Management Programs for
the Colorado Potato Beetle, Leptinotarsa decemlineata (Coleoptera:
Chrysomelidae)
ELIZABETH J. DANIELSONDepartment of EntomologyColorado State UniversityFort Collins, CO 80523email address:а bdaniel@lamar.colostate.edu ABSTRACT ааааааа The Colorado Potato Beetle, Leptinotarsa decemlineata, is one of the most destructive and devastating insect pests in North America.а Uncontrolled CPB populations can completely defoliate potato plants and can cause a complete loss of tuber production ( Hare 1980 in Hare 1990).а The CPB has developed widespread resistance to all currently registered synthetic insecticides (Fogash 1985 in Hare 1990). Therefore, the need for integrated pest management programs that incorporate chemical controls and cultural controls is great for this insect.а ааааааа Insecticides from plant origin and biorationals are two methods of control that provide alternatives to synthetic insecticides.а The use of insect antifeedants reduce pest management costs, lower the risks to the environment and to human health, and reduce or delay the development of insecticide resistance.а Current research shows that antifeedants are effective on populations of CPBs.ааааааа Limonoids are found in the plant families Meliaceae, Rutaceae, Cneroaceae, and Simaroubaceae.а Limonin and azadirachtin are the most potent limonoid antifeedants and have been studied the most extensively.а ааааааа Research conducted on sesquiterpenes has resulted in isolation of two compounds; siliphenes and bisabolenes.а Silphinene exhibits both antifeedant and toxic effects.аа Bisabolenes are effective antifeedants against several insect species including the CPB.ааааааа Neo-clerodane diterpenes are a promising new group of compounds that have affected feeding behavior of the CPB.а Neo-clerodanes have exhibited antifeedant activity and activity as feeding deterrents.ааааааа Common herbs such as basil, catnip, dill, rue, sage, and tansy have been tested for antifeedant activity.а Herbs have been shown to deter feeding to some extent.а ааааааа Bacillus thuringiensis is a biorational insecticide that is currently used in CPB management programs.а Bt has been tested in conjunction with antifeedants to reduce endotoxin concentrations and increase the efficacy of the endotoxin.ааааааа Research has been conducted on the use of cultural controls in conjunction with antifeedants.а Trap cropping, intercropping and crop rotation have all been studied for use with antifeedants.ааааааа The goals of this paper are 1) to review literature on the use of secondary plant compounds as antifeedants of the CPB and 2) to expound on the integration of antifeedants into CPB management programs. INTRODUCTIONHistory of Colorado potato beetle pest statusааааааа The Colorado potato beetle (Coleoptera: Chrysomelidae) has not always fed on the potato, Solanum tuberosum.а It is believed that the insect is native to Mexico and was introduced to the United States about 150 years ago.а After its introduction, it changed its food source to the cultivated potato and rapidly spread to areas where the potato is grown.а It has now become one of the most important and destructive insect pests of eastern North America (Hare 1990).ааааааа The Colorado potato beetle (CPB) overwinters in the soil as an adult.а CPBs become active in the spring.а Both larvae and adult lifestages feed on foliage of the cultivated potato and other suitable hosts including wild strains of potato, eggplant and tomato.а CPBs in uncontrolled populations can completely defoliate potato plants and can, at times, cause a total loss of tuber production (Hare 1980 in Hare 1990).а Due to this widespread destruction, insecticides were developed to control CPB populations. Resistance development ааааааа ааааааа The first large scale use of insecticides in an agricultural crop was for suppressionа of the CPB.а These insecticides used were very effective in their control.а The CPB was reduced to a minor pest until widespread resistance to DDT occurred in the 1950▓s (Hare 1990).а Since then, additional insecticides have been used to control populations, but the CPB has developed resistance to each insecticide used at a faster and faster rate.а The CPB is extremely adaptable to insecticide strategies used against it (Weisz et al 1994).а The CPBа has developed resistance to all registered synthetic insecticides by all three resistance mechanisms; decreased cuticular penetration, enhanced metabolic detoxification, and altered target site sensitivity (Forgash 1985 in Ferro and Lyon 1991, Rose et al in Weisz et al 1994). Figure 1. Taken from Bessin 1996а Control tacticsааааааа Management programs are being developed that implement integrated pest management techniques for the control of CPB.а Cultural controls such as intercropping, trap cropping, crop rotations, and the use of physical barriers are currently in use (Hoffman et al 1996).а The use of Biorational insecticides has been implemented in many CPB management programs.а Research is now being conducted on the use of secondary plant compounds as antifeedants in the hopes that commercial products can be made from the plant extracts.