced by the Dekalb cultivars; and a biochemically distinct Bt toxin, Cry9C, is produced by corn developed by AgrEvo (EPA 1998c). All the corn cultivars that produce Cry1A toxins have been approved for human consumption, but currently Cry9C corn has been approved only for cattle feed (EPA 1998c). That restriction by EPA has been established because the Cry9C toxin, unlike the Cry1A toxins, does not degrade rapidly in gastric fluids and is relatively more heat-stable; these characteristics of Cry9C raise concerns including those of allergenicity.

Novartis-produced Bt corn does not produce detectable levels of Bt toxin in the silks or corn kernels, so it does not effect the corn earworm, which feeds mostly on these two plant parts. The Cry9C toxin in AgrEvo corn is produced in the silk and corn ear, but it is not toxic to the corn earworm. The biological complexity of current Bt corn products is much greater than that of Bt potato and cotton; for example, only one company has commercialized Bt potato, and only one toxin type and seasonal tissue distribution are exhibited for each crop species.

Environmental impacts of Bt field corn must be judged against the typical corn system in which no insecticides are applied to control lepidopterans. Peer-reviewed studies have demonstrated adverse effects of Cry1Ab on predaceous lacewings (Hilbeck et al. 1998a, b), however, none of the studies conducted for EPA by the registrant has found adverse

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

effects on lacewings or other aboveground beneficial insects (EPA 1997a and 1998a).

It is difficult to reconcile the different findings of the studies conducted for EPA by Monsanto (for Cry1Ac and Cry1Ab toxins) and the studies by Hillbeck and colleagues (Cry1Ab). In the first study by Hilbeck et al. (1998a), lacewings were fed on small larvae of Bt-sensitive and Bt-nonsensitive herbivores that had eaten vegetative-stage Bt or non-Bt corn. The concentration of toxin to which the lacewings were exposed could have been above the 50 parts per million (50 ppm) expected in an ecologically realistic system. A total of 200 lacewings were used per treatment. The second Hillbeck et al. study (1998b) fed larvae purified bacterially-produced Bt at a concentration of 100 ppm in an artificial diet. In the Monsanto studies, the concentration of toxin was 20 ppm and involved coating lepidopteran eggs with bacterially produced toxin (Hoxter and Lynn 1992). In each Monsanto study, 30 lacewings were used per treatment. The Hilbeck et al. (1998a) and Monsanto studies followed larvae to pupation. The Hilbeck studies found more than a 50% increase in mortality; the Monsanto studies found no difference in mortality or lower mortality associated with Bt treatment. Because lacewings typically feed only on the internal content of the eggs, they may not have ingested much of the toxin which was deposited on the shells of the eggs in the Monsanto study. Given that Bt corn is already planted over millions of acres in the United States, it seems appropriate for EPA, USDA, or registrants to sponsor careful field tests to determine whether lacewings or other natural enemies of crop pests are adversely affected by Bt corn. One preliminary study of this type found no differences between Bt and non-Bt corn in effects on any natural enemies of crop pests (Pilcher et al. 1997), but more detailed studies would be useful. Likewise, the committee recommends that

EPA should provide guidelines for determining the most ecologically relevant test organisms and test procedures for assessing nontarget effects in specific cropping systems.

Peer-reviewed studies (for example, MacIntosh et al. 1990) demonstrated that the Bt toxin in corn could affect many lepidopteran species. A laboratory study showed that pollen from some Bt corn cultivars can kill and slow growth of monarch caterpillar larvae if enough pollen is placed on the milkweed leaves fed to the caterpillars (Losey et al. 1999)(see section 2.6.2). If monarchs are indeed being killed in nature by this pollen, the non-Bt corn planted as a refuge for susceptible pest insects could be planted around the edges of corn fields so that adjacent milkweed would be dusted only with pollen from non-Bt corn. It might also be possible to

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

shift to using Bt corn which does not produce biologically significant amounts of Bt in its pollen (EPA 1998b; Andow and Hutchinson 1998).

The potential for resistance to Bt toxin in a number of the target pests of Bt corn has been of concern to EPA, environmentalists, and university researchers (Ostlie et al. 1997; Andow and Hutchison 1998; Matten 1998; SAP 1998). In particular, the corn earworm may be especially vulnerable to evolving Bt resistance. Corn earworm is substantially less sensitive to Bt toxins than the primary target pest of Bt corn, the European corn borer. Bt corn varieties that express the toxin in the silks or corn kernels where corn earworm feed do not produce a high enough dose for corn earworm mortality. Corn earworm is also subject to selection pressure from Bt toxins in Bt cotton, since this pest feeds on a number of crops, including cotton, where it is known as the cotton bollworm (EPA and USDA 1999).

All the commercial cultivars provide substantial protection against the European corn borer (Ostlie et al. 1997). However, the Novartisproduced cultivars, which use green tissue and pollen-specific promoters to drive gene expression, have lower efficacy later in the season (Ostlie et al. 1997; Andow and Hutchison 1998). The lower late-season efficacy is also seen in the Dekalb-produced corn (Andow and Hutchison 1998). Lack of a high dose in these two types of Bt corn could undermine the high-dose refuge approach endorsed by EPA (Matten 1998; and section 2.9) and achievable with other Bt corn cultivars.

Cotton

Like potatoes, conventionally-grown cotton has been heavily treated with insecticides to control lepidopteran pests. Therefore, the introduction of Bt cotton can produce considerable environmental benefits. A 1998 survey indicated a general decrease in insecticide useage on Bt cotton (Mullins and Mills 1999). For example, in 66 comparisons in the Mississippi, Louisiana, and Arkansas region, the average number of insecticide sprays per field was 10.1 for non-Bt cotton and 7.9 for Bt cotton. Many of these insecticide treatments were made to control the boll weevil which is not affected by Bt. In 20 comparisons in the North Carolina, South Carolina, and Virginia region (where the boll weevil is not a pest), the average number of insecticide sprays was 3.7 for non-Bt cotton and 1.2 for Bt cotton. USDA's Economic Research Service found less clear patterns in changes in insecticide used on Bt cotton (USDA 1999d). Comparison of mean pesticide acre-treatments for 1997 showed that in only two of three regions surveyed did the adoption of Bt cotton reduce insecticide treatments normally used to control pests targeted by Bt. In one of three regions, total insecticide treatments for all other pests was higher for Bt adopters than for nonadopters (USDA 1999d). The results should be

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

interpreted with caution however, because, as stated in the USDA study, “attributing differences in yields, pesticide use, and profits between adopters and nonadopters observed in the data solely to adoption of genetically engineered crops is nearly impossible because many other factors also affect yield and pesticide use.” However, in summary the committee concludes that

The introduction of Bt cotton appears to be bringing environmental benefits through the reduced use of insecticides.

Monsanto has provided EPA with data on potential effects of the Cry1Ac toxin on nontarget species (for example, Sims 1994) and has made the information available to this committee. The range of nonpest species tested is similar to that of species tested for registrant 's corn, and the test protocols were also similar. None of the studies showed biologically significant impacts on nontargets other than lepidopterans. The only ecological study in which a comparison of corn and cotton testing produced contrasting results was the study of toxin degradation in soil (Ream 1994a; Sims and Sanders 1995). In the corn study, the half-life of Cry1Ab Bt toxin from pulverized corn placed in vials of soil was 1.5 days, and 90% of the activity was lost in 15 days. In the similar study of cotton plant material that produced Cry1Ac toxin, the half-life was 41 days, and 90% of the activity was expected to be lost in 136 days (on the basis of extrapolation). These differences may reflect differences in tissue degradation perhaps resulting from the differences in woody cotton tissue compared to com tissue. In comparison with insect-control practices now used in nontransgenic cotton, the possible minor effects of that Bt cotton may have on beneficial insects are expected to be insignificant.

