Cotesia Congregata Research Articles

Manipulation of fifth-instar host ( Manduca sexta) ecdysteroid levels by the parasitoid wasp Cotesia congregata

Although 5th (last) instar parasitized Manduca sexta larvae undergo developmental arrest and do not wander, they exhibit a small hemolymph ecdysteroid peak (300–400 pg/ μl) which begins one day prior to the parasitoid's molt to the 3rd (last) instar and concomitant emergence from the host. Ecdysteroids present in this peak were 20-hydroxyecdysone, 20,26-dihydroxyecdysone and one or more very polar ecdysteroids, as well as small amounts of 26-hydroxyecdysone and ecdysone. In parasitized day-1 and -2 5th instars ligated just behind the 1st abdominal proleg, hemolymph ecdysteroid levels increased in both anterior and posterior portions (100–500 pg/ μl), while in unparasitized larvae, hormone levels only increased in the anterior portion (100–350 pg/ μl). Thus, the ecdysteroid peak observed in host 5th instars was probably produced, at least in part, by the parasitoids. It may serve to promote Cotesia congregata's molt from the second to the third instar and/or to facilitate parasitoid emergence from the host. In parasitized day-1 and -2 5th instars ligated between the last thoracic and 1st abdominal segments, hemolymph ecdysteroid titers reached much higher levels (500–3500 pg/ μl) in the anterior portion (no parasitoids present) than in the posterior portion (150–450 pg/ μl). Therefore, it appears that the parasitoid's regulation of hemolymph ecdysteroid titers occurs at two levels. First, parasitization neutralizes the host's ability to maintain its normal hemolymph ecdysteroid levels. Second, in a separate action, the parasitoid manipulates the ecdysteroid-producing machinery so that hemolymph levels are maintained at the 200–400 pg/ μl characteristic of day 3–4 hosts. This is the first report of a parasitoid's ability to interfere with the normal inhibitory mechanisms which prevent prothoracic gland production of ecdysteroid at inappropriate periods of insect growth and development.

Tyrosine and catecholamine levels in the hemolymph of tobacco hornworm larvae, Manduca sexta, parasitized by the braconid wasp, Cotesia congregata, and in the developing parasitoids

Tyrosine and catecholamine levels in the hemolymph of tobacco hornworm larvae, Manduca sexta, parasitized by the braconid wasp, Cotesia congregata, and in the developing parasitoids

Feeding suppression in the tobacco hornworm, <i>Manduca sexta</i>: costs and benefits to the parasitic wasp <i>Cotesia congregata</i>

While the larvae of the parasitic wasp Cotesia congregata developed inside the tobacco hornworm, Manduca sexta, the host's behaviour was similar to that of unparasitized M. sexta. However, once the wasps had exited the host to spin their cocoons, M. sexta stopped feeding. Unparasitized M. sexta ate C. congregata cocoons, which suggests that cessation of feeding by parasitized M. sexta prevented them from consuming the wasps' cocoons. However, suppression of host feeding might be expected to lead to increased host mortality. Host mortality decreased wasp survival. Few cocoons (18%, n = 43) attached to dead hosts produced adult wasps, while 90% of cocoons on live hosts successfully produced adults (n = 20). In the field there was no significant increase in host mortality during wasp pupal development. Cotesia congregata appeared to benefit from the lack of feeding by M. sexta without incurring the increase in mortality that would result if the host died from its lack of feeding. Although cocoons that were d...

