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s Tactics for Prevention and Management THE FREQUENCY OF RESISTANCE in a pest population is in large part a result of selection pressure from pesticide use. Strategies to manage resistance aim to reduce this pressure to the minimum, using tactics designed to increase the useful life of a pesticide and to decrease the interval of time required for a pest to become susceptible to a given pesticide again (Chapter 31. Strategy is used here in the sense of an overall plan or methods exercised to combat pests, whereas tactic is used to mean a more detailed, specific device for accomplishing an end within an overall strategy. This chapter will focus on promising strategies and tactics. Judicious use of pesticides reduces the selection pressure on pest pop- ulations for developing resistance. Use of pesticides only as needed not only avoids or delays resistance but tends to protect nontarget beneficial species. These practices are an essential part of Integrated Pest Manage- ment (IPM), which implies the optimum long-term use of all pest-control resources available. Excessive use or abuse of pesticides for short-term gains (e.g., minor yield increase) may be the worst possible practice long- term because it may lead to the permanent loss of valuable, efficient, and often irreplaceable pesticides. Such practices represent a serious issue affecting all segments of society. Catastrophic events, such as the failure of an entire pesticide class against a target species, have in the past, and may again in the future, force dramatic changes in our crop production and pest-control practices. Genetic, biological, ecological, and operational factors influence devel- opment of resistance. Operational factors, including pesticide chemicals and how they are used, obviously can be controlled (Georghiou and Taylor, 1977; 313
314 TACTICS FOR PREVENTION AND MANAGEMENT Georghiou, this volume). The biological factors are considered beyond our control, but current studies in biotechnology and behavior have shown that components of genetic, reproductive, behavioral, and ecological factors may be manipulated and have potential for use in management (Leeper, this volume). While the basic principles of resistance management apply to all major classes of pests (insects, pathogens, rodents, and weeds), there are some important differences among these classes that influence the applicability of management strategies and tactics. Tactics are site and species specific. For example, many insects and plant pathogens have considerable mo- bility, whereas rodents and weeds generally have less. The usefulness of maintaining refuges can vary substantially among pest classes. Weed seeds, egg sacs of some nematodes, and the resting structures of some plant pathogenic fungi may remain dormant in soils for many years, thus pre- serving susceptible germ plasm. This does not occur for other classes of pests. Rates of reproduction, population pressure, and movement of sus- ceptible individuals from refuges into a treated area are often very high with plant pathogens, moderate to high with insects, and comparatively low for weeds and rodents (Greaves, this volume). The residual nature or persistence of pesticides varies greatly, which will affect the success of various tactics to manage resistance. Generally, the greater the persistence, the greater the probability of resistance. The number of target species being controlled with a given pesticide varies with the class of pest. Biological control agents are critical for many insect pests but have not yet become as important in control of pests in other classes. Other dif- ferences exist, but their strategic significance is poorly understood. Some of the most important issues that impinge on the development and selection of management tactics are: differences among classes of pests and pesticides; dynamics of resistance (differences between high- and low-risk pesticides, and variations in the rate of resistance development within species and geographic areas); complexes of pests on crops or locations requiring multiple pesticides for control; and lack of supporting data and validation in the field. Pesticides considered to be at high risk for resistance generally have a single site of toxic action and, in fungicides, are usually systemic, while low-risk compounds have multiple sites of action. Our current insec- ticides and most of our new systemic fungicides tend to have single sites and would, therefore, fall within the high-risk category. On the other hand, few plants have evolved resistance to herbicides, which also tend to have single sites of action. Although experience with inorganic insecticides (i.e., lead arsenate) shows that resistance can also develop to multisite compounds, such resistance is rare. The rate at which pesticide resistance develops is extremely variable among species as well as among different field populations of the same species.
