Rejection of host plant by larvae of cabbage butterfly: Diet-dependent sensitivity to an antifeedant

Garden nasturtium,Tropaeolum majus (Tropaeolaceae), is an acceptable host plant for the cabbage butterfly,Pieris rapae. Eggs are readily laid on the plant and hatching larvae feed and develop into normal pupae and adults. However, when second- to fifth-instar larvae were transferred from cabbage to nasturtium, they refused to feed and starved to death. Similar results were obtained when larvae were transferred from other host plants to nasturtium. However, larvae that were reared on nasturtium readily accepted cabbage as a new host plant. We have demonstrated the presence of strong antifeedants in nasturtium foliage and identified the most prominent active compound as chlorogenic acid. However, larvae reared on nasturtium had limited sensitivity, and larvae reared on a wheat germ diet were completely insensitive to the antifeedants. Larvae apparently develop sensitivity to the deterrent as a result of feeding on other host plants, whereas continuous exposure to the deterrent causes habituation or suppression of sensitivity development. The results demonstrate that dietary experience can dramatically affect the response of an insect to a potentially antifeedant compound in a plant.     

Feeding Deterrents and Sensitivity Suppressors for Pieris rapae Larvae in Wheat Germ Diet

Rejection of wheat germ diet by cabbage-reared Pieris rapae larvae is explained by the fact that the diet contains strong feeding deterrents to the larvae. These deterrents were present mainly in hexane and butanol (BuOH) fractions of diet extracts, but the post-BuOH water fraction also showed some deterrent activity. Although minor diet components such as sorbic acid had a weak deterrent effect, wheat germ was primarily responsible for the activity. Seven compounds isolated from the BuOH extract of wheat germ were deterrent to the larvae. UV spectra suggested that most of these compounds are apigenin-based flavones. Fractionation of the hexane extracts showed that most of the active compounds were methanol-soluble, and HPLC of the MeOH fractions revealed the presence of active compounds that were absent in the BuOH extract of wheat germ, as well as other compounds that were common to both fractions. Acceptance of nasturtium by P. rapae larvae reared on wheat germ diet is explained by a type of “cross habituation” of the larvae to feeding deterrents. When larvae were reared on cabbage leaves treated with chemical fractions from wheat germ diet, they readily accepted nasturtium, which is normally refused by cabbage-reared P. rapae larvae, due to the presence of strong feeding deterrents in the plant. A high percentage of diet-reared larvae also fed on Erysimum cheiranthoides and Iberis amara, which are normally rejected. The chemical constituents encountered in wheat germ can almost completely account for the larval acceptance of nasturtium. The suitability of wheat germ diet for rearing phytophagous insects for behavioral assays must be questioned.     

Role of volatile inforchemicals emitted by feces of larvae in host-searching behavior of parasitoidCotesia rubecula (Hymenoptera: Braconidae): A behavioral and chemical study

The role of volatile infochemicals emitted by feces of larvae in the host-searching behavior of the parasitoidCotesia rubecula was evaluated during single- and dual-choice tests inside a wind tunnel. The following treatments were tested: feces produced by second and fourth instars ofPieris rapae (preferred host), second instars ofP. brassicae (inferior host), second instars ofP. napi (nonhost), and wet feces of second instars ofP. rapae. During a single-choice situation females ofC. rubecula oriented to all types of feces tested. When a preference was to be made,C. rubecula preferred feces of second instars ofP. rapae over that of fourth, feces ofP. rapae over that ofP. brassicae, feces ofP. napi over that ofP. brassicae, and wet over normal host feces. No preference was exhibited between feces of second instars ofP. napi and that of second instars ofP. rapae. The relative importance of infochemicals from host feces versus plant damage caused by host larvae to the searching behavior ofC. rubecula was also evaluated. Plant damage was more important to the searching females than host feces when feces were present in specific concentrations in relation to damage. The volatiles released by normal and wet feces of second instars ofP. rapae, wet feces of fourth instars ofP. rapae, and normal and wet feces ofP. brassicae were collected and identified. Overall, 85 chemical compounds were recorded belonging to the following chemical groups: alcohols, ketones, aldehydes, esters, isothiocyanates, sulfides, nitriles, furanoids, terpenoids and pyridines. The blend of chemicals emitted by feces of different instars ofP. rapae and different species ofPieris exhibited an instar and species specificity in both quantity and quality. Wetting of normal feces increased the amount of volatile chemicals released, and it was also responsible for the appearance of new compounds. The role of feces of larvae in the host-seeking behavior ofC. rubecula is discussed.     

