Cardiochiles Nigriceps Research Articles

Optimizing Crops for Biocontrol of Pests and Disease.

Volatile compounds and extrafloral nectar are common defenses of wild plants; however, in crops they bear an as-yet underused potential for biological control of pests and diseases. Odor emission and nectar secretion are multigene traits in wild plants, and thus form difficult targets for breeding. Furthermore, domestication has changed the capacity of crops to express these traits. We propose that breeding crops for an enhanced capacity for tritrophic interactions and volatile-mediated direct resistance to herbivores and pathogens can contribute to environmentally-friendly and sustainable agriculture. Natural plant volatiles with antifungal or repellent properties can serve as direct resistance agents. In addition, volatiles mediating tritrophic interactions can be combined with nectar-based food rewards for carnivores to boost indirect plant defense.

Quantification of Information in a One-Way Plant-to-Animal Communication System

In order to demonstrate possible broader applications of information theory to the quantification of non-human communication systems, we apply calculations of information entropy to a simple chemical communication from the cotton plant (Gossypium hirsutum) to the wasp (Cardiochiles nigriceps) studied by DeMoraes et al. The purpose of this chemical communication from cotton plants to wasps is presumed to be to allow the predatory wasp to more easily obtain the location of its preferred prey—one of two types of parasitic herbivores feeding on the cotton plants. Specification of the plant-eating herbivore feeding on it by the cotton plants allows preferential attraction of the wasps to those individual plants. We interpret the emission of nine chemicals by the plants as individual signal differences, (depending on the herbivore type), to be detected by the wasps as constituting a nine-signal one-way communication system across kingdoms (from the kingdom Plantae to the kingdom Animalia). We use fractional differences in the chemical abundances, (emitted as a result of the two herbivore types), to calculate the Shannon information entropic measures (marginal, joint, and mutual entropies, as well as the ambiguity, etc. of the transmitted message). We then compare these results with the subsequent behavior of the wasps, (calculating the equivocation in the message reception), for possible insights into the history and actual working of this one-way communication system.

Tobacco as a Trap Crop for Heliothis virescens (F.) (Lepidoptera: Noctuidae) in Cotton

A 3-yr study evaluated tobacco as a trap crop for the tobacco budworm, Heliothis virescens (F.), in cotton. Small plot experiments were conducted on an experimental farm at Mississippi State University in Starkville, MS, in 1996 and on a commercial farm in Aliceville, AL, in 1997 to determine the ability of small strips of tobacco to trap H. virescens in cotton field plots. In the 1996 experiment, tobacco budworms eggs were significantly higher on tobacco than on cotton from 7 June through 19 June and from 10 July through 22 July. In 1997, H. virescens eggs were significantly higher on tobacco than on cotton for every sampling date throughout the growing season. The conclusion derived from these small plot experiments was that H. virescens females preferred tobacco over cotton as an ovipositional site. Therefore, in 1998, a large-scale field experiment was conducted to determine the effectiveness of tobacco as a trap crop for H. virescens in commercial cotton fields in Funston, GA. In this experiment, the number of H. virescens eggs was significantly lower in cotton fields with tobacco trap crops compared to control cotton fields without tobacco trap crops on 2 and 9 July. Also, economic threshold for H. virescens was not reached in these cotton fields with tobacco trap crops. In contrast, the economic threshold for this pest was met in cotton fields without tobacco trap crops on two dates during the growing season. For each year of the study, percentage total real mortality (rx) for eggs and larvae of H. virescens on tobacco was very high, ranging from 91.4–99.9%. Larval mortality was attributed in part to parasitization by Toxoneuron nigriceps (formerly Cardiochiles nigriceps) Viereck and Campoletis sonorensis Cameron and an infection by an ascovirus of H. virescens. Thus, tobacco served as a trap crop and sink for H. virescens in cotton in this study.

