Parasitoid Cotesia Marginiventris Research Articles

Maize terpene synthase 1 impacts insect behavior via the production of monoterpene volatiles β-myrcene and linalool

Plant-derived volatiles are important mediators of plant-insect interactions as they can provide cues for host location and quality, or act as direct or indirect defense molecules. The volatiles produced by Zea mays (maize) include a range of terpenes, likely produced by several of the terpene synthases (TPS) present in maize. Determining the roles of specific terpene volatiles and individual TPSs in maize-insect interactions is challenging due to the promiscuous nature of TPSs in vitro and their potential for functional redundancy. In this study, we used metabolite GWAS of a sweetcorn diversity panel infested with Spodoptera frugiperda (fall armyworm) to identify genetic correlations between TPSs and individual volatiles. This analysis revealed a correlation between maize terpene synthase 1 (ZmTPS1) and emission of the monoterpene volatiles linalool and β-myrcene. Electroantennogram assays showed gravid S. frugiperda could detect both linalool and β-myrcene. Quantification of headspace volatiles in a maize tps1 loss-of-function mutant confirmed that ZmTPS1 is an important contributor to linalool and β-myrcene emission in maize. Furthermore, pairwise choice assays between tps1 mutant and wild-type plants showed that ZmTPS1, and by extension its volatile products, aid host location in the chewing insect S. frugiperda, yet repel the sap-sucking pest, Rhopalosiphum maidis (corn leaf aphid). On the other hand, ZmTPS1 had no impact on indirect defense via the recruitment of the parasitoid Cotesia marginiventris. ZmTPS1 is therefore an important mediator of the interactions between maize and its insect pests.

Effects of Light Quality on Colonization of Tomato Roots by AMF and Implications for Growth and Defense.

Beneficial soil microbes can enhance plant growth and defense, but the extent to which this occurs depends on the availability of resources, such as water and nutrients. However, relatively little is known about the role of light quality, which is altered during shading, resulting a low red: far-red ratio (R:FR) of light. We examined how low R:FR light influences arbuscular mycorrhizal fungus (AMF)-mediated changes in plant growth and defense using Solanum lycopersicum (tomato) and the insect herbivore Chrysodeixis chalcites. We also examined effects on third trophic level interactions with the parasitoid Cotesia marginiventris. Under low R:FR light, non-mycorrhizal plants activated the shade avoidance syndrome (SAS), resulting in enhanced biomass production. However, mycorrhizal inoculation decreased stem elongation in shaded plants, thus counteracting the plant’s SAS response to shading. Unexpectedly, activation of SAS under low R:FR light did not increase plant susceptibility to the herbivore in either non-mycorrhizal or mycorrhizal plants. AMF did not significantly affect survival or growth of caterpillars and parasitoids but suppressed herbivore-induced expression of jasmonic acid-signaled defenses genes under low R:FR light. These results highlight the context-dependency of AMF effects on plant growth and defense and the potentially adverse effects of AMF under shading.

Parasitoid Causes Cascading Effects on Plant-Induced Defenses Mediated Through the Gut Bacteria of Host Caterpillars.

Koinobiont endoparasitoid wasps whose larvae develop inside a host insect alter several important facets of host physiology, potentially causing cascading effects across multiple trophic levels. For instance, the hijacking of the host immune responses may have effects on how insects interact with host plants and microbial associates. However, the parasitoid regulation of insect–plant–microbiome interactions is still understudied. In this study, we used the fall armyworm (FAW), Spodoptera frugiperda, and the braconid parasitoid Cotesia marginiventris to evaluate impacts of parasitism on the gut microbiome of FAW larvae, and respective maize plant defense responses. The level of reactive oxygen species and the microbial community in larval gut underwent significant changes in response to parasitism, leading to a significant reduction of Enterococcus, while elevating the relative abundance of Pseudomonas. FAW with parasitism had lower glucose oxidase (GOX) activity in salivary glands and triggered lower defense responses in maize plants. These changes corresponded to effects on plants, as Pseudomonas inoculated larvae had lower activity of salivary GOX and triggered lower defense responses in maize plants. Our results demonstrated that parasitism had cascading effects on microbial associates across trophic levels and also highlighted that insect gut bacteria may contribute to complex interrelationships among parasitoids, herbivores, and plants.