TACTICS FOR PREVENTION AND MANAGEMENT 315 Rate of reproduction, pest movement, relative fitness of resistant members of a population, mechanisms of resistance, etc., all contribute to the dy- namics of resistance and determine the severity of its effect on economic efficacy and the viability of continued use of a given compound. Therefore, the applicability of specific management tactics must be established on the basis of specific cases and locations. Although resistance poses a most serious threat to a pesticide's economic life and has resulted in total loss of previously valuable chemicals from some major pest-control programs, no pesticide has been lost from the marketplace solely because of resistance. Resistance is not absolute throughout a pest's range, and susceptible populations of some pests continue to exist. Further- more, in an area where resistance has occurred, a pesticide's continued use may be required to control other pests that are still susceptible. This may confound management attempts, but documented cases of resistance do not necessarily warrant removal of a pesticide. On the other hand, industry has a responsibility to adjust marketing plans (and perhaps propose label changes) to reflect a product's efficacy or inef- ficacy, leaving the marketplace to determine its actual value and life. In addition, public-sector research, extension, and regulatory programs have a key role to play in ensuring that growers are completely informed of resistance situations that are identified, so that rational decisions can be made among pest-control alternatives. Several major deficiencies in scientific understanding currently frustrate efforts to develop and implement tactics to manage resistance. Resistant strains of pests selected in the laboratory may differ from field strains in some ways, including fitness and number of alleles conferring resistance. Therefore, tactics should be validated for a wide range of pests under field as well as laboratory conditions. Monitoring technologies must be developed to evaluate the strategies, validate the tactics, accurately determine critical resistance frequencies for pests under different conditions, and guide the implementation of optimum tactics (Chapter 41. TACTICS FOR RESISTANCE MANAGEMENT Several concepts discussed below have been proposed as tactics for man- aging specific cases of resistance. Most of these tactics have been used, often inadvertently or without confirming data, in pest-control practices. Owing to lack of rigorous field and laboratory evaluations, our inability to establish and detect critical frequencies of resistance, and the limitations of space, no attempt is made here to detail the strengths and weaknesses of the tactics. Sweeping generalizations about the applicability or feasibility of specific tactics are not justified. These caveats must be kept in mind in interpreting the data presented in Table 1. The ratings are usually only valid within the
316 TACTICS FOR PREVENTION AND MANAGEMENT TABLE 1 Tactics for Management of Resistance to Pesticides and Their Suitability for Classes of Pests Tactics Insecticides Fungicides Herbicides Rodenticides + + (T) + + (T) + + (T) + + (T) + + (T) Variation in dose or rate Frequency of applications Local rather than areawide applications Treatments only to economic threshold Less persistent pesticides Life stages of pest Pesticide mixtures Alternations, rotations, or sequences of pesticide applications Pesticide formulation technology Synergists Exploiting unstable resistance Pesticide selectivity New toxophores with alternate sites of action Protection and use of natural enemies with pesticide tolerance Reintroduction of susceptible pests Code for Suitability Ratings: +++ + +++ + + + + + (T) + + (T) + + + + (T) ++ + + + + + (T) + + (T) + + + (T) + + + + +(T) + +(T) + + + + (T) + (T) +++ +++ + o O O o o o + + + o + + + (T) + + +++ +++ + (T) + + + + (T) o + O(-) O(-) +++ +++ ++ + O O + (T) o O O(-) + + + Very useful, generally supported by laboratory data and/or field experience. + + Moderately useful. + Minor use, in exceptional cases only, or supported by few data. O Not applicable or assumed to be of no value. (T) Supported by theoretical assumptions only. No data or experience. ( - ) May actually be detrimental to managing resistant populations. The suitability ratings presented in this table are very tenuous, may be theoretical or supported only by a few examples, and should not be assumed to be generally valid for each pesticide, pest, or tactic within each class. limitations of a few examples, often weakly supported, for each tactic within each pest group. Variation in Dose or Rate With this tactic, resistance may be delayed or minimized by preserving a sufficient population of susceptible individuals or alleles by using low rates