Intrinsic competition between larval parasitoids with different degrees of host specificity

Abstract. 1. Interspecific competition among parasitoids may play a key role in the community dynamics of tritrophic plant–herbivore–parasitoid systems and has important implications for management of herbivorous insect pests.2. A model system was used to explore the outcome of interspecific competition between parasitoids that differ in host specificity. The system included the lepidopteran pest Heliothis virescens, the generalist parasitoid Cotesia marginiventris, and two specialist parasitoids, Microplitis croceipes and Cardiochiles nigriceps.3. The generalist, C. marginiventris, dominated intrinsic competition when given an 8‐h developmental head start over C. nigriceps or when its oviposition was simultaneous with that of M. croceipes. Microplitis croceipes and especially C. nigriceps larvae prevailed when they were allowed to oviposit prior to C. marginiventris.4. Rates of host mortality prior to parasitoid emergence varied with parasitoid species composition and with the order of oviposition.5. Implications for integrated pest management and the adaptive significance of competition as related to host specialisation are discussed.

Transmission of Ascovirus fromHeliothis virescens(Lepidoptera: Noctuidae) by Three Parasitoids and Effects of Virus on Survival of ParasitoidCardiochiles nigriceps(Hymenoptera: Braconidae)

Abstract In field crops in the southeastern United States, larvae of Heliothis virescens (F.) are often infected with ascoviruses, especially toward the end of the growing season. Ascoviruses are unusual in that they are difficult to transmit per os, and several studies have provided data indicating that these viruses are vectored mechanically by parasitic wasps during oviposition. In Georgia, three parasitoids commonly parasitize H. virescens larvae: Cardiochiles nigriceps Viereck, Campoletis sonorensis (Cameron), and Microplitis croceipes (Cresson). In the current study, we investigated the transmission of ascovirus by these parasitoids by using females that were collected in the field or reared in the laboratory. After a single oviposition by C. nigriceps in an H. virescens larva with a 4-d-old ascovirus infection, all subsequent healthy larvae parasitized by this female developed ascovirus infection. After oviposition in an infected larva, examination of C. nigriceps by using transmission electron mic...

IS ATTRACTION FATAL? THE EFFECTS OF HERBIVORE-INDUCED PLANT VOLATILES ON HERBIVORE PARASITISM

We investigated the relationship between parasitoid attraction to herbivore-induced plant volatiles and larval parasitism rates of two closely related heliothine, noctuid moths. Heliothis subflexa Guenee is a specialist on plants in the genus Physalis, while Heliothis virescens Fabricius is an extreme generalist. In North America, these species serve as the only known hosts for the specialist parasitoid Cardiochiles nigriceps Vierick; oviposition into Helicoverpa zea, a non-host, does occur but results in lethal encapsulation of C. nigriceps' eggs. Heliothis virescens larvae are parasitised by C. nigriceps far more frequently than are H. subflexa larvae. Parasitoid attraction to volatiles emitted by tobacco in response to herbivory by H. virescens has previously been demonstrated. Using field experiments, we examined the possibility that pre-detection defenses against parasitoid attraction to herbivore-induced plant volatiles are responsible for H. subflexa's relatively low rates of parasitism by C. nigriceps. Herbivore-damaged plants were significantly more attractive to C. nigriceps than were larvae, larval frass, larval saliva, or damaged leaves alone. Plant species affected parasitoid attraction: tobacco was the most preferred plant species, followed by Physalis angulata, and then cotton. The parasitoid was also more attracted to host species (H. subflexa and H. virescens) than to the non-host species, H. zea. There was an interaction between plant species and herbivore species: each plant species was most attractive when infested by its typical herbivore (e.g., H. virescens on tobacco). We compared these data with those of a previously published experiment on field parasitism of H. virescens and H. subflexa, conducted at the same time and place. Our results indicate that differences in parasitoid attraction to herbivore-induced plant volatiles do not contribute to H. subflexa's relatively low parasitism rate. For the generalist, H. virescens, rates of attraction correspond with rates of parasitism; for the specialist, H. subflexa, they do not. Attraction to H. subflexa on P. angulata is greater than to H. virescens on P. angulata, yet parasitism of H. subflexa is much less than parasitism of H. virescens. These results indicate that pre-detection defenses are not responsible for H. subflexa's low parasitism on P. angulata.

