Regulation ofHelicoverpa zea larval behavior by the parasitoidEucelatoria bryani

The parasitoidEucelatoria bryani Sabrosky regulates the larval behavior of its hostHelicoverpa zea (Boddie). Parasitized third, fourth and fifth instars burrow into the soil 0.7–3.4 days earlier than unparasitized larvae that normally enter the soil to pupate at the end of the fifth and final larval instar. Parasitized third instars molt once then burrow as fourth instars, one instar earlier than normal. WhenE. bryani pupariated on the soil surface in the field, none survived to the adult stage. However,E. bryani adults emerged from 49.2% of hosts that had burrowed into the soil. By accelerating the timing ofH. zea burrowing behavior and causing host larvae to enter the soil before death,E. bryani ensures its pupariation in an environment with improved protection against natural enemies and lethal temperatures.     

Field cage performance of two tachinid parasitoids of the tomato fruitworm on insect resistant and susceptible tomato lines

Rates of parasitism ofHelicoverpa (=Heliothis) zea (Boddie) (Lepidoptera: Noctuidae) by two tachinid parasitoid species differing in larviposition habits were measured in field cages on tomato plant lines with or without methyl-ketone (2-tridecanone and 2-undecanone)-mediated insect resistance. Hosts were placed on resistant or susceptible plants, exposed to parasitoids for 24–48 h, then held on artificial diet for parasitoid emergence. Rates of parasitism byArchytas marmoratus (Townsend) (Diptera: Tachinidae), which larviposits on its host's food plant, were significantly reduced on the resistant plants, relative to those on the susceptible plants. Parastism by another Tachinid,Eucelatoria bryani Sabrosky, which larviposits directly into its host and does not directly contact the foliage of its hosts' food plant as a small larva, was not affected by the methyl-ketone mediated resistance.     

Production of a monoclonal antibody to the arylphorin of Heliothis zea

A species-specific monoclonal antibody was produced to whole plasma of fifth instar larvae of the corn earworm, Heliothis zea (Boddie) (Lepidoptera:Noctuidae). This antibody did not cross-react with proteins from the plasma of any of the other lepidopteran larvae tested, including other Heliothis spp. The antigen recognized by this antibody was characterized and found to be arylphorin, a prominent larval storage protein. This conclusion was based on the electrophoretic as well as immunological characteristics of the antigen. Polyacrylamide gel electrophoresis indicated that the antigen had an apparent native molecular weight of 460,000, and that it was composed of subunits having apparent molecular weights of 76,000. The isoelectric point of the antigen was 5.9, with some microheterogeneity being seen. Western blotting of arylphorin against the monoclonal antibody clearly identified the antigen as arylphorin. This protein was not found in egg homogenates or early instar larval plasma, but it was present in large quantities in pupal homogenates and, at trace levels, in adult homogenates. The efficient production and selection of hybridomas producing antibodies specific to H. zea arylphorin using unfractionated plasma as immunogen illustrates the fact that monoclonal antibody technology can produce highly specific antibodies using crude antigen preparations. We discuss the tradeoffs one must accept when choosing this strategy over one using purified immunogens.     

Molecular epidemiology ofCladosporium carrionii

The effects of cuticle from larvae ofTrichoplusia ni andHelicoverpa (=Heliothis)zea on expression of proteases and chitinases by germinating conidia ofNomuraea rileyi in submerged cultures were studied. Increasing the concentration ofT. ni orH. zea cuticle resulted in a 13- and 15-fold increase in protease activity, respectively. Endochitinase and N-acetylglucosaminidase activity on theT. ni andH. zea substrates increased as cuticular concentrations increased to 2.5%, then stabilized or decreased thereafter. The simultaneous expression of both proteases and chitinases suggests that they are controlled by a multiple-regulatory system.This article reports the results of research only. Mention of a proprietary product in this paper does not constitute a recommendation for use by US Department of Agriculture.     

