Hervibors Hosts Influence on Insecticide Resistance: a Review

Rev. Fac. Agron. (LUZ).1999,16: 127-140

Docurnent: Hervibors hosts influence on insecticide resistance: a review

Documento: La influencia del hospedero en la resistencia de hervíboros a insecticidas: una revisión

Abstract

Resistance is a worldwide problem, which if ignored or improperly niana- ged, will significantly reduce worldwide agricultura1 production and public health. Resistance is influenced by genetic factors but also there is an environmental effect, which in the case of phytofagas diseases is partially represented by the chemicals found in the host plants. Species with an evolutionary history of feeding on heavily chemically defended plant structures shoiild have elwated levels of enzyrnes that detoxify defensive chemicals, and therefore an enhrinced ability to evolve resistance to synthetic toxins. The role of host plant cheniistry on the expression and evolution of pesticide resistance is reviewed from the perspective of understanding the non-genetic factors influencing pesticide resist arice. This perspective is important since environmental factors may have re1ai;ively important effects influencing the activity of detoxification enzymes in anirnals, and hence, susceptibility to xenobiotics. Research on non-genetic factors influencing pesticide resistance must be undertaken if we are to iiicrease our confi- dence in proposed management strategies. Key words :detoxification enzymes, pesticide resistance, non-genetic factors, susceptibility to allelochemicals. Resumen

La resistencia de hervi%oros a insecticidas es un problema a nivel mundial, que si es ignorado o manejado inadecuadamente, reduciría significativame-ntela producción agrícola mundial y la salud pública. La resistencia está influenciada por factores genéticos pero también existe un efecto del medio ambiente, que en el caso de las plagas fitófagas está en parte representado por sustancias químicas

~écibidoel 19-12-1996 Aceptado el 24-09-1997 1. Cument address: Departamento Fitosanitario, Facultad de Agronomía, 1,UZ.Apartado Postal 15378.Maracaibo Edo. Zulia Vcnczuela Fax N (061) 596183 E-mail :[email protected]. (To whom correspondcnce should be addressed) 2. Department of Entomology, 501ASI, The Pennsylvania State University, Universitj Park, PA 16802, USA. Fax N(814) 865-3048 E-mail : BAMQ PSUVM.PSU.EDU. Domínguez y McPheron

presentes en las plantas hospederas. Las especies que se alimentan de estructv.ras de plantas muy bien defendidas químicamente deberían tener elevados nivele:; de enzimas que detoxifiquon las sustancias químicas usadas por las plantas para defenderse, y por lo tanto muestran una habilidad mejorada para desarrollar resistencia a las toxinas sintéticas. Se revisa el rol de la química de la planta hospedera, desde el punto de vista de entender el efecto de los factores no-genéticos que influyen en la resistencia a plaguicidas de los insectos herbívoros. Elsta perspectiva es importante ya que los factores del medio ambiente pueden llegar a tener un importante efecto en la actividad enzimática de detoxificación de los animales, y por lo tanto, la respectiva susceptibilidad a los xenobióticos. Investigación de los factores no-genéticos que influyen en la resistencia a plaguicidas debe ser llevada a cabo si queremos incrementar nuestra confianza en las estrategias de manejo propuestas. Palabras clave: enzimas, detoxificación, resistencia, plaguiciclas, factores no- genéticos, susceptibilidad, nleloquímicos. Introduction

The evolution of resistance to pes- pest populations, the phenotype upon ticides is an example of the evolution- which selection operates is a func-;ion ary process. The pesticicle is the selec- of both the genotype and the envilson- tion pressure, which creates a very ment. Relatively little research has strong fitness differential between sus- focused on the influence of envil-on- ceptible and resistant genotypes. The mental factors on the evolution of ?es- survival and subsequent reproduction ticide resistance. In t.he case of p!ant of resistant individuals leads to a pests, the chemical constituents of change in the frequency over time of plants are a significant part of the en- alleles conferring resistance. Wide- vironment, a part that has been spread application of pesticides has led shown to affect the action of many re- to a global resistance problem (12,13). sistance mechanisms. We review tlie Resistance compromise crops, animal role of host plant chemistry on the ex- production and human health pression and evolution of pesticid~ire- (through pesticide resistance in vectors sistance and show that this interac- of animal and human diseases and, tion must be considered if we are to drug resistance in the pathogens and develop rational pest managerrient parasites). While selection pressure strategies for safe and efficient i:rop acts to change allele Frequencies within production. Relationship between detoxification (Enzymatic Mechanisms) and host plants (Allelochemicals)