а These antifeedants are to be used either alone or as an integral part of an integrated pest management program for the control of CPBа populations. DISCUSSION Antifeedants and their mode of action ааааааа Antifeedants are ⌠substances which, when tasted by insects, result either temporarily or permanently, depending on potency, in the cessation of feeding■ (Kubo et al 1977 in Klocke et al 1989).а Three requirements exist for a substance to be classified as an antifeedant.а The insect must choose the untreated plant over the treated plant.а If an insect would die from starvation rather than eating the treated plant, the substance is an absolute antifeedant.а The second requirement is that if an insect is starving, it will choose any food, treated or untreated.а By inhibiting feeding only for a defined time, a substance with this type of effect is called a relative antifeedant.а The third requirement of an antifeedant is to have a toxic action if the insect ingests it.ааааааа Three modes of action exist for antifeedants (Murdock et al 1985 in Mendel et al 1991).а The two main modes of action by which secondary plant compounds act are deterrency and toxicity.а Deterrents prevent the insect from further feeding by affecting the peripheral nervous system.а Toxicants work once digested by the insect by disrupting cellular, biochemical, and physiological processes.а Most other insecticidal compounds act by affecting the insects central nervous system. Advantages and disadvantages of the use of antifeedantsааааааа Antifeedants offer many advantages over many current management techniques.а Insect antifeedants reduce pest management costs by reducing application rates and the frequency of chemical inputs.а They lower the risks both to the environment and to human health.а The use of antifeedants put additional stress on insect pests, such as the CPB, and can increase susceptibility to insecticides (Jermy 1990 in Murray et al 1993).а Antifeedants can reduce or delay development of insecticide resistance.а Antifeedants with both behavioral and physiological effects delay the development of resistance due to their multiple modes of action.ааааааа According to the Environmental Pollution Panel at the 1965 US President▓s Science Advisory Committee, antifeedants to not add much to the array of conventional insecticides available.а ⌠Repellents for plant-eating insects do not appear to offer any special advantage over conventional insecticides.а They require the same exhaustive study and development in order to prove their toxicological safety and have the same inherent problems as to application and confinement to the treatment area.а For pest species they only serve to spread the problem to neighboring untreated areas■ (Ascher 1970 in Schoonhoven 1982).а Antifeedants may cause aggregation of insects to unprotected areas of the plant where extensive damage will occur.а Because of the high structural complexity of many antifeedants, the synthesis of compounds for agricultural use may not be economically feasible (Bentley et al 1990). ааааааа The acceptance of antifeedants by growers may be slow because antifeedants do not suppress insect populations as rapidly as current synthetic insecticides.а Growers may also not realize or understand the value of different chemistries on resistance potentialа and their value in plant protection and may give up on them too soon (Hare 1983 in Hare 1990). Citrus limonoids and their role as antifeedants against the CPBааааааа Limonoidsа are tetranortriterpenes that are found in the plant families Meliaceae, Rutaceae, Cneoraceae, and Simaroubaceae.а Several citrus limonoids and limonin derivatives have been found to be insect antifeedants.а These include limonin, nomilin, obacunone, epilimonol and limonin diosphenol (Liu et al 1990).а Citrus limonoids are readily extracted from citrus seeds and are available in large quantities as waste products of the citrus industry (Klocke et al 1982 in Murray et al 1995).а Citrus limonoids work both as toxins and feeding deterrents.а In structure-activity studies of limonin, it has been determined that the furan ring and epoxide groups in the citrus limonoid structure are critical for the antifeedant activity of the limonoids against CPB larvae (Bentley et al 1988, Mendel et al 1991b).а Citrus limonoids have been shown to suppress adult CPB populations due to repellency or dispersal from treated plants (Murray et al 1993).а Limonoids inhibit ovipositon of CPBs due to nutritional disruption caused by limonoid induced antifeedant effects (Murray et al 1995).а Studies conducted on the effects of limonoid in reducing colonization and oviposition show that applications of limonoid are an effective means of controlling CPB populations.а Using natural products in this manner along with other adult disruption practices, may be effective in reducing CPB damage (Murray et al 1995).а Limonoids can be used as a pest management strategy.а Citrus limonoids can act to deter CPB populations away from the host crop into a trap crop where the CPBs could be destroyed using cultural or chemical methods (Murray et al 1995).ааааааа Limonin is the dominant citrus limonoid.а It has been used as the starting material to synthesize new antifeedants against the CPB (Bentley et al 1988).а CPB larvae feed readily on limonin treated foliage in several bouts.