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Pesticide Resistance: Strategies and Tactics for Management. 1986. National Academy Press, Washington, D.C. The Role of Cooperative Extension and Agncultural Consultants in Pesticide Resistance Management RAYMOND E. FRISBIE, PATRICK WEDDLE, and TIMOTHY I. DENNEHY Cooperative extension and private agricultural consultants can provide educationalprograms and service, respectively, in managing pesticide resistance. Training programs that include a background of pesticide resistance, identifying pests with a high resistance risk, recommending tactics that are compatible with integrated pest man- agement (IPM', and demonstrating techniques for measuring resis- tance frequency should be initiated by cooperative extension. Both cooperative extension and private consultants have a role to play in monitoring pest susceptibility to pesticides and establishing pesticide resistance management programs. Pesticide resistance management is an integral part of IPM. The USDA National Agricultural Pesticide Impact Assessment Program (NAPIAPJ should receive increased funding to expand pesticide resistance management through the state agricultural experiment stations and cooperative extension. INTRODUCTION The development of pesticide resistance, specifically insecticide resistance, was one of the most significant factors resulting in the formation of pest- management programs by cooperative extension. Pesticide resistance was also elemental in American farmers more readily accepting the services of professional agricultural consultants. Farmer organizations have rallied in support of integrated pest management (IPM), not because it offers any mystical solutions but because it represents a system of pest control that is 410
COOPERATIVE EXTENSION AND AGRICULTURAL CONSULTANTS 411 dynamic, economically sound, and offers multiple tactics that have potential for delaying the onset of pesticide resistance. The role of cooperative extension and its responsibility in implementing agricultural IPM programs is clear; it bears primary educational responsibility within the land-grant university system to inform American farmers and ranchers on the most advanced techniques, strategies, and tactics to manage pests. Because the way pesticides are selected and used strongly influences the implementation of IPM programs (Metcalf, 1980; Georghiou, 1983), cooperative extension and private consultants must assume an active role in pesticide management. Private agricultural consultants provide clientele with expert advice and service for the economic management of pests. For purposes of our discussion the role of cooperative extension will be viewed primarily as one of education and that of the private consultant as one of service. We fully recognize that there is no clear distinction between education and service, since both ex- tension and consultants frequently cross these lines in meeting their educa- tional and professional objectives. Extension specialists and private consultants deal with pest control on a day-to-day basis, in its most practical sense. Both are practitioners of pest management. Their programs and recommendations are based on information from state, federal, and private research and are tempered by their field experience and judgment. Because of their intimate contact with agricultural production, they are usually one of the first on the scene when pesticide resistance occurs. They often shoulder the responsibility of determining the preliminary causes of pesticide control failures; this activity usually requires close cooperation between research agencies and the pesticide industry. They are frequently faced with trying to seek options to regain control in a relatively short time when a pesticide control failure occurs. In most cases this is a reactive rather than a proactive encounter with resistance. From a survey of several states, we found that most deal with pesticide resistance on an ad hoc basis; that is, they go from one crisis to the next. The purpose of our discussion is to provide a framework for the development and application of resistance management programs by cooperative extension and private consultants. This framework emphasizes the importance of co- operative extension and agricultural consultants working with farmer orga- nizations, the chemical industry, federal and state research agencies, and state departments of agriculture to implement a reasonable and ongoing policy of pesticide resistance management. One has only to review the many reported cases of pesticide resistance to identify the large scope of the problem, as well as salient trends regarding the circumstances that have resulted in severe resistance problems. Metcalf (1980) recognized the importance of pesticides, but also stressed that "the only reasonable hope of delaying or avoiding pest resistance lies in IPM