Developmental inhibition ofSpodoptera litura (Fab.) larvae by a novel caffeoylquinic acid from the wild groundnut,Arachis paraguariensis (Chod et Hassl.)

A novel compound, 1-caffeoyl-4-deoxyquinic acid (1-CdQA) has been identified along with 3-caffeoylquinic acid (3-CQA) and 5-caffeoylquinic acid (5-CQA) (syn. chlorogenic acid) in the foliage ofArachis paraguariensis, a wild species of groundnut that is highly resistant to attack by the larvae ofSpodoptera litura. When neonate larvae were fed on diets treated with 3-CQA or 1-CdQA, their development was severely inhibited compared to larvae on untreated diets, and the effects were similar in nature and magnitude to those observed for larvae feeding on diets treated with 5-CQA, rutin, and quercetin. The effects of all the compounds were dose related, and their optimal concentration was approximately 3 mM, which corresponds to the total foliar concentration of both caffeoylquinic acids and quercetin diglycosides inA. paraguariensis. After 24 h, the development of third stadium larvae feeding on diets treated with 5-CQA, rutin and 1-CdQA was promoted compared to larvae on control diets, but after 96 hr larvae feeding on treated diets had gained significantly less weight than those on the control diets. 1-CdQA, 3-CQA, and quercetin dyglycosides (previously identified in the foliage ofA. paraguariensis) are considered to be valuable components in resistance of groundnuts toS. litura and may provide useful genetic markers in future breeding for resistance to this pest.     

Chemical constituents of an unacceptable crucifer,Erysimum cheiranthoides,deter feeding byPieris rapae

The wild cruciferErysimum cheiranthoides was found to contain extractable constituents that deterred feeding by larvae of the crucifer specialistPieris rapae when applied to cabbage leaf disks in both choice and nochoice bioassays. High-performance liquid chromatography was used to separate the extract into several fractions, two of which retained the feeding deterrent activity of the extract. UV-absorption spectra of the fractions suggested that one contained cardenolides similar or identical to those reported to deter oviposition byP. rapae onE. cheiranthoides. The other active fraction evidently contains a compound that deters larval feeding but not adult oviposition. The results suggest that the chemical defense ofE. cheiranthoides depends on two types of compounds acting on separate developmental stages of the insect.     

Reassessment of the role of gut alkalinity and detergency in insect herbivory

Previously it was reported that significant amounts of the tomato phenolic, chlorogenic acid, were oxidized in the digestive system of generalist feedersSpodoplera exigua andHelicoverpa zea. The covalent binding of the oxidized phenolic (i.e., quinone) to dietary protein exerts a strong antinutritive effect against larvae. In this study, we examined the fate of ingested chlorogenic acid in larvalManduca sexta, a leaf-feeding specialist of solanaceous plants. Significant amounts of chlorogenic acid were bound to excreted protein byM. sexta when larvae fed on tomato foliage. However, in the case ofM. sexta we suggest that the strong alkalinity and detergency of the midgut may minimize the antinutritive effects of oxidized phenolics. The solubility of tomato leaf protein is significantly greater at pH 9.7, representative of the midgut ofM. sexta, than at pH 8.0, representative of the midguts ofH. zea and S. exigua. We suggest that this increase in solubility would compensate for any loss in bioavailability of essential amino acids caused by the covalent binding of chlorogenic acid to amino acids. Furthermore, lysolecithin, a surfactant likely to contribute to the detergent properties of the midgut fluid, was shown to enhance protein solubility as well as inhibit polyphenol oxidase activity. The adaptive significance of gut alkalinity and detergency is discussed.Approved by the Director of the Arkansas Agricultural Experiment Station.     