Behavioral adaptations increase the value of enemy-free space for Heliothis subflexa, a specialist herbivore.

We investigated the importance of specialized behaviors in the use of enemy-free space by comparing the host-use behavior of two closely related moths, Heliothis subflexa Guenee and H. virescens Fabricius. Heliothis subflexa is a specialist on plants in the genus Physalis, whereas H. virescens is an extreme generalist, feeding on plants in at least 14 families. Heliothis subflexa uses the inflated calyx surrounding Physalis fruits as enemy-free space, and field rates of parasitism for H. subflexa on Physalis are much lower than for H. virescens on tobacco and cotton, common hosts found in the same habitat as Physalis. If Physalis, architecture were solely responsible for H. subflexa's low rates of parasitism on Physalis, we predicted that H. virescens larvae experimentally induced to feed on Physalis would experience parasitism rates similar to those of H. subflexa. We found, however, that specialized host-use and host-acceptance behaviors are integral to the use of enemy-free space on Physalis and strongly augment the effects of the structural refuge. In laboratory assays, we found considerable differences between the larval behavior of the specialist. H. subflexa, and the generalist, H. virescens, and these contributed to H. subflexa's superior use of enemy-free space on Physalis. We tested the importance of these behavioral differences in the field by comparing parasitism of H. virescens on Physalis, H. virescens on tobacco, and H. subflexa on Physalis by Cardiochiles nigriceps Vierick, a specialist braconid parasitoid. For H. virescens, a threefold decrease in parasitism occurred when feeding on Physalis (mean parasitism +/- SEM = 13 +/- 4%) rather than tobacco (43 +/- 4%), a difference we attribute to the structural refuge provided by Physalis. However, parasitism of H. virescens on Physalis was more than ten times as great as that of H. subflexa on Pliv.salis (1 +/- 4%), supporting the hypothesis that specialized behaviors have a substantial impact on use of Physalis as enemy-free space. Behavioral adaptations may be central to the use of enemy-free space by phytophagous insects and may act as an important selective force in the evolution of dietary specialization.

Effect of selected insecticides on the natural enemies Coleomegilla maculata and Hippodamia convergens (Coleoptera: Coccinellidae), Geocoris punctipes (Hemiptera: Lygaeidae), and Bracon mellitor, Cardiochiles nigriceps, and Cotesia marginiventris (Hymenoptera: Braconidae) in cotton.

We evaluated the toxicity of three insecticides (lambda cyhalothrin, spinosad, and S-1812) to the natural enemies Bracon mellitor Say, Cardiochiles nigriceps Viereck, Coleomegilla maculata De Geer, Cotesia marginiventris (Cresson), Geocoris punctipes (Say), and Hippodamia convergens Guérin-Méneville, in topical, residual, and field assays. Lambda cyhalothrin exhibited the greatest toxicity to the natural enemies. In topical toxicity tests, lambda cyhalothrin adversely affected each natural enemy species studied. Residues of lambda cyhalothrin on cotton leaves were toxic to B. mellitor, C. nigriceps, C. maculata, and C. punctipes. Interestingly, residues of this insecticide were not very toxic to C. marginiventris and H. convergens. Geocoris punctipes and C. maculata numbers in the field generally were significantly lower for lambda cyhalothrin treatments than for the other four treatments, substantiating the previous tests. Although cotton aphids began to increase over all treatments around the middle of the test period, the number of cotton aphids in the lambda cyhalothrin plots was significantly higher than the number in any of the other treatments. As cotton aphids increased in lambda cyhalothrin field plots, the predator H. convergens also increased in number, indicating that lambda cyhalothrin did not adversely affect it in accordance with the residual tests. Spinosad exhibited marginal to excellent selectivity, but was highly toxic to each parasitoid species and G. punctipes in topical toxicity tests and to B. mellitor in residual tests. Spinosad generally did not affect the number of G. punctipes, H. convergens, and C. maculata in the field except for one day after the second application for G. punctipes. S-1812 exhibited good to excellent selectivity to the natural enemies. Some reduction of G. punctipes occurred for only a short period after the first and second application of this insecticide in the field. H. convergens and C. maculata were affected very little by S-1812.