The lepidopteran pests that attack cotton differ by region. In North Carolina, South Carolina, and Virginia, the cotton bollworm, Helicoverpa zea, is a predominant pest. In the Midsouth, the tobacco budworm, Heliothis virescens, is typically the most important caterpillar pest. In Arizona, the pink bollworm, Pectinophora gossypiella, is the only lepidopteran pest of economic importance (Gould and Tabashnik 1998). Each of those pests has a different response to the one Bt toxin, Cry1Ac, that is present in commercial cotton cultivars. The concentration of this toxin in cotton decreases toward the end of the season but still seems to be enough throughout the season to fit the EPA SAP criteria (SAP 1998) of a “high dose” for the tobacco budworm. However, this same toxin concentration range causes only 60-95% mortality in the cotton bollworm and cannot be considered a high dose for this pest. Although high mortality in the pink bollworm has been reported in the field, at least some pink bollworms survive late in the season. Those differences in Bt cotton efficacy against the

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

cotton pest complex have caused concern for both short-term management and resistance management. From the perspective of short-term management, farmers have difficulty in distinguishing the tobacco budworm from the cotton bollworm during the egg and young caterpillar stages. In many areas, both species occur and farmers have trouble in determining when they can rely on Bt cotton for control and when they need to consider additional control tactics. That can lead to overuse of insecticides by farmers who are trying to decrease the probability of yield loss.

From the perspective of resistance management, the currently available cultivars of Bt cotton can fit well into the high-dose refuge approach to the tobacco budworm (section 2.9), but it falls short of producing adequate toxicity in the cotton bollworm. To slow the evolution of adaptation to Bt toxins by corn earworms, very large refuges of non-Bt cotton are needed (Gould and Tabashnik 1998).

The toxin titer in Bt cotton appears to be at least close to a high dose for the pink bollworm in early-season Bt cotton, but it is unlikely to achieve a high dose late in the season. Because Bt cotton has been so widely adopted by Arizona growers, refuges are on the verge of being too small, even if attainment of a high dose is assumed. Recent research indicates that pink bollworm has the genetic capacity to develop resistance to Bt cotton (Liu et al. 1999), so the overuse of Bt cotton in Arizona could lead to a rapid loss of the technology. Unlike the cotton bollworm, which is a pest on a wide variety of crops, the pink bollworm is specialized on cotton and its close taxonomic relatives. If pest resistance to Bt evolves in the pink bollworm, the problem will mostly be restricted to the cotton growers and is unlikely to have any impacts on organic or other farmers who rely on sprays of Bt bacterial formulations for the production of crops other than cotton.

3.1.3 Mammalian Toxicity Testing for Bt Crops

Only general summaries of the toxicologic assessments of Bt crops were available from EPA pesticide fact sheets. However, the Monsanto Company made available to the committee the toxicology data used in the registration process for its corn, cotton, and potatoes (see appendix B). The array of tests and the protocols used for the tests of Bt toxins from each of the three crops were generally similar. The committee therefore reviewed the three crops together. Three types of studies were conducted on each crop. The goals of the studies were

  • To establish equivalency of Bt proteins produced by the three crops and Bt proteins produced by appropriate B. thuringiensis strains or

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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engineered E. coli strains (Bartnicki et al. 1993a,b; Sammons 1994a; Lee et al. 1995).

  • To test for digestibility of the microbially-produced Bt proteins by simulated gastric and intestinal fluids (Bartnicki et al. 1993b; Keck et al. 1993; Ream 1994b,c).

  • To test for acute effects on mice of the microbially-produced Bt proteins (Naylor 1992, 1993a,b; Sammons 1994b).

Equivalency

Determining chemical or functional equivalence of two proteins is difficult, especially if there is any question about potential post-transcriptional modification of either protein. However, a battery of well-conceived biochemical and functional tests can provide reasonable assurance of equivalence. Tests performed on the plant and microbially-produced Bt proteins included:

  • Assessment of equal migration of the full length and trypsinated forms on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and binding of the migrated proteins to a polyclonal antibody.

  • Demonstration of lack of glycosylation.

  • Assessment of sequence similarity of about 10-15 amino terminal amino acids and up to three short internal protein sequences.

  • Determination of similar toxicities of the proteins to one or two insect species.

Protein reactivity with antibodies and protein mobility are ways to determine if two proteins are similar in structure. In SDS-PAGE immunoblotting tests of comigration, the plant- and microbially-produced Bt proteins always had similar, strong protein bands at the same gel position that reacted with the polyclonal antibody.2 However, in many cases, there were additional bands in the gel lanes with plant-expressed Bt protein or microbially-expressed Bt protein. These additional bands differed in migration, but they bound to the polyclonal antibody. That indicates that the antibody was not highly specific or that there were different Bt proteins in the two types of preparations. Inasmuch as there was less dissimilarity in the trypsinated preparations, some of the nonhomologous bands in the full-length preparations might have been biologically unimportant. In some comparisons for cotton, a non-Bt plant-protein extrac-

2  

The polyclonal antibody was raised against E. coli-produced recombinant Bt protein.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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tion was used as a control in the SDS-PAGE assessment; this offers useful information about potential binding to non-Bt proteins. A set of monoclonal (or polyclonal) antibodies with less cross-reactivity might prove useful in future tests. In general, good analytical tools to monitor Bt toxins and their concentrations are necessary for reliable human health and environmental testing.

Glycosylation appeared to be absent in all the Bt proteins on the basis of standard tests, and this result seems reasonably straightforward.

Because the DNA sequences of the Bt proteins were modified substantially in order to increase plant expression, there is always a question of whether the final amino acid sequences are identical (that is, cloning artifacts could lead to some amino acid substitutions). The sequencing of some of the terminal and internal amino acids cannot itself prove identity; however, combined with results of other tests, the finding that these sequences were identical offers additional support of equivalence.

Tests showing similar toxicity of the proteins to one or two insect species offer some support of equivalence. If such tests demonstrated substantial differences in toxicity, there would be strong evidence of nonequivalence. The finding of similar toxicity, however, does not prove equivalence. In the current studies, when two insect species were used, they were taxonomically closely related. Future tests with less related species would be useful. There is a question of how many biochemically distinct insect species should be tested in this kind of study.

Digestibility

The digestibility tests for each of the toxins were similar and reasonably straight forward. Microbially-produced Bt toxins were incubated with simulated gastric fluids and separately with simulated intestinal fluids. The gastric fluids caused the proteins to lose their biological activity against insect species and to lose their potential for binding with an antibody used for protein detection. In contrast, the intestinal fluids only truncated the full-length proteins to their active forms and did not affect the pretruncated proteins. Because the proteins reach the gastric fluids first, the lack of degradation by intestinal fluids would be unimportant unless an individual lacked active gastric digestion.

Acute Toxicity to Mice

Mice were given oral doses of the microbially-produced Bt toxins about 100-1000 times the acute dose that they would encounter in consuming one-tenth of their body weight in plant material. Ten 7-week-old

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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mice were exposed to each dose; in no case was significant mortality, weight loss, or a general adverse clinical symptom found.

Chronic Toxicity to Catfish

There is a limit to the amount of plant material that can be consumed by mice, and that limits testing to acute tests with purified material. In a novel approach (Jackson et al. 1995), Bt-producing corn (finely ground seed) was incorporated into the diet of catfish (100 fish per treatment) over a period of 66 days. The ground corn made up 35% by weight of the catfish diet. Non-Bt corn was used as a control. No significant effects on weight gain or rates of feed conversion were found. Although this is not a traditional toxicity test for human health assessment, the duration of the test, the sample size per treatment, and the amount of plant material consumed provide useful information. Catfish are not a close physiological model for humans, but forage- and grain-consuming mammals might be more appropriate models (see section 2.5).

Overall Findings

The committee concludes that

Although a number of the experiments performed in support of registration for transgenic pest-protected plants containing Bt proteins could be improved by modifications suggested above, the total weight of evidence from combined studies presented and previous knowledge about Bt proteins, provides reasonable support for the toxicological safety of crops containing the tested Bt proteins (that is, Cry1Ab, Cry1Ac, and Cry3A).

