Role of volatile inforchemicals emitted by feces of larvae in host-searching behavior of parasitoidCotesia rubecula (Hymenoptera: Braconidae): A behavioral and chemical study

Architecture, construction, retention, and repair of faecal shields in three tribes of tortoise beetles (Coleoptera, Chrysomelidae, Cassidinae: Cassidini, Mesomphaliini, Spilophorini)

Animal constructions are the outcomes of complex evolutionary, behavioural, and ecological forces. A brief review of diverse animal builders, the materials used, and the functions they provide their builders is provided to develop approaches to studying faecal-based constructions and faecal-carrying in leaf beetles (Coleoptera: Chrysomelidae). Field studies, rearing, dissections, photography, and films document shields constructed by larvae in two species in two tribes of the subfamily Cassidinae, Calyptocephalaattenuata (Spaeth, 1919) (Spilophorini), and Cassidasphaerula Boheman, 1853 (Cassidini). Natural history notes on an undetermined Cassidini species and Stolascucullata (Boheman, 1862) (Tribe Mesomphaliini) outline the life cycle of tortoise beetles and explain terms. Commonly, the cassidine shield comprises exuviae onto which faeces are daubed, producing a pyramidal-shaped shield that can cover most of the body (up to the pronotum). In Cal.attenuata the larval shield comprises only exuviae, while in Cass.sphaerula, instar 1 initiates the shield by extending its telescopic anus to apply its own faeces onto its paired caudal processes; at each moult the exuvia is pushed to the caudal process base but remains attached, then more faeces are applied over it. The larva's telescopic anus is the only tool used to build and repair the shield, not mouthparts or legs, and it also applies chemicals to the shield. Pupae in Cal.attenuata retain part of the exuviae-only shield of instar VI, while pupae in Cass.sphaerula retain either the entire 5th instar larval shield (faeces + all exuviae) or only the 5th larval exuvia. The caudal processes are crucial to shield construction, shield retention on the body, and as materials of the central scaffold of the structure. They also move the shield, though the muscular mechanism is not known. Altogether the faecal + exuviae shields may represent a unique morpho-behavioural synapomorphy for the crown-clade Cassidinae (10 tribes, ~ 2669 species) and may have been a key innovation in subsequent radiation. Defensive shields and domiciles may help explain the uneven radiation of chrysomelid subfamilial and tribal clades.

Parasitism causes changes in caterpillar odours and associated bacterial communities with consequences for host-location by a hyperparasitoid

Microorganisms living in and on macroorganisms may produce microbial volatile compounds (mVOCs) that characterise organismal odours. The mVOCs might thereby provide a reliable cue to carnivorous enemies in locating their host or prey. Parasitism by parasitoid wasps might alter the microbiome of their caterpillar host, affecting organismal odours and interactions with insects of higher trophic levels such as hyperparasitoids. Hyperparasitoids parasitise larvae or pupae of parasitoids, which are often concealed or inconspicuous. Odours of parasitised caterpillars aid them to locate their host, but the origin of these odours and its relationship to the caterpillar microbiome are unknown. Here, we analysed the odours and microbiome of the large cabbage white caterpillar Pieris brassicae in relation to parasitism by its endoparasitoid Cotesia glomerata. We identified how bacterial presence in and on the caterpillars is correlated with caterpillar odours and tested the attractiveness of parasitised and unparasitised caterpillars to the hyperparasitoid Baryscapus galactopus. We manipulated the presence of the external microbiome and the transient internal microbiome of caterpillars to identify the microbial origin of odours. We found that parasitism by C. glomerata led to the production of five characteristic volatile products and significantly affected the internal and external microbiome of the caterpillar, which were both found to have a significant correlation with caterpillar odours. The preference of the hyperparasitoid was correlated with the presence of the external microbiome. Likely, the changes in external microbiome and body odour after parasitism were driven by the resident internal microbiome of caterpillars, where the bacterium Wolbachia sp. was only present after parasitism. Micro-injection of Wolbachia in unparasitised caterpillars increased hyperparasitoid attraction to the caterpillars compared to untreated caterpillars, while no differences were found compared to parasitised caterpillars. In conclusion, our results indicate that host-parasite interactions can affect multi-trophic interactions and hyperparasitoid olfaction through alterations of the microbiome.