Contrasting insect attraction and herbivore-induced plant volatile production in maize.

The maize inbred line W22 has lower herbivore-induced volatile production than B73 but both fall armyworm larvae and the wasps that parasitize them prefer W22 over B73. Maize inbred line W22 is an important resource for genetic studies due to the availability of the UniformMu mutant population and a complete genome sequence. In this study, we assessed the suitability of W22 as a model for tritrophic interactions between maize, Spodoptera frugiperda (fall armyworm) and the parasitoid wasp Cotesia marginiventris. W22 was found to be a good model for studying the interaction as S. frugiperda prefers W22 over B73 and a higher parasitism rate by C. marginiventris was observed on W22 compared to the inbred line B73. W22 also produced lower amounts of many herbivore-induced volatile terpenes and indole emission upon treatment with S. frugiperda oral secretions. We propose that some of the major herbivore-induced terpene volatiles are perhaps impeding S. frugiperda and C. marginiventris preference and that as yet unidentified compounds are produced at low abundance may be positively impacting these interactions.

Bt cotton producing Cry1Ac and Cry2Ab does not harm two parasitoids, Cotesia marginiventris and Copidosoma floridanum

Cabbage looper, Trichoplusia ni (Hübner) is an important lepidopteran pest on many vegetable and greenhouse crops, and some field crops. Although there are no commercial transgenic Bt vegetable or greenhouse crops, T. ni is a target of Bollgard II cotton, which produces Cry1Ac and Cry2Ab. We expand on previous work that examined the effect of Bt crops on parasitoids using Bt-resistant lepidopteran populations as hosts. Cry1Ac/Cry2Ab-resistant T. ni larvae were used to eliminate host quality effects and to evaluate the direct effects of Bt cotton on the parasitoids Copidosoma floridanum (Ashmead) and Cotesia marginiventris (Cresson). These tri-trophic studies confirm that Bt cotton had no significant impact on development, success of parasitism, survival and adult longevity of C. marginiventris when using Bt-resistant T. ni fed on Bt cotton. Similarly, this Bt cotton had no significant impact on the development, mummy weight and the number of progeny produced by C. floridanum. Our studies verified that lyophilized Bt crop tissue maintained its insecticidal bioactivity when incorporated into an artificial diet, demonstrating that hosts and parasitoids were exposed to active Cry proteins. The egg-larval parasitoid C. floridanum, or similar species that consume their entire host, should be considered useful surrogates in risk assessment of Bt crops to non-target arthropods.

Duration of plant damage by host larvae affects attraction of two parasitoid species (Microplitis croceipes and Cotesia marginiventris) to cotton: implications for interspecific competition.

Volatile organic compounds (VOCs) released by herbivore-damaged plants can guide parasitoids to their hosts. The quantity and quality of VOC blends emitted by plants may be affected by the duration of plant damage by herbivores, which could have potential ramifications on the recruitment of competing parasitoids. We used two parasitoid species, Microplitis croceipes and Cotesia marginiventris (Hymenoptera: Braconidae), to address the question of whether duration of plant damage affects parasitoid use of plant VOCs for host location. Both wasp species are larval endoparasitoids of Heliothis virescens (Lepidoptera: Noctuidae), an important pest of cotton. Attraction of the two parasitoid species to odors emitted by undamaged (UD), fresh (6 h infestation) damage (FD), and old (24 h infestation) damage (OD) cotton plants infested by H. virescens larvae was investigated using a headspace volatile collection system coupled with four-choice olfactometer bioassay. Both sexes of M. croceipes showed a preference for FD- and OD-plant odors over UD-plants. On the other hand, more C. marginiventris females were attracted to UD- and FD-plants than to OD-plants. GC/MS analyses showed qualitative and quantitative differences in the VOC profiles of UD, FD, and OD-plants, which may explain the observed preferences of the parasitoids. These results suggest a temporal partitioning in the recruitment of M. croceipes and C. marginiventris to H. virescens-damaged cotton, and may have potential implications for interspecific competition between the two parasitoid species.

Volatiles produced by soil-borne endophytic bacteria increase plant pathogen resistance and affect tritrophic interactions.

