Chemical Defense in <Emphasis Type="Italic">Elodea nuttallii</Emphasis> Reduces Feeding and Growth of Aquatic Herbivorous Lepidoptera

The submersed macrophyte Elodea nuttallii (Hydrocharitaceae) is invasive in Europe and frequently found in aquatic plant communities. Many invertebrate herbivores, such as larvae of the generalist aquatic moth, Acentria ephemerella (Lepidoptera, Pyralidae), avoid feeding on E. nuttallii and preferably consume native species. First instar larvae exhibited a high mortality on E. nuttallii compared to the native macrophyte Potamogeton perfoliatus. Mortality of older larvae was also high when fed E. nuttallii exposed to high light intensities. Growth of older larvae was strongly reduced on E. nuttallii compared to pondweeds (Potamogeton lucens). Neither differences in nitrogen nor phosphorus content explained the different performance on these submerged macrophytes, but plants differed in their flavonoid content. To investigate whether plant-derived allelochemicals from E. nuttallii affect larval performance in the same way as live plants, we developed a functional bioassay, in which Acentria larvae were reared on artificial diets. We offered larvae Potamogeton leaf disks coated with crude Elodea extracts and partially purified flavonoids. Elodea extracts deterred larvae from feeding on otherwise preferred Potamogeton leaves, and yet, unknown compounds in the extracts reduced growth and survival of Acentria. The flavonoid fraction containing luteolin-7-O-diglucuronide, apigenin-7-O-diglucuronide, and chrysoeriol-7-O-diglucuronide strongly reduced feeding of larvae, but did not increase mortality. The concentrations of these compounds in our assays were 0.01–0.09% of plant dry mass, which is in the lower range of concentrations found in the field (0.02–1.2%). Chemical defense in E. nuttallii thus plays an ecologically relevant role in this aquatic plant–herbivore system.     

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.     

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.     

Honeydew analysis for detecting phloem transport of plant natural products

Analysis of honeydew excreted by various sap-sucking insects indicated the presence of certain plant secondary constituents in the phloem of their host plants. Honeydew excreted by mealybugs (Pseudococcus longispinus), living onCastanospermum australe, contained the indolizidine alkaloid castanospermine, a potent -glucosidase inhibitor. Similarly, honeydew from green peach aphids (Myzus persicae), feeding onSenecio vulgaris flower buds, contained the pyrrolizidine alkaloid senecionine, itsN-oxide, and hydrolytic products including retronecine. Cardenolides were detected in the honeydew of oleander aphids (Aphis nerii) feeding on oleander (Nerium oleander), indicating that these compounds are translocated in the phloem. On the other hand, honeydew from greenbugs (Schizaphis graminum), feeding on barley, lacked gramine or related indole metabolites. Similarly MBOA, the breakdown product of DIMBOA, was not detected in the honeydew of greenbugs living on DIMBOA-containing wheat.     

The Effects of Nectar–Nicotine on Colony Fitness of Caged Honeybees

Nectar of many bee flowers contains secondary compounds, which are considered toxic for honeybees on repeated exposure. Although many anecdotal reports indicate the toxicity of secondary compounds to bees, only a few studies have tested the extent of toxicity at different honeybee ages, especially at the larval stages. Honeybees encounter nicotine at trace concentrations (between 0.1 and 5 ppm) in floral nectar of a few Nicotiana spp. and in Tilia cordata. Adult honeybee workers tolerate these nicotine concentrations. In controlled nonchoice feeding experiments with caged bees, we investigated the effect of nicotine on hatching success and larval and forager survival. Naturally occurring concentrations of nectar–nicotine did not affect hatching success of larvae or their survival, but the latter was negatively affected by higher concentrations of nicotine (50 ppm). Concentrations of nicotine in fresh honey samples from the hives were 90% lower than the concentrations in the offered experimental sucrose solutions. Our results indicate that honeybees can cope with naturally occurring concentrations of nicotine, without notable mortality, even when consumed in large quantities for more than 3 weeks.     