Response of Parasitic Wasp Cotesia glomerata L. (Hymenoptera: Braconidae) to Cabbage Plants of Two Varieties: Olfactory Spectra of Males and Females

The parasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) oviposits in larvae of the large cabbage white butterfly (Pieris brassicae L.). Many parasitoids are attracted by volatile organic compounds (VOCs) emitted by the plant that the host insects feed on. The objectives of the study were to identify the VOCs in leaves of two varieties of cabbage (white cabbage, Brassica oleracea var. capitata f. alba and cauliflower, B. oleracea var. botrytis) damaged by P. brassicae caterpillars which elicit antennal responses of C. glomerata, and characterize the olfactory spectra of females and males. Leaf extracts were analyzed by using gas chromatography equipped with an electroantennographic detector (GC-EAD) and GC-mass spectrometry. In total, 32 olfactory-active compounds for C. glomerata in cauliflower and 24 in white cabbage were revealed. The females perceived more compounds than males. Hexan-1-ol, (E)-3-hexen-1-ol, (E)-2-octenal, benzylcianide, tetradecanal, and two unidentified compounds elicited EAG responses in females but not in males. Females were more sensitive to (E)-3-hexenal, (Z)-3-hexenal, and pentadecenal, whereas males showed higher sensitivity to (Z)-3-hexenyl butanoate, heptanal, (Δ)-2-pentenal, (E)-2-hexenol, and octanal. The olfactory spectrum of C. glomerata was expanded from 18 to 41 VOCs emitted by different varieties of cabbage damaged by P. brassicae caterpillars. Eight EAG-active VOCs were common for all cabbage varieties. In tritrophic interactions, benzylcyanide can serve as an important signal for C. glomerata females indicating damage of cabbage caused by P. brassicae caterpillars. The data are useful for development of a push-pull strategy for P. brassicae control, based on parasitoid behavior regulation by VOCs.

The Pupal Parasitoid Trichopria drosophilae Is Attracted to the Same Yeast Volatiles as Its Adult Host

There is increasing evidence that microorganisms, particularly fungi and bacteria, emit volatile compounds that mediate the foraging behaviour of insects and therefore have the potential to affect key ecological relationships. However, to what extent microbial volatiles affect the olfactory response of insects across different trophic levels remains unclear. Adult parasitoids use a variety of chemical stimuli to locate potential hosts, including those emitted by the host's habitat, the host itself, and microorganisms associated with the host. Given the great capacity of parasitoids to utilize and learn odours to increase foraging success, parasitoids of eggs, larvae, or pupae may respond to the same volatiles the adult stage of their hosts use when locating their resources, but compelling evidence is still scarce. In this study, using Saccharomyces cerevisiae we show that Trichopria drosophilae, a pupal parasitoid of Drosophila species, is attracted to the same yeast volatiles as their hosts in the adult stage, i.e. acetate esters. Parasitoids significantly preferred the odour of S. cerevisiae over the blank medium in a Y-tube olfactometer. Deletion of the yeast ATF1 gene, encoding a key acetate ester synthase, decreased attraction of T. drosophilae, while the addition of synthetic acetate esters to the fermentation medium restored parasitoid attraction. Bioassays with individual compounds revealed that the esters alone were not as attractive as the volatile blend of S. cerevisiae, suggesting that other volatile compounds also contribute to the attraction of T. drosophilae. Altogether, our results indicate that pupal parasitoids respond to the same volatiles as the adult stage of their hosts, which may aid them in locating oviposition sites.