Similar tests of other Bt proteins would be appropriate in the future, but a question remains about the strength of data that should be required when a class of plant-defensive substances has not been previously characterized as well as Bt proteins have been. As stated above, biochemical and functional equivalence is difficult to prove. Functional activity can be affected by single amino acid substitutions that would be difficult to detect with current methods. Although the sequence of the cloned gene is usually known and will largely remain intact during plant expression, modifications such as amino acid substitutions, proteolytic processing, and glycosylation are all possible. Therefore, it would be helpful to use plant-produced defensive substances in as many tests as is feasible. However, because it may often be difficult to extract and purify pest-protective

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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proteins from the plant itself, EPA should provide guidelines for determining when a similar protein produced by another organism can be considered equivalent to the plant-produced protein and be used in toxicological and environmental testing (see recommendation in chapter 2, section 2.5.1).

Research should be conducted to assess the relevance of using forageand grain-feeding organisms in chronic toxicity studies (see section 2.5.2). Such studies have historically been conducted in evaluating new conventionally-bred forage-crop varieties and have often detected effects of new varieties on animal performance traits (Hanson et al. 1973; Reitz and Caldwell 1974).

3.1.4 Virus-resistant Squash and Papaya

Many crops are damaged by viruses, sometimes to the extent that these pathogens limit the regions where the crop can be grown. Viruses are often transmitted by aphids and other insects that are difficult or impossible to control effectively with pesticides. Conventional breeding for virus-protection is sometimes possible, but naturally occurring protective genes are not always available in the crop or related species. Furthermore, the rapid evolution and spread of new viral strains often thwarts efforts to achieve durable protection in the crop.

Transgenic methods can provide much-needed protection from viruses by transferring pieces of the viral genome into plants (Sanford and Johnston 1985; Powell-Abel et al. 1986). In the future, it might be possible to avoid viral infections by developing plants that have transgenic protection from aphids and other vectors. Here the committee discusses the first transgenic virus-protected crops to be granted nonregulated status and sold commercially: Asgrow's crookneck squash varieties (deregulated in 1994 and 1996) and Cornell University' s papaya (deregulated in 1996). Virus-protected potato has also been approved for marketing, and more than 20 other domesticated species have been field-tested to evaluate transgenic protection against viruses (chapter 1, table 1.4 and table 1.5).

Squash

Varieties of domesticated Cucurbita pepo—commonly known as zucchini, yellow crookneck squash (summer squash), or acorn squash—are widely cultivated in the United States for human consumption. In some regions and some years, viral infections stunt and mottle, or deform the squash, causing major economic losses and even complete failure of the crop. The outbreaks are often intermittent, however, and therefore difficult to predict (for example, Schultheis and Walters 1998). Some of the

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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most common cucurbit viruses in the United States are watermelon mosaic virus 2 (WMV2), cucumber mosaic virus (CMV), zucchini yellow mosaic virus (ZYMV), and papaya ringspot virus (PRSV, formerly WMV1). Each of those viruses has many host species in the curcurbit family and others, and all are transmitted by aphids (Price 1940; USDA 1994b). Infections in any given area are sporadic, and by the time symptoms are visible it is often too late to control the aphids with pesticides (for example, Tricoli et al. 1995).

Protection from viruses in cucurbit crops has been improved somewhat by the use of genes from wild and cultivated species (for example, Gilbert-Albertini et al. 1993). Cucurbits are well known for their ability to hybridize (Wilson 1990), but species barriers have often made it difficult to transfer desirable genes into commercial varieties. Genetic protection from WMV2 and ZYMV was developed nearly simultaneously by both transgenic and conventional breeding. The Harris Moran company released a conventionally bred zucchini known as Tigress with protection from WMV2 and ZYMV (USDA 1994b; Schultheis and Walters 1998), and Upjohn/Asgrow used viral coat protein genes to achieve similar objectives in transgenic yellow crookneck squash. Upjohn/Asgrow's first commercial transgenic product was Freedom II, which was protected from WMV2 and ZYMV and was deregulated in 1994; it exhibits strong protection from the targeted viruses (Fuchs and Gonsalves 1995; Tricoli et al. 1995).

In a 1995 field experiment in North Carolina, both the conventional Tigress and the transgenic Freedom II exhibited better protection from viruses than most other varieties tested (Schultheis and Walters 1998). Asgrow then produced another transgenic pest-protected squash, known as CZW-3 (deregulated in 1996), with a marker gene for resistance to the antibiotic kanamycin and coat-protein genes for protection from three cucurbit viruses. Squash plants with transgenic protection from as many as five viruses have been field-tested (USDA 1999c) and might eventually be considered for deregulation.

The health concerns about transgenic virus-protected squash have been related to both viral and bacterial genes that are expressed in all the plant's cells, including the edible portions. Human or animal consumption of plants with viral coat proteins is widely considered to be safe, on the basis of common exposure to these proteins in nontransgenic types of food. However, Asgrow's 1996 virus-protected squash also had a marker gene for resistance to the antibiotic kanamycin. Some people have proposed that the widespread use of antibiotic-resistance genes as markers for transgenic traits could exacerbate current losses of antibiotic effectiveness due to overuse. However, for markers in current commercial use, that risk is extremely small (Nap et al. 1992; Fuchs et al. 1993), and the

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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Food and Drug Administration (FDA) has approved many crops with transgenic kanamycin resistance. FDA continues to examine the safety of antibiotic-resistance genes and issued the following draft statement in 1998 (FDA 1998):

FDA acknowledges that the likelihood of transfer of an antibiotic resistance marker from plants to microorganisms in the gut or in the environment is remote and that such transfer, if any, would likely be insignificant when compared to transfer between microorganisms and, in most cases, would not add to existing levels of resistance in bacterial populations in any meaningful way.

Nevertheless, in the future, it may not be necessary to take even the small risks associated with the use of antibiotic resistance markers, since other types of markers could be substituted in future transgenic crops (for example, Kunkel et al. 1999).

The major environmental risks that have been discussed in connection with virus-protected crops pertain to effects of viral coat protein genes on the pathogenicity of other viruses (Falk and Bruening 1994) and consequences of crop-to-wild gene flow that could allow beneficial transgenes to move into feral-crop plants or closely related weeds, as described in section 2.7. The first issue was studied experimentally at Cornell University by Dennis Gonsalves, who helped Asgrow to develop the virus-protected squash; he and his collaborators concluded that the risks that other viruses would become transmissible (from heteroencapsidation) or that the nonpathogenic viruses would become more virulent (from recombination) were exceedingly small (Fuchs et al. 1998; also see section 2.8).

The second issue—whether wild relatives could benefit from disease-protection genes —was more controversial. An important precedent for USDA in dealing with this potential problem was established in the following example. To get more information on the ecology and systematics of weedy, free-living Cucurbita pepo (FLCP), the USDA Animal and Plant Health Inspection Service (APHIS) commissioned a report on the risks that may be posed by crop-to-wild gene flow by Dr. Hugh Wilson, an expert on cucurbit taxonomy and ecology. Wilson concluded that FLCP is a significant agricultural pest that might benefit ecologically from protection from ZYMV and WMV2 (Wilson 1993). Key information about squash and its weedy North American relatives is summarized below with an evaluation of USDA 's conclusions.

Volunteer squash plants are not known to spread and become weeds, but C. pepo crosses freely with a wild weedy species that is variously known as C. texana (Texas gourd) or, more recently, wild or free-living C. pepo (FLCP), and C. pepo subspecies ovifera (same species as zucchini and

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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crookneck squash; Decker 1988, Wilson 1993). FLCP is an agricultural weed in cotton and soybean fields, where its tough gourds interfere with harvesting machinery (Harrison et al. 1977; McCormick 1977; Oliver et al. 1983; Bridges 1992). Cultivated squash and free-living squash co-occur in many regions of Texas, Louisiana, Alabama, Mississippi, Missouri, and Arkansas (Wilson 1993). The bouyant gourds of FLCP are widely dispersed during floods, and this leads to periodic infestations of crops along river floodplains. Once FLCP becomes established in an area, it can be difficult to eradicate, because some seeds germinate throughout the growing season, other seeds remain dormant for long periods, and the plants spread laterally by branching and producing roots along the nodes of their long, viny stems. USDA asserts that FLCP is only a minor weed that can be easily controlled with herbicides (such as Cobra, bromoxynil, and glyphosate; USDA 1994b and 1996b). Nonetheless, FLCP is clearly a weed that merits close attention from regulatory agencies. It was previously listed as one of the top 10 most important weeds in Arkansas (McCormick 1977). Additional fitness-related traits such as virus-protection could potentially increase the agricultural impacts of this weed.