Plant-natural enemy association in the tritrophic system,Cotesia rubecula-Pieris rapae-brassiceae (cruciferae): I. Sources of infochemicals

The role of airborne infochemicals in host selection by the parasitoidCotesia rubecula (Marshal) (Hymenoptera: Braconidae) was examined in a wind tunnel. To elucidate the role of volatile chemicals in attractingC. rubecula to cabbage infested by the host [Pieris rapae L. (Lepidoptera: Pieridae)], the potential sources of volatiles related toP. rapae infestation on cabbage were tested individually. The responses of females to nonhost plant species, bean and geranium, as well as to frass of a nonhost lepidopteran were also examined.C. rubecula was attracted to cabbage previously infested byP. rapae and to frass and regurgitate ofP. rapae. No attraction was observed to larvae ofP. rapae alone. Females were also attracted to mechanically damaged cabbage, cabbage previously infested byPlutella xylostella L. (Lepidoptera: Plutellidae) (a nonhost lepidopteran herbivore), and cabbage previously infested by snails (a nonhost, noninsect herbivore). Intact cabbage, bean, and geranium plants elicited no attraction. A low frequency of attraction was observed to mechanically damaged bean and geranium. Attraction was also observed to frass ofP. xylostella. Volatiles from cabbage related to damage, and volatiles from frass and regurgitate of the host seem to play an important role in guidingC. rubecula to plants infested by its host.     

Larval exposure to oviposition deterrents alters subsequent oviposition behavior in generalist,Trichoplusia ni and specialist,Plutella xylostella moths

The present study was undertaken to determine the effects of larval feeding experience on subsequent oviposition behavior of the resulting moths. Larvae of the cabbage looper (Trichoplusia ni, Noctuidae) and the diamondback moth (Plutella xylostella, Plutellidae) were exposed to the phenylpropanoid allelochemical trans-anethole (at 100 ppm fw in artificial diet) or the limonoid allelochemical toosendanin (10 ppm sprayed on cabbage leaves). Both compounds had been shown to deter oviposition in naïve moths in previous choice tests. Moths developing from experienced larvae (both sexes) showed a decrease in oviposition deterrence response when given a choice between control and treated leaves, unlike naïve moths. This phenomenon, analogous to habituation to feeding deterrents in lepidopteran larva, occurred irrespective of duration of feeding on the deterrent compound. We also observed that F₁larvae resulting from experienced moths (previously exposed to toosendanin as larvae) grew as well on toosendanin-treated foliage as on control foliage. In contrast, growth of F₁larvae from naïve moths was significantly impaired by toosendanin. These results demonstrate that host-selection behavior in cabbage looper (a generalist) and diamondback moth (a specialist) may be shaped by feeding experience according to Hopkins' Host Selection Principle in addition to chemical legacy.     

Chemical stimulants and deterrents regulating acceptance or rejection of crucifers by cabbage butterflies

GravidPieris rapae butterflies oviposit on many, but not all, crucifers. Rejection ofErysimum cheiranthoides andCapsella bursa-pastoris was initially explained by the presence of chemical deterrents in the plants. Analyses and bioassays of plant extracts indicated the absence of oviposition stimulants inC. bursa-pastoris, but similar chemical separation ofE. cheiranthoides extracts revealed the presence of stimulants as well as deterrents. Choice tests illustrate how acceptance or rejection of a plant by an insect may depend on the balance of positive and negative chemical stimuli within the plant.     