Volatile Infochemicals from Rhyzopertha dominica Larvae and Larval Feces Involved in Theocolax elegans Host Habitat Location

Simple Summary

Food protection is a key issue to guarantee food security. One of the major criticisms is related to insect pests, which can severely damage stored products. Control of stored product pests widely relies on synthetic pesticides, which are potentially harmful to human health and the environment. In this context, the application of chemicals during post-harvest should be limited, and natural enemies, like parasitoid wasps, might be useful for biologically based pest management programs. The effectiveness of this approach under field conditions is still uncertain, and more information about parasitoid biology and behavior can be valuable to improve mass rearing and control strategies. This study investigates the host habitat location behavior of Theocolax elegans, a generalist parasitoid attacking several stored product pests, including Rhyzopertha dominica, a key pest species of stored grains. Bioassays demonstrated that the parasitoid females were not attracted by intact kernels; indeed, the parasitoid females were strongly attracted by infested wheat and by the host feces, locating the suitable hosts through the characteristic volatile infochemicals emitted by these substrates. Results from the present research are encouraging and suggest that biological control agents might be used to reduce the amount of synthetic insecticides.

Abstract

The development of biologically based approaches for stored product pest control is needed to reduce chemical inputs. Bioassays were performed to investigate host habitat location in the trophic interaction durum wheat/Rhyzopertha dominica/Theocolax elegans. GC-MS analyses were carried out to identify some chemical compounds produced by the host-related substrates. Choice and no-choice experiments demonstrated that female parasitoids were poorly attracted to intact kernels with respect to the infested substrates. Furthermore, T. elegans females performed longer residence time on infested wheat, and they generally displayed a short-term like fidelity for this cue, remaining on it during the whole observation. Infested wheat emitted one chemical (fenchone), which is possibly linked to host larvae presence, while the feces produced by host larvae emitted some other characteristic compounds. Results demonstrated that the presence of host larvae is a key factor for T. elegans host habitat location, considering that the attractiveness of the undamaged wheat is irrelevant. Furthermore, parasitoid females tended to stay on attractive cues, limiting the risk of contamination of other commodities. Biological control tools may be useful to improve the efficiency of pest management programs, but their application should be carefully evaluated.

Foraging behaviour of the exotic wasp Vespula germanica (Hymenoptera: Vespidae) on a native caterpillar defoliator

Vespula germanica is a social wasp and an opportunistic predator. While foraging, these wasps learn and integrate different kinds of cues. They have successfully invaded many parts of the world, including native Nothofagus and Lophozonia forests located in the Andean-Patagonian region, where they forage on native arthropods. Perzelia arda, a lepidopteron defoliator of Lophozonia obliqua, uses the foliage to hide in and feed on. The purpose of this work is to study whether V. germanica use olfactory cues when foraging on P. arda. To do this, we used a Y-tube olfactometer and established three treatments to compare pairs of all combinations of stimuli (larvae, leaves with larval traces, and leaves without larval traces) and controls. Data were analysed via two developed models that showed decisions made by V. germanica and allowed to establish a scale of preferences between the stimuli. The analysis demonstrates that V. germanica wasps choose P. arda as larval prey and are capable of discriminating between the offered stimuli (deviance information criterion (DIC) null model = 873.97; DIC simple model = 84.5, n = 152). According to the preference scale, V. germanica preferred leaves with traces of larvae, suggesting its ability to associate these traces with the presence of the prey. This may be because, under natural conditions, larvae are never exposed outside their shelters of leaves and therefore V. germanica uses indirect signals. The presence of V. germanica foraging on P. arda highlights the flexible foraging behaviour of this wasp which may also act as a positive biological control, reducing lepidopteran populations.

Adaptive advantages of appearance: predation, thermoregulation, and color of webbing built by New Zealand's largest moth

Prey are often difficult to locate visually, which may help them avoid predators. However, an animal's appearance might also evolve in response to the abiotic environment. Here, we investigate the processes that determine the appearance of silk webbing built by New Zealand's largest endemic moth Aenetus virescens (Lepidoptera: Hepialidae), whose larvae burrow into the trunks of native trees. Larvae cover tunnel entrances with silk webbing, detritus, and epiphytes, giving them a similar appearance to tree bark. First, we conducted spectral analyses of webbing and background bark in avian tetrahedral color space to test whether webbing made larvae less visible to predatory parrots. Next, we manipulated the spectral contrast of webbing and background bark and assessed its effect on predation by parrots for over 2 yr. Last, we measured the effect of webbing on tunnel temperatures and quantified how temperatures within tunnels affected larval growth. Results indicate that webbing made larvae less visible to predatory parrots. However, webbing contrast to background bark did not affect predation by parrots. Instead, webbing increased temperatures within tunnels and facilitated more rapid larval growth. Overall results indicate that the appearance of organisms that are difficult to locate visually may not always result from selection by predators.