Tyrosine and catecholamine levels in the hemolymph of tobacco hornworm larvae,Manduca sexta, parasitized by the braconid wasp,Cotesia congregata, and in the developing parasitoids

Parasitism of fifth instar Manduca sexta larvae by the gregarious parasitoid Cotesia congregata prevented normal storage of tyrosine in the hemolymph, whereas total tyrosine levels increased over eight times in the hemolymph of unparasitized larvae by day 4. Tyrosine glucoside, the hemolymph storage form of tyrosine and the precursor for pupal cuticle sclerotizing agents, was found only in trace amounts in parasitized larvae at the time of parasitoid emergence, but had increased to over 6 mM in hemolymph of unparasitized larvae. Concentrations of dopamine and N-β-alanyldopamine (NBAD), precursors for melanization and sclerotization of cuticle, respectively, had approximately doubled in the hemolymph of parasitized larvae by the day of parasitoid emergence, but not in unparasitized larvae. Catecholamine biosynthesis may be transiently stimulated for wound-healing, as black melanic pigmentation appeared around the wasp emergence holes in the host integument. C. congregata larvae accumulate tyrosine, dopamine, and NBAD by the time of emergence and cocoon spinning, either by direct uptake or by synthesis from precursors obtained from the host. NBAD increased in parasitoid larvae close to pupation, suggesting it functions as the main precursor for pupal cuticle tanning. Both dopamine and NBAD increased dramatically in pharate adult wasps just before eclosion and N-acetyldopamine (NADA) appeared for the first time. Dopamine was highest in concentration and total amount, and it can serve both as a precursor for black melanic pigmentation of adult wasp cuticle and for synthesis of NADA and NBAD, the precursors for cuticle sclerotization. Arch. Insect Biochem. Physiol. 38:193–201, 1998. © 1998 Wiley-Liss, Inc.

Excision of the polydnavirus chromosomal integrated EP1 sequence of the parasitoid wasp Cotesia congregata (Braconidae, Microgastinae) at potential recombinase binding sites.

Cotesia congregata polydnavirus (CcPDV) is essential for successful parasitism of Manduca sexta larvae by the braconid wasp Cotesia congregata. To determine the molecular mechanisms for the vertical transmission of CcPDV in the wasps, we analysed the different forms of the virus sequences containing the gene encoding the early parasitism-specific protein 1 (EP1). By a detailed molecular analysis, we demonstrated that the EP1 sequences are present in wasp DNA in two forms: a circular form as seen in the virus particles and a form integrated into the wasp genome. Moreover, we showed that the integrated form of the EP1 sequences is able to excise in the ovary cells. A fragment corresponding to an EP1 'empty locus' (without the viral sequence) was PCR-amplified from ovarian DNA. Comparison of the sequences isolated from the EP1 circle, the integrated form and the empty locus revealed that the extremities of the EP1 genomic sequences constitute a direct repeat. Strikingly, these sequences contain a potential binding site for a recombinase of the Hin family located in close vicinity to the position where the DNA strand exchange occurs. Thus, the data bear upon the possibility that the bracovirus circles are excised via a mechanism related to the Hin mediated Conservative Specific-Specific Recombination (CSSR) of prokaryotes.

Developmental changes in teratocytes of the braconid wasp Cotesia congregata in larvae of the tobacco hornworm, Manduca sexta

The endoparasitic wasp Cotesia congregata develops in the hemocoel of larval stages of the tobacco hornworm, Manduca sexta. Teratocytes were released from the serosal membrane during hatching of the first instar wasp larva at 2–3 days after oviposition; about 160 cells were released per embryo. The cells increased in diameter from about 10 to >200 μm prior to wasp emergence. Nascent microvilli, visible on the cell surface before hatching of the first instar larva, rapidly increased in length and number following release of the cells. Irrespective of when the wasps were due to emerge, or how many parasitoids were present in the host, dramatic cytological changes occurred in the cells during the last instar of the host's development. Many of these morphological and ultrastructural changes were symptomatic of the cytological features of degenerating or apoptotic cells, and large numbers of vesicles appeared interspersed amongst the microvilli. The nucleus developed extensive dentritic ramifications, and the chromatin condensed in large clumps on the inner nuclear membrane. At the final stages of the wasps' development, the nucleus occupied the bulk of the interior of the cell. The cytoplasm gradually grew dramatically more electronluscent and less granular, as did the nucleoplasm, which is also indicative of impending cell death. Following the parasites' emergence, many of the cells underwent extensive blebbing of the cell surface. Teratocytes within a host appeared heterogeneous with respect to their morphological appearance. Analysis of the proteins secreted by teratocytes in vitro following labelling with 35S-methionine showed that many (>30) polypeptides were synthesized de novo and secreted by the cells; some proteins were clearly targeted for secretion. We presume that the cells likely secrete a large number of proteins in vivo as well as in vitro.