Regulation of host endocrine system by the endophagous braconid cardiochiles nigriceps and its polydnavirus

Regulation of host endocrine system by the endophagous braconid cardiochiles nigriceps and its polydnavirus

<I>Toxoneuron</I> (Hymenoptera: Braconidae): Designation of a Neotype for Its Type Species, <I>T. viator</I> and Discovery of a Possible Senior Synonym of its Best Known Species, <I>T. nigriceps</I>

A neotype is designated for Bracon (Toxoneuron) viator Say and a detailed redescription is provided. A lectotype for Toxoneuron bicolor Szépligeti is also designated, and the transfer of Cardiochiles nigriceps Viereck to the genus Toxoneuron is discussed. Morphological differences between T. bicolor and T. nigriceps are very minor, suggesting a potential synonymy, with bicolor as the senior synonym. Though biological data on T. bicolor are lacking, distributional data indicate that differences in host specificity could be used to elucidate the relationship between these 2 nominal species.

Cardiochiles nigriceps polydnavirus: molecular characterization and gene expression in parasitized Heliothis virescens larvae

Cardiochiles nigriceps Viereck is an endophagous parasitoid of larval stages of the tobacco budworm, Heliothis virescens (F.). This hymenopteran parasitoid, belonging to the family Braconidae, is associated with a polydnavirus (CnPDV), injected at oviposition along with the egg. The infection of various tissues by CnPDV determines the suppression of the host immune system and the developmental arrest of mature host larvae. In this study, CnPDV has been characterized at the structural and molecular level. The negatively stained nucleocapsids show evident ‘end structures’ and a tail-like appendage. The CnPDV genome is typically segmented, with circular dsDNA molecules, ranging in size from 2.5 kb to more than 23 kb. The early expression pattern of CnPDV in parasitized hosts has been analysed and viral clones, genomic and cDNAs, identifying genes expressed within 48 h after parasitization have been isolated. The molecular organization of one of these genes, named CnPDV1, and its putative protein product have been determined. Significant sequence homologies with other known proteins were not detected. In situ hybridization experiments indicated that this gene is expressed in the prothoracic glands of parasitized host mature larvae. A functional analysis of CnPDV1 gene product is required to assess its possible role in the regulation of parasitoid-induced alterations of host larvae.

Intrinsic and extrinsic competitive interactions between two larval parasitoids of Heliothis virescens

Summary1. Competition between parasitoid species may be a key factor in the community dynamics of plant–herbivore‐parasitoid systems and is an important consideration in the selection and management of effective biological control agents.2. Interspecific competition can occur between adult parasitoids searching for hosts (extrinsic competition) and between multiple parasitoid larvae developing within a single host individual (intrinsic competition). A model system comprising the lepidopteran pest Heliothis virescens and two key hymenopteran endoparasitoids, Microplitis croceipes and Cardiochiles nigriceps, was employed to explore parasitoid host‐location strategies and the consequences of intrinsic and extrinsic competitive interactions between parasitoid species.3. The less specialised of the two parasitoids, M. croceipes, was found to have a shorter hatching time and to dominate intrinsic competition, except when its oviposition followed that of the more specialised parasitoid, C. nigriceps, by 16 h or more. This interval corresponded to the differential in hatching time between the two species.4. Cardiochiles nigriceps, however, displayed superior host‐searching efficiency that may compensate for its disadvantage in intrinsic competition. This parasitoid was more effective at detecting host infestation sites via airborne odours and at locating and attacking early instar host larvae than was M. croceipes.