Biological activity of maize leaf extracts against the African armyworm,Spodoptera exempta

Maize (Zea mays L.) leaf tissue of cv Bastille and cv Michoacan 12 was extracted with n-hexane. The extracts were bioassayed against 5th instar African armyworm,Spodoptera exempta (Walker)(Lepidoptera: Noctuidae), by feeding the larvae on agar based media or sucrose impregnated glass fibre discs. The hexane extract of the ‘resistant’ cv Bastille exhibited feeding deterrency and toxicity which were not shown by the ‘susceptible’ cv Michoacan 12. The hexane extract of cv Bastille was adsorbed onto silica gel, the solution filtered off and the adsorbed component taken up into ethyl acetate. Bioassay of these fractions indicated that the toxic and deterrent action was retained in the ethyl acetate fraction. Preparative thin layer chromatography of the ethyl acetate fraction isolated two biologically active constituents. These were both growth inhibitors and lethal by ingestion to the 5th instar African armyworm. Implications for resistance in maize varieties to insect pests are discussed.     

Parasite regulation of host insect metamorphosis: a new form of regulation in pseudoparasitized larvae ofTrichoplusia ni

Summary When eggs ofTrichoplusia ni (Lepidoptera) are stung by a parasitic wasp,Chelonus sp., the developing host larvae precociously initiate metamorphosis ten days later. Precocious initiation of metamorphosis occurs even in ‘pseudoparasitized’ stung hosts which contain no living parasites at the time of symptoms of host regulation by the parasite. In feeding, penultimate instar, pseudoparasitized hosts, the corpora allata activity, hemolymph juvenile hormone esterase activity, in vivo rates of juvenile hormone metabolism and changes in hemolymph protein composition all follow the pattern of the normal last instar. This and other evidence suggests the entire developmental pattern of the last larval instar is precociously expressed in penultimate instar, pseudoparasitized hosts. The cause of precocious expression of the developmental program leading to metamorphosis is a significant decrease in the critical size parameter that, in normal larvae, signals attainment of the last instar. The induction, in preultimate instar larvae, of the entire feeding stage developmental program leading to metamorphic commitment, using either biochemical, surgical or parasitic experimental probes, has not been previously reported. The results have important implications for the study of host-parasite endocrine interaction, of normal insect metamorphosis and even of human puberty.     

The resistance of Pteridium aquilinum (L.) Kuhn to insect attack by Trichoplusia ni (Hübn.)

Summary Resistance of Pteridium aquilinum to insect attack was studied by incorporating air dried bracken leaf meal and extracts of bracken leaf meal into an artificial diet for Trichoplusia ni (Lepidoptera: Noctuidae). When larvae are reared on diets containing 6% bracken leaf meal, they do not mature past the second instar and after 16 days the average weight is approximately 1 mg compared to 70 mg for control larvae. Feeding initiation studies indicate that a feeding deterrent is present in bracken fern but feeding rates and food utilization efficiency studies suggest that either the deterrent or another compound also functions as a toxin. This toxin does not affect growth, feeding rates, or utilization efficiency for the first 4 days after third instar larvae are transferred to a diet containing the water extract of bracken leaf meal; thereafter growth is terminated and feeding is greatly reduced. The active factor is water soluble, heat labile, and non-volatile and this partial characterization indicates that neither the bracken ecdysones or the anti-thiamine factor of bracken is involved in the resistance of this fern to insect attack by T. ni.     

Development of the polyembryonic parasitoid Copidosoma floridanum in Trichoplusia ni

Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) parasitized by the polyembryonic egg-larval parasitoid Copidosoma floridanum (Ashmead) (Hymenoptera: Encyrtidae) attained significantly larger final weights and head capsule widths than unparasitized controls. The difference in weight between parasitized and unparasitized hosts was not entirely accounted for by the weight of the C. floridanum brood. The head capsule widths of all parasitized and unparasitized fifth instars used in the study exceeded the critical threshold of 1.66 mm previously established for T. ni metamorphosis. The critical ratios associated with each T. ni instar of: 1) maximum weight within the instar:head capsule width and 2) maximum weight within the instar:weight at the beginning of the instar differed between parasitized and unparasitized larvae. Development of C. floridanum was synchronized with that of its host. Germ band formation and gastrulation of morulae destined to produce reproductive larvae invariably coincided with the host molt to the ultimate, fifth instar. Reproductive larvae had two instars. Eclosion from the egg to the first instar occurred during day 2 of the host's fifth instar, and ecdysis from the first to the second instar was synchronized with host cocoon spinning. Conversely, embryogenesis of morulae destined to produce precocious larvae began during the host first instar, continued through the second and third instar and ceased during the penultimate, fourth instar. Precocious larvae never molted and died when the host was consumed by the reproductive larvae.