It became obvious relatively early els of insecticide resist.ance rather rap- in the history of insecticide use that idly. Gordon (15) suggested that the polyphagous species develop high lev- natural exposure of polyphagous spe- Rev. Fac. Agron. (LUZ). 1999,16: 127-140

cies to a wide variety of plant in the esterases are acetylcholinest- allelochemicals had resulted in high erase, important in the proper t rans- capacities for their detoxification, mission of nerve siglials, and juveriile which would now enable the insects to horrnone esterase, wluch helps to regu- develop resistance to synthetic insec- late the process of metamorpkiosis. ticides. This idea directly implicated These enzymes work, in general, by plant allelochemicals as the natural breaking carboxylester and substrates for insecticide detoxifying phosphorotriester bonds. The:~are enzymes for the first time. active against many types of in:;ecti- Plant allelochemicals modify lev- cides, especially organophosphates and els of detoxifjmg enzymes in herbivores pyrethroids. Much of the evidence for and, therefore, their susceptibility to a role of esterases in insecticide iaesis- insecticides (4, 6, 26, 29, 34, 41, 45). tance comes from nssays of general Insects have detoxification mecha- esterase activity using model sub- nisms to deal with plant chemicals and strates. However, this is only an indi- also often the same mechanisms are rect measure of the role of esterases. involved in pesticide resistance. It is Some studies with synergists havc?also important to understand the interac- implicated esterases. Esterase genes tion of plant allelochemicals with the associated with insecticide resistance detoxification system. have been identified in mosquitoes 'and Three systems OS detoxification aphids. enzymes e., polysubstrate Mullin and Croft (33) fo-r ex- monooxygenases (PSMO),general es- ample, found large differences in gen- terases (GE),and glutathione S-trans- eral esterase activity relative to ferases (GST) are commonly regarded snapbean (ranging from 0.4-fold on a as the most important biochemical mint to 2.4-fold on umbellifers) for mechanism for the metabolism of Tetranychus urticae fed 13 difforent xenobiotics (7, 45) including host-adapted strains. allelochemicais (53)and pesticides (17). Lindroth (25) studied the ef'fects Xenobiotics may act as inducers by of food plant on larval performance and stimulating enzyme synthesis (53). midgut detoxification enzymes iii lar- Insects induced by dietary vae of the luna moth, Actias luna. He allelochemicals or host plants appar- found that larva1 food plants (1)lack ently increase metabolism of severa1 cherry, cottonwood, quaking aspen, synthetic pesticides, as demonstrated white willow, red oak, white oal.;, tu- by their increased tolerance to these lip tree, paper birch, black walnut, compounds (17). Insecticide resistant butternut, shagbark hickory) affected strains of insects often have greater the activities of solul>leesterases and detoxikng enzyme activities (43),and were 1.8-fold higher in larvae fed wal- in at least one example, enzyme induc- nut, than in larvae fed birch. Microso- ibility was greater than in a suscep- mal esterases exhibited an opposite tible strain (37). trend in activity, with lowest v:ilues Esterases. This is a very large in larvae fed walnuf;, and highest in family of related enzymes. Included those fed birch. Domínguez y McPheron