THE INFLATED CALYX OFPHYSALIS ANGULATA: A REFUGE FROM PARASITISM FORHELIOTHIS SUBFLEXA

Heliothis subflexa and H. virescens are the only known hosts of the parasitoid Cardiochiles nigriceps. H. subflexa, a specialist on plants in the genus Physalis, appears to have a lower parasitism rate compared to the polyphagous H. virescens. Physalis is characterized by producing fruits that are concealed within a paper-like lantern, within which H. subflexa larvae feed. We conducted an experiment to determine whether H. subflexa is protected from parasitism by feeding within the lanterns of Physalis. We compared parasitism rates of wild H. subflexa larvae on normal P. angulata plants to parasitism rates of larvae on plants that had the tips of all the lanterns removed to allow parasitoids access to the larvae. A higher percentage of larvae were parasitized on plants with cut lanterns (48.25%) than uncut lanterns (6.74%). A second experiment was conducted to determine whether volatiles released from cut lanterns influenced the searching of C. nigriceps. This experiment consisted of the two treatments from the previous experiment and a third in which the tips of lanterns were removed and the resulting hole was stapled shut. Cutting plants did not make the plants more attractive to searching parasitoids. We conclude that H. subflexa gains protection from parasitism by feeding within the inflated calyx of Physalis.

Analyses of two parasitoids with convergent foraging strategies

We compared the foraging strategies of two key braconid endoparasitoids of the tobacco budworm (Heliothis virescens Fab.), Cardiochiles nigriceps Vier. and Microplitis croceipes Cresson, that differ in host and habitat range but otherwise share comparable, overlapping niches. The most important host-location cues by far for both species were materials associated with damaged plants. Both species demonstrated a significant preference for volatiles released from plants damaged by H. virescens larvae over those released from undamaged tobacco and cotton plants. In choice experiments with damaged tobacco versus cotton, M. croceipes showed a significant preference for cotton plants. In contrast, C. nigriceps preferred damaged tobacco plants. Plant compounds provoked a strong response even when released from systemically induced plants (from which damaged leaves, host, and host by-products were removed). C. nigriceps appears to have a much keener ability to locate hosts over long distances than M. croceipes. This observation may be related to the highly specialized nature of this parasitoid. The possible adaptive significance of the foraging behaviors of these two parasitoids is discussed.

Prothoracic gland inactivation in Heliothis virescens (F.) (Lepidoptera:Noctuidae) larvae parasitized by Cardiochiles nigriceps Viereck (Hymenoptera:Braconidae)

Heliothis virescens (F.) last instar larvae parasitized by the endophagous braconid Cardiochiles nigriceps Viereck fail to attain the pupal stage, due to a parasitoid-induced alteration of ecdysteroid biosynthesis and metabolism. Currently available information on host prothoracic gland inactivation in this host-parasitoid system is reported here. Prothoracic glands of H. virescens mature larvae show a depressed biosynthetic activity, without undergoing gross morphological disruption. The ultrastructure of gland cells is characterized by minor parasitoid-induced changes, with the rough endoplasmic reticulum appearing more developed and electrondense than in nonparasitized controls. Eventually, the cells of prothoracic glands of parasitized host last instar larvae die but maintain their structural integrity. The inactivation of pupally committed host prothoracic glands is achieved through the disruption of the PTTH signal transduction pathway. The second messenger cAMP appears to be normally produced in response to PTTH stimulation of glands explanted from parasitized host larvae, however the downstream activation of the cAMP-dependent protein kinase does not appear to occur. In fact, a marked underphosphorylation of regulatory target proteins is observed. This underphosphorylation is associated with a significant reduction in general protein synthesis, which appears to be blocked at the translational level, to a redirection of specific protein synthesis and to a drastic suppression of ecdysteroidogenesis. These parameters appeared to be correlated in a kinetic time-course study, confirming their functional link. C. nigriceps polydnavirus (CnPDV) plays a major role in the inactivation of pupally committed host prothoracic glands, while putative factors occurring in the host haemolymph do not seem to be of particular importance at that developmental stage. Southern blot hybridization indicates the occurrence of PKI(protein kinase inhibitor)-like genes in the C. nigriceps genome, which, in contrast, are undetectable in H. virescens.