Hybridization between cultivated squash and FLCP is known to occur across distances of 1 km or more. Cultivated squash and wild squash have single-sex flowers that make them dependent on insect pollinators for seed set, and bees are known to carry crop pollen to wild plants as far as 1.3km (Kirkpatrick and Wilson 1988). Although cultivated squash hybridizes and backcrosses with weedy FLCP, USDA stated that “there is no scientific or anecdotal evidence that supports the contention that hybrids between yellow crookneck squash and FLCP plants are weeds and are persistent” (USDA 1994b). That statement appears to overlook the fact that after the first hybrid generation, spontaneous backcrossing with wild plants can allow crop genes to spread into FLCP populations, which are clearly persistent weeds. Crop-wild hybrids and their offspring are vigorous and fertile, so neutral or beneficial transgenes could probably persist in wild or weedy populations (Fuchs and Gonsalves 1999). Furthermore, virus-protection transgenes confer strong protection in crop-wild hybrids and backcrossed generations, as expected (Fuchs and Gonsalves 1999). The environmental question posed by this situation is “will genes for virus-protection be beneficial enough to cause this weed (FCLP) to become more common?”

Wild C. pepo and cultivated C. pepo are susceptible to the same viruses (Provvidenti et al. 1978). To check for viral infections in FLCP populations, Asgrow conducted a survey in 1993. Its analyses of 14 FLCP patches in nine locations did not detect viral infections in wild plants; one plant was sampled at each location (USDA 1994b). No information was given on whether cultivated squash in these areas were infected with ZYMV or

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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WMV2; that information would have shown whether the viruses were present that year. Because of a severe drought, no FLCP plants were sampled in Texas. On the basis of anecdotal reports and this one-season survey of only nine locations, USDA-APHIS concluded (USDA 1994b):

Given the available knowledge, it is unlikely that resistance to ZYMV and WMV2 infection will confer a selective advantage or be maintained in the FLCP populations. Surveys of natural FLCP populations for the incidence and severity of ZYMV and WMV2 infections suggest that resistance to these viruses will confer little, if any, selective advantage, because disease caused by these viruses is apparently not among the factors important to the survival or reproductive success of FLCP.

The issue merits further empirical study, especially because selectively neutral crop genes are often maintained in the gene pools of wild and weedy plants (for example, Whitton et al. 1997). Also, the selective benefit of such genes could vary geographically and over time, and a small-scale survey like Asgrow's could easily miss infections that affect long-term population dynamics. No studies other than the one just mentioned have been conducted to determine whether viral diseases are important to the survival or reproductive success of FLCP.

Questions about the weediness of FLCP were addressed again when Asgrow requested deregulation of the CZW-3 squash in 1995. The CZW-3 squash is resistant to CMV (cucumber mosaic virus), as well as to ZYMV and WMV2. To the committee's knowledge, USDA-APHIS did not obtain any new, original data on the agroecological factors that regulate FLCP populations in their geographic range (USDA 1996b). Instead, the 1996 deregulation of the CZW-3 again relied on the 1993 Asgrow survey and anecdotal evidence from interviews with several weed scientists in Arkansas (USDA 1996b). Some of the reasoning in the permit document is not well supported. For example, USDA states that the arrival of ZYMV in the United States in the 1980s did not lead to decreases in FLCP populations, as would be expected if ZYMV suppressed FLCP populations. That statement is puzzling in light of the statement that FLCP populations apparently have become less of a weed problem in the 1990s, presumably because of changes in available herbicides (USDA 1996b). Without any studies of FLCP populations, one cannot rule out the possibility that viral diseases and other factors (such as frequency of floods) have also played a role in suppressing FLCP.

In summary, the committee concludes that

USDA's assumption that transgenic resistance to viruses will not affect the weediness of wild C. pepo might be correct, but longer-term empirical studies are needed to determine whether this is true.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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In addition, the cumulative effects of additional transgenic protective traits on this weedy crop relative deserve further scrutiny, especially if the traits are linked and inherited as a unit. Protection from several common cucurbit viruses is expected to have a greater effect on weedy populations than resistance to only two or three.

In conclusion,

USDA's assessment about how the spread of virus-protective transgenes will affect free-living C. pepo populations is not well supported by scientific studies.

A precedent was set with the deregulation of Freedom II, which had protective traits similar to those of a conventionally bred variety. On the basis of its approval of Asgrow's CZW-3 squash, USDA seems unconcerned about incremental increases in the number of viral-protection traits that will be transferred to weedy wild relatives of squash. In cases like this, the committee recommends that

USDA should require original data to support agency decision-making concerning transgenic crops when published data are insufficient.

In cases when crucial scientific data are lacking about the potential impacts of gene flow on wild or weedy relatives, the committee recommends delaying approval of deregulation pending sufficient data (for example, surveys from several years over several regions), establishing a scientifically rigorous monitoring program in key areas to check for undesirable effects of resistance transgenes after the transgenic pest-protected plant is commercialized, or restricting the initial areas where the plants can be grown.

Restricting the areas where the squash can be initially grown would be preferable to unconditional deregulation, at least until more data are available.

Papaya

Papaya (Carica papaya) is an important fruit crop in lowland regions of many subtropical and tropical countries, including Brazil, India, Mexico, Thailand, Vietnam, and Australia. It is a fast-growing, tree-like herbaceous plant that bears fruit in its first year and is usually replanted after 2 years, when the fruits are too high to harvest easily. In commercial plantations, papaya is often infected by the common, aphid-transmitted papaya ringspot virus (PRSV), which causes severe stunting and low fruit

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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yields (Gonsalves 1998). The effects of this virus are so severe that year-round cultivation might not be profitable and infected plantations are often abandoned. Efforts to control the aphids, or to conventionally breed for protection from the virus have been unsuccessful. Tolerance of the virus can sometimes be achieved by deliberately infecting the plants with a mild strain of PRSV, but this is laborious, unpopular with farmers, and only partially effective (Gonsalves 1998).

Papaya was introduced to the Hawaiian island of Oahu in the 1940s and PRSV began infecting the plantations in 1945. The industry then moved to the island of Hawaii, as did the virus; the small, isolated region of Puna remained virus-free until 1992. Meanwhile, Dennis Gonsalves and his colleagues at Cornell University began developing transgenic papaya with protection from PRSV in the hope of rescuing Hawaii's tenuous papaya industry. First they fused a coat protein gene from a mild strain of PRSV to a kanamycin-resistance marker. The linked transgenes were then inserted into the genomes of two local cultivars, christened UH Rainbow and SunUp. The cultivars were highly protected from the Hawaiian strains of the virus and were deregulated in 1996; this allowed the industry to begin recovering from its complete collapse. The virus is unlikely to disappear, however, because it also infects cucurbits and other hosts that occur nearby. Farmers expect transgenic protection to be a boon to the local economy, but the boon could be temporary if the pathogen evolves a way to circumvent the plants' protective mechanism. Another concern is the possibility that other races of PRSV will reach Hawaii. The transgenic papaya is not protected from isolates of the PRSV from outside Hawaii, but further research by the Gonsalves group suggests that protection from a broader spectrum of PRSV races can be obtained (Gonsalves 1998). Parallel research projects are now under way in other papaya-growing countries.

The human health risks that were evaluated before deregulation of transgenic papaya are similar to those described above for squash. As with Asgrow's squash, the papaya's viral coat protein is not expected to jeopardize human health, because consumers already ingest this compound in nontransgenic food. Gene flow to feral or wild relatives was not an issue because no wild relatives occur in Hawaii, Puerto Rico, or Florida, and the crop itself is not weedy.