Activities of microsomal cis- and actions and many substrates. Each trans-epoxide hydrolase in northern particular enzyme hris a broad, but corn rootworm, Diabrotica barberi unique, pattern of substrate spekfic- Smith& Lawrence, were significantly ity. Most of our knowledge of the increased by diet shifts from corn ear monooxygenase system comes fisom to squash blossom and sunflower in- studies on mammalian liver. How- florescence, while levels of these en- ever, some recent gerietic studie:; in zymes in the western corn rootworm, insects are beginning to add to our D. virgifera uirgifera LeConte \vere understanding. It is now clear tha3ta unaffected (42). specific cytochrome P,,,-dependent Susceptible larvae from artificial monooxygenase is responsible for the diet had significantly higher nonspe- ability of black swallowtail butterfly cific esterase activity than susceptible caterpillars to deal with certain ch~mi- larvae fed apple, gorse, broom, and cals in their diet. In the case of m 3ny blackberry. Furthermore, activity of organophosphate insecticides, cerí ain nonspecific esterases of resistant lar- monooxygenase enzymes actu:tlly vae fed blackberry was significantly make the insecticide more toxic to the lower than activities in resistant lar- insect by substituting an oxygen for a vae fed artficial diet, gorse, apple, or sulfur atom. Even so, this enzjrme broom, and not significantly different family appears to be responsible f,r a from nonspecific esterase activities of number of cases of resistance to insecti- susceptible larvae reared on artificial cides, based upon synergism studies. diet, gorse, apple, blackberry, or broom Monooxygenase activity appears to be (40). partially responsible for Colorado po- Esterases afford protection from tato beetle resistance to abamectiii. phenolic glycosides to Papilioglaucus The first evidence that plant canadensis, and general esterase ac- allelochemicals could induce the tivity was elevated 22% after consump- PSMO system was reported by tion of a phenolic glycoside diet (27). Brattsten et al. (8). They found that The induction capacity of hydrolytic larvae of the polyphzigous southern enzyme systems (e.g., esterases, ep- arrnyworm were induced rapidly l>ya oxide hydrolases) is generally marginal variety of allelochemicals. The larvae in comparison to that of PSMOs and with induced enzymes were less sus- glutathione transferases (28). ceptible to the toxic tobacco alkaloid Cytochrome Pao-dependent nicotine. That induced activity did ~o- monooxygenases (PSMO). These vide general protection was indic~ted enzymes are linked to the electron by the fact that allelochemical-mrdi- transport system of the cell. They add ated induction often reduced the rius- oxygen to the substrates, and the sub- ceptibility of insects to insecticides (6). strate is then more easily excreted. A study with 35 species of lier- There is usually a farnily of cytochrome bivorous Lepidoptera larvae (23) gave P4ao-dependent monooxygenase en- rise to the idea that polyphagous cat- zymes present in each individual or- erpillar~had higher detoxification en- ganism to deal with many types of re- zyme activity than oligophagous snd Rev. Fac. Agron. (LUZ). 1999,16: 127-140 monophagous species. The need for tolerance to carbaryl and metbomyl higher PMFO leve1 was in agreement was greater than with larvae fed the with the greater risk for generalists other plants (11). in contacting plants potentially richer Yu (48) demonstrated PSM O in- in allelochemical diversity and concen- duction by plants in fa11 armyvonn tration compared with specialists, larvae. Alfalfa, sorghum, peanuts, which usually are well-adapted via a cabbage, cowpeas, cotton, single detoxification mechanism to the Bermudagrass and corn al1 stimulated host's specific defensive chemicals. enzyme activity, with corn being the In another lepidopteran, the var- strongest inducer. Millet and soybean iegated cutworm, Peridroma saucia leaves induced no more activity than (Hubner), feeding on peppermint in- the artificial diet control. In tests,with duced midgut PSMO activity up to 45- eight insecticides, fa11 armyworrn lar- fold compared with activity in larvae vae were more tolerant after feeding fed a basic control diet (54). Mint-fed on corn than on soyhean leaves. larvae were more tolerant of the in- Glutathione S-transferases secticide, carbaryl, than were bean-fed (GST).Glutathione transferases work larvae. Yu et al. (54) suggested the by adding the tripeptide glutathione to possibility that plant species differ in a substrate. The subsequent cleavage the degree to which they stimulate of the substrate leacls to easier E xcre- such enzymes and that an insect's tion. As with the otlier detoxification ability to detoxi6 insecticides may de- enzymes, there are multiple genes for pend on the nature of its host plant. glutathione transferase proteins, and In a similar study, Berry et al. each protein has a unique specijicity. (4) investigated the influence of pep- Many studies suggesting a role for glu- permint, alfalfa, snap beans, garden tathione transferase in insecticide re- beets, curly dock, and artificial diet on sistance have used enzyme assays with the midgut microsomal oxidase activ- model substrates. However, in liouse ity of variegated cutworm larvae and flies, conjugation of glutathione to in- on its susceptibility to different insec- secticides has been demonstrated. ticides. They found that tolerance to Also, DDT dehydrochlorinase, a acephate, methomyl, and malathion mechanism of resistcancein hous~tflies, was greater when larvae were fed pep- has been shown to be a glutat2ione permint leaves than in those fed bean transferase. leaves. Midgut enzyme activity was Plants and plant allelocheniicals increased up to 9 times when larvae also induced glutatliione transferase fed on peppermint leaves. activities in fa11 armyworm (49, 52). With last instar cabbage looper, Parsnip caused a 39-fold increasc com- Trichoplusia ni (Hubner), larvae fed pared with activity in fa11 annyworm peppermint, alfalfa, broccoli, cabbage, larvae fed artificial diet. Marked in- or artificial diet, only peppermint-fed duction of this enzyme was als,o ob- larvae had a four-fold increase in mid- served in larvae fed on turnip ancl cow- gut aldrin epoxidase activity. Bioas- peas, but nine other host plants (pea- says of induced larvae indicated that nuts, cotton, corn, cucumber, potato, Domínguez y McPheron