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Résumé T. ni Hübner parasité par le parasitoïde ovo-larvaire C. floridanum Ashmead à développement polyembryonnaire atteint un poids final signficativement plus élevé avec une capsule céphalique plus grosse que les témoins non parasités, sans subir de mues surnuméraires. La différence de poids entre noctuelles parasitées ou non ne correspondait pas entièrement au poids des C. floridanum. Les largeurs des capsules céphaliques de tous les T. ni du cinquième stade dépassaient toutes le seuil critique de 1,66 mm lié à la métamorphose, mais les seuils critiques de taille du corps:largeur de la capsule céphalique et/ou taille et corps, taille initiale du corps au début du stade associé à la mue, différaient chez T. ni parasités ou non. Les développements de T. ni et de C. floridanum étaient synchrones. La formation de la bande germinative et la gastrulation de la morula produisant la multiplication des larves ont coïncidé invariablement avec la mue de l'hôte donnant le dernier stade. Les larves polyembryonnaires ont présenté deux stades. L'éclosion des oeufs s'est produite le deuxième jour du cinquième stade de T. ni, et le passage du premier au second était synchrone de la formation du cocon de l'hôte. Réciproquement, l'embryogenèse de la morula qui donnait des larves précoces commençait pendant le premier stade de l'hôte et se poursuivait à travers les second et troisième, pour cesser pendant le quatrième et pénultième stade.

     

Immunological Detection of Hymenopteran Parasitism inHelicoverpa zeaandHeliothis virescens

We have developed a monoclonal antibody that recognizes the larval and adult stages ofMicroplitis croceipes(Cresson) (Hymenoptera: Braconidae) andCotesia marginiventris(Cresson) (Hymenoptera: Braconidae), two principal endoparasitoids of the polyphagous pestsHelicoverpa zea(Boddie) (Lepidoptera: Noctuidae) andHeliothis virescens(F.) (Lepidoptera: Noctuidae). The antibody has been incorporated into an indirect enzyme-linked immunosorbent assay that can be used to distinguish parasitized from unparasitized pests. The antibody cross-reacts with several different hymenopteran parasitoids, but not with any of the noctuid pests we have assayed. An immunoassay based on a broadly reactive antibody such as this one enables comprehensive detection of hymenopteran parasitism with a single antibody reagent.     

Effects of the ecdysteroid agonists RH 5849 an RH 5992, alone and in combination with a juvenile hormone analogue, pyriproxyfen, on larvae ofSpodoptera exigua

Biological activity assays with RH 5849 and RH 5992 indicated that both compounds affected growth and development of last-instar larvae ofSpodoptera exigua (Hübner) (Lepidoptera: Noctuidae) in a dose-dependent manner. Within the first 24 h after treatment by continuously offering leaves dipped in a water solution of ≥50 mg/l RH 5849 and ≥0.5 mg/l RH 5992, symptoms of a prematurely induced larval moult and head capsule apolysis were visible. Intoxicated larvae died shortly afterwards, showing signs of unsuccessful ecdysis. LC₅₀-values of RH 5849 and RH 5992 for fifth-instarS. exigua larvae were 110 and 2.5 mg/l, respectively. Pyriproxyfen alone affected the larval stage and disturbed normal metamorphosis. One supernumerary larval instar occurred occasionally. LC₅₀-value for pyriproxyfen was 1.7 mg/l. Larvae simultaneously treated with RH 5849 or RH 5992 and pyriproxyfen, continued to grow until they attained a size and weight about 2–3 times that of the controls. This growth was accompanied by at least one and sometimes two supernumerary moults. Concerning thein vivo imaginal wing disc growth and development, only in larvae treated with 10 and 50 mg/l RH 5849 or 0.5 mg/l RH 5992, tracheole migration was observed earlier than in the controls. When applying 300 mg/l RH 5849 or 3–7 mg/l RH 5992, the discs remained small and no signs of tracheole migration were observed. In larvae simultaneously treated with RH 5849 or RH 5992 and pyriproxyfen, tracheole migration was not prematurely induced and a pupal cuticle was produced in the discs of larvae, undergoing a supernumerary moult. No clear signs of evagination were observed.     