Identification of neuropeptides in the midgut of parasitized insects: FLRFamides as candidate paracrines

Parasitism of Manduca sexta (Lepidoptera: Sphingidae) larvae by the braconid wasp Cotesia congregata (Hymenoptera: Braconidae) leads to accumulation of peptides in host neurons and neurosecretory cells of the central nervous system (CNS) and neurons and endocrine/paracrine cells of the midgut. This accumulation has now facilitated the characterization of two new members of the FLRFamide family from midguts of parasitized larvae. The peptides, given the names F24 and F39, are 24 and 39 amino acids in length with the sequences VRDYPQLLDSGMKRQDVVHSFLRFamide and YAEAAGEQVPEYQALVRDYPQLLDSGMKRQDVVHSFLRFamide. The sequence of F24 is identical to the C-terminal 24 amino acids of F39. The C-terminal 10-mer of each is identical to a previously characterized decapeptide neurohormone (F10). This sequence is preceded by a potential processing site. In nonparasitized insects F39 was present at several-fold the amount of F24. In parasitized insects F24 and F39 accumulate in the middle and posterior regions of the midgut, which are enriched in endocrine/paracrine cells reacting with FLRFamide antisera. In the combined brain and subesophageal ganglion F39 was not detected and the amount of F24 never exceeded 2 fmol per Br/SEG. Of the three peptides, only F10 was found in the hemolymph. Thus, F24 and F39 may be intermediates in the biosynthesis of F10 and may themselves be released locally from endocrine/paracrine cells in the midgut epithelium.

Parasitism-induced Effects on Host Growth and Metabolic Efficiency in Tobacco Hornworm Larvae Parasitized by Cotesia congregata

Parasitism by the braconid wasp Cotesia congregata affects the growth of Manduca sexta larvae in a parasitoid ‘dose-dependent’ fashion. Following parasitization of fourth-instar larvae, more heavily parasitized larvae grew larger compared to those containing fewer parasitoids due to an increase in host dry weight. The differences in host mass appeared to arise after oviposition. A ‘dose-dependent’ enhancement of host dry weight would appear nutritionally beneficial for the parasitoids developing in more ‘crowded‘ hosts. The efficiencies of conversion of ingested and digested food to body mass and the approximate digestibility of the diet ingested by the host caterpillar did not vary significantly with clutch size although parasitoids took slightly longer to develop in the more heavily parasitized hosts. Larval parasitoids developing in the presence of many competitors weighed up to 50% less than those developing in hosts with fewer endoparasitoids, although the weight of adult female parasitoids did not vary significantly with wasp clutch size. The maximum number of emerging wasps was 200 parasitoids, possibly representing the host’s ‘carrying capacity’ for larvae parasitized in the fourth-instar. The ratio of emerging to non-emerging parasitoids decreased as parasitoid clutch size increased, with few or none emerging from very heavily parasitized hosts containing more than 400 parasitoids. © 1997 Elsevier Science Ltd. All right reserved

Effects of Parasitism by the Braconid Wasp Cotesia congregata on Metabolic Rate in Host Larvae of the Tobacco Hornworm, Manduca sexta