Volatile organic compounds (VOCs) released by soil microorganisms influence plant growth and pathogen resistance. Yet, very little is known about their influence on herbivores and higher trophic levels. We studied the origin and role of a major bacterial VOC, 2,3-butanediol (2,3-BD), on plant growth, pathogen and herbivore resistance, and the attraction of natural enemies in maize. One of the major contributors to 2,3-BD in the headspace of soil-grown maize seedlings was identified as Enterobacter aerogenes, an endophytic bacterium that colonizes the plants. The production of 2,3-BD by E. aerogenes rendered maize plants more resistant against the Northern corn leaf blight fungus Setosphaeria turcica. On the contrary, E. aerogenes-inoculated plants were less resistant against the caterpillar Spodoptera littoralis. The effect of 2,3-BD on the attraction of the parasitoid Cotesia marginiventris was more variable: 2,3-BD application to the headspace of the plants had no effect on the parasitoids, but application to the soil increased parasitoid attraction. Furthermore, inoculation of seeds with E. aerogenes decreased plant attractiveness, whereas inoculation of soil with a total extract of soil microbes increased parasitoid attraction, suggesting that the effect of 2,3-BD on the parasitoid is indirect and depends on the composition of the microbial community.

An ecogenomic analysis of herbivore‐induced plant volatiles in Brassica juncea

Upon herbivore feeding, plants emit complex bouquets of induced volatiles that may repel insect herbivores as well as attract parasitoids or predators. Due to differences in the temporal dynamics of individual components, the composition of the herbivore-induced plant volatile (HIPV) blend changes with time. Consequently, the response of insects associated with plants is not constant either. Using Brassica juncea as the model plant and generalist Spodoptera spp. larvae as the inducing herbivore, we investigated herbivore and parasitoid preference as well as the molecular mechanisms behind the temporal dynamics in HIPV emissions at 24, 48 and 72 h after damage. In choice tests, Spodoptera litura moth preferred undamaged plants, whereas its parasitoid Cotesia marginiventris favoured plants induced for 48 h. In contrast, the specialist Plutella xylostella and its parasitoid C. vestalis preferred plants induced for 72 h. These preferences matched the dynamic changes in HIPV blends over time. Gene expression analysis suggested that the induced response after Spodoptera feeding is mainly controlled by the jasmonic acid pathway in both damaged and systemic leaves. Several genes involved in sulphide and green leaf volatile synthesis were clearly up-regulated. This study thus shows that HIPV blends vary considerably over a short period of time, and these changes are actively regulated at the gene expression level. Moreover, temporal changes in HIPVs elicit differential preferences of herbivores and their natural enemies. We argue that the temporal dynamics of HIPVs may play a key role in shaping the response of insects associated with plants.

Eliminating host-mediated effects demonstrates Bt maize producing Cry1F has no adverse effects on the parasitoid Cotesia marginiventris

The fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is an important pest of maize in the United States and many tropical areas in the western hemisphere. In 2001, Herculex I(®) (Cry1F) maize was commercially planted in the United States to control Lepidoptera, including S. frugiperda. In 2006, a population of S. frugiperda was discovered in Puerto Rico that had evolved resistance to Cry1F maize in the field, making it the first well-documented case of an insect with field resistance to a plant producing protein from Bacillus thuringiensis (Bt). Using this resistant population, we conducted tri-trophic studies with a natural enemy of S. frugiperda. By using resistant S. frugiperda, we were able to overcome possible prey-mediated effects and avoid concerns about potential differences in laboratory- or field-derived Bt resistance. We used the Cry1F-resistant S. frugiperda to evaluate effects of Cry1F on Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae), a larval endoparasitoid of S. frugiperda, over five generations. Our results clearly demonstrate that Cry1F maize does not affect development, parasitism, survivorship, sex ratio, longevity or fecundity of C. marginiventris when they parasitize Cry1F maize-fed S. frugiperda. Furthermore, the level of Cry1F protein in the leaves was strongly diluted when transferred from Bt maize to S. frugiperda and was not detected in larvae, cocoons or adults of C. marginiventris. Our results refute previous reports of C. marginiventris being harmed by Bt proteins and suggest that such results were caused by prey-mediated effects due to using Bt-susceptible lepidopteran hosts.