Increase in respiratory rate during feeding in larvae of the armyworm, Spodoptera exempta

Oxygen consumption was measured in fifth and sixth instar larvae of Spodoptera exempta (Walker) (Lepidoptera, Noctuidae) at 25d?C using Warburg manometry. The mean rate of oxygen consumption while feeding on maize or sorghum leaves was approximately twice that at rest. The increment in oxygen consumption attributable to feeding was 583±207 μlg-¹ h-^l (mean±95% confidence limits) or, for maize, 6.1±3.5 μl O₂ per mg of leaf eaten.     

Movement of proteins across the digestive system of the tobacco budworm, Heliothis virescens

Bovine serum albumin (BSA) and anti‐BSA polyclonal antibody were used as model polypeptides to examine the movement of foreign proteins across the insect digestive system and their accumulation in hemolymph of fourth stadium tobacco budworms, Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae). Hydrateable meal pads were developed in these studies as a method for easily introducing compounds into the insect digestive system. When insects were allowed to feed continuously on hydrated meal pads containing 0.8 mg of anti‐BSA per gram diet, the level of antibody found in hemolymph was 2.4 ± 0.1 and 3.4 ± 0.1 µg ml^?1 (average  1 SEM) after 8 and 16 h, respectively, as determined by enzyme‐linked immunosorbant assay (ELISA). Continuous feeding on hydrated meal pads containing the same concentration of BSA produced hemolymph concentrations of 1.5 ± 0.1 and 1.6 ± 0.1 µg ml^?1 hemolymph at 8 and 16 h, respectively. Western blot analyses demonstrated that BSA and anti‐BSA both retained their primary and multimeric structure and that anti‐BSA maintained its antigenic activity in the meal pads and after movement from meal pads into the hemolymph. When 1 µg of anti‐BSA or BSA was injected into the hemocoel of fourth instars, the concentrations decreased with time and 120 min after injection were 20% and 0.6% of the original concentration, respectively. When added at the same concentration to plasma in vitro, the decrease was 81.5% and 57.5%, respectively, at 2 h. The accumulation of native anti‐BSA and BSA protein in insect hemolymph is the result of their rate of movement across the gut and their rate of turnover in hemolymph. Movement of anti‐BSA and BSA across the digestive system was also noted in Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), Acheta domesticus (L.) (Orthoptera: Gryllidae), and Gromphadorhina portentosa (Schaum) (Blattaria: Blattellidae). Anti‐BSA and BSA were not detected in the hemolymph of Manduca sexta (L.) (Lepidoptera: Sphingidae) after feeding.     

Toxicity and anticholinesterase activity of the fungal metabolites territrems to the corn earworm, Helicoverpa zea

The toxicity and anticholinesterase activity of tremorgenic fungal metabolites, territrems, to the corn earworm, Helioverpa zea (Boddie) (Lepidoptera, Noctuidae) were examined. In oral toxicity studies, territrem A significantly inhibited growth by 40% at 25 ppm and by 89% at 250 ppm. Territrem B and an epoxy-derivative significantly inhibited growth by ca. 45% at 250 ppm. Piperonyl butoxide administered orally synergized the toxicity of the territrems tested. In topical toxicity studies, the epoxy-derivative caused 26% mortality and 25% growth retardation at 10 mg/gm insects. Territrem A and B were not significantly lethal, but did reduce growth by 15–20% at 10 mg/gm insect. Paraoxon tested in the same way caused 100% mortality at 25 ppm orally and 10 mg/gm topically. However, all territrems tested in vitro as inhibitors of H. zea head acetylcholinesterase were at least comparable to or were more active than paraoxon. Topically administered epoxy-territrem B also inhibited H. zea head acetylcholinesterase. The potential for development of new insecticides from a territrem lead structure is discussed.     