Bermudagrass, millet, sorghum, soy- those fed on the susceptible variety, bean) caused little or no effect com- Emika (24). Furtherinore, the activ- pared to artificial diet. Fa11 armyworm ity of GST in aphid tissues was sig- larvae fed for two days on cowpeas were nificantly correlated with the conceii- twice as tolerant to diazinon, tration of allelochemicals in the wE ent metamidophos, and methyl parathion on which they had fed (24). as those fed on soybeans, one of the Host plant did afTect larval detoxi- less active plant inducers of the en- fication enzyme activity in both the zyme. resistant and susceptible strairi of Induction of GST also occurs in Platynota idaeusalis. Glutathime deciduous tree-feeding insects. transferase and esterase activities, Lindroth (25) have demonstrated that both implicated in P. idoeusalis resis- GST activities in the luna moth (Actias tance to azinphosmethyl, varied sig- luna) larvae fed black walnut, butter- nificantly between strains and among nut and shagbark hickory were 2 to 3- hosts. Diets of apple and plantain ap- fold higher than in those fed paper peared to inhibit both enzyme systi?ms birch. compared to artificial diet in both in- Severa1 allelochemical inducers sect strains (10). of GST in fa11 armyworm larvae (51, Hunter et al. (19) determined 52) did not induce GST in diamond- that an apple allelochemical, p'llo- back moth larvae. Among the host ridzin, influenced detoxification ac1,ivi- plants investigated, rape was most ties of larval P. idaeusalis. Phloriclzin active in inducing GST in diamond- decreased GST activity in both suscep- back moth larvae (53). tible and resistant P. idaeusalis. PJso, GST activity was significantly phloridzin inhibitecl esterase ,ind higher in cereal aphids, Sitohion aniline hydroxylation of the susceptible avenae (F.), fed on the nioderatcly re- larvae, but induced higher esterase sistant wheat variety, Grana, than in activity in resistant lnrvae. Relationship between insecticide toxicity and host plants (Allelochemical variation)

Detoxification mechanisms dis- pesticides, plus the daiiger that the few cussed in previous section are often pesticides that are presently available very important for insecticide resis- will become ineffective because o re- tance. Because of the interaction of sistance, preserving pest susceptibil- those plant chemical with detoxifica- ity to currently available pesticides is tion mechanisms it is important to valuable until we have other IPM-com- review the evidences that plant chemi- patible control measures. Thus, it is cals can change patterns of insecticide important to consider what factor:; in- resistance. Furthermore, due to the fluence the loss of pesticide suscepti- rapidly accelerating cost and difficulty bility and to obtain a basic underskmd- in discovering and registering new ing of non-genetic infliiences (e.g., rlict, Rev. Fac. Agron. (LUZ). 1999,16: 127-140