3.2 ANALYSIS OF THE 1994 AND 1997 PROPOSED ENVIRONMENTAL PROTECTION AGENCY RULES FOR PLANT-PESTICIDES

The above case studies described the type of data and information used for regulatory review of transgenic pest-protected plants. This sec-

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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tion describes the overarching proposed EPA rule for reviewing such plants and its scientific basis.

In 1994, EPA published a proposed rule on the regulatory status of plant-produced pesticides (EPA 1994a), stating that “the substances plants produce to protect themselves against pests and diseases are considered to be pesticides under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) definition of pesticide” and that “These substances ”, not the plants, “along with the genetic material necessary to produce them, are designated ‘plant-pesticides'.” “ Recognizing the unique characteristics of plant-pesticides”, EPA proposed to “create a new part in the [Code of Federal Regulations] for regulations unique to plant-pesticides” (see also section 1.5.3). In 1997, EPA published additional material (EPA 1997b) related to the 1994 proposed rule to comply with the Food Quality Protection Act (FQPA) (Public Law 104-170, EPA 1997b).

EPA proposed to regulate “a pesticidal substance that is produced in a living plant and the genetic material necessary for the production of the substance, where the substance is intended for use in the living plant.” That definition excludes pesticidal substances such as pyrethrum and neem, that are produced by plants but are then extracted from the plants before being used although substances used in this way are subject to EPA regulation as conventional pesticides. The genetic material considered necessary for producing such a pesticidal substance includes genes that encode the substance itself (for example, genes coding for pesticidal proteins) or lead to the production of the substance (for example, genes coding for enzymes that convert compounds that are naturally present in the plant into pesticidal compounds). The necessary genes also include promoter, enhancer, and terminator sequences that regulate expression of the encoding genes.

EPA justified regulation of the genetic material, in addition to the pesticidal substance because the substance might not be present in all plant stages, such as seeds and pollen, although the capacity to produce the substance would be in the seeds and the pollen. Another justification for examining both encoding and regulatory DNA sequences is that variation in the concentration of the pesticidal substance in plant parts depends on specific characteristics of both the encoding and the regulatory sequences.

EPA initially proposed to regulate what it referred to as “inert ingredients” such as selectable markers that enabled researchers to test for the presence of introduced genes but were not necessary for production of the pesticidal substance. It later requested comment on dropping the regulation of inert ingredients (EPA 1996); therefore, selectable markers for transgenic pest-protected plants would be evaluated only by FDA, which has evaluated the use of these markers in the past (FDA 1994a).

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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The proposed rule casts a wide net that captures all plant-produced products that mitigate pest damage. EPA recognized that not all these pest-protected plant products required regulatory scrutiny. Therefore, a major portion of the 1994 proposed rule involved justification of exemptions of specific classes of pesticidal gene products from regulation under FIFRA and FFDCA. The rationale for the exemptions involves the balance between human health and ecological concerns and the positive contributions expected from pest-protected cultivars. The 1994 document states that “EPA finds that the plant-pesticides it is proposing to exempt have a low probability of risk and have potential benefits associated with them (for example, economic benefits to farmers and reducing the need for chemical pesticides) that outweigh any potential risks associated with them, and that the low probability of risk does not justify the cost of regulation.” For most classes of exemptions, the scientific logic used by EPA is clear, for other classes it is questionable.

3.2.1 Exemption for Sexually Compatible Genes
Exemption from FIFRA and FFDCA

The largest class of plant-pesticides proposed for exemption by EPA includes all cases in which the “genetic material that encodes for a pesticidal substance or leads to the production of a pesticidal substance is derived from plants that are sexually compatible with the recipient plant and has never been derived from a source that is not sexually compatible with the recipient plant” (EPA 1994c, p. 60537). According to the 1994 document (EPA 1994c, p. 60537-60538), sources are

[C]apable of forming a viable zygote through the fusion of two gametes and can include the use of bridging crosses and the use of wide cross breeding techniques such as surgical alteration of the plant pistil, bud pollination, mentor pollen, immunosuppressants, in vitro fertilization, pre- and post-pollination hormone treatments, manipulation of chromosome numbers, and embryo culture. Wide crosses, for the purpose of this exemption, also include ovary and ovule cultures.

As long as the genetic material comes from a sexually compatible plant, the plant-pesticide is exempt, regardless of whether the method of transferring genetic material uses sexual crosses or transgenic technology.

The committee recognizes the realistic limitations of overseeing the pesticidal substances in conventional pest-protected plants and, given their history of safe use, recognizes that there are practical reasons for exempting those substances. However, the committee questions the scientific basis used by EPA for this exemption because no strict dichotomy

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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or new categories appear to exist between the risks to health and the environment that might be posed by conventional and transgenic pest-protected plant products (section 2.2.1).

The categorical exemption also applies to transgenic pest-protected plant products that contain transgenes from sexually compatible species, and the committee questions the scientific basis for this exemption as well, specifically because the genes and gene products can be expressed at concentrations far greater than the concentrations at which they are naturally expressed (section 2.4.1 and section 2.5.2). Even though the risks of many transgenic pest-protected plants containing genes from sexually compatible species are expected to be low and would justify exemption, lack of experience with these products and public concern over genetic engineering suggest that a blanket exemption for them is inadvisable.

Major portions of the 1994 and 1997 documents explain the scientific rationale for the categorical exemption. The 1994 document states that “since traits can be passed through a plant population by sexual recombination, it is reasonable to predict that, in a sexually compatible population, new exposures of organisms that associate with plants in the population to the pesticidal substance are unlikely.” It might be appropriate to exempt those plant protectants, but the rationale in the above statement and in other statements in the 1994 document disregards the following:

  • Even for sexually compatible populations that are theoretically capable of natural cross-fertilization in the wild, there is no substantial passing of traits between populations unless the populations are in close proximity. Plants used as sources of new traits for commercial cultivars may come from small plant populations grown in remote locations or may be plants that are not commonly eaten by humans. New exposures could result if genes from such plants were used in commercial transgenic pest-protected plants.

  • A body of scientific literature demonstrates that some populations of a plant species contain toxic compounds not found in other populations of the same species (Dirzo and Harper 1982) or contain these toxic compounds in much higher concentrations than other populations (Gould 1983 and 1988b). Therefore, even though these populations are sexually compatible, the transfer of genetic material from one population to another could result in novel or increased exposures of humans and other nontarget species.

  • EPA is considering specifically traits that enhance the pesticidal nature of a crop. If those traits were not providing novel mechanisms of toxicity to or deterrence of some pests, plant breeders

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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would not search for them and invest years of work moving them into commercial cultivars. Thus, there is reason to expect that organisms in US agroecosystems and humans could be exposed to new toxins when they associate with or eat these plants.

Plants with Pest-Protection due to Structural Transgenes from Sexually Compatible Relatives

In addition to exempting a plant-pesticide when all the genetic sequences associated with its production are from a sexually compatible plant, the 1994 proposed regulations indicate that as long as the coding sequence (that is, structural gene) is from a sexually compatible plant, the “regulatory regions and non-coding, nonexpressed nucleotide sequences may be derived from any source” and still merit a categorical exemption. Therefore, transgenic pest-protectants under the control of promoters that lead to transgene overexpression would be exempt if the structural gene for the protectant is derived from a sexually compatible species. Exempting plant-pesticides developed in this manner on the basis of an assumption of no new exposure seems to be flawed. Many coding sequences in plants are naturally expressed at very low concentrations or only in specific plant parts. If under normal conditions a plant protectant were produced only in the roots of a corn plant because of the specificity of the natural promoter sequence, a new exposure would occur if the coding sequence were spliced to a constitutive promoter that caused the plant protectant to be produced throughout the plant. Even if the plant protectant were naturally expressed throughout the plant, use of a novel promoter could increase its concentration dramatically.