Differential Performance of a Specialist and Two Generalist Herbivores and Their Parasitoids on Plantago lanceolata

The ability to cope with plant defense chemicals differs between specialist and generalist species. In this study, we examined the effects of the concentration of the two main iridoid glycosides (IGs) in Plantago lanceolata, aucubin and catalpol, on the performance of a specialist and two generalist herbivores and their respective endoparasitoids. Development of the specialist herbivore Melitaea cinxia was unaffected by the total leaf IG concentration in its host plant. By contrast, the generalist herbivores Spodoptera exigua and Chrysodeixis chalcites showed delayed larval and pupal development on plant genotypes with high leaf IG concentrations, respectively. This result is in line with the idea that specialist herbivores are better adapted to allelochemicals in host plants on which they are specialized. Melitaea cinxia experienced less post-diapause larval and pupal mortality on its local Finnish P. lanceolata than on Dutch genotypes. This could not be explained by differences in IG profiles, suggesting that M. cinxia has adapted in response to attributes of its local host plants other than to IG chemistry. Development of the specialist parasitoid Cotesia melitaearum was unaffected by IG variation in the diet of its host M. cinxia, a response that was concordant with that of its host. By contrast, the development time responses of the generalist parasitoids Hyposoter didymator and Cotesia marginiventris differed from those of their generalist hosts, S. exigua and C. chalcites. While their hosts developed slowly on high-IG genotypes, development time of H. didymator was unaffected. Cotesia marginiventris actually developed faster on hosts fed high-IG genotypes, although they then had short adult longevity. The faster development of C. marginiventris on hosts that ate high-IG genotypes is in line with the “immunocompromized host” hypothesis, emphasizing the potential negative effects of toxic allelochemicals on the host’s immune response.

Effects of Rewarding and Unrewarding Experiences on the Response to Host-induced Plant Odors of the Generalist Parasitoid Cotesia marginiventris (Hymenoptera: Braconidae)

Associative learning is known to modify foraging behavior in numerous parasitic wasps. This is in agreement with optimal foraging theory, which predicts that the wasps will adapt their responses to specific cues in accordance with the rewards they receive while perceiving these cues. Indeed, the generalist parasitoid Cotesia marginiventris shows increased attraction to a specific plant odor after perceiving this odor during contact with hosts. This positive associative learning is common among many parasitoids, but little is known about the effects of unrewarding host searching events on the attractiveness of odors. To study this, preferences of female C. marginiventris for herbivore-induced odors of three plant species were tested in a six-arm olfactometer after the wasps perceived one of these odors either i) without contacting any caterpillars, ii) while contacting the host caterpillar Spodoptera littoralis, or iii) while contacting the non-host caterpillar Pieris rapae. The results confirm the effects of positive associative learning, but showed no changes in innate responses to the host-induced odors after “negative” experiences. Hence, a positive association is made during an encounter with hosts, but unsuccessful host-foraging experiences do not necessarily lead to avoidance learning in this generalist parasitoid.

Attraction of the parasitoid Cotesia marginiventris to host (Spodoptera frugiperda) frass is affected by transgenic maize

We assessed in the laboratory the attraction of the parasitoid Cotesia marginiventris (Cresson) toward odors emitted by conventional maize (Zea mays L. ssp. mays) and Bt (Bacillus thuringiensis) maize seedlings following actual or simulated injury by Spodoptera frugiperda (Smith), the parasitoid's host, and emitted by the host's frass, produced following consumption of conventional or Bt maize seedlings. Females of C. marginiventris exhibited similarly strong responses to conventional and Bt maize seedlings injured by the host or with simulated injury, and these were stronger than responses to clean air. In contrast, the responses of C. marginiventris females were consistently weaker toward host frass derived from Bt maize tissue compared to frass derived from conventional maize tissue. We hypothesized that the weakened response was due to a detrimental effect of Bt endotoxins, present in the Bt maize tissue, on the bacterial community present in the host's gut and frass, including bacteria that produce odors attractive to C. marginiventris. As an initial test of our hypothesis, we compared between the responses of C. marginiventris females to host frass produced following consumption of Bt maize and frass produced from conventional maize which had been treated with an antibiotic (tetracycline) to eliminate host gut bacteria. Our results showed that C. marginiventris females responded similarly weakly to host frass derived from conventional maize tissue treated with antibiotic and to frass derived from Bt maize tissue, treated or untreated with antibiotic, while they responded strongly to frass derived from conventional maize untreated with antibiotic, so provided initial, partial support for our hypothesis. We discussed the weakened response of C. marginiventris females to host frass derived from Bt maize in the context of plausible impacts of transgenic crop cultivars on parasitoid foraging and populations, and the implications for biological control of non-target, polyphagous pests, such as S. frugiperda.