TACTICS FOR PREVENTION AND MANAGEMENT 317 of a given pesticide so as not to select against heterozygotes where resistance is recessive. On the other hand, the use of high doses has also been proposed, but as a means of eliminating or reducing the frequency of heterozygotes where resistance is dominant. While laboratory studies have supported the latter approach with insecticides, there is limited evidence to confirm its success under field conditions with the possible exception of some pests of stored grain. Using fungicides at dosage rates giving less than 100 percent control may minimize the threat of resistance, if low levels of disease can be tolerated or if a high level of resistance may occur (e.g., benomyl or metalaxyl). If resistance is linked to decreased fitness, however, or if low levels of resistance are likely to occur (e.g., dicarboximides), high dose rates might be recommended to control all individuals in the populations. Also, because of the explosive reproductive capacity of some pathogens or the high premium paid for a totally disease- and insect-free crop (e.g., apple scab and coaling moth), some disease situations require virtually total control. There is no proof that herbicide-use rate has any effect on the development of resistance in weeds, although circumstantial evidence indicates that high rates may favor resistance. Because of the short generation time of rodents, any treatment that leaves significant numbers of survivors fosters selection for resistance. Both low concentrations in baits or inadequate applications fit this category. Unfortunately, specific field data are lacking. Frequency of Application Fewer or less frequent applications, which reduce the selection pressure over time, should reduce the rate and probability of resistance development. This tactic is assumed to be valid for management of resistance to insecticides, but it is unconfirmed. Circumstantial-evidence indicates that in areas where a fungicide is used only once or twice a season, the threat of resistance development is reduced compared to full season programs. For example, in northern Europe, resistance quickly developed when metalaxyl was used full season to control late blight. Based on limited experience, it may be possible to continue cautious use of such fungicides even after resistance has devel- oped. A specific herbicide is most commonly used only once per crop season. Postemergent herbicides or those having brief soil activity could be applied several times, especially in perennial crops, but this would tend to increase selection pressure for resistance. Paraquat-resistant weeds have occurred in a few areas following frequent applications of this herbicide. If applications of rodenticides are made monthly (as by a Pest Control Operator EPCOl), the selection pressure would be persistent and could speed selection. Treat- ments once or twice a year (as with urban rat control programs) would be nearly as efficient in selecting for resistance, however, because removing susceptible individuals from each generation as it reaches reproductive age speeds selection.
318 TACTICS FOR PREVENTION AND MANAGEMENT Local Rather Than Areawide Applications Control of a pest with a particular pesticide in a single field or site, rather than over a large area, can leave refuges in surrounding areas to thwart resistance development; this is believed to be a useful tactic, especially with insecticides. Susceptible individuals move into previously treated areas, thus diluting the frequency of resistance. The success of this tactic may vary with insect species, refuges, and other factors. In some cases, an areawide ap- plication of the right insecticide can severely reduce a particular generation of specific insects when properly timed, thereby reducing or eliminating the need for further applications. Plants, even weeds with seeds that are easily spread, are not sufficiently mobile to allow this tactic to be very successful with herbicides; seeds probably serve more often as a way of introducing alleles conferring resistance than of moving in large enough numbers of susceptible alleles to swamp those conferring resistance. Some plant pa- thologists feel that this tactic is not appropriate for airborne pathogens with potential for resistance under high population pressure. For example, resis- tance has often occurred when metalaxyl was used to stop heavy infestations of late blight (potatoes) or blue mold (tobacco). When metalaxyl has been used over a wide area as a preventive treatment before the disease started, however, resistance has not developed in these pathogens, at least in North America. On the other hand, some experts suggest that we should "confuse" the pathogen by localized use of two or more fungicides having different mechanisms of action, together with multiple cultivars that have a number of alleles conferring resistance to the pathogen (although the latter tactic assumes a single fungicide is used in the area). To the extent that resistant rodents are considered less fit competitors (the British view), localized control would result in islands of resistance that would not readily spread. Areawide control, however, is likely to result in areawide resistance (as in Denmark) (Greaves, this volume). Treatments Based on Economic Threshold This tactic delays pesticide applications until the economic threshold is reached and may allow a certain level of crop damage to occur. This is a means of reducing the selection pressure for resistance. The success of this tactic in managing insecticide resistance varies with the insect pest and con- ditions. The establishment of valid economic thresholds and the use of pes- ticides only when the threshold is exceeded is a major principle of IPM. The economic threshold often varies because it depends on commodity prices. The benefit of this tactic in managing resistance to fungicides is generally unconfirmed. It may be applicable with less virulent or localized plant path- ogens, when total disease control is not necessary, or when the disease occurs