age, development, temperature, nutri- azinphosmethyl(4, 53). The r. .en- ents) on the expression of insecticide tration of phloridzin, a majoi ple resistance. For instance, plants can allelochemical(l8,20), in artificial diet influence the toxicity of insecticides to changed P. idaeusalis susceptibility to herbivorous insects indirectly by induc- azinphosmethyl(l9). Susceptible third ing higher activities of insecticide- instar larvae fed artificial diet were detoengenzyrnes or inhibiting these even more susceptible to enzymes by limiting the energy avail- azinphosmethyl in the presence of pl-ilo- able to the insects to perform detoxifi- ridzin, while resistant larvae fed arti- cation reactions (6). Furthermore, the ficial diet with or without phloridzin diversity and variability in composi- did not change in their responses to tion and concentration of plant azinphosmethyl(l9). allelochemicals (e.g., plant variety, The susceptibility of the south- growth condition, plant part, and sea- ern armyworm to nrsenicals wns in- son) may impose a corresponding phe- fluenced when different host-plant fo- notypic and genotypic diversity and liage was treated and fed to larva.-.(30, flexibility on detoxikng capabilities of 44). Brattsten et al. (8),workii.,; with the insects (6). the same species, found that mixed- It has long been known that feed- function oxidases were induced rapidly ing on certain host plants can alter the by a variety of allelochemicals. Lar- susceptibility of the herbivore to insec- vae with induced activity were less ticides (4, 54). This altered response susceptible to the toxic tobacco .alka- to insecticides is often due to a direct loid nicotine. induction of the insect's detoxification Feeding on peppermint induced system by exposure to plant chernicals. the midgut polysubstrnte There is evidence that herbivorous in- monooxygenase (PSMO) activity of the sects metabolize and detoxify insecti- variegated cutworm, Peridroma cides using the same enzymes that are saucia (Hubner), up to 45-fold com- involved in the metabolism of ingested pared with activity in larvae fed a ba- plant allelochemicals (2,5). Further- sic control diet (54). Larvae given pep- more, induction of a detoxification en- permint leaves for 2 days were less zyme system as a result of feeding on susceptible to a 0.55%carbaryl treat- particular host plants can alter the ment than bean leaf-fed larvae exposed susceptibility of insects to pesticides (4, to a 0.1% dose. They suggestecl the 5,8, 11,30,38, 48,49, 54). possibility that plant species diflkr in Brattstenet al. (8) reported that the degree to which they stimu:.ated some naturally occurring substances such enzymes and that an insect's in host plants increased the activity of ability to detoxify insecticides may de- mixed function oxidases, thereby re- pend on the nature of its host plant ducing the susceptibility of larvae of (54). Berry et al. (4) reported that tol- southern armyworm, Spodoptera erance to acephate, methomyl, and eridania (Cramer), to insecticides. malathion was greater in variegated Plant secondary chemicals have been cutwonn larvae fed peppermint leaves shown to have an effect on toxicity of than in those fed bean leaves. Domínguez y McPheron

Larvae of fa11 armyworm, He demonstrated an adverse intei~ac- Spodoptera frugiperda (J. E. Smith), tion between plant resistance lind reared on millet were 6-fold more sus- chemical control wherein the phy- ceptible to trichlorfon than larvae tochemical responsible for resistance reared on bermudagrass, corn, cotton to one pest species, at concentrations or soybean, while larvae reared on present in resistant plants, induces bermudagrass and millet were more insecticide tolerance in another pest susceptible to carbaryl and permethrin species on the same crop. Moreolrer, than larvae reared on corn, cotton, or treatment of the tohacco budworm, soybean (47). In tests with eight in- Heliothis virescens F., larvae witli 2- secticides, Yu (48) found that fa11 ar- tridecanone resulted in increased tol- myworm larvae were more tolerant erance to diazinon (39).They also foimd after feeding on corn, the strongest that tobacco budworm larvae were inducer among ten hosts tested, than over four-fold more tolerant to diazi-ion on soybean leaves, one of the least ac- when fed leaves of wild tomato tlian tive inducers. In addition, fa11 army- when fed artificial diet (39). worm larvae fed for two days on cow- Third instar corn earworm ar- peas were twice as tolerant to diazinon, vae fed on a haricot benn diet were sig- methamidophos, and met.hyl parathion nificantly less susceptible to topictilly as those on soybeans. Arnong last in- applied cis-cypermethrin than larvae star cabbage looper, Trichoplusia ni fed a wheat genn diet (31). Larvae fed (Hubner), larvae fed peppermint, al- on an alfalfa diet were of intermediate falfa, broccoli, cabbage, or artificial susceptibility. Likewise, larvae fecl on diet, only peppermint fed larvae had a the wheat germ diet were approxi- four-fold increase in midgut aldrin mately twice as susceptible to topicidly epoxidase activity. Bioassays of in- applied carbaryl as tliose fed on the duced larvae indicated that tolerance haricot been diet. Furthermore, sixth to carbaryl and methomyl was greater instar corn earworm larvae fed on diot than with larvae fed the other plants containing coumariri required 7.5 (11). times as much carbaryl to achieve the Experiments conducted by same LD,, as those fed on a control tiiet Kennedy (21) with corn earworm, (31). Helicoverpa zea (Boddie), larvae and Muehleisen et al. (35) investi- one tomato allelochemical (2- gated the effects of cotton plant tridecanone), which plays an important allelochemicals fed to corn earwclrrn role in the resistance of wild tomato to larvae on their response to insecticides Manduca sexta (L.) and Colorado po- and levels of detoxifYing enzynies. tato beetle, Leptinotarsa decemlineata They reported increased toleranci? to (Say), showed an induction of mixed methyl parathion in 6-day-old c3rn function oxidase activity in corn ear- eanvorm larvae fed a cotton flower 11ud worm larvae in the presence of this diet. Their data suggested that the compound. Bioassays of induced lar- response of insects to insecticides niay vae indicated an enhanced ability of be greatly modified by the presence the insect to metabolize carbaryl(21). and concentration of host pl:.int Rev. Fac. Agron. (LUZ). 1999,16: 127-140