Nitrogen Deficiency Affects Bottom-Up Cascade Without Disrupting Indirect Plant Defense

Nitrogen (N) is an important macronutrient for plants and insects alike, and the availability of this critical element may considerably modify bottom-up effects in tritrophic systems. By using hydroponically cultured Glycine max, we investigated the impact of N deficiency on plant growth, photosynthetic efficiency, primary metabolism, and herbivore-induced volatile (VOC) emission. Cascading effects of N deficiency on higher trophic levels were assessed by measuring the performances of the herbivore Spodoptera frugiperda and its parasitoid Cotesia marginiventris. In addition, we studied the volatile-guided foraging behavior of C. marginiventris to explore whether nutrient stress affects the plant's indirect defense. Our results show that photosynthetic efficiency, leaf N, and soluble protein content were significantly reduced in N deficient plants whereas root biomass was increased. Nitrogen starved plants emitted the same range of herbivore-induced VOCs as control plants, but quantitative changes occurred in the release of the main compound and two other volatiles. Herbivore growth and the performance of parasitoids developing inside the affected hosts were attenuated when caterpillars fed on N deficient plants. The behavioral response of C. marginiventris to induced VOCs from N deficient hosts, however, remained unaffected. In summary, N stress had strong bottom-up effects over three trophic levels, but the plant's indirect defense remained intact.

Predicting population dynamics of the parasitoid Cotesia marginiventris (Hymenoptera: Braconidae) resulting from novel interactions of temperature and selenium

Changes in trophic level interactions due to global climate change and the increasing occurrence of pollution are likely to have consequences for natural enemies. Specifically, information regarding the effects of these factors on insect parasitoids is relatively sparse. We examined the individual and joint effects of temperature and the pollutant selenium on the fitness correlates of the parasitoid wasp Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae), parasitizing Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae). Our specific objective was to determine in a factorial experiment how three temperatures (constant 28.6°C, constant 33°C and a fluctuating temperature between 28.6 and 33°C) and three concentrations of seleno-dl-methionine (0.00, 21.21, and 42.42 µg/g) affected the parasitoid's fitness and life history. Parasitoids failed to complete development at the constant 33°C, but developed significantly faster at the fluctuating temperature compared to the constant 28.6°C. There were significant declines due to increased temperature, but not selenium, on C. marginiventris survival time, adult body weight, body size, hind tibia length, female life span and number of progeny that survived to adulthood. Mean generation times and the intrinsic rate of increase (r) further show that both of these life table statistics declined under conditions of increased temperatures. We discuss the implications of these results in helping to understand and predict the effectiveness of biological control programs and pest management strategies as climate changes in the presence of metal and metalloid pollution.

Strong Attraction of the Parasitoid Cotesia marginiventris Towards Minor Volatile Compounds of Maize

Plants infested with herbivorous arthropods emit complex blends of volatile compounds, which are used by several natural enemies as foraging cues. Despite detailed knowledge on the composition and amount of the emitted volatiles in many plant-herbivore systems, it remains largely unknown which compounds are essential for the attraction of natural enemies. In this study, we used a combination of different fractionation methods and olfactometer bioassays in order to examine the attractiveness of different compositions of volatile blends to females of the parasitoid Cotesia marginiventris. In a first step, we passed a volatile blend emitted by Spodoptera littoralis infested maize seedlings over a silica-containing filter tube and subsequently desorbed the volatiles that were retained by the silica filter (silica extract). The volatiles that broke through the silica filter were collected on and subsequently desorbed from a SuperQ filter (breakthrough). The silica extract was highly attractive to the wasps, whereas the breakthrough volatiles were not attractive. The silica extract was even more attractive than the extract that contained all herbivore-induced maize volatiles. Subsequently, we fractioned the silica extract by preparative gas-chromatography (GC) and by separating more polar from less polar compounds. In general, C. marginiventris preferred polar over non-polar compounds, but several fractions were attractive to the wasp, including one that contained compounds emitted in quantities below the detection threshold of the GC analysis. These results imply that the attractiveness of the volatile blend emitted by Spodoptera-infested maize seedlings to C. marginiventris females is determined by a specific combination of attractive and repellent/masking compounds, including some that are emitted in very small amounts. Manipulating the emission of such minor compounds has the potential to greatly improve the attraction of certain parasitoids and enhance biological control of specific insect pests.