We examined growth rates, gas exchange patterns and energy metabolism of tobacco hornworm (Manduca sexta) larvae parasitized by the braconid wasp Cotesia congragata. Larvae parasitized at the beginning of the fourth-instar had reduced growth compared to unparasitized larvae of the same age and short-term differences in metabolism (measured as rates of CO 2 production, V ̇ CO 2) were apparent almost immediately after wasp oviposition. However, over the growth period between parasitization and the last part of the fifth-instar, there was no significant difference between parasitized and unparasitized hosts as seen in the relationship between mass and V ̇ CO 2. One day prior to parasitoid emergence, host larvae stopped eating, ceased spontaneous locomotor activity and showed a dramatic decline in metabolism. The 60% decline of V ̇ CO 2 at this time is consistent with lack of specific dynamic action because the animals were not feeding. Gas exchange became highly cyclical on the day of parasitoid emergence, but the cause and significance of this phenomenon, which disappeared by the third day following emergence, are not clear. This pattern of cycling was not induced by starving nonparasitized larvae for 6 days, nor by immobilizing nonparasitized larvae with tetrodotoxin. Ecdysteroid levels in the host's hemolymph significantly increased on the day when parasitoids completed their L2–L3 molt and began emerging, but not during the wasps' L1–L2 molt which occurred a few days earlier. Contrary to our initial expectation that hemolymph ecdysteroid titers might be linked to alterations in the host's metabolic rate, we observed no such correlation.

Inhibition of testicular growth and development in Manduca sexta larvae parasitized by the braconid wasp Cotesia congregata

Tobacco hornworm larvae parasitized by the gregarious larval endoparasitoid Cotesia congregata exhibited an inhibition in testicular growth and development, the extent of which was determined by the age and developmental stage of the host at the time of parasitization. The degree of parasitic castration, as assessed by measurements of testicular volume, was correlated with the stadium in which parasitization occurred. A mathematical formula requiring the measurement of testicular length, width and depth was used to calculate testicular volume. The use of the depth parameter revealed a negative correlation between host weight and testicular volume in parasitized larvae. Testicular volumes of fifth instar hosts, which had been parasitized in the first stadium, were significantly smaller than those originally parasitized as fourth or fifth instar larvae and were not correlated with parasitoid load. Effects of natural parasitism were not duplicated by injections of C. congregata polydnavirus and venom, topical treatment with the juvenile hormone analog methoprene, or starvation of nonparasitized larvae. Larvae receiving virus plus venom or methoprene grew larger due to delayed wandering and had larger testes than controls. Deleterious effects on host testes may be due to the effects of nutrient competition between the developing parasitoid progeny and the gonads, combined with the juvenilizing effects believed to be caused by the polydnavirus.

Correlation between changes in host behaviour and octopamine levels in the tobacco hornworm Manduca sexta parasitized by the gregarious braconid parasitoid wasp Cotesia congregata

The parasitoid wasp Cotesia congregata lays its eggs within the body of its host, the larval form of the tobacco hornworm Manduca sexta. Host behaviour appeared normal until approximately 8 h prior to the emergence of the parasitoids from their host at which time M. sexta feeding and locomotion declined irreversibly. This change in host behaviour may be to the advantage of the wasp since unparasitized M. sexta presented with wasp pupae ate them. Despite the decline in feeding and locomotion, hosts with emerged parasitoids had normal reflexes and showed no other signs of debilitation. Concomitant with the change in host behaviour, octopamine concentration measured using high-performance liquid chromatography with electrochemical detection (HPLC-ED) increased from 22.2±2.1 pg µl-1 to 143.7±7.8 pg µl-1 in the haemolymph of the host. In unparasitized M. sexta, however, increased octopamine levels were correlated with increased activity. We discuss possible explanations for the co-occurrence of high haemolymph octopamine levels and low behavioural arousal in parasitized M. sexta.