Herbivore-infested plants selectively attract parasitoids

1-3 . Here we use chemical and behavioural assays to show that these plant emissions can transmit herbivore-specific infor- mation that is detectable by parasitic wasps (parasitoids). Tobacco, cotton and maize plants each produce distinct volatile blends in response to damage by two closely related herbivore species, Heliothis virescens and Helicoverpa zea. The specialist parasitic wasp Cardiochiles nigriceps exploits these differences to distinguish infestation by its host, H. virescens, from that by H. zea. The production by phylogenetically diverse plant species and the exploitation by parasitoids of highly specific chemical signals, keyed to individual herbivore species, indicates that the interaction between plants and the natural enemies of the herbi- vores that attack them is more sophisticated than previously realized. Herbivore-induced plant signals are important for the foraging success of parasitoids and for the plants' defence 3-8 . The production and release of volatiles is triggered at least in part by substance(s) in the oral secretion of herbivores 2,9 ; in cotton, this is known to be an active process in which several terpenoids are synthesized de novo in response to insect feeding 10 . The chemical composition of released volatiles varies among plant tissues (cotton leaves, flowers and bolls, for example) 11 , varieties 12 and cultivars 13 . Although volatile emis-

Regulation ofHeliothis virescens prothoracic glands byCardiochiles nigriceps polydnavirus

Heliothis virescens (F.) Larvae parasitized by the endophagous braconid Cardiochiles nigriceps Viereck fail to attain the pupal stage. This developmental alteration is caused by both an inactivation of prothoracic glands of last-instar larvae and an altered ecdysone metabolism. Decrease in ecdysteroidogenesis in vitro was already evident in glands explanted from larvae that have attained the early cell formation stage (day 4 of fifth instar), 6 h after parasitoid oviposition. Ecdysteroidogenesis nearly ceased by 24 h after parasitoid oviposition. The degree of this biosynthetic depression increased as the time between parasitization and gland dissection increased. A time-course study allowed us to determine if both the degree of phosphorylation of regulatory target proteins, the rate of general protein synthesis and ecdysteroidogenesis decreased in concert over time. The results provide further evidence in support of the hypothesis that these cellular activities in prothoracic gland cells are functionally correlated in steroidogenic responses. Treatment with calyx fluid and venom of C. nigriceps duplicates the parasitism-induced inactivation of host prothoracic glands. A 6-h conditioning in vitro of pupally committed host prothoracic glands with these parasitoid female reproductive secretions resulted in a significant depression of their ecdysteroid production. However, glands lost their sensitivity to calyx fluid and venom treatment when explanted from hosts that had already attained the cell formation stage. This was further supported by the fact that nearly all the host larvae parasitized on day 4 of fifth instar (cell formation stage) pupated, while parasitization on day 3 resulted in only 11% pupation. The coupled trioxsalen/UV irradiation treatment of C. nigriceps calyx fluid and venom eliminated their negative effect on biosynthetic activity in vitro by host prothoracic glands. This result indirectly demonstrates that C. nigriceps polydnavirus is the major regulating factor involved in the host prothoracic gland inactivation. Arch. Insect Biochem. Physiol. 38:1–10, 1998. © 1998 Wiley-Liss, Inc.

Biochemical and ultrastructural alterations in prothoracic glands of Heliothis virescens (F.) (Lepidoptera: Noctuidae) last instar larvae parasitized by Cardiochiles nigriceps Viereck (Hymenoptera: Braconidae)