Influence of cabbage proteinase inhibitorsin situ on the growth of larvalTrichoplusia ni andPieris rapae

Trypsin and chymotrypsin inhibitors are proteins that are developmentally regulated in foliage of cabbage plants, appearing at high concentrations in young foliage on mature plants. This temporal and spacial regulation of foliar proteinase inhibitors is synchronized with the appearance and distribution of foliar feeding Lepidoptera. When insects were allowed to select their feeding sites, larvalPieris rapae fed on the young foliage of cabbage plants, while larvalTrichoplusia ni fed on the mature foliage on cabbage plants. LarvalP. rapae that fed on mature plants were significantly smaller than larvae feeding on young plants, while there was no significant difference between larvalT. ni feeding on mature plants and those feeding on young plants. Thus, there was a significant inverse correlation between the level of proteinase inhibitory activity in cabbage foliage and larval growth. WhenP. rapae andT. ni were provided with an artificial diet containing total protein (including significant levels of proteinase inhibitors) that was extracted from cabbage foliage, there was a significant reduction in growth and development of both species of Lepidoptera.     

Plant-natural enemy association in tritrophic system,Cotesia rubecula-Pieris rapae-brassicaceae (Cruciferae). III: Collection and identification of plant and frass volatiles

To elucidate the identity of the volatile compounds that could be involved in the searching behavior of the parasitoidCotesia rubecula Marshall (Hymenoptera: Braconidae), the volatiles released by cabbage and frass of Lepidoptera feeding on cabbage were collected and analyzed using a gas chromatograph-mass spectrometer. The volatiles emitted by intact cabbage were -pinene, -pinene, myrcene, 1,8-cineole,n-hexyl acetate,cis-3-hexen-1-yl acetate, and dimethyl trisulfide. Mechanical damage on an intact plant induced the release of two more compounds,trans-2-hexenal and 1-methoxy-3-methylene-2-pentanone. Current feeding by larvae ofPieris rapae L. (Pieridae) induced the plant to release all the compounds released after mechanical damage and additionally 4-methyl-3-pentenal and allyl isothiocyanate. Current feeding by larvae ofPlutella xylostella L. (Plutellidae) induced the plant to release all the compounds present after mechanical damage and additionally allyl isothiocyanate. The volatiles emitted after feeding by the lepidopterans had ceased were the same as those emitted by cabbage damaged by mechanical means. The blend of volatiles emitted by frass was comprised of plant chemicals, mainly sulfur compounds. Frass ofP. rapae emitted allyl isothiocyanate, methyl isothiocyanate, methyl propyl sulfide, dimethyl trisulfide,S-methyl methane thiosulfinate, 4-methyl-3-pentenal,trans-2-hexenal, and 2,3-dihydro-4-methyl furan. Frass ofP. xylostella emitted only dimethyl trisulfide andS-methyl methane thiosulfinate. The blend of volatiles emitted by frass is herbivore-species specific.     

Generalization of a Habituated Feeding Deterrent Response to Unrelated Antifeedants Following Prolonged Exposure in a Generalist Herbivore, Trichoplusia ni

The possibility of generalization of habituated response to unrelated feeding deterrents following prolonged exposure was examined in third instar Trichoplusia ni (Lepidoptera: Noctuidae) larvae by rearing them on antifeedants and then testing with other unrelated antifeedants. We introduced neonate larvae (<24-hr old) onto cabbage leaves treated with crude seed extracts of Melia volkensii (Meliaceae) or oil of Origanum vulgare (“oregano”) (Lamiaceae) and allowed them to feed until early in the third instar. Naïve larvae were reared on cabbage leaves treated with carrier solvent alone. Both experienced and naïve larvae were tested for feeding deterrent response with the same and the different extracts in a leaf disc choice bioassay. Habituation was generalized to both M. volkensii and oregano following prolonged exposure to either plant extract and also to a pure allelochemical, thymol, following prolonged exposure to either digitoxin or xanthotoxin. However, there was no generalization of the habituated response to oregano following prolonged exposure to digitoxin or thymol, or to thymol or xanthotoxin following prolonged exposure to oregano or M. volkensii. Our results demonstrate that habituated response to feeding deterrents in a polyphagous insect herbivore can be generalized among and between plant extracts and pure allelochemicals, but not in all situations. The implications of such behavioral plasticity in herbivorous insects for the use of antifeedants as crop protectants or for host plant shifts is discussed.     