412 MANAGEMENT OF RESISTANCE TO PESTICIDES programs that decrease the frequency and intensity of genetic selection by reduced reliance on insecticides and alternatively rely on multiple inter- ventions in insect population control by natural enemies, insect diseases, cultural manipulations, and host plant resistance. " Therefore, the principle of using alternatives to chemicals for suppression of pests is well estab- lished, although this does not entirely supplant chemical controls in many IPM systems. We have observed a positive and general trend toward increased use of biologically and ecologically sound alternatives to chemicals in many crop- ping systems (Huffaker, 1980; Metcalf and Luckman, 1982; Croft and Hoyt, 1983), but we feel that measures are generally not being taken to promote maintained efficacy of the diminishing number of IPM-compatible pesticides available to agriculture. Resistance management is not a major consideration of practitioners in the selection and use of pesticides. Therefore, we suggest that pesticide management, and more specifically pesticide resistance man- agement, deserves more intensive attention from extension, private consul- tants, research, regulatory agencies, the chemical industry, and farmers, especially with respect to the management of resistance to IPM-compatible . . pesticides. In order for pesticide resistance management to become an integral part of education and service programs, certain factors must be considered. First, we must understand the rationale or psychology of pesticide users and those who recommend the use of pesticides. Although farmers adopt- ing IPM practices have made significant strides in the selection, timing, and application of pesticides, evidence suggests that pesticides are still used as indiscriminate mortality factors. Those who recommend or use pesticides are generally not concerned with the class of pesticide, mode of action, potential risk as a candidate for resistance, and in some cases may still not be concerned with their impact on natural enemies. Coop- erative extension, therefore, must become more aware of resistance man- agement strategies and must work to further develop such strategies into ongoing IPM programs. The role of extension in pesticide resistance management should be one of educational leadership. Private consultants should learn about and practice pesticide resistance management and share this information with their cli- entele and other agricultural consultants. Chemical producers should promote product stewardship by interfacing with research and extension and aug- menting and supporting resistance management. Regulatory agencies should consider resistance problems in their evaluations of benefit and risk associated with specific compounds and should consider the impact that regulations may have on resistance management programs. Farmers should strive to stay abreast of information relating to resistance and resistance prevention that could be of value in their farming operations.
COOPERATIVE EXTENSION AND AGRICULTURAL CONSULTANTS TRAINING COOPERATIVE EXTENSION AND CONSULTANTS 413 It is imperative that cooperative extension and private consultants have a basic understanding of the various strategies in chemical management and resistance. This is a first step toward creating an awareness of the problem. Once this is accomplished, educational programs can be targeted for agri- cultural producers, thus creating a more informed and productive relationship with the pesticide industry as well as with research and regulatory agencies. Cooperative extension IPM programs must stress that the choice of a pesticide is extremely important, not only in the short term but for the maintenance and use of the chemical over time. Professional improvement training programs must emphasize pesticide resistance management and pro- vide understanding of (1) the major mechanisms of resistance, (2) the key pesticides most likely to induce resistance, (3) the factors leading to cross/ multiple resistance, (4) the techniques for monitoring specific resistance problems, and (5) management strategies. Private consultants should take an active part in planning and conducting training programs. Training sessions with cooperative extension and private consultants could be held jointly or in parallel. A professional improvement training program for cooperative extension should be identified as a priority by the Extension Committee on Organization and Policy Technical Advisory Committee on IPM. By pooling training material developed in individual states by cooperative extension, consultants, research, industry, and regulatory agencies, a general outline of the pertinent training elements can be identified. The states would then have the choice of selecting and modifying those elements that most appropriately fit their conditions for in-state training programs. We suggest that a series of pesticide resistance management workshops be established for cooperative extension training. The development of a pesticide resistance management training program, as well as the overall implementation of the program, should include experienced representatives from at least state research and extension, state regulatory agencies, private consultants, and the agricultural chemical in- dustry. Pesticide resistance management training sessions or workshops could be the initial binding force to bring these groups together to discuss problems and learn from them in open forums. Certain basic elements should be included in resistance management work- shops. Background of Pesticide Resistance Training should start with an overview that stresses the known factors that influence selection for pest resistance: genetic, biological (biotic and behav- ioral), and operational (chemical and application technology) (Georghiou,