а After several feeding bouts the larvae slow down their feeding activity resulting in feeding reductions (Mendel et al 1991a).аа Limonin causes cessation of feeding, reduced locomotor activity, and regurgitation at high levels in larvae (Alford et al 1987).а Increased levels of antifeedant activity were seen when greater amounts of limonin were ingested by CPBs and when the overall feeding activity of the beetle larvae was high (Alford et al 1987).а According to Bentley et al 1990, the depression in feeding rate that occurs after limonin treatment may be due to a toxicity effect taking place after the plant material is ingested.а Figure 2.а The conversion of limonin to limonin double salt.ааааааа Limonin in its double salt form has been studied to determine its antifeedant effects against the CPB.а Limonin salt is taken up through the potato leaves and translocated.а Studies show that limonin salt by itself is not an effective antifeedant, but that at low pH levels it can be converted to limonin with in the potato leaves and result in larval feeding and growth reductions.а At high doses, limonin salt can be detrimental to potato leaves(Liu et al 1991).ааааааа Epilimonol, a limonin derivative, is a potent antifeedant against CPB larvae and adults.а Epilimonol reduces feeding in adult CPBs.а As the length of exposure to epilimonol increases, the amount of foliage consumed by adult beetles decreases (Liu et al 1989).а Long term exposure results in decreased vigor and higher mortality of adults.а Oviposition of CPBs is suppressed due to the feeding reduction and through starvation (Liu et al 1989). Figure 3.а Molecular structure of Epilimonolааааааа Obacunone, nimolin, and limonin diosphenol are also effective antifeedants.а According to Mendel et al 1991b, obacunone and nomilin are more effective than limonin.а Nomilin and obacunone act primarily as toxins against CPB populations.а Limonin diosphenol has been shown to be very effective in the depression of feeding, growth, and development of CPB larvae (Liu et al 1990). Figure 4.а Molecular structures of obacunone, nomilin, and limonin diosphenol Azadirachtin and its effect against CPB populationsааааааа Azadirachtin is a tetranortriterpenoid that is isolated from the seeds of the neem tree, Azadirachta indica.а Neem tree seed extracts are reported to have antifeedant activity against over 200 insect species, including the CPB (Jacobsen 1989).а Research indicates that neem extracts induce morphogenetic defects in CPB larvae, reduce fecundity in adults, and lead to increased feeding inhibition and mortality in both larvae and adults (Kaethner 1992).а Zehnder et al 1988 found that CPB colonization decreased following neem applications.а In adults, neem extracts cause avoidance or reduced acceptance of treated foliage.а Larvae show decreased vigor or mortality due to neem extracts.а This reduction in foliage consumption may be due to the antifeedant properties of neem or due to the toxic effects that weaken the larvae and make them less able to feed (Zehnder et al 1988).а ааааааа The advantages of neem include having only small side effects on beneficial organisms, decreased reliance on broad spectrum insecticides, and a decreased selection pressure for insecticide resistant CPB populations (Zehnder et al 1988).а One disadvantage is that the effect of neem depends on the predominant life stage at application.а Field applications of neem are most effective when small larvae are the predominant life stage (Zehnder et al 1988).а This gives the applier only a short window of opportunity in which to apply neem to get the greatest reduction in CPB larvae.а Another disadvantage is that although potato plants sprayed with neem extracts are commercially acceptable, the average yields are lower than the untreated potato yields (Zehnder et al 1988). Figure 5.аа Molecular structure of Azadirachtin Sesquiterpenes as CPB antifeedantsааааааа Sesquiterpenes are a group of compounds that have shown antifeedant effects against many insect species.а Sesquiterpenes isolated from the familyа Asteraceae are an important source of CPB antifeedants (Hough-Goldstein 1990).а Gonzalez-Coloma et al 1995 discovered that an ethanolic extract of Senecio palmensis had strong antifeedant activity against CPB larvae. Two compounds have been isolated from Senecio palmensis, one from the chemical class of bisabolenes and the other a silphinene sesquiterpene (Gonzalez-Coloma et al 1995).а Both of these chemicals may alter the host selection process through adult behavioral avoidance because adults are highly mobile and are the primary finders of host plants (Hough-Goldstein 1990).а Bisabolenes serve as effective antifeedants by causing feeding inhibition.а The modes of action for siliphene sesquiterpenes are both antifeedant and toxic effects.а These modes of action result in detrimental effects such as a decreased growth rate in CPB larvae (Gonzalez-Coloma et al 1995). Clerodane diterpenes as CPB antifeedantsааааааа neo-clerodane diterpenes are a promising new group of compounds that have affected the feeding behavior of CPBs.а The most abundant sources of neo-clerodane diterpenes are from plants in the genus Teucrium.а According to Merritt et al 1992, approximately 150 neo-clerodanes have been isolated (In Ortega et al 1995).