allelochemicals (35). lowest LD,,'s among the cereal zulti- Abd-Elghafar et al. (1 found that vars examined were 3.5:l for third-and fifth-instar tobacco bud- deltamethrin in grasshoppers reared wonn larvae became less susceptible on 'Cascade' oats and 'Gazelle' rye, 'md to methyl parathion after one day of 1.6:l for dimethoate in grasshoppers feeding on wild tomato or peppermint fed 'Bonanza' barley and 'Fidler' onts; plants compared to larvae fed on arti- such differences would represent. sub- ficial diet. Furthermore, fifth-instar stantial differences in the amoL nt of budwonn larvae fed wild tomato leaves insecticide required in the field. were more tolerant to methyl parathion Robertson et al. (40) exaniiried than those fed peppermint leaves, the effects of host plants and noth whereas, overall, third-instar larvae genotypes on susceptibility to were less tolerant than fifth-instar lar- azinphosmethyl in the light brown vae (1). apple moth, Epiphyas postvittana Susceptibility of western corn (Walker). Their results demonst-ated rootworm, Diabrotica virgifera that resistant larvae fed black *asp- virgifera LeConte, adults to aldrin in- berry and susceptible larvae fed on creased seven or nine-fold when main- artificial diet were similar. Moreover, tained on squash blossom and sun- resistant larvae fed black raspl~erry flower, respectively, instead of corn were significantly less resistant than (42). Northern corn rootworm, resistant larvae fed apple, artificial Diabrotica barberi Smith & Lawrence, diet, broom, or gorsc, whereas suscep- exhibited only slight modification of tible larvae reared on artificial diet aldrin susceptibility among the three were significantly more tolerant com- host diets (corn, squash, sunflower) pared with susceptible larvae ré ared (42). on any of the host plant species. In another coleopteran, the tox- Platynota idaeusalis is a highly icity of perrnethrin was significantly polyphagous species, which utilin es at greater to Colorado potato beetle reared least 17 plant families (32). Larva1 on eggplant than to those reared on populations have been found on a wide tomato (14). variety of herbaceous plant species be- Beny et al. (3) determined that neath host apple, penr, peach, nectar- larvae of gypsy moth, Lymantria ine, and cherry trees (22). Theréfore, dispar (L.), reared on Douglas-fir were there is a high probability for this in- significantly more tolernnt to both topi- sect to encounter and deal with an cally and orally adnlinistered abundance of plant allelochemjcals. diflubenzuron than were those raised Knight and Hull(22) noted that luiowl- on white alder. edge ofP. idaeusalis biology outsjde of Hinks and Spurr (19) found that apple, on ground cover withiii or- host plants can significantly affect the chards, would be extremely useful in susceptibility of neonate migratory an IPM program. If the same enz:mes grasshoppers, Melanoplus sanguinipes that are involved in the metabolism of (F.), to deltamethrin and dimethoate. plant allelochemicals are also involved The ratios between the highest and in metabolism and detoxification of Domínguez y McPheron pesticides (2,19,35),then this maybe ents, number of days that the lar~ae a major non-genetic influence on re- were allowed to feed on the hosts, tom- sistance. Non-overlap of 95% confi- perature, and composition of artificial dence limits at the LD,,level suggested diet (19, 28, 36, 46, 50, 55). If erivi- that the overall effect of host plants on ronmental fnctors have relatively im- toxicity of azinphosmethyl to P. portant effects, as these results sug- idaeusalis was significant (9, 10). gest, then differences in susceptibility When susceptible lnrvae of P. between larvae or adults collected fiom idaeusalis were fed different hosts, different field populations must be they were subsequently found to have based on genetic and/or environmen- different levels of susceptibility to taldifferences. Thus, bioassaysoffic?ld- azinphosmethyl.the resistant strain collected adults, which eliminate labo- responded to artificial diet and plan- ratory rearing, may not provide use- tain with a large increase in the leve1 ful infonnation nnd could produce nus- of resistance compared to the suscep- leading conclusions nbout resista-ice tible strain, demonstrating that resis- (19,401. Since assays of field-collected tance in P. idaeusalis wns genetically insects are efficient and widely used based. Resistant larvae appear resis- to monitor resistance, the poten1;ial tant if they eat plantain or dandelion, types of environment:ll effects, envi- but appear susceptible if they eat black ronment x genotype interactions, and raspberry or, to some extent, apple. In their effect on resistance merit furt'ner contrast, susceptible larvae appear consideration (36). Choice oflarval host susceptible if they eat black raspberry plant could have a dramatic effect on or plantain, but appear resistant if the apparent OP resistance of P. they eat dandelion; apple is interme- idaeusalis. It appears that feeding.on diate in effect (9). The results of this apple and black raspberry plants niay study differ in part from those of a be inhibiting the genetic resistance similar study by Robertson et al. (40) present in the resistarit P. idaeusctlis on another tortricid species, light strain. In contrast, susceptible P. brown apple moth. They found that idaeusalis appear resistant if they 63ed susceptible larvae reared on artificial on apple or dandelion (9). diet were significantly more tolerant Hunter et al. (19) studied the ef- compared with susceptible larvae on fect of phloridzin, a major apple any of the natural host plant species allelochemical(18,19~~on the toxicity they tested. Because of the many dif- of azinphosmethyl to susceptible ~lnd ferences between these experiments, resistant P. idaeusalis. In their as- however, comparisons between stud- say of third instar resistant and sus- ies must be approached cautiously. ceptible P. idaeusalis strains by c.iet Among many factors that could explain incorporation of azinphosmethyl, tliey the different results are the following: showed that mortality of third instar different insect species, weight and susceptible larvae was higher in ;he instar of the larvae at the time bioas- presence of phloridzin in the d: et. says, variety and root stock of apple Third instar resistant larvae reared on trees, growth stage of plants, nutri- artificial diet with or without pkilo- ridzin were not significantly different 136 Rev. Fac. Agron. (LUZ). 1999,16: 127-140 in their responses to azinphosmethyl. found that differences in susceptibility The effects ofgenotype, host plant, to acephate between the resistant and and age on susceptibility to acephate susceptible colonies were geneticnlly in the B biotype of sweetpotato white- based and that responses of each colony fly, Bemisia tabaci (Gennadius) were were not significantly affected by dif- examined by Omer et al. (36). In con- ferences in the three host plants stud- trast to studies discussed above, they ied (pole bean, tomato, zucchini). Conclusions