A role for volatiles in intra- and inter-plant interactions in birch

One of the first observations that plants might utilize cues released by damaged neighbors under natural conditions was made in birch forests in 1985. However, the mechanisms underlying the observations were not determined, and birch (Betula spp.) has been neglected as a study system for inter-plant interaction ever since. Volatiles released by vegetative plant parts in response to herbivore damage play important roles as signals in plant-to-plant interactions in a range of woody and herbaceous plant species, and also have been shown to mediate signaling between branches of the same plant that have limited vascular connection. We established greenhouse experiments to assess: 1) whether exposure to plant volatiles from herbivore-damaged birches primes defense responses in undamaged neighbors; and 2) whether defenses also are primed in undamaged parts of the same plants with limited vascular connection. We observed a priming of defense responses, which were manifested in an augmented emission of terpenes and aromatic compounds in undamaged conspecific neighbors, and also an augmented emission of green leaf volatiles in systemic branches. Our work provides strong evidence of inter-plant signaling by volatiles, and an intra-plant systemic response in birch. However, the responses are specific, with emissions of different groups of plant volatiles typifying the primed response. This work complements and extends the previous work conducted with a natural population of birches.

Body odors of parasitized caterpillars give away the presence of parasitoid larvae to their primary hyperparasitoid enemies

Foraging success of parasitoids depends on the utilization of reliable information on the presence of their often, inconspicuous hosts. These parasitic wasps use herbivore-induced plant volatiles (HIPVs) that provide reliable cues on host presence. However, host searching of hyperparasitoids, a group of parasitoids that parasitize the larvae and pupae of other parasitoids, is more constrained. Their hosts do not feed on plants, and often are even concealed inside the body of the herbivore host. Hyperparasitoids recently have been found to use HIPVs of plants damaged by herbivore hosts in which the parasitoid larvae develop. However, hyperparasitoids that search for these parasitoid larvae may be confronted with healthy and parasitized caterpillars on the same plant, further complicating their host location. In this study, we addressed whether the primary hyperparasitoid Baryscapus galactopus uses caterpillar body odors to discriminate between unparasitized herbivores and herbivores carrying larvae of parasitoid hosts. We show that the hyperparasitoids made faster first contact and spent a longer mounting time with parasitized caterpillars. Moreover, although the three parasitoid hosts conferred different fitness values for the development of B. galactopus, the hyperparasitoids showed similar behavioral responses to caterpillar hosts carrying different primary parasitoid hosts. In addition, a two-chamber olfactometer assay revealed that volatiles emitted by parasitized caterpillars were more attractive to the hyperparasitoids than those emitted by unparasitized caterpillars. Analysis of volatiles revealed that body odors of parasitized caterpillars differ from unparasitized caterpillars, allowing the hyperparasitoids to detect their parasitoid host.

Effects of glucosinolates on a generalist and specialist leaf-chewing herbivore and an associated parasitoid