Do Foliar Phenolics Provide Protection to Heliothis virescens from a Baculovirus?

The effects of chlorogenic acid on larval survival and growth of the tobacco budworm Heliothis virescens and larval susceptibility to Helicoverpa zea nucleopolyhedrovirus (HzSNPV) were studied with an artificial diet and transgenic tobacco Nicotiana tabacum. Survival of neonates on over- or underexpressed phenylalanine ammonia-lyase (PAL) transgenic tobacco lines was positively correlated with the level of chlorogenic acid. Larval weight was not correlated with the level of chlorogenic acid in tobacco foliage. On the other hand, larval weights on artificial diet supplemented with chlorogenic acid were negatively correlated with chlorogenic acid concentration. Second instars treated on virus-treated (10 occlusion bodies/larva) tobacco leaf disks for 24 hr and then reared on tobacco leaves showed higher survival time and increased mortality on lines with increasing levels of chlorogenic acid. However, viral-related larval mortality on artificial diet was negatively correlated with dietary chlorogenic acid. These results illustrate the inadequacy of artificial diet studies in establishing causal relationships among plant chemistry, herbivores, and natural enemies.     

Comparative Analysis of Headspace Volatiles from Different Caterpillar-Infested or Uninfested Food Plants of Pieris Species

Plants that are infested by herbivores emit volatile cues that can be used by the natural enemies of the herbivores in their search for hosts. Based on results from behavioral studies, we investigated to what extent intact and herbivore-infested plant species and varieties from the food plant range of Pieris herbivore species differ in the composition of the volatile blends. Parasitoids of Pieris species, Cotesia glomerata and C. rubecula, show differential responses towards various herbivore-infested food plants, whereas differences in responses to plants infested by other herbivore species were less clear. Chemical analysis of the headspace samples of red cabbage, white cabbage, and nasturtium plants that were infested by P. brassicae or P. rapae larvae, or that were intact, yielded 88 compounds including alcohols, ketones, aldehydes, esters, nitriles, terpenoids, sulfides, (iso)thiocyanates, carboxylic acids, and others. The analysis revealed that herbivore-infested plants emit the largest number of compounds in the highest amounts. The plant species affected the volatile blend more than did the herbivore species, and differences between plant varieties were less pronounced than differences between plant species. Differences in headspace composition between plants infested by P. brassicae or P. rapae were mainly of a quantitative nature. Herbivore-infested nasturtium differed considerably from the cabbage varieties in a qualitative way. Headspace compositions of red and white cabbage varieties were comparable to that of the food plant Brussels sprouts (Brassica oleracea gemmifera cv. Titurel) as determined in earlier studies in our laboratory. With respect to plant response to herbivory, nasturtium differed considerably from the cabbage varieties analyzed so far and shows resemblance with Lima bean, cucumber, and corn. These plant species produce a greater quantity and variety of volatiles under herbivore attack than intact plants. The results of this study are discussed in relation to behavioral observations on C. glomerata and C. rubecula.     