Importance of predation by Coleomegilla maculata larvae in the natural control of the corn earworm in sweet corn

The corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) is a serious pest of many crops in North America, particularly sweet corn. The larvae of Coleomegilla maculata (DeGeer) (Coleoptera: Coccinellidae) are the most frequently observed predators of H. zea eggs in sweet corn. Companion cropping strategies have been developed to augment densities of C. maculata in sweet corn plantings. In this study, the importance of predation by C. maculata larvae was evaluated in experiments when larval abundance was manipulated using hand removal and physical exclusion. In 2003 and 2004, sentinel H. zea eggs were exposed on corn for 48 h. Egg mortality was 13.8% greater in the treatment where C. maculata larvae were present. In 2004 and 2005 eggs were exposed on corn under the same hand removal and physical exclusion techniques and monitored at 3-h intervals for 24 h to directly observe predation on sentinel eggs. Coleomegilla maculata larvae were the most frequently observed predators of the eggs, accounting for 45.9% of 85 total observed predation events in the control. Whereas in the treatment where C. maculata larvae were reduced, egg survival was 26.13% greater and only 37 total predation events were observed. All pest eggs were eliminated from individual ears 22.1% more often when C. maculata larvae were present at natural densities indicating that predation by this predator stage lessens crop damage by H. zea. The results of this study suggest that C. maculata larvae are an irreplaceable source of natural mortality for H. zea eggs on sweet corn.     

Dietary self-selection for vitamins and lipid by larvae of the corn earworm, Heliothis zea

Last instar larvae of the corn earworm, Heliothis zea (Boddie) (Lepidoptera: Noctuidae), require lipid and certain vitamins in their diet in order to complete larval development. When experimental larvae were offered two nutritionally incomplete diets, each lacking a different one of the requirements, they invariably ate from both diets, self-selecting a mixture that was nutritionally superior to either diet alone.

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Résumé Les chenilles de dernier stade d' H. zea (Boddie) (Lep.:Noctuidae) ont besoin de lipides et de certaines vitamines dans leur régime alimentaire, probablement parce qu'elles n'en contiennent pas assez provenant des stades précédents. Quand deux régimes incomplete ont été fournis aux chenilles (chacun privé différemment de l'un de ces consituants: lipide, chlorure de choline ou un ensemble de vitamines), elles ont consommé systématiquement des deux régimes et autosélectionné un aliment de qualité supèrieure à celle de chaque régime séparément. D'un autre côté, les chenilles témoins à qui on a offert deux régimes identiques et nutritionnellement complets, ont choisi apparemment leur aliment au hasard, et presque la moitié d'entre elles ont consommé exclusivement on presque exclusivement l'un des régimes, tandis que l'autre moitié a choise exclusivement or presque exclusivement l'autre.

     

Field evaluation of insect resistance in a wild tomato and its effects on insect parasitoids

Populations ofHelicoverpa (=Heliothis) zea (Boddie),Heliothis virescens (F.),Manduca sexta (L.) andM. quinquemaculata (Haw.) and their egg and larval parasitoids were sampled in field plots of the: insect-resistant wild tomato,Lycopersicon hirsutum f. glabratum C. H. Mull, accession PI 134417; susceptible commercial tomato cultivar ‘Better Boy’; F₁ hybrid; and selected, moderately resistant backcross genotype. Densities ofH. zea andH. virescens eggs and small larvae were higher on resistant genotypes than on susceptible genotypes, but densities of large larvae were similar on all genotypes. Densities ofManduca spp. larvae were too low to permit similar analyses of the effects of plant genotype. Rates of egg parasitism byTrichogramma spp. andTelenomus sphingis (Ashmead) were reduced on insect-resistant genotypes. Rates of parasitism by the larval parasitoidsCampoletis sonorensis (Cameron) andCotesia congregata (Say) were reduced on resistant genotypes. No consistent effects on parasitism rates byCotesia marginiventris (Cresson) were observed and parasitism rates byCardiochiles nigriceps Viereck were unaffected.     