Inhibitory effects of parasitism by the gregarious endoparasitoid Cotesia congregata on host testicular development

Inhibitory effects of parasitism by the gregarious endoparasitoid Cotesia congregata on host testicular development

Inhibitory effects of parasitism by the gregarious endoparasitoidCotesia congregata on host testicular development

Cotesia congregata is a gregarious larval endoparasitoid of the tobacco hornworm, Manduca sexta. Parasitized larvae exhibit a variety of physiological and developmental aberrations, the most obvious of which is the induction of developmental arrest characterized by the absence of wandering behavior and suppression of pupation. This arrest appears attributable to continued maintenance of an elevated titer of juvenile hormone and reduced levels of hemolymph juvenile hormone esterase activity. Injection of the wasp's polydnavirus into nonparasitized larvae also causes arrest and the larvae eventually form larval-pupal intermediates instead of normal pupae, indicating the virus may be partially responsible. Aside from causing arrested host development, parasitism also inhibits the normal development and differentiation of testes in male host larvae, so that the testes atrophy instead of growing synchronously with other larval tissues. Here we report that parasitism has pronounced disruptive cytological effects on the developing reproductive organs of male hosts, in addition to causing them to atrophy. Parasitism results in a reduction in testicular volume attributable to a reduction in the number of developing germ cells. Microscopy revealed that the structural integrity of the sheaths surrounding the testicular follicles also is disrupted, so that the tissues appear grossly abnormal compared to those of nonparasitized larvae. Intrahemocoelic injection of purified C. congregata polydnavirus in combination with venom into nonparasitized fourth instar larvae, or topical application of 100 μg of methoprene to fourth instar larvae, also alters sheath integrity and reduces the numbers of developing germ cells, but not to the same degree as the pattern observed in truly parasitized hosts. The occurrence of cell death in the male gonad was documented using the vital dyes acridine orange and ethidium bromide. Arch. Insect Biochem. Physiol. 36:95–114, 1997. © 1997 Wiley-Liss, Inc.

When good bugs go bad: intraguild predation by Jalysus wickhami on the parasitoid, Cotesia congregata

AbstractWe report a case of direct intraguild predation involving an insect predator and parasitoid in an agricultural system. The spined stilt bug, Jalysus wickhami Van Duzee, feeds on eggs of the tobacco hornworm, Manduca sexta L., and also on prepupal and pupal stages of the gregarious hornworm parasitoid, Cotesia congregata (Say). In two separate trials, mean mortality of attached parasitoids was significantly lower (66%, 73%) than that of their detached siblings (97%, 96%) after a 3 day exposure to stilt bugs, demonstrating that attachment to the host offered some protection against predation. In no‐choice experiments, prepupal parasitoids suffered greater mortality (0 day‐old = 61%, 1 day‐old = 65%) than pupal parasitoids (2 day‐old = 50%, 3 day‐old = 14%). When offered in combination with 0 or 2 day‐old hornworm eggs, respective mortality of 0,1,2 and 3‐day‐old pupal parasitoids showed a similar pattern (67%, 63%, 33% and 23%). In another experiment, mortality of 0‐day‐old pupal parasitoids (64%) was greater than that of 3 day‐old pupal parasitoids (38%). Mortality of pupal parasitoids was not affected by the availability of hornworm eggs, a highly acceptable food. Younger pupal parasitoids (= prepupae) probably suffered greater mortality because they were more easily fed on by stily bugs than older (pupated) ones. Because C. congregata overwinters in the prepupal stage, it may be particularly vulnerable to attack late in the season when stilt bug populations are large and hornworm eggs are relatively uncommon.

Temporal pattern of parasitism-induced immunosuppression in Manduca sexta larvae parasitized by Cotesia congregata