Glucosinolates (GLS) are secondary plant metabolites that as a result of tissue damage, for example due to herbivory, are hydrolysed into toxic compounds that negatively affect generalist herbivores. Specialist herbivores have evolved specific adaptations to detoxify GLS or inhibit the formation of toxic hydrolytic products. Although rarely studied, GLS and their breakdown products may also affect parasitoids. The objectives were to test the effects of GLS in a multitrophic system consisting of the generalist herbivore Spodoptera exigua, the specialist herbivore Pieris rapae, and the endoparasitoid Hyposoter ebeninus. Three ecotypes of Arabidopsis thaliana that differ in their GLS composition and concentrations and one transformed line that constitutively produces higher concentrations of aliphatic GLS were used, the latter allowing a direct assessment of the effects of aliphatic GLS on insect performance. Feeding by the generalist S. exigua and the specialist P. rapae induced both higher aliphatic and indole GLS concentrations in the A. thaliana ecotypes, although induction was stronger for indole than aliphatic GLS. For both herbivores a negative correlation between performance and aliphatic GLS concentrations was observed. This suggests that the specialist, despite containing a nitrile-specifier protein (NSP) that diverts GLS degradation from toxic isothiocyanates to less toxic nitriles, cannot completely inhibit the formation of toxic GLS hydrolytic products, or that the costs of this mechanism are higher at higher GLS concentrations. Surprisingly, performance of the parasitoid was positively correlated with higher concentrations of aliphatic GLS in the plant, possibly caused by negative effects on host immune responses. Our study indicates that GLS can not only confer resistance against herbivores directly, but also indirectly by increasing the performance of the parasitoids of these herbivores.

Trichomes as dangerous lollipops: do lizards also use caterpillar body and frass odor to optimize their foraging?

When attacked by herbivores, plants produce toxic secondary metabolites that function as direct defenses, as well as indirect defenses that attract and reward predators of the offending herbivores. These indirect defenses include both nutritive rewards such as extra floral nectar, as well as informational rewards, such as the production and release of volatile compounds that betray the location of feeding herbivores to predators. Herbivory of Nicotiana attenuata by the tobacco hornworm (Manduca larvae) alters the volatile profiles of both the plant and larval headspace. Herbivory-elicited specific changes in the volatile profiles are detected by arthropod predators of Manduca larvae. The known predators that perceive volatile cues induced by Manduca herbivory of N. attenuata are insects that target Manduca at early developmental stages, when the larvae are still small; large, late-instar larvae may have outgrown these predation risks. However, here we offer evidence that branched chain aliphatic acids derived from the digestion of plant O-acyl sugars from trichomes may betray Manduca larvae to lizard predators during late developmental stages as well.

Trichome-derived O-acyl sugars are a first meal for caterpillars that tags them for predation

Plant glandular trichomes exude secondary metabolites with defensive functions, but these epidermal protuberances are surprisingly the first meal of Lepidopteran herbivores on Nicotiana attenuata. O-acyl sugars, the most abundant metabolite of glandular trichomes, impart a distinct volatile profile to the body and frass of larvae that feed on them. The headspace composition of Manduca sexta larvae is dominated by the branched chain aliphatic acids hydrolyzed from ingested O-acyl sugars, which waxes and wanes rapidly with trichome ingestion. In native habitats a ground-hunting predator, the omnivorous ant Pogonomyrmex rugosus, but not the big-eyed bug Geocoris spp., use these volatile aliphatic acids to locate their prey.

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.

Formation of simple nitriles upon glucosinolate hydrolysis affects direct and indirect defense against the specialist herbivore, Pieris rapae

The glucosinolate-myrosinase system, found in plants of the order Brassicales, has long been considered an effective defense system against herbivores. The defensive potential of glucosinolates is mainly due to the products formed after myrosinase-catalyzed hydrolysis upon tissue damage. The most prominent hydrolysis products, the isothiocyanates, are toxic to a wide range of organisms, including herbivorous lepidopterans. In contrast, little is known about the biological activities of alternative hydrolysis products such as simple nitriles and epithionitriles that are formed at the expense of isothiocyanates in the presence of epithiospecifier proteins (ESPs). Here, we used transgenic Arabidopsis thaliana (Brassicaceae) plants overexpressing ESP (35S:ESP plants) to investigate the effects of simple nitriles on direct and indirect defense against the specialist cabbage white butterfly Pieris rapae L. (Lepidoptera, Pieridae). In the 35S:ESP plants, glucosinolates are hydrolyzed mainly to simple nitriles upon tissue disruption, while isothiocyanates are the predominant hydrolysis products in Columbia-0 (Col-0) wild-type plants. The parasitoid Cotesia rubecula (Hymenoptera, Braconidae), a specialist on P. rapae larvae, was significantly more attracted to P. rapae-infested 35S:ESP plants than to P. rapae-infested Col-0 wild-type plants in a wind tunnel setup. Furthermore, female P. rapae butterflies laid more eggs on Col-0 wild-type plants than on 35S:ESP plants when the plants had been damaged previously. However, when given a choice to feed on 35S:ESP or Col-0 plants, caterpillars did not discriminate between the two genotypes. Growth rate and developmental time were not significantly different between caterpillars that were reared on 35S:ESP or Col-0 plants. Thus, the production of simple nitriles instead of isothiocyanates, as catalyzed by ESP, can promote both direct and indirect defense against the specialist herbivore P. rapae.