414 MANAGEMENT OF RESISTANCE TO PESTICIDES 1983~. Appropriate literature that deals with resistance in major pest classes should be included, such as Georghiou and Saito (1983), LeBaron and Gressel (1982), and Delp (19801. Case studies should be examined to develop a knowledge base and expectations based on general trends that have occurred in the development of resistance (e.g., insect resistance to chlorinated hy- drocarbons, organophosphates, and synthetic pyrethroids; fungal resistance to benomyl; and weed resistance to triazine herbicides). Identification of High-Risk Pests and Pesticides Cooperative extension IPM specialists in each state should take the time to identify those pests and pesticides that have high probabilities for resistance development. Special attention should be given to pests that have demon- strated the ability to develop severe degrees of cross-resistance. For example, the tobacco budworm Heliothis virescens has developed cross-resistance. We are faced with a high risk of resistance to synthetic pyrethroids with this pest. It might be appropriate to draw upon the experience and expertise of IPM specialists when developing a training session for management of re- sistance to synthetic pyrethroids in H. virescens. Resistance Prevention Tactics Reducing selection pressure is one of the most obvious and IPM-compatible measures that can be used to thwart the development of resistance (Metcalf, 1980~. Precise timing of insecticide applications, based on field scouting data and economic thresholds, has had a major impact in certain IPM pro- grams by reducing the frequency and extent of treatments (Lacewell and Taylor, 1980; Frisbie and Adkisson, 19851. The intent of most sound IPM programs is to stress nonpesticide control measures. Crop rotation, cultural practices, resistant cultivars, scouting, eco- nomic thresholds, and other management tactics that reduce the need for pesticides should be stressed. The IPM concept dictates that pesticides be resorted to as the last alternative and after nonchemical control measures have been maximized. Despite the use of nonpesticide control tactics, how- ever, pests frequently develop populations that can cause economic loss. In such cases the choice of pesticides should be carefully made. Evaluating the relative appropriateness of pesticides for IPM programs is a very difficult and subjective task. Metcalf (1980) assigned pest management ratings to a group of insecticides, and in doing so provided generalizations regarding their IPM compatibility. Pesticides that are short-lived and do not have a prolonged environmental persistence should be identified (Georghiou, 1983~. Once these pesticides are identified they should be promoted as preferred alternatives in an IPM
COOPERATIVE EXTENSION AND AGRICULTURAL CONSULTANTS 415 program. Pesticides that have long residual lives should be avoided when possible. We fully recognize that short-residual pesticides may not be avail- able or economically feasible for all pest species. Chemicals with short residual lives may impact fewer life stages than long-residual chemicals. If possible the exposure of all life stages of a pest to pesticides should be avoided. The resistance management training session should include all available crop-specific information concerning rotations, mixtures, or sequences of chemicals to prevent or manage resistance. This information should be de- veloped for key pests that have a high probability of developing resistance to certain pesticides. Conscientious use of appropriate operational strategies, such as using alternative rotations or mixtures of chemicals, should become an integral part of IPM systems. Demonstration of Available Techniques Cooperative extension personnel should know of direct or indirect methods that are available for estimating the frequency of resistant pests at a specific location. These may involve bioassays of living subjects or biochemical assays of prepared samples of field-collected subjects. Use of such methods should be accompanied by an understanding of the frequency-dependent nature of resistance problems. Therefore, action thresholds must be discussed to emphasize that there are frequencies below which resistant pests do not seriously threaten chemical efficacy. In many cases action thresholds will need to be based on field experience, since research data in this area is often lacking. IMPLEMENTATION Of the pesticide resistance management programs that have been developed in the United States and around the world, extension and private consultants have been involved in one way or another, working with university and federal research or industry. The following outline is one approach in for- malizing the role of cooperative extension and private consultants in pesticide resistance management. Susceptibility Monitoring The experience of research scientists, extension specialists, and consultants should be drawn upon to develop a list of candidate pests and pesticides for which resistance problems appear highly probable. These pests should be included in a formalized monitoring program to determine pesticide suscep- tibility levels (Brown, 19761. Historically, the approach has been to wait