Differencial toxicity of particular diet effects. For many pests it is diffi- allelochemicals to phytophagous in- cult to control the diet. Even in cases sects can now be explained on the ba- where we got the bioassay froni just sis of differences in the enzyme activ- one plant species, the chemical \.aria- ity of insects (28). Differences in en- tion among the individual host plant zyme activity may be genetically could affect resistance. Thus, bioas- linked, but may also occur due to says of field-collected adults, vlrhich changes in individual insects as a con- eliminate laboratory rearing, mE.y not sequence of a host of intrinsic and ex- provide useful information and could trinsic factors. The role of particular produce misleading conclusions :lbout enzyme systems in the detoxication resistance (19, 40). Since asseys of and comparative toxicity of specific field-collected insects are efficien t and allelochemicals needs further study. widely used to monitor resistance, the We know little about how al1 enzyme potential types of environmental ef- systems are altered by extrinsic fac- fects, environment and genotype inter- tor~such as diet (28). In order to be actions, and their effect on resis tance able to reduce the problem of resistance merit further consideration (36:. De- it is important to monitor pest popula- riving appropriate rates of insecticides tions for evidences of resistance. Ac- for one host plant species and extend- curate results require the control of ing these rates to related host plants, many variables as possible when con- as is current practice, probably results ducting bioassays. The examples dis- in instantes of excessive or inadequate cussed above demonst.rate the poten- use of pesticides. The efficacy of in- tia1 effect of plant chemicals in the insecticides might be increased Ey ad- sect diet on patterns o€insecticide re- justing rates of application to riatch sistance. Therefore, whenever possible pest species response on specific: host we should attempt to control for this plants (16). Literature cited

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