The present study describes both the ultrastructural and the biochemical alterations induced by the parasitoid Cardiochiles nigriceps Viereck on the prothoracic glands of the last (fifth) instar larvae of its host Heliothis virescens (F.). Gross morphological degeneration or signs of necrosis of the prothoracic gland cells were not observed. A rough endoplasmic reticulum more developed and electrondense than in non-parasitized controls was the only ultra-structural change occurring in the prothoracic glands of parasitized host last instar larvae. However, although structurally undegraded, 5 days after parasitoid oviposition into newly moulted host last (fifth) instar larvae, the prothoracic gland cells appeared to be functionally dead and became permeable to trypan blue. In contrast, cells of the prothoracic glands explanted on day 4 of the fifth instar, 3 days after parasitism, performed as above, were still alive and did not absorb the trypan blue, as observed in synchronous non-parasitized controls. These glands were used to study in vitro the major biochemical alterations at cellular level induced by the parasitoid. Ecdysteroid production, RNA synthesis, protein synthesis and phosphorylation were studied in response to crude prothoracicotropic hormone (PTTH) extract, forskolin and dibutyryl cAMP (dbcAMP), these latter compounds mimicking the PTTH action at different levels of the signal transduction pathway. The prothoracic glands of H. virescens last instar larvae, parasitized by C. nigriceps, showed a severely depressed biosynthesis of ecdysteroids, which was not resumed with any of the stimulating treatments performed. This severely depressed biosynthetic activity was associated with a remarkable reduction in the rate of total protein synthesis and to a marked underphosphorylation of those proteins normally phosphorylated when healthy glands are challenged with dbcAMP. However, the early steps of the PTTH signal transduction pathway in the host prothoracic glands are apparently not altered by C. nigriceps parasitism. This is supported by the identical effect of PTTH and dbcAMP treatments on RNA synthesis in the prothoracic glands of both parasitized and non-parasitized larvae. In fact, in both cases a reduced level of [ 3H]uridine incorporation into the total RNA synthesized in vitro was observed in stimulated glands. These experimental data would suggest the occurrence of a parasitoid-induced block of protein synthesis translation in host prothoracic glands, partly determined by the marked underphosphorylation of key target regulatory proteins.

Interaction of Resistant Plants and Wasp Parasitoids of Tobacco Budworm (Lepidoptera: Noctuidae)

The interactions of 2 species of parasitoid wasps, their host, Heliothis virescens (F.), and transgenic tobacco producing a low concentration of Bacillus thuringiensis Berliner toxin were investigated in field tests. Campoletis sonorensis (Cameron) and toxic plants each decreased survival of larvae during the first 6 d on plants. C. sonorensis and toxic plants acted synergistically in combination, decreasing larval survival beyond the level expected for an additive interaction. Cardiochiles nigriceps Viereck did not significantly reduce 6-d survival of host larvae and did not interact with plant toxicity. Parasitism of H. virescens exposed for 1 d to natural populations of the same parasitoids was highly variable from day to day, and also varied between years. In 1990, C. sonorensis was the more prevalent species; in 1991, C. nigriceps was dominant. In 1990, single-day parasitism of neonates by C. sonorensis was lower on toxic plants than nontoxic plants. This result, which contradicted previous findings of increased parasitism among larvae exposed for several days on toxic plants, probably was caused by differences in initial behavior of larvae on toxic versus nontoxic plants. In 1991, position of larvae on plants did not affect parasitism by C. sonorensis ; however, parasitism by C. nigriceps was significantly higher among larvae collected from exposed positions on leaves compared with apical buds. Patterns of daily parasitism of 1- to 8-d-old larvae were extremely different for the 2 parasitoids: vulnerability to C. sonorensis among host larvae on nontoxic plants began to decline after 3 d; whereas, parasitism by C. nigriceps increased over the 1st few days and remained high through 8 d.

Functional Response of Microplitis croceipes and Cardiochiles nigriceps (Hymenoptera: Braconidae) to Variation in Density of Tobacco Budworm (Lepidoptera: Noctuidae)

The functional response of Microplitis croceipes (Cresson) and Cardiochiles nigriceps Vierick to variation in tobacco budworm, Heliothis virescens (F.), density was determined in large field cages over squaring cotton. Wasps were released in cages containing various host densities. Three methods—logit analysis, logistic regression, and comparative curve fitting with 3 foraging models—were used for the analysis of functional response data. For both parasitoid species, the Rogers functional response model fit the data well. The search rate for M. croceipes (33.12 m2/d) was not significantly different than that for C. nigriceps (24.37 m2/d). Handling time for M. croceipes (≍7 min) was shorter than that for C. nigriceps (≍91 min). The functional response curves were significantly different between species for the Rogers model, and C. nigriceps females parasitized a greater proportion of H. virescens larvae on caged cotton than did M. croceipes females. Of the 2 species, C. nigriceps should out perform M. croceipes in a cotton field.