416 MANAGEMENT OF RESISTANCE TO PESTICIDES until there has been a serious economic failure due to resistance before susceptibility tests are initiated. In most cases this has amounted to post- mortem descriptions of resistance. We do not propose that every key pest be included in monitoring programs only those for which resistance poses a serious threat to the profitable production of a commodity and, therefore, where support is likely to be generated for such a program. Resistance-risk candidates can be selected for monitoring programs based on resistance in- formation from other areas of the country and the world. Such was the case when the apple scab fungus Venturia inaequalis developed resistance to dodine in New York in the late 1960s (Delp, 19801. Plant pathologists in the northeastern, Midwestern, and mid-Atlantic apple-producing states im- mediately began to monitor for resistance in their orchards. A similar scenario was experienced when apple scab developed resistance to benomyl (Hickey and Travis, 1984~. In certain instances, however, resistance may occur so unexpectedly that susceptibility monitoring programs may not have had time to be initiated. This was found with the horn fly Haematobia irritans resis- tance to pyrethroid-impregnated ear tags in the southeastern United States and in Texas (Shackelford, 19841. We recommend that pests be monitored on a regional basis and on a scale appropriate for the particular system under study. Sample collections should complement existing monitoring programs and may be obtained in the course of existing field monitoring. Cooperative extension personnel and private consultants can provide biological samples from a wide geographical and ecological range. Laboratory tests may be conducted by university, inde- pendent, and possibly chemical industry laboratories, depending on the spe- cifics and economics of the particular cropping system. Once tests are conducted susceptibility information should be recorded and mapped. Management Programs When the susceptibility tests indicate a decrease in pest susceptibility to a certain pesticidets), the following steps may be undertaken. Pest Samples Pest samples should be collected from fields suspected of containing high frequencies of resistant pests, and laboratory bioassays should be conducted to estimate the frequency of resistant individuals and the degree of resistance (LCso, LD50, LTso' etc.) (Teetes et al., 1975; Delp, 1980; Truelove and Hensley, 1982; Dennehy et al., 1983; El-Guindy, 19841. Rou- tine monitoring of susceptibility to pesticides may be necessary. IPM prac- titioners should verify resistance problems through laboratory toxicologists. A working resistance management program depends on a close working relationship between these parties.
COOPERATIVE EXTENSION AND AGRICULTURAL CONSULTANTS 417 Field Tests Field tests should be conducted to assess the degree to which the laboratory resistance bioassay reflects loss of efficacy under typical treat- ment conditions in the field. Standard field trials should be performed in areas with susceptible populations and in areas with resistant populations. The laboratory bioassay must have relevance to the field such that individuals shown to be resistant in the laboratory bioassay actually do contribute to a substantial loss of efficacy under the treatment conditions in the field. Data Reliability The reliability of field-susceptibility data must be de- termined. In the worst possible case one might obtain susceptibility estimates from two different locations within the same field and find that one sample contained only resistant individuals while the other contained only susceptible individuals. Although this scenario is highly unlikely, it is obvious that, given such circumstances, a single susceptibility estimate from fields would be meaningless. One way to estimate how well a single susceptibility estimate reflects average susceptibility throughout a location is to perform numerous susceptibility estimates within single locations. Dennehy and Granett (1984) did this to estimate the within-field variability in estimates of spider mite susceptibility to dicofol. Procedures should be standardized for collecting, culturing. and bioassavina test organisms. ~ ,, ~-~ , ~ Designation Criteria Criteria must be established for designating pop- ulations (fields) as either susceptible or resistant. Therefore, action thresholds for resistance must be developed that describe the frequency of resistant types at which a field should be designated as resistant and for which appropriate resistance management strategies should be initiated (Dennehy and Granett, 19841. Action thresholds must incorporate available information on bioassay reliability, within-field variability in susceptibility estimates, and reduction in field efficacy with increasing frequency of resistance. Dennehy and Granett (1984) established an action threshold for dicofol-resistant spider mites in cotton; when the frequency of resistant spider mites at any location was greater than or equal to 10 percent of the population, the use of alternative miticides was recommended. Geographic Extent Once a routine susceptibility screening program dis- covers and validates the presence of a resistance problem, the geographic extent of resistant populations should be determined as quickly as possible using the extension IPM program network and supported, when available, by information from private consultants. Dennehy and Granett (1984) de- veloped a wide-scale monitoring program for dicofol-resistant Tetranychus spp. in the cotton-producing region of the San Joaquin Valley of California. Similarly, the geographic range of the greenbug Schizaphis graminum resis- tant to disulfoton was determined in a cooperative effort between the Texas