The 1997 EPA Federal Register document addresses plant-pesticides derived from sexually compatible plants in terms of FQPA requirements (EPA 1997b). The focus is therefore mainly on human health considerations associated with FFDCA tolerance requirements. The document states (EPA 1997b, p. 27136) that

EPA has extensively evaluated whether quantitative changes in levels of the pesticidal substances that are the subject of the proposed exemption would warrant regulation by the setting of a food tolerance. EPA has determined that changes in the levels of these pesticidal substances present a reasonable certainty of causing no harm because the highest levels likely to be attained in plants are not likely to result in overall significantly different dietary exposure.

The exemption includes all genes derived from sexually compatible plants and promoter sequences from any origin. This statement by EPA is based on the following findings stated in the 1997 document:

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×
  1. That there are few documented cases of new plant cultivars causing food safety problems despite the large numbers of new varieties introduced into commerce each year, is a reflection of the effectiveness of this process (of conventional breeding with sexually compatible plants).

    (EPA 1997b, p. 27135)

  2. Because knowledge of human consumption of food derived from sexually compatible plants was available and adequately addressed the issues of hazard and exposure, the Agency did not use, for the proposed exemption (59 FR 60535), data gathered in the laboratory through animal testing.

    (EPA 1997b, p. 27137)

  3. They (the exempted pesticidal substances) are part of the metabolic cycles of these plants. They are thus subject to the processes of degradation and decay that all organic matter undergoes. They are unlikely to persist in the environment or bioaccumulate in the tissues of living organisms. Because they do not persist, the potential for new exposures to the residues to occur, beyond direct physical exposures to the plant, would be limited.

    (EPA 1997b, p. 27135)

  4. The amount of pesticidal substance produced by plants normally varies among members of a closely related population (even within a single variety), because of the effects of conditions such as genetic constitution and environment (for example, weather) on trait expression. This variation in turn leads to differences in the levels and types of exposure to the pesticidal substance. Since such variation is a natural phenomenon common to all plants, humans have been and always are exposed to varying levels of the pesticidal substances that are subject of this exemption when they consume food from plants.

    (EPA 1997b, p. 27135-27136)

  5. Greatly increased levels of a pesticidal substance would, in general, only be accomplished at the expense of expressing other agriculturally desirable traits (for example, yield). EPA does not believe that levels of pesticidal substances that are the subject of the proposed exemption (59 Fed. Reg. 60535) will be increased to a point that will result in an adverse dietary effect.

    (EPA 1997b, p.27136)

  6. There is no evidence that such pesticidal substances, as a compo-

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

nent of food, present a different level of dietary risk for infants and children than they would for the adult population.

(EPA 1997b, p. 27136)

  1. EPA is not aware of any other substances outside of the food supply that may have a common mechanism of toxicity with the residues of the pesticidal substances that are the subject of the proposed exemption (59 FR 60535), although it cannot rule out the possibility.

    (EPA 1997b, p. 27138)

  2. [T]he potential for causing adverse health effects may be more circumscribed than for traditional pesticides because, in many cases, the only significant route of human exposure may be oral.

    (EPA 1997b, p. 27136)

The committee questions the scientific basis of the categorical exemption of plant-pesticides from sexually compatible plants and EPA's rationale. Although the committee agrees that there are few documented cases of new plant cultivars causing food safety problems (point 1), the committee does not believe that this provides a scientific basis for a categorical exemption of plant-pesticides from sexually compatible plants in light of the examples provided in this report. EPA's third point is questioned on the basis of evidence of indirect effects on nontarget organisms and data on the persistence of some naturally-occurring plant secondary compounds (see section 2.6). EPA's points four and five are questioned because transgenic methods can create a situation where a gene product is not regulated by the normal regulatory systems in the plant (for example, use of constitutive promoters). Additionally, information in chapter 2 indicates that there is not sufficient data on chronic effects on humans (point 2), and that some of these compounds (for example, alkaloids) share a similar mechanism of activity as do organophosphates (point 7).

Although the same scientific arguments can be made for the risks posed by conventional pest-protected plants, which are not subject to regulation under the coordinated framework, lack of experience with transgenic pest-protected products and public concern with these products constitute practical reasons for not granting a categorical exemption to transgenic pest-protectants derived from sexually compatible species. In summary, the committee recommends that

Given that transfer and manipulation of genes between sexually compatible plants could potentially result in adverse effects in some cases (for example, modulation of a pathway that increases the concentration

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

of a toxicant) and given the public controversy regarding transgenic products, EPA should reconsider its categorical exemption of transgenic pest-protectants derived from sexually compatible plants.

3.2.2 Exemption of Viral Coat Proteins

In addition to exempting plant-pesticides derived from sexually compatible plants, the 1994 and 1997 EPA documents propose a number of more specific exemptions. EPA generally provides more reasonable scientific justification for these exemptions. One specific class of plant products that was proposed for categorical exemption was viral coat proteins (VCPs). VCPs are already present in foods because of natural virus infections of crops and have not caused obvious medical problems, so health concerns are considered minimal. The EPA exemption of VCPs is also based on considerations that “include the low potential for adverse effects to nontarget organisms and the potential benefits (environmental and economic) of utilizing VCP (virus coat protein) mediated resistance.” The committee, in general, agrees with this assessment of the minimal health and nontarget effects posed by VCP expression in crop plants (see also section 3.1.4) and concludes that

Viral coat proteins in transgenic pest-protected plants are not expected to jeopardize human health because consumers already ingest these compounds in nontransgenic food. However, the committee questions the categorical exemption of all viral coat proteins under FIFRA due to concerns about outcrossing with weedy relatives.

Although ecological concerns are discussed and a more restrictive exemption that considers outcrossing is presented, the proposed rule favors complete exemption of VCPs.

EPA should not categorically exempt viral coat proteins from regulation under FIFRA. Rather, EPA should adopt an approach, such as the agency 's alternative proposal (as stated below in Option 2), that allows the agency to consider the gene transfer risks associated with the introduction of viral coat proteins to plants.

Option 2: Exemption of coat proteins form plant viruses produced in plant with low potential for outcrossing to wild relatives. Under this exemption the Agency would limit its exemption of VCP-mediated resistance coat proteins to those viral coat protein/plant combinations that would have the least potential to confer selective advantage on free-living wild relatives.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

In section 2.7 and section 3.1.4, the committee explains why the more restrictive exemption should be considered.

3.2.3 Exemption for Nontoxic Modes of Action

The 1994 EPA document requested comments on a proposal to exempt plant-pesticides that acted primarily by affecting plants and “that act through nontoxic modes of action.” The types of substances that clearly are in this category are structural barriers such as plant hairs; substances that inactivate or resist toxins that are produced by pests; and substances that decrease chemical components needed for pest growth. As discussed in chapter 2 (section 2.4 and section 2.5), these exemptions are unlikely to result in any new human exposure to harmful substances.

However, within the same category the 1994 EPA document also discusses exempting plant hormones. Plant hormones often cause multiple changes in plants, including changes in secondary metabolites that might be toxic, so the scientific basis of such an exemption is questionable.

As with the exemption of VCPs, the categorical exemption of substances that act through nontoxic modes of action mostly considers human health effects. As outlined in previous sections of our report ( section 2.6 and section 2.7) there is a need to consider separately the impact of such substances on nontarget species and the potential for the genes that code for these substances to move to feral populations or weedy relatives of the crop, where they could increase recipient plants' fitness. Categorical exemption under FIFRA might not be scientifically justifiable.

3.2.4 Oversight for Pleiotropic Effects

The 1994 EPA document states that

any food safety questions beyond those associated with the plant-pesticide, such as those involving changes to food quality or raised by unexpected or unintended compositional changes, are under FDA's jurisdiction. Similarly, food safety issues associated with alterations in levels of a substance with pesticidal properties, or the appearance of a substance with pesticidal properties, that occur as an unintended consequence of modifications to a non-pesticidal trait would also fall under FDA's authority.

That is an important statement and shifts an important component of pest-protected plant assessment to FDA.