Responses of Queenright and Queenless Workers of <Emphasis Type="Italic">Apis Cerana</Emphasis> to 9-keto-2(<Emphasis Type="Italic">E</Emphasis>)-decenoic Acid,a Pheromonal Constituent of the Mandibular Gland

In dequeened honeybee colonies ovarian activation occurs in some workers, and the pheromonal bouquets of these laying workers become more queen-like. In the Asiatic honeybee, Apis cerana, we compared the amount of 9-keto-2(E)-decenoic acid (9-ODA), a mandibular gland pheromone component, between non-laying workers from queenright colonies and laying workers from queenless colonies, and further, applied synthetic 9-ODA to workers to determine whether they discriminate workers with activated ovaries based on the level of this compound. Levels of 9-ODA were higher in laying workers from dequeened colonies than in non-laying workers from queenright colonies. In both queenright and queenless colonies, workers attacked more workers treated with 9-ODA than control-treated workers. These results suggest that detection of pseudoqueens in A. cerana is mediated by changes in 9-ODA.     

Fate of iridoid glycosides in different life stages of the Buckeye,Junonia coenia (Lepidoptera: Nymphalidae)

The buckeye butterfly,Junonia coenia (Lepidoptera: Nymphalidae), specializes on plants that contain iridoid glycosides. To determine the fate of these compounds in larvae, pupae, and adults of this species, we reared larvae on artificial diets with and without iridoid glycosides, and on leaves of a host plant,Plantago lanceolata (Plantaginaceae). Quantification by gas chromatography showed that newly molted third-, fourth-, and fifth-instar larvae reared on leaves ofP. lanceolata contained means of 5.13, 2.88, and 6.83% dry weight iridoid glycoside. In contrast, the mean iridoid glycoside concentration of actively feeding fifth-instar larvae was 0.28% dry weight, that of pupae was 0.19% dry weight iridoids, and adults contained no detectable iridoids. Feeding experiments suggested that this reduction in actively feeding larvae was due to the metabolism of iridoid glycosides.P. lanceolata leaves in these experiments contained a mean of 1.00% dry weight iridoid glycoside, with a 2:1 ratio of aucubin to catalpol. Calculation of iridoid consumption and utilization indices showed that larvae fed artificial diets consumed, digested, and sequestered aucubin and catalpol in similar ways. When these indices were calculated for larvae fed leaves ofP. lanceolata, catalpol was sequestered twice as efficiently as aucubin.     

Laboratory Evaluation of <Emphasis Type="Italic">Artemisia annua</Emphasis> L. Extract and Artemisinin Activity against <Emphasis Type="Italic">Epilachna paenulata</Emphasis> and <Emphasis Type="Italic">Spodoptera eridania</Emphasis>

Ethanolic extract of aerial parts of Artemisia annua L. and artemisinin were evaluated as anti-insect products. In a feeding deterrence assay on Epilachna paenulata Germ (Coleoptera: Coccinellidae) larvae, complete feeding rejection was observed at an extract concentration of 1.5 mg/cm² on pumpkin leaf tissue. The same concentration produced a feeding inhibition of 87% in Spodoptera eridania (Cramer) (Lepidoptera: Noctuidae). In a no-choice assay, both species ate less and gained less weight when fed on leaves treated with the extract. Complete mortality in E. paenulata and 50% mortality in S. eridania were observed with extract at 1.5 mg/cm². Artemisinin exhibited a moderate antifeedant effect on E. paenulata and S. eridania at 0.03–0.375 mg/cm². However, a strong effect on survival and body weight was observed when E. paenulata larvae were forced to feed on leaves treated at 0.03 and 0.075 mg/cm². Artemisia annua ethanolic extract of aerial parts at 1.5 mg/cm² showed no phytotoxic effect on pumpkin seedlings.     