Photodynamics of Porphyric Insecticides

The discovery of porphyric insecticides was a direct fallout of the discovery and development of photodynamic herbicides. Tetrapyrrole-dependent photodynamic herbicides are compounds that force green plants to accumulate undesirable amounts of metabolic intermediates of the chlorophyll and heme metabolic pathways, namely, tetrapyrroles. In light, the accumulated tetrapyrroles photosensitize the formation of singlet oxygen that kills treated plants by oxidation of their cellular membranes. Demonstration of the potential for tetrapyrrole accumulation in insects was achieved by spraying T. ni larvae with δ-aminolevulinic acid (ALA) and 2,2-dipyridyl (Dpy). Treated larvae were placed overnight in darkness at 28°C in order to allow for tetrapyrrole accumulation. Extraction of treated, dark-incubated larvae with ammoniacal acetone, followed by spectrofluorometric examination of the larval extract, revealed the accumulation of massive amounts of protoporphyrin IX (Proto). A high degree of correlation was observed between Proto accumulation in darkness and larval death in the light. A few hours after exposure to light, the larvae became sluggish and flaccid due to loss of body fluids. Death was accompanied by extensive desiccation. Because control of insects by ingestion is as viable an option as control by spraying, and offers certain advantages under household conditions, studies were conducted to determine whether combinations of ALA and porphyric insecticide modulators would be effective if ingested with the food. The effect of ALA and 1,10-phenanthroline (Oph) were determined by incorporating them into the diet of T. ni larvae. After exposure to light, following 17 h of dark incubation, larvae underwent violent convulsions and vomiting and died within 20 to 40 s. Tetrapyrrole analysis of the treated larvae immediately after dark incubation revealed significant amounts of Proto and Zn-Proto accumulation. Correlation between tetrapyrrole accumulation and larval death was significant. Similar results were obtained when ALA and Dpy were administered to the larvae with the diet. The above results indicated that in addition to contact via spraying, porphyric insecticides had the potential to be very potent when ingested. For a more thorough understanding of the mode of action of porphyric insecticides, the phenomenology of tissue, cellular, and subcellular sites of tetrapyrrole accumulation in representative insect species was investigated. In T. ni larvae, on a unit protein basis, about 59% of the accumulated Proto was observed in the hemolymph, 35% in the gut, and 6% in the integument. Further understanding of the response of insect organs and tissues to porphyric insecticide treatment was obtained by investigating the response of isolated organs and tissues to incubation with ALA + Dpy or ALA + Oph in adult Blattella germanica (German cockroach), adult Anthonomus grandis (cotton boll weevil), fifth instar larvae of Heliothus zea (corn earworm), and fifth instar larvae of T. ni (cabbage looper). In T. ni, and H. zea, significant Proto accumulation was observed in incubated midgut and fat bodies. Proto accumulation occurred when tissues were incubated with Dpy, ALA + Dpy, Oph, and ALA + Oph (2). No response to treatment with ALA alone was observed. In cockroaches, more of the Proto appeared to accumulate in the male and female guts than in their abdomen. As in T. ni and H. zea, the response was elicited by each of the treatments that included Dpy or Oph. Cotton boll weevil abdomens appeared to be less responsive than the abdomens of the other three species. To determine whether Proto accumulation resulted in photodynamic damage of incubated tissues, T. ni midguts were incubated in darkness either in buffer, with ALA, or with Oph + ALA. Oxygen consumption of the tissue was monitored before and after exposure to 2-h of illumination. A 30% decrease in O₂ consumption was observed in midguts treated with Oph or with ALA + Oph after 2 h in the light. The decrease in oxygen consumption observed in isolated T. ni midguts was shown to be caused by photodynamic damage to mitochondrial enzymes. Finally, structure-function photodynamic insecticidal studies led to the identification of 36 compounds belonging to 10 different chemical families that were effective (>70% mortality) against at least one insect species. Of the 36 modulators, 10 exhibited potent activity toward cockroaches.     