418 MANAGEMENT OF RESISTANCE TO PESTICIDES Agricultural Experiment Station and the Texas Agricultural Extension Service IPM Program (Teetes et al., 1975~. Greenbug resistance to disulfoton was at first restricted to five counties in the Texas High Plains area in 1974. It was later determined that disulfoton-resistant greenbug populations were pres- ent in the northern portion of the Texas High Plains, Oklahoma, and South Dakota. Resistance of the apple scab pathogen to benomyl was anticipated and then determined in North Carolina (Sutton, 1978~. Subsequently, a three- year (1976-1979) monitoring program nicely outlined the geographical dis- tribution of resistant fungi (Sutton, 1983~. Once the geographical distribution of resistance has been determined, cooperative extension should immediately provide this information to the growers through its educational channels. Rapid delineation of the areas with resistant populations should deter a wholesale abandonment of the use of a product based on exaggerated reports of resistance and should lengthen the life of the compound. Cooperative extension and private consultants must work closely with research laboratories and chemical producers to develop this information. Bioassay Techniques Rapid, practitioner-assessable bioassay techniques should be developed and distributed, and cooperative extension IPM spe- cialists and agricultural consultants should be educated in their use. A rapid bioassay method has been developed for detection of dicofol-resistant spider mites in cotton (Dennehy et al., 1983) and is currently being evaluated by field personnel in California. Techniques have also been used for benomyl- and dodine-resistant apple scab (fungal) populations (Sutton, 1978, 1983~. A rapid bioassay technique was used to determine pyrethroid resistance levels of hornflies in Texas (Shakelford, 19841. . . Management Strategies Management strategies that consider all useful information on the stability and inheritance of resistance (cross- and multiple resistance) and relevant information on insect ecology, biology, and toxi- cology must be developed. A management strategy should be recommended when the frequency of resistant individuals in a population is greater than or equal to the accepted action threshold. The most common management strategy used, once the distribution and frequency of resistant types are determined, is an immediate switch to an alternative pesticide, and often to one that possesses a different mode of toxicological action. It is the respon- sibility of the extension service, and indeed that of private consultants, to notify agricultural producers of effective alternative chemicals and to rec- ommend appropriate rates and timing of applications. Growers can manip- ulate resistant populations or deter resistance development by rotating groups of chemicals, alternating pairs of chemicals, using mixtures of chemicals, or adding synergists to pesticides. In addition growers may influence resis
COOPERATIVE EXTENSION AND AGRICULTURAL CONSULTANTS 419 lance by the manner in which materials are applied (rate, volume, equipment used, etc.) or by influencing migration, overwintering, dispersion, or other aspects of pest biology/ecology. Recommendations must result from field and laboratory investigations of the system in question and should recognize the differences between cropping systems and different geographical regions of the same cropping system. EDUCATION It is a definite responsibility of the Cooperative Extension Service to provide educational information and training programs to make farmers and ranchers aware of resistance management. This further emphasizes the im- portant role of pesticide resistance management within the context of ongoing IPM programs. The topics that have been previously discussed should be packaged in such a way that agricultural clientele gain a basic understanding of tactics in managing pesticide-resistant populations. Agricultural consul- tants who have experience in pesticide resistance management should be invited to participate in and, if possible, help develop training programs. As with most IPM training programs, close coordination should be maintained with the state agricultural experiment station, the USDA, and the state de- partment of agriculture. CONCLUSION The Cooperative Extension Service should develop initiatives in the area of pesticide resistance management that would be part of ongoing state, regional, and national IPM programs in the United States. Other countries may consider these for their respective agricultural agencies. Cooperative extension should serve as the coordinating body to seek the appropriate form and function of pesticide resistance management training activities from pri- vate consultants, research and regulatory agencies, chemical industry, and IPM farmer organizations. Three specific recommendations follow: 1. Pesticide resistance management should be a high-priority area in co- operative extension and USDA federal extension IPM educational programs. The Extension Committee on Organization and Policy-Technical Advisory Committee on IPM is a logical body to address the issue. 2. The joint role of cooperative extension, private consultants, state and federal research, and the state departments of agriculture should be identified to develop techniques for sampling, bioassay, decision (action) thresholds, and appropriate management strategies for implementing pesticide resistance management programs. To achieve this we recommend that additional funding be provided