TACTICS FOR PREVENTION AND MANAGEMENT 319 only occasionally. It is probably not useful for the more virulent and systemic diseases. Under current management practices, this tactic is only of marginal benefit in limiting resistance in weeds and rodents. Introduction of the newer postemergence herbicides, however, provides the potential to exploit this tactic to control weeds, based on the number of weeds that compete for resources with the cultivated plant. Use of Less Persistent Pesticides The selection and use of pesticides or formulations having a lower bio- logical persistence can be a useful tactic for managing resistance. Insecticides with short residual lives tend to slow the development of resistance due to reduced exposure, but success may depend on the nature of the insect and insecticide. Persistence of a fungicide will always prolong the period of selection pressure and thus favor the build-up of resistance. It is important to point out that a less persistent fungicide applied more frequently will have the same effect on resistance (e.g., a 14-day treatment schedule of one fungicide versus a 7-day schedule of another with half the persistence). Relatively long persistence and excellent control of most weeds are believed to be mainly responsible for the numerous occurrences of triazine-resistant weeds. This tactic is not considered to be applicable to rodenticides. Life Stages of Pest This tactic is based on using a pesticide against the life stage of the target pest that is not so likely to develop resistance. For example, in some lepi- dopterous species the adults and/or very early larval stages (instars) are apparently less able to metabolize insecticides than are late instars. In theory, the rate of developing resistance would be lessened by targeting insecticides against the adults or early instars, thereby reducing the selection pressure on later instars that have a higher resistance risk due to their greater enzymatic activity for pesticide metabolism. Applying a fungicide during the sexual stage theoretically should increase the chance of selecting for a higher level of resistance in the fungus. On the other hand, when fungicides have been applied during the asexual stages (e.g., late blight and apple scab), resistance has developed very rapidly. This tactic is not applicable to herbicides or rodenticides. Mixtures Simultaneous use of two or more pesticides having differing mechanisms of action or target sites (Chapter 2) has been and will continue to be a very important tactic to avoid and manage resistance. Certain limitations and
320 TACTICS FOR PREVENTION AND MANAGEMENT conditions must apply for this tactic to be successful in managing resistance in insects and other pests. The use of mixtures must start early before resis- tance occurs to one of the components (unless negatively correlated toxicity or enhanced susceptibility is present), each component must have similar decay rates (preferably short stability), and they must have different modes and sites of action or different resistance mechanisms (with fungicides, sim- ilar translocation). Nevertheless, resistance to two or more different insec- ticides can develop by the same process as with a single pesticide it just takes longer. Mixing chemicals sometimes leads to potentiation, rather than merely additive effects, thus delaying or preventing resistance even further. In case of established resistance, potentiation may become the only means of controlling the pest (V. Dittrich, Ciba-Geigy Corporation, Baste, personal communication). Mixtures are assumed to be an important tactic in avoiding or delaying the development of resistance to single-target-site fungicides by plant pathogens. Limited laboratory data show that mixed populations of resistant and susceptible Phytophthora infestans shifted to the resistant pop- ulations more slowly when mixtures were used. On the other hand, some reports indicated that resistance to a specific site-inhibitor fungicide can continue to increase when one is used in combination with a multisite fun- gicide, due in part to the pathogen population's not being controlled by the multisite inhibitor (e.g., lack of translocation). The use of mixtures has been a major tactic in preventing both the development and spread of weed re- sistance to herbicides. Resistant weeds have not usually occurred where herbicide mixtures are used, but triazine-resistant weeds have often developed after 5 to 10 years where this class of herbicide has been applied alone and frequently. Once resistant weeds have developed in an area, they usually take over completely if the single-problem herbicide is used exclusively. The use of mixtures has not been a usual tactic for rodenticides. To mix an acute poison with an anticoagulant is illogical. Mixing of warfarin and vitamin D (calciferol) in England seems not to have enhanced efficacy significantly. Alternations, Rotations, or Sequences of Pesticide Applications The use of pesticides of differing classes or modes and sites of action in rotation, alternation, or sequence to control the same pests has been much studied and accepted to avoid resistance. It assumes that the number of generations or length of time between uses of any one material is sufficient to allow resistance to decline below a critical frequency (Georghiou, 1980; Georghiou et al., 19831. Whether this tactic is superior to pesticide mixtures and the optimum sequence, frequency, and rate of each component will likely vary according to the pest, pesticide, and other factors. It is based on the relative instability of particular resistance mechanisms and is especially viable when it is known that cross-resistance does not occur. Annual rotation or