Fecundity and oviposition ofEucelatoria bryani, a gregarious parasitoid ofHelicoverpa zea andHeliothis virescens

We examined longevity, fecundity, and oviposition strategies ofEucelatoria bryani Sabrosky (Diptera: Tachinidae), a gregarious endoparasitoid ofHelicoverpa zea (Boddie) andHeliothis virescens (F.) (Lepidoptera: Noctuidae). Longevity of adult femaleE. bryani was not related to body size. In contrast to longevity, largerE. bryani females had greater potential fecundity than smaller females, as determined by the number of embryonated eggs present in the common oviduct. However, female parasitoid size did not affect primary clutch size (number of eggs deposited in a host). Because embryos in eggs located in the ovisac were larger than those located elsewhere in the common oviduct, maximum primary clutch size may be physiologically limited by the number of fully mature eggs a female has available at one time.E. bryani females adjusted primary clutch size in response to host size, for bothH. zea andH. virescens. This adjustment appears to be adaptive because females did not overexploit hosts by depositing more larvae than a host could support. Adult emergence was not related to host size. Although host weight positively influencedE. bryani progeny weight, increases in progeny size with host size were counterbalanced by increases in primary clutch size with host size.     

Biological control of tobacco cutworm,Spodoptera litura Fabricius with entomopathogenic nematodes

The efficacies of several entomopathogenic nematodes ofSteinernema andHeterorhabditis spp. were examined against tobacco cutworm,Spodoptera litura Fabricius.H. bacteriophora HY showed 100% mortality after 20 h against 2nd instar of tobacco cutworm. In the case of 3–4th instar,S. carpocapsae PC.,H. bacteriophora HY andS. monticola CR showed 100% mortality after 47 h. In the case of 5–6th instar,S. carpocapsae PC proved more effective than the others. Generally, the number of nematodes harvested increased as their size decreased. Also, the highest number of nematodes was obtained in the 5–6th instar ofS. litura byH. bacteriophora HY, showing about 1.3×10⁶ nematodes per larva.In vitro culturedS. carpocapsae PG showed 100% mortality after 73 h against 5–6th instar tobacco cutworm, indicating that nematodes producedin vitro can be potentially used for the biological control ofS. litura instead of nematodesin vivo.     

A host-plant specialist, Helicoverpa assulta, is more tolerant to capsaicin from Capsicum annuum than other noctuid species

Plant secondary compounds not only play an important role in plant defense, but have been a driving force for host adaptation by herbivores. Capsaicin (8-methyl-N-vanillyl-6-nonenamide), an alkaloid found in the fruit of Capsicum spp. (Solanaceae), is responsible for the pungency of hot pepper fruits and is unique to the genus. The oriental tobacco budworm, Helicoverpa assulta (Lepidoptera: Noctuidae), is a specialist herbivore feeding on solanaceous plants including Capsicum annuum, and is one of a very few insect herbivores worldwide capable of feeding on hot pepper fruits. To determine whether this is due in part to an increased physiological tolerance of capsaicin, we compared H. assulta with another specialist on Solanaceae, Heliothis subflexa, and four generalist species, Spodoptera frugiperda, Heliothis virescens, Helicoverpa armigera, and Helicoverpa zea, all belonging to the family Noctuidae. When larvae were fed capsaicin-spiked artificial diet for the entire larval period, larval mortality increased in H. subflexa and H. zea but decreased in H. assulta. Larval growth decreased on the capsaicin-spiked diet in four of the species, was unaffected in H. armigera and increased in H. assulta. Food consumption and utilization experiments showed that capsaicin decreased relative consumption rate (RCR), relative growth rate (RGR) and approximate digestibility (AD) in H. zea, and increased AD and the efficiency of conversion of ingested food (ECI) in H. armigera; whereas it did not significantly change any of these nutritional indices in H. assulta. The acute toxicity of capsaicin measured by injection into early fifth instar larvae was less in H. assulta than in H. armigera and H. zea. Injection of high concentrations produced abdominal paralysis and self-cannibalism. Injection of sub-lethal doses of capsaicin resulted in reduced pupal weights in H. armigera and H. zea, but not in H. assulta. The results indicate that H. assulta is more tolerant to capsaicin than the other insects tested, suggesting that this has facilitated expansion of its host range within Solanaceae to Capsicum after introduction of the latter to the Old World about 500 years ago. The increased larval survival and growth due to chronic dietary exposure to capsaicin suggests further adaptation of H. assulta to that compound, the mechanisms of which remain to be investigated.