Twenty-four hours after parasitization by the braconid parasitoid Cotesia congregata, Manduca sexta larvae showed a strong suppression of their encapsulation response to positively charged polysaccharide (Sephadex A-25) beads. In contrast, by 8 days post-parasitization, the frequency of encapsulation of injected beads was no longer distinguishable from that observed in nonparasitized controls. This pattern of induction of encapsulation suppression followed by recovery was temporally correlated with morphological abnormalities occurring in host hemocytes. The immunosuppressive and morphological hemocyte changes were mimicked by injection of unparasitized M. sexta larvae with filter-purified C. congregata polydnavirus, indicative of viral mediation of these changes. In transplantation experiments, unparasitized fourth-instar M. sexta larvae received first instar C. congregata larvae which had been excised from their original hosts 8 days post-parasitization. Half of these M. sexta larvae encapsulated the transplanted parasitoids, but died in the wandering stage. The remaining half of the larvae did not encapsulate the parasitoids, and exhibited growth inhibition and a failure to molt to the fifth instar. Transplanted parasitoids emerged and pupated from 43% of these larvae, all of which were developmentally arrested in the fourth instar. Thus, hosts exhibit a recovery of their hemocytic encapsulation capabilities over the course of parasitism, and the later stage parasitoids appear capable of avoiding the host immune response and completing development in the absence of the preconditioning effects of the polydnavirus, as well as other factors normally present in naturally parasitized hosts, such as ovarian proteins, venom, and teratocytes.

Parasitism-induced accumulation of FMRF amide-like peptides in the gut innervation and endocrine cells of Manduca sexta

Manduca sexta larvae that are parasitized by the braconid wasp Cotesia congregata enter a state of developmental arrest following emergence of the wasp larvae from the host. These fifth instar hosts linger for 2 to 3 weeks without resuming feeding, molting, or metamorphosis once the wasps emerge. Immunohistochemical staining with antiserum against FMRFamide revealed dramatic accumulation of FMRFamide-like peptide(s) in the gut nervous and endocrine systems of the developmentally arrested larvae when compared to that observed in unparasitized feeding or starved larvae. Specifically, the number of immunopositive cells and the intensity of staining was enhanced in the neurons of the frontal ganglion, the axons and axon terminals on the midgut surface, and in the gastric endocrine cells. These results were confirmed using ELISA to show that the relative amounts of FMRFamide-like peptides in midgut extracts were highly elevated in the parasitized larvae relative to the fed or starved unparasitized larvae. These data suggest that FMRFamide-like peptides in developmentaly arrested larvae are produced in a significantly larger number of gastric endocrine cells, and that the rate of release of the peptides may be suppressed, or the rate of their synthesis may be elevated. Localization of FMRFamide-like peptides in the gastric endocrine cells of C. congregata is also described.

Accumulation of neuropeptides in the cerebral neurosecretory system of Manduca sexta larvae parasitized by the braconid wasp Cotesia congregata.

Fifth instar larvae of Manduca sexta that were parasitized by the braconid wasp Cotesia congregata failed to develop after the parasitoid larvae emerged, and these host larvae lingered for 2-3 weeks in a quiescent, nonfeeding state without initiating a larval molt or metamorphosis. This study was focused on the neuroendocrine changes associated with the host's developmental arrest. Immunohistochemical studies suggested that the host brain neurosecretory cells as well as their axon terminals in the corpora cardiaca-corpora allata complex accumulated multiple neuropeptides. The extent of accumulation in cells and axons increased with time, so that hosts examined 7-14 days after the wasps emerged showed the most intense staining with antibodies against prothoracicotropic hormone, bombyxin, allatotropin, allatostatin, diuretic hormone, eclosion hormone, proctolin, and FMRFamide. Increased levels of prothoracicotropic hormone and FMRFamide-like peptides in the brains of parasitized larvae were confirmed using Western blots and enzyme-linked immunosorbent assay (ELISA), respectively. Starvation of the unparasitized larvae induced some accumulation of the neuropeptides; however, the intensity of staining and number of immunopositive cells and axons were in most cases clearly higher in the parasitized larvae. Our results suggest that accumulation of the neuropeptides is associated with developmental arrest of parasitized larvae. Because a similar developmental arrest occurs in a wide range of parasitized insects, our findings may have relevance for many other species. Moreover, these data illustrate the potential value of using parasitized M. sexta larvae as a model for studying the mechanisms governing the rates of neuropeptide expression, processing, packaging, and release, as well as providing a rich source of neuropeptides, thus facilitating their isolation and characterization.