Toxicity of allelochemicals from wild insect-resistant tomatoLycopersicon hirsutum f.glabratum toCampoletis sonorensis,a parasitoid ofHeliothis zea

Greenhouse-grown plants of five tomato lines varying in their level of 2-tridecanone-mediated resistance toManduca sexta (L.) andLeptinotarsa decemlineata (Say) were found to adversely affect larvae ofCampoletis sonorensis (Cameron), a larval endoparasitoid ofHeliothis zea (Boddie), in a manner directly related to their level of resistance. The parasitoid larvae, which emerge as fifth instars from their host and construct a cocoon on the foliage of their hosts' host plant, suffered extensive mortality during cocoon spinning on highly resistant foliage. Mortality was greatest (82%) on the highly resistant plants ofLycopersicon hirsutum f.glabratum (accession PI 134417) and an F₁ backcross [(L. esculentum × PI 134417) × PI 134417] selection. Mortality was intermediate (40 and 28%, respectively) on backcross selections with moderate and low levels of resistance and least (8%) on susceptibleL. esculentum. Removal of the glandular trichomes, which contain 2-tridecanone in their tips, from the foliage eliminated differences in parasitoid mortality among plant lines.Bioassays of 2-tridecanone indicated that it is acutely toxic to fifth instarC. sonorensis larvae at the quantities associated with highly resistant foliage and produces symptoms identical to those observed on resistant foliage. 2-Undecanone, a second methyl ketone present in the glandular trichomes of resistant foliage, was also toxic toC. sonorensis larvae, but significantly less so than 2-tridecanone. The results support the hypothesis that 2-tridecanone is responsible for the observed mortality ofC. sonorensis larvae during cocoon construction on resistant foliage.     

Systemic induction of feeding deterrents in cotton plants by feeding ofSpodoptera SPP. Larvae

Cotton,Gossypium hirsutum L., has been shown to exhibit systemic induced resistance to arthropods under certain conditions. We conducted experiments to determine the effects of previous feeding ofSpodoptera exigua Hübner andSpodoptera littoralis (Boisd.) larvae on feeding behavior, growth, and survival of larvae subsequently feeding on cotton. In one feeding choice test,S. exigua larvae preferred young leaves from undamaged control plants to undamaged young leaves from a previously damaged plant. Feeding deterrence was noticeable after only 6 hr of initial feeding damage by larvae, and there was almost complete deterrence after 30 and 54 hr of continuous feeding. In a second feeding choice test,S. littoralis larvae fed more on mature leaves from undamaged control plants than on undamaged mature leaves from previously damaged plants. In no-choice tests, third instars ofS. littoralis fed undamaged young leaves from damaged plants did not gain weight and died by the seventh day, whereas larvae fed young leaves from undamaged control plants gained weight and pupated within 11 days. Sixth instars ofS. littoralis fed either old damaged leaves, old undamaged leaves, or young undamaged leaves all from previously damaged plants gained weight slowly and took more than 12 days to pupate, whereas larvae fed young leaves from undamaged plants gained weight rapidly and pupated within five days of the beginning of the experiment.     

Activation of plant foliar oxidases by insect feeding reduces nutritive quality of foliage for noctuid herbivores

The foliage and fruit of the tomato plantLycopersicon esculentum contains polyphenol oxidases (PPO) and peroxidases (POD) that are compartmentally separated from orthodihydroxyphenolic substrates in situ. However, when leaf tissue is damaged by insect feeding, the enzyme and phenolic substrates come in contact, resulting in the rapid oxidation of phenolics to orthoquinones. When the tomato fruitwormHeliothis zea or the beet army-wormSpodoptera exigua feed on tomato foliage, a substantial amount of the ingested chlorogenic acid is oxidized to chlorogenoquinone by PPO in the insect gut. Additionally, the digestive enzymes of the fruitworm have the potential to further activate foliar oxidase activity in the gut. Chlorogenoquinone is a highly reactive electrophilic molecule that readily binds cova-lently to nucleophilic groups of amino acids and proteins. In particular, the —SH and —NH₂ groups of amino acids are susceptible to binding or alkylation. In experiments with tomato foliage, the relative growth rate of the fruitworm was negatively correlated with PPO activity. As the tomato plant matures, foliar PPO activity may increase nearly 10-fold while the growth rate of the fruitworm is severely depressed. In tomato fruit, the levels of PPO are highest in small immature fruit but are essentially negligible in mature fruit. The growth rate of larvae on fruit was also negatively correlated with PPO activity, with the fastest larval growth rate occurring when larvae fed on mature fruit. The reduction in larval growth is proposed to result from the alkylation of amino acids/protein byo-quinones, and the subsequent reduction in the nutritive quality of foliage. This alkylation reduces the digestibility of dietary protein and the bioavailability of amino acids. We believe that this mechanism of digestibility reduction may be extrapolatable to other plant-insect systems because of the ubiquitous cooccurrence of PPO and phenolic substrates among vascular plant species.     