TACTICS FOR PREVENTION AND MANAGEMENT 321 alternation is probably not a good strategy for many high-risk fungicides because resistance can develop within one growing season, but sequences of applications of different fungicides is often quite useful. Rotation of lower- risk compounds (e.g., ergosterol biosynthesis inhibitors) may be an accept- able way to prolong the life of a fungicide. As with some of the other tactics, voluntary compliance or enforceability often prevents the general use or success of this tactic in management of resistance to fungicides. Although no direct evidence documents the effectiveness of annual rotations for man- agement of resistance to herbicides, abundant circumstantial data support the use of annual rotations or alternations of herbicides. This has not been used as a resistance-avoiding tactic, but has been used inadvertently due to the very common practice of rotating crops, mainly for other reasons, which usually requires different herbicides to avoid crop phytotoxicity and to max- imize control of the different weeds. Variable sequences of different her- bicides during a crop season are often used to control or manage resistant weeds once they have developed. Mixing or alternating anticoagulants is ineffective because of cross-resistance in rodents. However, the use of an acute rodenticide alternately (or periodically) in a control program with an- ticoagulants is thought to be the best way to prevent resistance from being selected (Greaves, Jackson, this volume). Pesticide Formulation Technology Although additional research is needed to substantiate this tactic, formu- lation technology can be used in several ways to combat pesticide resistance. It can reduce the dose or rate of pesticide applications. Synergists, adjuvants, penetrants, and materials that improve bait attractancy can be incorporated into pesticide formulations. If resistance is due to differential penetration of an insecticide, the adjuvants or penetrants used in the formulation could be useful to delay or reduce resistance. Changing the attractant in an insect bait could modify the effectiveness or potential resistance to a less effective attractant. Controlled release or longer residual type formulations might en- hance the rate of resistance development due to longer selection pressure, but this has not been sufficiently tested and would depend on other factors, such as the life span of the target pest species and the effect of low levels of the insecticide on insect reproduction. No data are available, but the same factors would likely apply to fungicides and herbicides, except for bait at- tractants. Poorly formulated rodenticide baits could enhance the selection for anticoagulant resistance because these compounds require multiple feedings to be effective. Baits with low palatability will be insufficiently consumed, thus leaving significant numbers of survivors and fostering the selection for resistance. Other factors discussed above also would likely apply (Jackson, this volume).
322 TACTICS FOR PREVENTION AND MANAGEMENT Synergists The use of pesticide synergists as a tactic for resistance management has been of special interest, but further study is required to evaluate the practical use of this tactic. It is generally based on the use of a second chemical that counteracts or inhibits the mechanism responsible for resistance to the pes- ticide. Insecticide synergists inhibit specific detoxification enzymes and thus can reduce or eliminate the selective advantage of individuals possessing such enzymes. Synergists as inhibitors of oxidases (e.g., piperonyl butoxide), dehydrochlorinase (e.g., chlorfenethol), esterase (e.g., DEF), and other more recent enzyme inhibitors have found some use in field applications. Their utility to inhibit the evolution of resistance would depend on the absence of an efficient, alternative mechanism of resistance in the target population. The relatively high cost of the synergist, formulation problems, the potential synergism of mammalian toxicity, and the high level of biochemical adap- tation in some major insects (e.g., house fly), have militated against their use. Increased rate of metabolism by the target pathogen is not a common mechanism of fungicide resistance, but it does occur in a few cases. Fur- thermore, it has been shown that synergism may counteract development of resistance (e.g., a fungicide that inhibits respiration has increased the uptake of fenarimol by a fenarimol-resistant strain, thereby making the resistant strain again sensitive). The use of synergists may not be applicable with herbicides. Some synergistic interactions between herbicides (e.g., atrazine and tridiphane) have been reported due to reduced metabolism of atrazine triggered by an enzyme inhibition from tridiphane. Herbicide resistance, however, has not been due to enhanced metabolism of the herbicide by the resistant weeds. A combination of antibiotic (to reduce production of vitamin K by gut bacteria) with anticoagulant (Prolin@) appeared to give no field advantage to the formulation and would not be expected to impact on resis- tance development with rodenticides. Other synergist-type compounds have not been suggested. Exploiting Unstable Resistance Pesticide resistance often carries with it, especially during its original development, some deficiencies in fitness, vigor, behavior, or reproductive potential. These characteristics often make the resistant biotype of the target pest more susceptible to other control measures. Unstable resistance can be exploited by using other insecticides or control programs to control resistant insects preferentially or selectively until resistance diminishes. Resistant plant pathogens may be unstable at time of initial mutation or development and should be more easily controlled then. It is important to determine if resistance is stable, fit, and genetically based. By use of fungicides in which resistance