Cotton Plant, Gossypium hirsutum L., Defense in Response to Nitrogen Fertilization

Plants respond to insect herbivory by producing dynamic changes in an array of defense-related volatile and nonvolatile secondary metabolites. A scaled response relative to herbivory levels and nutrient availability would be adaptive, particularly under nutrient-limited conditions, in minimizing the costs of expressed defensive pathways and synthesis. In this study, we investigated effects of varying nitrogen (N) fertilization (42, 112, 196, and 280 ppm N) on levels of cotton plant (Gossypium hirsutum) phytohormones [jasmonic acid (JA) and salicylic acid (SA)], terpenoid aldehydes (hemigossypolone, heliocides H(1), H(2), H(3), and H(4)), and volatile production in response to beet armyworm (Spodoptera exigua) herbivory. Additional bioassays assessed parasitoid (Cotesia marginiventris) host-searching success in response to cotton plants grown under various N fertilizer regimes. At low N input (42 ppm N), herbivore damage resulted in significant increases in local leaf tissue concentrations of JA and volatiles and in systemic accumulation of terpenoid aldehydes. However, increased N fertilization of cotton plants suppressed S. exigua-induced plant hormones and led to reduced production of various terpenoid aldehydes in damaged mature leaves and undamaged young leaves. While increased N fertilization significantly diminished herbivore-induced leaf volatile concentrations, the parasitism of S. exigua larvae by the parasitoid C. marginiventris in field cages did not differ among N treatments. This suggests that, despite significant N fertilization effects on herbivore-induced plant defenses, at short range, the parasitoids were unable to differentiate between S. exigua larvae feeding on physiologically different cotton plants that share large constitutive volatile pools releasable when damaged by herbivores.

Molecular and genomic basis of volatile‐mediated indirect defense against insects in rice

Rice plants fed on by fall armyworm (Spodoptera frugiperda, FAW) caterpillars emit a blend of volatiles dominated by terpenoids. These volatiles were highly attractive to females of the parasitoid Cotesia marginiventris. Microarray analysis identified 196 rice genes whose expression was significantly upregulated by FAW feeding, 18 of which encode metabolic enzymes potentially involved in volatile biosynthesis. Significant induction of expression of seven of the 11 terpene synthase (TPS) genes identified through the microarray experiments was confirmd using real-time RT-PCR. Enzymes encoded by three TPS genes, Os02g02930, Os08g07100 and Os08g04500, were biochemically characterized. Os02g02930 was found to encode a monoterpene synthase producing the single product S-linalool, which is the most abundant volatile emitted from FAW-damaged rice plants. Both Os08g07100 and Os08g04500 were found to encode sesquiterpene synthases, each producing multiple products. These three enzymes are responsible for production of the majority of the terpenes released from FAW-damaged rice plants. In addition to TPS genes, several key genes in the upstream terpenoid pathways were also found to be upregulated by FAW feeding. This paper provides a comprehensive analysis of FAW-induced volatiles and the corresponding volatile biosynthetic genes potentially involved in indirect defense in rice. Evolution of the genetic basis governing volatile terpenoid biosynthesis for indirect defense is discussed.

Plant surface wax affects parasitoid’s response to host footprints

The plant surface is the substrate upon which herbivorous insects and natural enemies meet and thus represents the stage for interactions between the three trophic levels. Plant surfaces are covered by an epicuticular wax layer which is highly variable depending on species, cultivar or plant part. Differences in wax chemistry may modulate ecological interactions. We explored whether caterpillars of Spodoptera frugiperda, when walking over a plant surface, leave a chemical trail (kairomones) that can be detected by the parasitoid Cotesia marginiventris. Chemistry and micromorphology of cuticular waxes of two barley eceriferum wax mutants (cer-za.126, cer-yp.949) and wild-type cv. Bonus (wt) were assessed. The plants were then used to investigate potential surface effects on the detectability of caterpillar kairomones. Here we provide evidence that C. marginiventris responds to chemical footprints of its host. Parasitoids were able to detect the kairomone on wild-type plants and on both cer mutants but the response to cer-yp.949 (reduced wax, high aldehyde fraction) was less pronounced. Experiments with caterpillar-treated wt and mutant leaves offered simultaneously, confirmed this observation: no difference in wasp response was found when wt was tested against cer-za.126 (reduced wax, wt-like chemical composition) but wt was significantly more attractive than cer-yp.949. This demonstrates for the first time that the wax layer can modulate the detectability of host kairomones.