Sequestration of host plant-derived compounds by geometrid moth, Milionia basalis, toxic to a predatory stink bug, Eocanthecona furcellata

A predatory stink bug, Eocanthecona furcellata, died after feeding on Milionia basalis larvae. The compounds toxic to E. furcellata were isolated from the hemolymph of M. basalis larvae and identified as inumakilactone A, nagilactone C, and nagilactone C glucoside. The concentrations of inumakilactone A, nagilactone C, and nagilactone C glucoside in the hemolymph of the final instar larvae were 130, 50, and 770 g/ml, respectively. Nagilactone C showed the highest insecticidal activity against second-instar nymphs of E. furcellata, while nagilactone C glucoside showed the lowest, one twentieth of that of nagilactone C. When mixed compounds were given at the same concentrations as those in hemolymph of M. basalis, all nymphs of E. furcellata died with in three days. Inumakilactone A and nagilactone C were found to be in the leaves of podocarp, Podocarpus macrophyllus, the only host plant of M. basalis, at concentrations of 13 and 175 g/g fresh weight, respectively. However, no nagilactone C glucoside was detected in the leaves of this species. These results suggested that M. basalis may transform nagilactone C to its glucoside.     

Composition of larval secretion ofChrysomela lapponica (Coleoptera,Chrysomelidae) and its dependence on host plant

The defensive secretion ofChrysomela lapponica larvae, which is produced by nine pairs of exocrine dorsal glands, has been chemically analyzed. TheC. lapponica larvae were kept in the laboratory on leaves of either birch (Betula pendula), alder (Alnus glutinosa), or willow (Salix fragilis). Larvae developed normally on birch and willow, whereas those on alder died within a few days. GC-MS analyses of the secretion of larvae on birch and willow revealed that the composition of this secretion differs distinctly from the known ones of several otherChrysomela species feeding exclusively on Salicaceae. In the exocrine secretion of larvae on birch, 69 compounds were identified, which included the main components isobutyric acid, 2-methylbutyric acid, and esters of the two. Several of the esters have not been reported previously from nature. The alcoholic components of the esters may be hydrolysis products ofBetula glycosides. Most components of the secretion of larvae feeding on birch were also found in the secretion of larvae feeding on willow. In addition, major amounts of benzoic acid and salicylalcohol were present in the secretion of the larvae feeding on willow.C. lapponica obviously acquires salicylalcohol by hydrolysis of salicin from willow leaves. However, in contrast to otherChrysomela species,C. lapponica larvae oxidize only traces of salicylalcohol to salicylaldehyde. The repellent activity of single authentic compounds of the secretion of larvae feeding on birch and willow, respectively, was tested in laboratory bioassays with ants (Myrmica sabuleti). Biosynthetic pathways to some identified compounds are suggested and discussed under evolutionary and functional aspects.     

Effects of two plant secondary metabolites,cineole and gallic acid,on nightly feeding patterns of the common brushtail possum

We investigated effects of two plant secondary metabolites (PSMs), cineole and gallic acid, on the nightly feeding behavior of the common brushtail possum (Trichosurus vulpecula), a generalist folivore. We tested whether possums altered their feeding behavior in response to increasing levels of cineole, a dietary terpene. Possums were fed artificial diets containing three levels of cineole; zero (basal diet), medium (6.8% of total dry matter, DM), and high (15.3% DM). In another experiment, we introduced gallic acid, a dietary phenolic, into the diets. Possums were offered a Choice PSM diet (cineole and gallic acid diets simultaneously) or a No-Choice PSM diet (containing either cineole or gallic acid). Detoxification products of cineole and gallic acid were examined in urine to determine that different detoxification pathways were utilized in the elimination of each compound. With increasing cineole levels, possums ate less, had smaller feeding bouts, and had a lower rate of intake, but did not extend their total nightly feeding time. Possums offered the Choice PSM diet, compared with the No-Choice diets, ate more, had larger feeding bouts, and tended to increase their rate of intake. Results from the urinary analysis indicated that gallic acid and cineole were not involved in competing detoxification pathways in brushtail possums. There was also a significant sex effect: females ate more overall, ate more per feeding bout, and ate at a higher rate than males. These results indicate that PSMs not only constrain overall intake, but that possums alter their feeding behavior in response to them. Altered feeding patterns may reduce the negative influence of PSMs on intake.     