TACTICS FOR PREVENTION AND MANAGEMENT 323 is associated with a lack of fitness, the resistant mutants would not survive when the selection pressure is removed. Weed biotypes resistant to herbicides are usually less fit or competitive than the susceptible population and may be more easily controlled with alternate herbicides. In England resistant rats reportedly have higher vitamin K demand than normal rats and thus do not survive well (Greaves, this volume), although resistance in monitored English populations seems to have reached an equilibrium point rather than decreasing toward extinction. Pesticide Selectivity Selective insecticides often eliminate the pest species while preserving or causing less injury to the predators and beneficial insects. This is an IPM approach and will help to delay or prevent resistance development by pro- viding additional mortality factors for resistant pests. A selective pesticide is often a specific single-target-site chemical with a higher resistance risk, but this danger might be alleviated somewhat by using less specific pesticides applied more selectively, for example in baits, as systemic insecticides in furrow, or as seed treatments. This approach is the most useful in management of resistance in insects and mites. The use of compounds with multisite action has not been a tactic to manage resistance in weeds or rodents. New Toxophores with Alternate Sites of Action The discovery and development of new pesticides has often been viewed as a major approach to management of resistance to earlier pesticides. Re- placing older pesticides with new ones because of pest resistance has never been the primary objective of this predominately industrial activity, however. It is obvious that future priorities in pesticide development should give more attention to new or alternate target sites that will have lower risk of resistance development. While we need to encourage new discoveries, we must do everything possible to preserve all of our present pesticides. This strategy is a vital and relatively long-term solution to the control of pests resistant to current pesticides, but it can never be a permanent solution. Pests are likely to evolve various means to survive any new pesticides and other control measures. It is also becoming more difficult and expensive to make new and novel chemical discoveries. We are fortunate to have available many types of herbicides with different modes of action, but we can still benefit from breakthroughs in new chemistry to control resistant or problem weeds in certain crops. Development of new types of rodenticides has contributed to resistance management in recent years. New materials include bromethalin, vitamin D3, and alphachlorhydrin.
324 TACTICS FOR PREVENTION AND MANAGEMENT Protection and Use of Natural Enemies with Pesticide Tolerance The intentional protection of natural enemies of pests or the introduction of such predators, especially those with natural or induced tolerance to the specific pesticide, has become a tactic of much interest in resistance man- agement. The development and release of predators with some level of re- sistance has shown promising results in managing insecticide resistance. Such predator resistance is usually intentionally developed by laboratory exposure during several generations. Genetic engineering offers even more potential in this area. It should be pointed out that the use of several of the previously described tactics will tend to interfere with or counteract this tactic. The use of resistant beneficials has not been used to manage fungicide resistance successfully. Laboratory data indicate that it could be a useful tactic where populations of soil antagonist strains of microorganisms (e.g., Trichoderma and Gliocladium) are used in an IPM approach. The use of resistant bene- ficials is not applicable to herbicides and is often not compatible with ro- denticide use. With most rodents, their predators are slow breeding, are unable to match the rapid build-up of mouse and rat numbers, and are ineffective in structured urban environments. Reintroduction of Susceptible Pests Increasing or encouraging the immigration of susceptible pest genotypes can be effective in dealing with a small insect population with a high pro- portion of resistant individuals. This tactic shifts the population away from a critical frequency of resistance. The reintroduced susceptibles must be numerous enough to swamp the endemic, resistant population, thereby re- ducing the likelihood of mating between resistant individuals (Suckling, 1984~. This tactic is often most applicable where pest control is not intensive. It is not likely to be an appropriate tactic for managing resistant fungi, weeds, or rodents. RECOMMENDATIONS 1. Efforts should be expanded to develop IPM systems and steps taken to encourage their use as an essential feature of all programs to manage resistance. 2. Increased research and development emphasis should be directed to- ward laboratory and field evaluation of strategies and tactics for preventing or slowing resistance development, including efforts to: a. Develop models, to be tested in laboratory and field experiments, to assist in formulating hypotheses on managing resistance.