As discussed previously in this report (section 2.4.1 and section 2.5.2), genetic changes that result in production of a specific plant protectant can result in production of biologically active compounds other than the in-

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

tended plant protectants. Such pleiotropic effects are sometimes difficult to predict. Furthermore, as outlined in previous (section 2.4.2 and section 2.5), many approaches to producing plant protectants through the use of plants that are sexually compatible with the crop plant can result in crops that produce new compounds owing to linkage between the genes for the plant protectants and genes for the other compounds. FDA needs to address these “unintended compositional changes” carefully during their consultation process with the plant producers. USDA and EPA should also be aware of those unintended changes in evaluating the potential agricultural and ecological effects of pest-protected plants.

The committee recommends that

EPA, FDA, and USDA collaborate on the establishment of a database on natural plant compounds of potential dietary or other toxicologic concern.

The database would be publicly available and updated regularly. The following guidelines should be considered: initial emphasis should be on obtaining baseline profiles for food plants that are known to have toxic constituents and on the commonest varieties; differences among varieties, developmental stages, tissues and environmental conditions are important and should be analyzed after initial average baselines have been established; only information based on state-of-the-art chemistry and analytic methods should be incorporated; and potential information should be peer-reviewed by a committee of experts before it is added to the database (see also section 3.4.1).

3.3 SUGGESTED QUESTIONS FOR OVERSIGHT

Given the above concerns with the scientific basis of proposed oversight, the committee proposes that federal agencies use the following questions as a guide in developing their review process. These decision keys leave sufficient room for agency judgment case by case. For the most part, the agencies are following a similar logic in their decision-making, but there are some points where current decision-making does not agree with the following questions; these discrepancies are pointed out in the text.

Because the Coordinated Framework for the Regulation of Biotechnology was designed for transgenic products (see chapter 1) and the agencies do not actively assess conventional pest-protected plant products, the following questions focus on transgenic pest-protected plant products. However, the questions could be adapted and applied to nonregulatory safety assessments of conventional pest-protected plants, as the underly-

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

ing concerns are not dependent on the method used to produce the plant (section 2.2.1).

3.3.1 Health Concerns: Guiding Principles

The principles in the following questions could be used to determine when a detailed analysis of health risks is warranted for transgenic pest-protected plants.

  1. Is the substance found in plant parts that consumers3 eat or workers come into contact with?

    1. Yes or Unknown—go to 2.

    2. No—exempt from health concerns.

  2. Is the substance known to have general chemical and physical properties common to many allergens?

    Note: Criteria outlined in figure 2-1 could offer components for this type of evaluation.

    1. Yes or Unknown—subject to safety assessment.

    2. No—go to 3.

  3. Is the substance similar to substances that people now eat or come into contact with, and can confident predictions of safety based on the similarities be made?

    1. Yes—go to 4.

    2. No or Unknown—subject to safety assessment.

  4. Is the expected exposure to the substance substantially greater than current exposures?

    1. Yes or Unknown—subject to safety assessment.

    2. No—go to 5.

  5. Is there a reasonable chance, based on known properties of the substances, that its production will lead to harmful concentrations of toxicants or allergens that consumers eat or workers come into contact with? 4

    1. Yes or Unknown—subject to safety assessment.

    2. No—exempt from health concerns.

3  

Including human and non-human consumers, such as food animals or pets.

4  

Pleiotropic effects.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

EPA exempts from FFDCA and FIFRA pesticidal substances in transgenic pest-protected plants that are derived from transgenes from sexually compatible species. The committee's questions are not in accordance with that categorical exemption. Given that transfer and manipulation of genes between sexually compatible plants could potentially result in adverse effects (for example, modulation of a pathway increases the concentration of a toxicant), the categorical exemption of pest protectants solely on the basis of derivation from sexually compatible plants could be scientifically unsound in some cases.

FDA's policy for foods derived from new plant varieties is designed to address questions 1 through 5 with respect to dietary exposure to substances that are not regulated by EPA as pesticides. For pesticidal substances, EPA may consult with FDA on allergenicity issues (see chapter 4).

3.3.2 Ecological Concerns: Guiding Principles

Nontarget effects and hybridization with weedy relatives are subjects of concern for transgenic pest-protected plants. The committee suggests that a particular pest-protected plant needs to be exempt from both of these ecological concerns in order to avoid safety assessments.

Nontarget Effects: Guiding Principle

Nontarget effects are often unknown or difficult to predict. Along with standard screens for toxicity to nontarget species, comparison with agricultural practices that would occur if the transgenic pest-protected plant were not used could be made. For example, nontarget effects of transgenic Bt cotton could be compared with nontarget effects from nontransgenic cotton and the accompanying pesticide use needed to compensate for the lack of the transgenic trait. Broader environmental consequences such as changes in soil quality, wildlife habitat, or the use of fertilizers or water could be used to determine the contribution of the new variety to the sustainability of the agricultural system in which it is grown (Cook 1999). Such general environmental considerations could have effects on nontarget organisms.

However, it is important to point out that there is disagreement among scientists, including within the committee, as to whether comparison to currently used pest control practices should be the determining factor for allowing commercialization of a transgenic pest-protected plant. Most agree that it is one of many important factors. Therefore, both toxicity testing and field tests comparing agricultural methods are suggested. The committee recognizes that the question below leaves much room for agency judgment.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×
  1. Is it reasonable to expect that commercialization of plants with the transgenic resistance trait will have more substantial adverse effects on nontarget organisms than current pest control5 has on these organisms?

    1. Yes or More data needed to make a determination—subject to nontarget considerations.

    2. No—exempt from nontarget considerations.

Hybridization with Wild or Weedy Relatives: Guiding Principles

The following guiding principles regarding hybridization with wild or weedy relatives are suggested for reviewing transgenic pest-protected plants. These guidelines are designed for annual crop plants and may require modification in order to address perennials. EPA's categorical exemptions of transgenic plants that have sexually compatible, nontoxic, and viral coat proteins are not in agreement with these principles in some cases. USDA analyzes these concerns according to risks posed to agriculture, so weedy relatives with agricultural effects are of concern; its methods are similar to the following questions, although original data are not always used. FDA does not provide oversight for ecological concerns.

  1. Does the cultivated plant occur in feral populations or hybridize with related species in the United States?6

    1. Yes or More data needed—go to 2.

    2. No—exempt from weedy-relative considerations.

  2. Have feral populations or wild relatives been reported as weedy or invasive in the United States or have a reasonable potential to become weedy?7

    1. Yes or More data needed—go to 3.

    2. No—exempt from weedy-relative considerations.

5  

Current pest control methods could include both the use of chemical insecticides or other non-chemically based methods.

6  

Hybridization refers to any naturally occurring gene flow that results in permanent introgression of genes from cultivated plants into noncultivated populations. Annual crops that persist for 1 or 2 years as volunteers are not considered to be feral populations.

7  

Applies to plants in both managed and unmanaged habitats. A species does not have to be included on the Federal Noxious Weed List to qualify as weedy or invasive, but it should be mentioned in peer-reviewed journals or other professional publications.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×
  1. Does the gene for resistance confer a specific type of resistance or a greatly enhanced degree of resistance that is not found in feral populations or sexually compatible wild relatives in the United States? 8

    1. Yes or More data needed—go to 4.

    2. No—exempt from weedy-relative considerations.

  2. Is it reasonable to expect that this trait could have a substantial impact on the population dynamics of feral plants or wild relatives and will lead to increased abundance?9

    1. Yes or More data needed—subject to weedy-relative considerations.

    2. No—Exempt from weedy-relative considerations.

In addition to the recommendations in section 3.1.4, the committee recommends that

USDA should research, publicize, and periodically revise lists of plant species with feral populations or wild relatives in the United States in order to evaluate the impacts of outcrossing.

3.4 RESEARCH NEEDS

The committee realizes that there remain some uncertainties regarding the use of pest-protected plants, including transgenic pest-protected plants. These uncertainties can lead to ambiguities in regulation and often force agencies to base their decisions on minimal data sets. Additional research should continue to refine and improve the risk assessment methods and procedures and continue to develop additional data on both conventional and transgenic pest-protected plant products. Research along the following lines should be given priority to aid in decisionmaking. These categories have been chosen on the basis of the discussions in chapter 2 and this chapter. Many of these research needs are also highlighted in the executive summary (section ES.5).