420 MANAGEMENT OF RESISTANCE TO PESTICIDES through the USDA National Agricultural Pesticide Impact Assessment Pro- gram (NAPIAP), which is responsible for determining the benefits and use of pesticides. This program is conducted through the state agricultural ex- periment stations and the Cooperative Extension Service and deals specifi- cally with pesticides; therefore, it is a logical place~to initiate a broader national program on pesticide resistance management. The NAPIAP has identified pesticide resistance as a project area. This area must be strengthened and expanded into pesticide resistance management. The designated NAPIAP state specialist could work closely with state agricultural experiment station scientists and Cooperative Extension Service specialists, along with the state department of agriculture, consultants, and grower groups to organize a state- based pesticide resistance management program. This program should be designated within each state as a major branch of their current IPM program and become a key area for identifying and consolidating pesticide resistance management efforts. 3. IPM research and educational programs must be emphasized. It is only through IPM programs that we can effectively reduce selection pressure leading to resistance. We believe the only reasonable course to follow is the development of alternative strategies, either chemical or nonchemical, within the context of IPM. REFERENCES Brown, A. W. A. 1976. Epilogue: Resistance as a factor in pesticide management. Pp. 816-824 in Proc. 15th Int. Congr. Entomol., Washington, D.C. College Park, Md.: Entomological Society of America. Croft, B. A., and S. C. Hoyt, eds. 1983. Integrated Management of Insect Pests of Pome and Stone Fruits. New York: John Wiley and Sons. Delp, C. J. 1980. Coping with resistance to plant disease control agents. Plant Dis. July:652-657. Dennehy, T. J., and J. Granett. 1984. Monitoring dicofol resistant spider mites (Acari: Tetranychidae) in California cotton. J. Econ. Entomol. 77:1386-1392. Dennehy, T. J., J. Granett, and T. F. Leigh. 1983. Relevance of slide-dip and residual bioassay comparison to detection of resistance in spider mites. J. Econ. Entomol. 76:1225-1230. El-Guindy, M. A. 1984. The phenomenon of resistance to insecticides in agricultural pests. Pp. 12-13 in Proc. Symp. Integrated Pest Manage. Ration. Pestic. Use in Arab Countries. League of Arab States: Arab Organization for Agricultural Development. Frisbie, R. E., and P. L. Adkisson. 1985 IPM: definitions and current status in U.S. agriculture. Pp. 41-52 in Biological Control in Agricultural Integrated Pest Management Systems, M. H. Hoy and D. C. Herzog, eds. New York: Academic Press. Georghiou, G. P. 1983. Management of resistance in arthropods. Pp. 769-792 in Pest Resistance to Pesticides, G. P. Georghiou and T. Saito, eds. New York: Plenum. Georghiou, G. P., and T. Saito, eds. 1983. Pest Resistance to Pesticides. New York: Plenum. Hickey, K. D., and J. W. Travis. 1984. Management of resistant strains of orchard pathogens. P. Fruit News 63:60-62. Huffaker, C. B. ed. 1980. New Technology of Pest Control. New York: John Wiley and Sons. Lacewell, R. D., and C. R. Taylor. 1980. Benefit-cost analysis of integrated pest management
COOPERATIVE EXTENSION AND AGRICULTURAL CONSULTANTS 421 programs. Pp. 283-301 in Proc. Sem. and Workshop Pest and Pesticide Management in the Caribbean, Vol. 2, E. G. R. Gooding, ed. CICP/USAID. LeBaron, H. M., and J. Gressel, eds. 1982. Herbicide Resistance in Plants. New York: John Wiley and Sons. Metcalf, R. L. 1980. Changing role of insecticides in crop protection. Annul Rev. Entomol. 25:219- 256. Metcalf, R. L., and W. H. Luckman, eds. 1982. Introduction to Insect Pest Management, 2nd ed. New York: John Wiley and Sons. Shackelford, K. 1984. Is time running out for insecticide ear tags? The Cattleman July:59-65. Sutton, T. B. 1978. Failure of combinations of benomyl with reduced rates of non-benzimidazole fungicides to control Venturia inaequalis resistant to benomyl and the spread of resistant strains in North Carolina. Plant. Dis. Rep. 62:830-834. Sutton, T. B. 1983. Disease management strategies and techniques. Pp. 103-104 in Integrated Pest and Orchard Management Systems for Apples in North Carolina, G. C. Rock and J. L. Apple, eds. N. C. Agric. Res. Serv. N. C. State Univ. Tech. Bull. No. 276. Teetes, G. W., C. A. Schaefer, J. R. Gipson, R. C. McIntyre, and E. E. Latham. 1975. Greenbug resistance to organophosphorous insecticides on the Texas High Plains. J. Econ. Entomol. 68(2):21= 216. Truelove, B., and J. R. Hensley. 1982. Methods for testing herbicide resistance. Pp. 117-131 in Herbicide Resistance in Plants, H. M. LeBaron and J. Gressel, eds. New York: John Wiley and Sons.