TACTICS FOR PREVENTION AND MANAGEMENT 325 b. Develop and validate sampling and bioassay techniques for mon- itoring low levels of resistance. c. Identify chemicals with negatively correlated cross-resistance and develop rotations or mixtures based on this information. d. Determine stability of resistance in pest populations to specific pesticides. e. Evaluate alternations and rotations of pesticides shown by research findings or field experience to have high potential as tactics to manage resistance. Design rotation schedules that will maintain acceptable levels of susceptibility. f. Investigate inlaboratory end geld basic genetic, toxicological, and ecological factors that influence the rate of resistance devel- opment. g. Use traditional and biotechnological genetic methods to produce pesticide-resistant biological control agents and herbicide-resistant crop plants. h. Investigate pest migration and the factors that influence it to de- termine the potential for assessing the spread of resistant forms to new areas and the reinvasion of resistant populations by susceptible pests from refuges. 3. Population biologists, toxicologists, and modelers should be involved in designing and executing research and validation efforts. 4. The private sector, extension personnel, and regulatory agencies should encourage the use of promising tactics to manage resistance, while attempting to confirm or validate their usefulness (Davies, 1984~. 5. As part of overall IPM strategy to manage resistance, increase efforts to understand and use components of those genetic, reproductive, behavioral, and ecological factors that may minimize the need for pesticide use and reduce resistance development. 6. The traditional method for dealing with resistance has been to switch to a new pesticide. This does not address the problem of resistance, but at best simply delays its recognition and may exacerbate it through cross-re- sistance. For these reasons and because further discovery and development of new and better pesticides is uncertain, greater efforts must be made to conserve existing materials as finite resources. 7. Do not depend totally or too much on any one pesticide or means to control any pest, especially with high-risk pesticides against major pests. 8. When resistance occurs, move promptly to take necessary actions and apply the best tactics to manage the resistance with all tools and technology we have available. 9. Encourage the use of crop rotations so that different herbicides will be used in successive seasons on different crops. 10. Industry should continue to search for and develop new toxophores
326 TACTICS FOR PREVENTION AND MANAGEMENT and, in some cases, new synergists, with emphasis on new mechanisms or approaches (e.g., behavioral-type insecticides, multisite fungicides, etc.), rather than to kill the pest by direct, immediate, and single-site action. REFERENCES Davies, R. A. H. 1984. Insecticide resistance: An industry viewpoint. Pp. 593-600 in 1984 Proc. Br. Crop Prot. Conf. Pests and Dis. Georghiou, G. P. 1980. Insecticide resistance and prospects for its management. Residue Rev. 76:131-145. Georghiou, G. P., A. Lagunes, and J. D. Baker. 1983. Effect of insecticide rotations on evolution of resistance. Pp. 183-189 in IUPAC Pesticide Chemistry, Human Welfare and the Environment, J. Miyamoto et al., eds. Oxford: Pergamon. Georghiou, G. P., and C. E. Taylor. 1977. Operational influences in the evolution of insecticide resistance. J. Econ. Entomol. 70:653-658. Suckling, D. M. 1984. Insecticide resistance in the light brown apple moth: A case for resistance management. Pp. 248-252 in Proc. 37th N.Z. Weed and Pest Control Conf. WORKSHOP PARTICIPANTS Tactics for Prevention and Management HOMER M. LEBARON (Leader), Ciba-Geigy Corporation DANIEL ASHTON, Bowling Green State University AHMED NAss~R BALLA, Agricultural Research Corp., The Sudan FAUSTO C~sNERos, The International Potato Center, Peru R. A. H. DAv~Es, ICI Plant Protection Division, Great Britain DoNA~D E. Davis, Auburn University JOHAN DEKKER, Agricultural University, Wageningen, The Netherlands TIMOTHY J. DENNEHY, Cornell University VOLKER DITTRICH, Ciba-Geigy, Ltd., Switzerland GEORGE P. GEORGHIOU, University of California, Riverside EDWARD H. G~Ass, New York State Agricultural Experiment Station, Cornell University KENNETH S. HAGEN, University of California, Albany WAYNE HARNISH, FMC Corp. WILLIAM B. JACKSON, Bowling Green State University JOHN R. LEEPER, E. I. du Pont de Nemours and Company JAMES V. PAROCHETTI, U.S. Department of Agriculture FRED W. SEIFE, University of Illinois HOWARD WEARING, DSIR, New Zealand