The role of host plant species in the phenotypic differentiation of sympatric populations of Aleiodes nolophanae and Cotesia marginiventris

AbstractThe ecology of parasitoids is strongly influenced by their host plant species. Parasitoid fitness can be affected by a variety of plant traits that could promote phenotypic differentiation among populations of parasitoids. Generalist parasitoids are expected to be more affected by plant traits (e.g., plant defensive traits) than specialist parasitoids. Data are presented on phenotypic differences of two braconid parasitoid wasps ovipositing on the same insect host species on two different host plant species. Adult mass, adult longevity, and percent parasitism are compared for the generalist parasitoid Cotesia marginiventris Cresson and the specialist parasitoid Aleiodes nolophanae Ashmead (both Hymenoptera: Braconidae) emerging from green cloverworms, Hypena scabra Fabricius (Lepidoptera: Noctuidae), feeding on two host plant species, alfalfa (Medicago sativa L.) and soybean (Glycine max L. Merr.) (both Fabaceae), at three locations. Specialist wasps that parasitized the green cloverworm on alfalfa had a significantly larger mass than the ones that parasitized the green cloverworm on soybean at the three study sites. Generalist wasps that parasitized green cloverworms on alfalfa had a larger mass than wasps parasitizing green cloverworms on soybean only at one of the study sites (i.e., Prince George's County, MD, USA). Similarly, both specialist and generalist wasps lived longer when parasitizing green cloverworms on alfalfa than when parasitizing them on soybean at only one of the study sites (i.e., Prince George's County). In Prince George's County, percent parasitism on alfalfa by the specialist parasitoid was higher than on soybean for three consecutive years and percent parasitism by the generalist parasitoid was the same on alfalfa and soybean every year. Thus, phenotypic differences among populations associated with different host plant species vary geographically (i.e., parasitoid phenotype associated with different host plant species differ at some sites while it is the same at other sites). The implications of geographic variation for biological control are discussed.

Isolation and Characterization of a Baculovirus Associated with the Insect Parasitoid Wasp,Cotesiamarginiventris, or Its Host,Trichoplusia ni

A multiple nucleopolyhedrovirus (MNPV) was isolated from Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) larvae that had been stung by the parasitoid Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae). The wild type virus was plaque purified by infecting a Heliothis subflexa (BCIRL- HsAM1) cell line and isolating several clones. The mean estimated genomic size of this virus based on PstI, BstEII, StyI, HindIII restriction profiles was estimated to be 106 ± 2.5 kbp (mean±SE). A clone designated as TnMNPV/CmBCL9 was used in bioassays against several lepidopteran pests and in comparative studies with the baculoviruses AcMNPV, AgMNPV, AfMNPV, PxMNPV and HzSNPV of Autographa califomica, Anticarsia gemmatalis, Anagrapha falcifera, Plutella xylostella, and Helicoverpa zea, respectively. Infectivity studies showed that TnMNPV/CmBCL9 was highly infectious for Heliothis subflexa and T. ni, with an LC50 value 0.07 occlusion bodies/mm2 in both species and also infectious for H. zea and Heliothis virescens with LC50 values of 0.22 and 0.27 occlusion bodies/mm2, respectively. Restriction endonuclease analysis of the isolate and selected baculoviruses revealed profiles that were very similar to AfMNPV but different from the restriction endonuclease profiles of the other baculoviruses. Hybridization studies suggest that the TnMNPV/CmBCL9 was closely related to AfMNPV and AcMNPV-HPP. Further support for this comes from a phylogenetic analysis employing a split-graphs network, comparing the polh, egt, and p10 genes from TnMNPV/CmBCL9 with those from other baculoviruses and suggests that this virus is closely related to the AcMNPV variants, AfMNPV and RoMNPV of Rachiplusia ou.