Defecation behavior and ecology of insects

Whereas foraging has been a major focus of ecological and entomological research, its obligate partner, defecation, has been comparatively neglected. Insects exhibit a range of intriguing behavioral and morphological adaptations related to waste disposal in a range of contexts, including predator-prey interactions, hygiene, habitat location, reproduction, feeding, and shelter construction. Some insects, for example, make use of their own excrement as a physical or chemical defense against natural enemies, while others actively distance themselves from their waste material. Internally feeding insects, fluid-feeders, and social insects face particular challenges because their feeding behavior and/or site fidelity makes them especially vulnerable to problems associated with waste accumulation. As is true for foraging, ecological interactions involving defecation may have far-reaching evolutionary consequences and merit further study.

Variation In Plant Volatiles and Attraction Of The ParasitoidDiadegma semiclausum(Hellén)

Differences in allelochemistry of plants may influence their ability to attract parasitoids. We studied responses of Diadegma semiclausum (Hellén), a parasitoid of the diamondback moth (Plutella xylostella L.), to inter- and intraspecific variation in odor blends of crucifers and a non-crucifer species. Uninfested Brussels sprout (Brassica oleracea L. gemmifera), white mustard (Sinapis alba L.), a feral Brassica oleracea, and malting barley (Hordeum vulgare L.) were compared for their attractivity to D. semiclausum in a Y-tube bioassay. Odors from all plants were more attractive to the parasitoid than clean air. However, tested against each other, parasitoids preferred the volatile blend from the three cruciferous species over that of malting barley. Wasps also discriminated between uninfested crucifers: mustard was as attractive as feral B. oleracea, and both were more attractive than Brussels sprout. Attractivity of uninfested plants was compared with that of plants infested by larvae of the host P. xylostella. Host-infested mustard and Brussels sprout were more attractive than uninfested conspecifics. Interestingly, the volatile blends of uninfested white mustard and infested Brussels sprout were equally attractive. We also compared the volatile composition of different plant sources by collecting headspace samples and analysing them with GC-MS. Similarities of volatile profiles were determined by hierarchic clustering and non-metric scaling based on the Horn-index. Due to the absence of several compounds in its blend, the volatile profile of barley showed dissimilarities from blends of crucifers. The odor profile of white mustard was distinctly different from the two Brassicaceae. Feral Brassica oleracea odor profile was different from infested Brussels sprout, but showed overlap with uninfested Brussels sprout. Odor blends from infested and uninfested Brussels sprout were similar, and mainly quantitative differences were found. D. semiclausum appears to discriminate based on subtle differences in volatile composition of odor blends from infested and uninfested plants.

A role for isothiocyanates in plant resistance against the specialist herbivore Pieris rapae

We experimentally reanalyzed the classic interaction between Pieris rapae, a specialist lepidopteran herbivore, and isothiocyanates (mustard oils) that are characteristic phytochemicals of the Brassicaceae. Previous investigations have suggested that P. rapae is unaffected by isothiocyanates. Using whole plants, root extracts, and a microencapsulated formulation of allyl isothiocyanate, we now show that isothiocyanates reduce herbivore survival and growth, and increase development time, each in a dose-dependent manner. Neither the substrate allyl glucosinolate, nor myrosinase, the enzyme that results in the breakdown of glucosinolates, negatively affected P. rapae. Thus, we present strong evidence for a role for isothiocyanates in plant resistance against the specialist herbivore P. rapae.