An Abundantly Expressed Hemolymph Glycoprotein Isolated from Newly Parasitized Manduca sexta Larvae Is a Polydnavirus Gene Product

Cotesia congregata polydnavirus (CcPDV) is essential for successful parasitism of Manduca sexta larvae by the braconid wasp C. congregata. CcPDV virions are present in large numbers in the oviducts of C. congregata and injected with eggs into the hemocoel of M. sexta larvae during parasitization. Injection of sucrose density purified virions into nonparasitized larvae causes several of the parasitism-induced alterations in the physiology of host larvae that occur as a result of natural parasitism, including the synthesis of novel hemolymph proteins and abrogation of the host's immune response against the developing parasites. One of these proteins, early-expressed protein 1 (EP1), is a 190-kDa molecule which constitutes up to 5% of the total hemolymph protein by 24 hr following oviposition by the wasp. Using N-terminal sequence data for EP1 to construct primers for use in the polymerase chain reaction, we amplified and cloned a cDNA corresponding to the gene encoding EP1. This cDNA hybridized to DNA of the CcPDV genome, but not to DNA isolated from M. sexta larvae, suggesting that EP1 is a CcPDV gene product. A cDNA clone was isolated from an expression library generated from RNA extracted from newly parasitized M. sexta larvae. Sequence analysis of the cDNA clone revealed the presence of an open reading frame of 819 bp encoding a protein of 30.7 kDa. In vitro transcription/translation of the cDNA clone produced a protein of approximately 31 kDa, which was immunoprecipitated by EP1-specific polyclonal antiserum generated against purified deglycosylated EP1. EP1-like sequences also were amplified from male wasp genomic DNA, suggestive of integration of EP1-like sequences in the genome. This report constitutes the first evidence that a specific protein isolated from a parasitized host insect is a wasp polydnavirus gene product.

Behavioral responses to host foodplants of two populations of the insect parasitoid Cotesia congregata (Say).

To test the hypothesis that natural enemy populations differ in their behavioral responses to plants or to plant allelochemicals, we compared two populations of the gregarious larval endoparasitoid, Cotesia congregata (Say) (Hymenoptera: Braconidae) that differed in their historical and present exposure to tobacco. The major hosts for both populations were Manduca sexta L. and M. quinquemaculata (Haworth) (Lepidoptera: Sphingidae), but these hosts were typically encountered on tobacco by parasitoids in one population (Upper Marlboro) and on tomato by parasitoids in another population (Wye). Early in the season, Wye parasitoids preferred to oviposit in M. sexta on tomato rather than on tobacco and Upper Marlboro parasitoids showed no preference; neither population showed any preference later in the season. Neither of the strains originating from the two populations showed a landing preference for tobacco or tomato in flight chamber trials, but Upper Marlboro parasitoids searched longer on tobacco than on tomato, and Wye parasitoids searched longer on tomato. When nicotine solutions were applied to tobacco leaf, searching responses of Upper Marlboro parasitoids were enhanced by 0.001-1.0% nicotine, and searching responses of Wye parasitoids were decreased by 0.01-1.0% nicotine. We speculate that population differences in searching responses to tobacco and nicotine may explain the differential parasitism responses found early in the season.

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

AbstractPopulations of Helicoverpa (= Heliothis) zea (Boddie), Heliothis virescens (F.), Manduca sexta (L.) and M. 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 of H. zea and H. 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 of Manduca spp. larvae were too low to permit similar analyses of the effects of plant genotype. Rates of egg parasitism by Trichogramma spp. and Telenomus sphingis (Ashmead) were reduced on insect‐resistant genotypes. Rates of parasitism by the larval parasitoids Campoletis sonorensis (Cameron) and Cotesia congregata (Say) were reduced on resistant genotypes. No consistent effects on parasitism rates by Cotesia marginiventris (Cresson) were observed and parasitism rates by Cardiochiles nigriceps Viereck were unaffected.