Isolation and identification of oviposition deterrents to cabbage butterfly,Pieris rapae,fromErysimum cheiranthoides

Avoidance of some crucifer species by the crucifer specialist,Pieris rapae, has been attributed to the presence of oviposition deterrents in these plants. Studies on one such unacceptable plant,Erysimum cheiranthoides, have resulted in the isolation ofn-butanol-soluble deterrents from the alcoholic extract of foliage. The active fraction contained three cardiac glycosides, which were isolated by reversed-phase HPLC and by open column chromatography on silica gel. Chemical and spectral evidence (UV, [¹H]NMR, and FAB-MS) led to the characterization of these compounds as erysimoside (1), erychroside (2), and erycordin (3). Erysimoside and erychroside were strongly deterrent toPieris rapae, but erycordin was inactive. Both active compounds have the same aglycone, strophanthidin (5) and the inner sugar in both cases is a 2,6-dideoxy hexose to which the outer sugar is attached at position C-4. These structural features, which are absent in the inactive compound (3), may represent specific requirements for oviposition deterrent activity.     

Cotesia glomerata Female Wasps Use Fatty Acids from Plant–Herbivore Complex in Host Searching

Cotesia glomerata parasitizes early instars of the cabbage butterfly,Pieris rapae, in Japan. Female wasps antennatedRorippa indica leaves damaged by feeding ofP. rapae larva, but ignored artificially damaged leaves. Females also antennated filter paper containingR. indica leaf juice plusP. rapae regurgitant. Chemical analysis revealed five compounds in higher amounts in the infested edges of leaves than in artificially damaged edges. Among them, we identified palmitic acid, oleic acid, and stearic acid. Female wasps antennated filter paper containing each of these three acids. We discuss the function of these acids in the tritrophic context.     

Protective action of midgut catalase in lepidopteran larvae against oxidative plant defenses

Catalase activity was detected in the midgut tissues and regurgitate of several lepidopteran pests of the tomato plant. Greatest activity in the midgut was detected in larvalHelicoverpa zea, followed bySpodoptera exigua, Manduca sexta, andHeliothis virescens. We present evidence that catalase, in addition to removing toxic hydrogen peroxide, may inhibit the oxidation of plant phenolics mediated by plant peroxidases. Small amounts of larval regurgitate significantly inhibited foliar peroxidase activity via removal of hydrogen peroxide. Treatment of foliage with purified catalase nearly eliminated peroxidase activity and was superior as a larval food source compared to untreated foliage. Tomato foliar peroxidases oxidize an array of endogenous compounds including caffeic acid, chlorogenic acid, rutin, coumaric acid, cinnamic acid, and guaiacol. The oxidized forms of these compounds are potent alkylators of dietary and/or cellular nucleophiles (e.g., thiol and amino functions of proteins, peptides, and amines). When tomato foliar protein was pretreated with peroxidase and chlorogenic acid and incorporated in artificial diet, larval growth was reduced compared to larvae fed untreated protein. Thus, the diminution of peroxidase activity and removal of hydrogen peroxide by catalase may represent an important adaptation to leaf-feeding. The secretion of catalase in salivary fluid during insect feeding is also suggested to be a potential mechanism for reducing hydrogen peroxide formation as an elicitor of inducible plant defenses.