Leaf-tying by tortricid larvae as an adaptation for feeding on phototoxicHypericum perforatum

Hypericin, an anthrone dimer inHypericum perforatum (Guttiferae), is shown for the first time to be phototoxic to generalist lepidopteran larvae; survivorship was reduced and development prolonged forHeliothis zea (Noctuidae) andPlatynota flavedana (Tortricidae) that consumed hypericin diets in the presence of light. Survivorship for both species is enhanced when photoactivating wavelengths are excluded by filters. In natureP. flavedana successfully survives onH. perforatum by tying together leaves and feeding inside the ties. Shielded from light, the larvae are protected by their mode of feeding from phototoxic compounds.     

Hemicellulose is an important leaf-feeding resistance factor in corn to the fall armyworm

The fall armyworm,Spodoptera frugiperda (J. E. Smith), (FAW) is a major pest of corn,Zea mays L., in the southeastern United States. The damage to pretassel corn is caused by larvae feeding primarily on immature inner whorls. In this study, resistant lines were found to contain more crude fiber in whorls, mostly hemicellulose and cellulose. While hemicellulose, chiefly an arabinoxylan, was higher in resistant (R) lines than in susceptible (S) lines, the distribution of constituent neutral sugars was very similar in the lines. Both lines also containedp-coumaric and ferulic acids. These phenolic acids are known to occur both in the free state and in the cell wall as complexes bound by ester linkages to the arabinose moiety of the arabinoxylan.¹³C NMR data showed that the intensity of the carbonyl carbon (184 ppm) in resistant hemicellulose was stronger, indicating a greater degree of cross-linking. Thus, resistant hemicellulose is both structurally different from susceptible hemicellulose and present in greater quantities. In two of three laboratory dietary tests, FAW larval weight gains were significantly higher on diets with (S) hemicellulose incorporated at the same level as (R) hemicellulose. Therefore, resistance to the FAW appears to be correlated with both a greater amount and a higher degree of cross-linking of the hemicellulose of (R) lines.Mention of a trademark, proprietary product, or vendor does not constitute a guarantee of warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable.     

Response of generalist and specialist insects to qualitative allelochemical variation

We examined the effects of a set of four biosynthetically related iridoid glycosides, aucubin, catalpol, loganin, and asperuloside, on larvae of a generalist,Lymantria dispar (Lymantriidae), the gypsy moth, and an adapted specialist, the buckeye,Junonia coenia (Nymphalidae). In general,L. dispar grew and survived significantly less well on artificial diets containing iridoid glycoside, compared to a control diet without iridoid glycosides. In choice tests, previous exposure to a diet containing iridoid glycosides caused larvae subsequently to prefer iridoid glycoside-containing diets even though they were detrimental to growth and survival. In contrast,J coenia larvae grew and survived better on diets with aucubin and catalpol, the two iridoid glycosides found in the host plantPlantago lanceolata (Plantaginaceae), than on diets with no iridoid glycoside or with loganin and asperuloside. The results of choice tests of diets with and without iridoid glycosides and between diets with different iridoid glycosides reflected these differences as well. These results are discussed in terms of (1) differences between generalists and specialists in their response to qualitative variation in plant allelochemical content, (2) the induction of feeding preferences, and (3) the evolution of qualitative allelochemical variation as a plant defense.     

Antifeedants to Armyworms, Spodoptera lituraand Pseudaletia separata,from Bitter Gourd Leaves, Momordica charantia

Bitter gourd, Momordica charantia, was less palatable to two species of armyworms, Spodoptera litura and Pseudaletia separata, than two other cucurbitaceous plants. A methanol extract of M. charantia leaves inhibited feeding of the armyworm larvae. The two most active fractions obtained by silica gel chromatography were purified by HPLC. Momordicine II, a triterpene monoglucoside, was identified as an antifeedant compound from the more active of these fractions. The second active fraction led to the isolation of a new triterpene diglucoside. Fresh leaves of M. charantia contained ca. 0.3% of momordicine II. Momordicine II showed significant antifeedant effects on P. separata at concentrations of 0.02, 0.1, and 0.5% in artificial diets. Momordicine II caused a significant feeding reduction in S. litura only at the highest concentration (0.5%) tested. The difference in feeding response of the two armyworms to momordicine II may be related to the diversity in their host range.     