а Ortega researched the effects of four natural neo-clerodanes on the feeding behavior of CPB larvae.а All four neo-clerodanes (eriocephalin, teucrin-A, teuscorolide, and teucvin) affected feeding behavior.а Eriocephalin and teucvin showed antifeedant activity.а Teucrin-A was determined to be a toxic antifeedant.а Teuscorolide was found to act as a feeding deterrent. Common herbs and their effects as antifeedantsааааааа Common herbs such as basil, catnip, dill, rue, sage, and tansy have been studiedа for the control of CPB because they have been associated with insect control in folklore or have been involved in scientific studies (Hough-Goldstein et al 1990).а All six of the afore mentioned herbs deter feeding to some extent.а Adult CPBs are more selective on tansy, sage, rye and basil.а Adults and larvae elicit similar responses to catnip and dill (Hough-Goldstein et al 1990).а Rue contains a variety of alkaloids, furanocoumarins, and several ketones and flavonol glycosides.а Sage, basil, catnip, and dill contain a variety of monoterpenoids, some of which have exhibited insecticidal activity (Hough-Goldstein et al 1990).а Catnip, when interplanted with a host crop, has decreased CPB populations by 58-83% (Panasiuk 1984).ааааааа Of the common herbs, tansy has been studied the most extensively.а Tansy has been used as a general purpose insect repellent (Duke 1985 in Hough-Goldstein et al 1990). Tansy oil produces avoidance behavior in CPBs (Panasiuk 1984).аа CPB populations have decreased from 60-100% when the host plant has been interplanted with tansy (Panasiuk 1984).а Tansy contains approximately 100 different compounds, primarily being monoterpenes such as thujone, camphor, and borneol (Schearer 1984 in Hough-Goldstein et al 1990).а Tansy oil components include alpha terpinene, thujone, dihydrocarvone, and carvone (Panasiuk 1984). The effect of Bacillus thuringiensis as a biorational insecticideааааааа Bacillus thuringiensis (Bt) is a biorational insecticide that is currently being used in many CPB management programs.а Biorationals are generally less toxic and less harmful to natural enemies and the environment than synthetic insecticides (Jacobsen 1989).а Bacillus thuringiensis var. san diego, also referred to as Bacillus thuringiensis var. tenebrionis, is an endotoxin that acts as a poison and a teratogen against CPB larvae.а Bt works by causing gut paralysis which leads to the cessation of feeding and death a few days later (Hare 1990).а The timing of application of Bt is crucial.а The efficiency of Bt is strongly related to temperature.а Applications when temperatures range from approximately 28 to 33 degrees Celsius are the most effective (Ferro et al 1991).ааааааа Hough-Goldstein et al 1991 researched the effects of M-One, a commercial formulation of Bt, for the control of first, second, and early third instars of CPB larvae.а They found that M-One was not as effective against late instars as it was against early instars (Ferro et al 1991).а Feeding stimulants were used to help CPBs locate the Bt treated foliage.а Feeding stimulants such as sucrose, Coax, and Entice were used, but were found insufficient to significantly increase CPB mortality above that of M-One used alone (Hough-Goldstein et al 1991).ааааааа Murray et al 1993 researched the interactive effects of citrus limonoids with Bacillus thuringiensis var. san diego.а The use of citrus limonoids does not interfere with the insecticidal activity of the Bt endotoxin.а Larvae treated with an endotoxin following a limonoid treatment resulted in the most delayed larval development.а CPB adult populations treated with endotoxin followed by a citrus limonoid treatment were significantly lower than the control populations (Murray et al 1993).а The use of endotoxins and antifeedants such as limonoids together in integrated pest management systems is an effective strategy to lessen the heavy reliance on synthetic chemical insecticides. CONCLUSIONааааааа Increased rates of resistance development to currently available insecticides has resulted in the need for additional control techniques against the CPB.а Integrated pest management programs offer an array of methods, both cultural and chemical, to control CPB populations. Considerable potential exists for natural plant products that inhibit insect feeding and their use in CPB management programs.а Antifeedants can be used alone, in combination with biorationals, in combination with synthetic insecticides, or in addition to cultural methods to reduce populations.а The use of natural insecticides as a component of CPB management programs ensures greater longevity of insecticides through multiple modes of action and reductions in the amount and frequency of chemical applications.а Natural insecticides are less toxic to humans, less harmful to the environment, and protect natural enemies.а The use of natural insecticides to control CPB populations can not be ignored.а Research to date reveals that the use of natural insecticides is effective as a means of crop protection. Continued research is essential to the mass production of additional natural insecticides available to the consumer for use as a component in CPB management programs. 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