Effect of conspecific and heterospecific feces on foraging and oviposition of two predatory ladybirds: role of fecal cues in predator avoidance

Growing evidence suggests a flow of chemical information from higher to lower trophic levels that affects foraging and oviposition of 'prey' in response to potential risks from predators. This was investigated in two species of ladybird predators of aphid, Harmonia axyridis and Propylea japonica. H. axyridis is known to be the stronger intraguild predator and P. japonica to be the more frequent intraguild prey in interactions of these two species. These ladybirds share aphid prey on mugworts, hibiscus, and Italian ryegrasses in fields of northern Japan but largely avoid each other on the same plant. Fecal cues of these ladybird predators were found to contribute in their assessment of predation risk from conspecific and heterospecific competitors in common habitats. Gravid females of H. axyridis reduced rates of feeding and oviposition when exposed to feces of conspecifics, but not when exposed to feces of P. japonica. In contrast, gravid females of P. japonica reduced feeding and oviposition when exposed to feces of both H. axyridis and its own species. Females of both ladybird species exhibited similar behavior in response to water extracts of feces. For P. japonica, the influence of heterospecific feces was greater than that of conspecific feces. Our results demonstrate that feces of ladybirds contain odors that have the potential to deter the feeding and oviposition activities of conspecific as well as heterospecific ladybirds. Such deterrence allows these insects to avoid predation risk. Differences in responses of the two predators are discussed.

Experimental studies of the host-finding behavior of Trogus pennator, a parasitoid of swallowtail butterflies

The parasitic wasp Troguspennator (Hymenoptera: Ichneumonidae) attacks larvae in two genera of Papilionidae, Eurytides and Papilio, on plants in a variety of families. The female wasps' responses to food plants, feeding damage, and frass were examined in a series of experiments designed to test the hypothesis that parasitic wasps that specialize on host taxa and seek their hosts in a variety of habitats exhibit fixed responses to host-derived cues and more flexible responses to cues associated only with the hosts' food plants. Naive T pennator females showed no preferences when offered either a choice between two papilionid food plants or a choice between a food plant and a plant not used for food by Papilionidae. After experience with hosts in the presence of a particular food plant, however, wasps preferred that plant. Naive wasps did prefer plants damaged by host larvae over plants damaged by nonhost (saturniid) larvae and also preferred methylene chloride extracts of host frass over extracts of frass from saturniid larvae fed on the same plant species, results indicating that the responses of T. pennator females to host-derived cues are innate. The chemical compositions of the extracts of frass from several papilionid and one saturniid species were also examined, and the significance of the finding that no host-specific patterns were detected among the major components of the extracts is discussed.

Differential attractiveness of induced odors emitted by eight maize varieties for the parasitoid cotesia marginiventris: is quality or quantity important?

Herbivore-induced plant volatiles can function as indirect defense signals that attract natural enemies of herbivores. Several parasitoids are known to exploit these plant-provided cues to locate their hosts. One such parasitoid is the generalist Cotesia marginiventris, which is, among others, attracted to maize volatiles induced by caterpillar damage. Maize plants can be induced to produce the same blend of attractive volatiles by treating them with regurgitant of Spodoptera species. We collected and analyzed the regurgitant-induced emissions of two plant species (cowpea and maize) and of eight Mexican maize varieties and found significant differences among their volatile emissions, both in terms of total quantity and the quality of the blends. In a Y-tube olfactometer. the odors of the same artificially induced plant species and Mexican varieties were offered in dual choice experiments to naive mated females of C. marginiventris. Wasps preferred cowpea over maize odor and, in 3 of 12 combinations with the maize varieties, they showed a preference for the odors of one of the varieties, A comparison of the odor collection with results from the behavioral assays indicates that not only the quantity of the volatile emissions, but also the quality (composition) of the volatile blends is important for attraction of C. marginiventris. The results are discussed in the context of the possibility of breeding crop varieties that are particularly attractive to parasitoids.