Quantitative and qualitative effects of larval diet on male scent secretions ofEstigmene acrea,Phragmatobia foliginosa,andPyrrharctia isabella (Lepidoptera: Arctiidae)

In feeding experiments with insects reared in the laboratory, the presence of the dihydropyrrolizines hydroxydanaidal and danaidal in the male scent organs (coremata) of the arctiids,Estigmene acrea (Drury),Phragmatobia fuliginosa (L.), andPyrrharctia isabella (J.E. Smith), was shown to depend on the presence of a source of pyrrolizidine alkaloids (PAs) in the larval diet.Phragmatobia males given an artificial diet supplemented with the powdered roots of the PA-containing plantSymphytum officinale L. (comfrey) produced more hydroxydanaidal than danaidal, whereas males given an artificial diet supplemented with dried whole plants of another PA-containing species,Senecio vulgaris L., produced more danaidal than hydroxydanaidal.Pyrrharctia males produced hydroxydanaidal with little if any danaidal, whether the source of PAs was comfrey orS. vulgaris. A behavioral bioassay showed that the coremata of PA-deniedPyrrharctia male progeny of PA-denied parents were pheromonally inactive, whereas those of PA-denied male progeny of PA-supplied parents (male and/or female) were often active. This indicates that a small amount of pheromone is made from PAs transferred from the female to her eggs and that males effect copulatory transfers of PAs that are, in turn, passed to the eggs by the mated female. Field observations ofPhragmatobia andPyrrharctia larvae feeding on sources of PAs were reported. The PA monocrotaline was shown to be a feeding stimulant forPyrrharctia larvae.     

Hydroxamic acid glucosides in honeydew of aphids feeding on wheat

DIMBOA glucoside (2-O-/gb-D-glucopyranosyl-4-hydroxy-7-meth-oxy-1,4-benzoxazin-3-one), the main hydroxamic acid (Hx) in intact wheat plants, was detected in the honey dew ofRhopalosiphum padi feeding on seedlings of six wheat cultivars that differed in their concentration of Hx, suggesting that the chemical circulates in the phloem. Neither the aglucone (DIMBOA) nor its main breakdown product were found in any of the honeydew samples. Honey dew production by aphids caged on seedlings of the wheat cultivars and DIMBOA glucoside concentrations in the honeydew followed biphasic curves when plotted against Hx concentration, suggesting passive ingestion of the chemical from the phloem at low Hx concentrations and limited ingestion due to feeding deterrency by Hx in mesophyll cells at high Hx concentrations. The presence of plant toxins such as Hx glucosides in the phloem sap, the main ingesta of aphids, and in the mesophyll cells, has major implications for plant defense, through a feeding deterrent effect during stylet penetration, and deterrency (antixenosis) along with antibiosis during feeding.     

Effects ofHymenaea courbaril leaf resin on the generalist herbivoreSpodoptera exigua (beet armyworm)