8  

The frequency of the resistance trait might vary among populations. If the resistance trait is regarded as rare, go to 4. Also, go to 4 if the resistance trait is found only in geographically isolated populations.

9  

This will require agency judgment.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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3.4.1 Health Effects Research

Methods for more efficiently and accurately identifying potential food allergens in transgenic pest-protected plants should be developed. Criteria of digestibility and overall homology with known allergens can be good indicators of allergenicity (Metcalfe 1996a), but the identification of specific protein sequences (or epitopes) involved in allergic responses, the further development of tests with human immune-system endpoints, and the development of more-reliable animal models should be pursued (section 2.5.1).

The committee suggests the establishment of a database on natural plant-defensive compounds of potential dietary or other toxicologic concern. Information needed for this database includes a clear list of what plants are used, phenotypic variation in the substances in different parts of plants, and genetic variations in different varieties. Research is needed to determine the baseline concentrations of secondary compounds in plant species of potential dietary or other toxicological concern and to determine how these compounds may vary depending on the genetic background and environmental conditions (see section 2.5.2 and recommendations in section 3.2.4).

For longterm toxicity testing, research should be conducted to examine whether longterm feeding of transgenic pest-protected plants to animals whose natural diets consist of large quantities and the type of plant material being tested (for example, grain or forage crops fed to livestock) could be a useful method for assessing potential human health impacts (see section 2.5.1).

3.4.2 Plant Breeding and Molecular Biology Research

Research on the mechanisms of pest-protection in both conventional and transgenic pest-protected plants should be encouraged so that we can produce crops that are only minimally affected by diseases and pests, deploy pest-protection strategies that have only minimal impact on the environment, and produce crops that can be consumed or used safely by humans and animals.

A major goal of current and future development of conventional and transgenic pest-protected plants should be to decrease the potential for ecological and health problems associated with some types of pest-protected plants (section 2.2.1). That includes developing breeding approaches and assays for avoiding the development of varieties with unintended high concentrations of potential toxins or decreased concentrations of essential nutrients, controlling expression of transgenes that have potential adverse nontarget effects to only nonedible plant tissues, and

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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eliminating expression of transgenes that encode resistance factors in pollen. In addition, development of strategies that enhance the effective life span, or durability, of transgenic pest-protection mechanisms is vital. Research to develop better promoters that restrict expression of transgenes to non-edible plant tissues could lead to decreased potential for food safety problems with some pest-protected plants. Research could also lead to the more efficient use of non-constitutive promoters that result in more durable pest-protection or environmental safety. Transgenic or other techniques to decrease the potential for the spread of transgenes into wild populations should be explored.

For conventional pest-protected plants and for transgenes moved by breeding to new cultivars, the linkage of pest-protection traits to other traits carried inadvertently by the breeding process should be investigated for commercial cultivars, and more research should be conducted on potential health and ecological impacts of such linkage (section 2.4.2). Recent advances in plant genomics should help to identify the biochemical and physiological function of linked genes. Similarly, research is needed to better understand potential pleiotropic effects of pest-protection genes.

3.4.3 Ecological Research

Research to increase our understanding of the population biology, genetics, and community ecology of the target pests should be conducted, so that more ecologically and evolutionarily sustainable approaches to pest management with pest-protected plants can be developed ( section 2.6). Knowledge of pests' roles in the larger biological community (for example, their role as food sources for nontarget organisms or their roles as predators of other agriculturally relevant pests) will allow us to anticipate better the indirect effects of declines in the pests due to both conventional and transgenic pest-protected plants. Knowledge of the pest population biology will enable prediction of the types of pest-protection mechanisms that would most efficiently reduce a target organism' s pest status (Kennedy et al. 1987) and would help us to design more accurate resistance management plans (Gould 1998).

Research to assess gene flow and its potential consequences should be conducted (section 2.7). A list of plants with wild or weedy relatives in the United States should be established in an accessible public database (see section 3.3). This database should include the geographic locations of these relatives and could be used to determine which crop-weed complexes should be regulated. For weed species of concern (plants that might hybridize with transgenic pest-protected plants), more ecological and agricultural research is needed on the following: weed distribution

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×

and abundance (past and present), key factors that regulate weed population dynamics in managed and unmanaged areas, the likely impact of specific, novel resistance traits on weed abundance in managed and unmanaged areas, and rates at which resistance genes from the crop would be likely to spread among weed populations.

Because it is sometimes difficult to predict ecosystem level effects from small scale laboratory and field tests, longterm monitoring of pest-protected crops should be conducted after commercialization of these crops. EPA and USDA's Agriculture Research Service and Animal Health Plant and Inspection Service should encourage long-term monitoring for ecological impacts. Also, more rigorous field comparisons should be conducted to determine the relative impacts of conventional and transgenic pest-protected crops compared to impacts of standard and alternative agricultural practices on nontarget organisms.

Further studies are needed to determine the distances and densities of biologically active Bt corn pollen in the vicinity of a crop. More information is needed about the timing of pollen release, the types of insect species that would be harmed by ingesting pollen at observed concentrations, and the magnitude of mortality due to pollen versus other factors that limit nontarget populations.

3.5 RECOMMENDATIONS

  • EPA should provide guidelines for determining the most ecologically relevant test organisms and test procedures for assessing nontarget effects in specific cropping systems.

  • The USDA should require original data to support agency decision-making concerning transgenic crops when published data are insufficient.

  • In cases when crucial scientific data are lacking about the potential impacts of gene flow on wild or weedy relatives (for example, squash case study), the committee recommends delaying approval of deregulation pending sufficient data (for example, surveys from several years in several regions), establishing a scientifically rigorous monitoring program in key areas to check for undesirable effects of resistance transgenes after the transgenic pest-protected plant is commercialized, or restricting the initial areas where the plants can be grown.

  • USDA should research, publicize, and periodically revise lists of plant species with feral populations or wild relatives in the United States in order to evaluate the impacts of outcrossing.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
×
  • The EPA, FDA, and USDA should collaborate on the establishment of a database for natural plant defensive compounds of potential dietary or other toxicological concern.

  • Given that transfer and manipulation of genes between sexually compatible plants could potentially result in adverse effects in some cases (for example, modulation of a pathway that increases the concentration of a toxicant), and given public controversy regarding transgenic products, EPA should reconsider its categorical exemption of transgenic pest-protectants derived from sexually compatible plants.

  • EPA should not categorically exempt viral coat proteins from regulation under FIFRA. Rather, EPA should adopt an approach, such as the agency 's alternative proposal, that allows the agency to consider the gene transfer risks associated with the introduction of viral coat proteins to plants.

  • EPA should review exemptions of transgenic pest-protected plant products to ensure that they are consistent with the scientific principles elucidated in this report.

Suggested Citation:"3. Crossroads of Science and Oversight." National Research Council. 2000. Genetically Modified Pest-Protected Plants: Science and Regulation. Washington, DC: The National Academies Press. doi: 10.17226/9795.
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Next: 4. Strengths and Weaknesses of the Current Regulatory Framework »
Genetically Modified Pest-Protected Plants: Science and Regulation Get This Book
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This book explores the risks and benefits of crops that are genetically modified for pest resistance, the urgency of establishing an appropriate regulatory framework for these products, and the importance of public understanding of the issues.

The committee critically reviews federal policies toward transgenic products, the 1986 coordinated framework among the key federal agencies in the field, and rules proposed by the Environmental Protection Agency for regulation of plant pesticides. This book provides detailed analyses of:

  • Mechanisms and results of genetic engineering compared to conventional breeding for pest resistance.
  • Review of scientific issues associated with transgenic pest-protected plants, such as allergenicity, impact on nontarget plants, evolution of the pest species, and other concerns.
  • Overview of regulatory framework and its use of scientific information with suggestions for improvements.
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