The hypothesis that leaf resin ofHymenaea courbaril (Leguminosae) functions as a defence against herbivory was tested in a series of experiments with the generalist herbivore, beet armyworm (Spodoptera exigua Hübn). Pure leaf resin obtained by steam distillation was incorporated into an artificial diet at 0.0%, 0.16%, 0.50%, and 1.60% (dry weight) in one experiment and 0.0%, 1.0%, and 3.2% (dry weight) in a second experiment;S. exigua larvae were reared on these diets. In four palatability experiments the amount of leaf area eaten was compared between pairs of different leaf material. Mortality due to viral infection of larvae in the first feeding experiment showed a dose-response to leaf resin concentration. In the second experiment larvae showed a dose-response in the reduction of pupal weight and delay of time to pupation. These are interpreted as a dose-related stress onS. exigua which occurs with increased resin concentration in the diet. In the palatability tests,S. exigua strongly preferred untreated to resintreated bean leaf disks, bean disks toH. courbaril immature leaf disks,H. courbaril immature leaf toH. courbaril mature leaf, and bean leaf disks toH. courbaril newly emerged leaf disks. From these results, we conclude thatH. courbaril leaf resins possess toxic and feeding-deterrent properties which make them effective as an herbivore defense.     

Mechanisms of resistance in wild riceOryza brachyantha to rice leaffolderCnaphalocrocis medinalis (Guenée) (Lepidoptera: Pyralidae)

A wild rice,Oryza brachyantha, was rated as highly resistant to rice leaffolder,Cnaphalocrocis medinalis (Guenée), while its F₁ hybrid from a cross with a high-yielding, leaffolder-susceptible rice, IR31917-45-3-2, was rated resistant in a standard screening test. In comparison with IR31917-45-3-2,O. brachyantha and the F₁ hybrid were unsuitable for oviposition, survival, and growth ofC. medinalis. However, growth and survival of larvae in artificial diets containing lyophilized leaf powder of IR31917-45-3-2 andO. brachyantha were comparable. Studies on the feeding and settling preference of the larval stages suggested that the mechanism of resistance ofO. brachyantha is that of antixenosis type. Olfactometer tests with first-instar larvae and electroantennogram responses of adults indicated a lack or low levels of volatile attractants inO. brachyantha and the F₁ hybrid in comparison with IR31917-45-3-2. Bioassays of sequential solvent extracts of these plants indicated that larval preference for IR31917-45-3-2 and nonpreference forO. brachyantha and the F₁ hybrid were due partly to chemical factors present in hexane and methylene chloride extractables. Closer arrangement of silica cells in the epidermal layer ofO. brachyantha and a higher mandibular wear in larvae reared onO. brachyantha suggested that physical resistance due to silica may be an additional cause of resistance. It was hypothesized that the high levels of resistance observed inO. brachyantha may be due to an additive or synergistic action of the absence of attractants or feeding stimulants, the presence of deterrents, and the physical resistance offered by silica. The significance of these results to a successful wide hybridization program aimed at transferring resistance factors from wild rice to cultivated rice is discussed.     

Differential effect of tomatine and its alleviation by cholesterol on larval growth and efficiency of food utilization inHeliothis zea andSpodoptera exigua

The effect of tomatine on larval growth ofHeliothis zea andSpodoptera exigua was assessed by rearing larvae on diets with different concentrations of the chemical added. When reared from neonates, linear dose-response relationships were obtained for both species, withS. exigua being three times more sensitive to tomatine thanH. zea. Tomatine toxicity was completely alleviated inH. zea by the addition of equimolar cholesterol into the diet; however, inS. exigua some toxicity was maintained. Larvae ofS. exigua that were started on control diet were insensitive to tomatine after five days; larvae started on diet with an EC₅₀ of tomatine and then switched to control diet after five days failed to recover from toxicosis. Larval growth ofH. zea, on the other hand, was affected both at the neonate and third-instar stage, but normal growth resumed when the larvae were transferred to control diet. Tomatine had little or no affect on food consumption, assimilation, or dietary utilization of the food by third-instar larvae ofS. exigua, except at a concentration 10 times the EC₅₀. In contrast, the efficiency of food utilization ofH. zea larvae decreased with increasing tomatine concentrations. Assimilation of the food tended to increase, although not significantly, as tomatine levels increased. Food consumption ofH. zea larvae also increased when the tomatine concentration was greater than an EC₅₀. The addition of equimolar cholesterol to diets with an EC₅₀ of tomatine restored weight gain and nutritional indices values to control values. These results are related to the utility of using tomatine in host-plant resistance programs.