Host habitat assessment by a parasitoid using fungal volatiles

BACKGROUND: The preference – performance hypothesis predicts that oviposition preference of insects should correlate with host suitability for offspring development. Therefore, insect females have to be able to assess not only the quality of a given host but also the environmental conditions of the respective host habitat. Chemical cues are a major source of information used by insects for this purpose. Primary infestation of stored grain by stored product pests often favors the intense growth of mold. This can lead to distinct sites of extreme environmental conditions (hot-spots) with increased insect mortality. We studied the influence of mold on chemical orientation, host recognition, and fitness of Lariophagus distinguendus, a parasitoid of beetle larvae developing in stored grain. RESULTS: Volatiles of wheat infested by Aspergillus sydowii and A. versicolor repelled female parasitoids in an olfactometer. Foraging L. distinguendus females are known to be strongly attracted to the odor of larval host feces from the granary weevil Sitophilus granarius, which may adhere in remarkable amounts to the surface of the grains. Feces from moldy weevil cultures elicited neutral responses but parasitoids clearly avoided moldy feces when non-moldy feces were offered simultaneously. The common fungal volatile 1-octen-3-ol was the major component of the odor of larval feces from moldy weevil cultures and repelled female parasitoids at naturally occurring doses. In bioassays investigating host recognition behavior of L. distinguendus, females spent less time on grains containing hosts from moldy weevil cultures and showed less drumming and drilling behavior than on non-moldy controls. L. distinguendus had a clearly reduced fitness on hosts from moldy weevil cultures. CONCLUSION: We conclude that L. distinguendus females use 1-octen-3-ol for host habitat assessment to avoid negative fitness consequences due to secondary mold infestation of host patches. The female response to fungal volatiles is innate, suggesting that host-associated fungi played a crucial role in the evolution of host finding strategies of L. distinguendus. Research on the role of host-associated microorganisms in the chemically mediated orientation of parasitoids is still at the beginning. We expect an increasing recognition of this issue in the future

Chemical Ecology of the Parasitoid Wasp Genus Nasonia (Hymenoptera, Pteromalidae)

The use of chemical cues and signals is essential for communication in insects. Wasps of the genus Nasonia (Hymenoptera, Pteromalidae) are gregarious parasitoids that lay their eggs into puparia of cyclorrhaphous flies. During their life cycle, various kinds of semiochemicals are used: (1) a male abdominal sex pheromone that attracts females and induces site fidelity in males, (2) a female-derived contact sex pheromone eliciting courtship behavior in males, (3) an oral male aphrodisiac eliciting receptivity signaling in females and causing a switch in the females' olfactory preferences, (4) chemicals derived from host habitat and host puparia used in olfactory host finding by female wasps, and (5) chemicals used by females to assess the quality and parasitization status of potential hosts. We review the literature on the chemical ecology of Nasonia spp. following the wasps' life cycle from emergence to oviposition. We depict biosynthetic pathways where available, discuss ecological implications, highlight differences among Nasonia species, summarize insights into their olfactory perception and associative learning abilities, and point out gaps in our understanding of the chemical ecology of these parasitoids to be addressed in future studies

Similar Is Not the Same – Mate Recognition in a Parasitoid Wasp

Finding and recognizing a suitable mate is a key prerequisite to reproductive success. Insects often recognize prospective mates using chemical cues and signals. Among these, cuticular lipids commonly serve for mate recognition at close range. The lipid layer on the surface of insects is comprised predominantly of cuticular hydrocarbons (CHCs), though more polar compounds may also be present. While the composition of the cuticular profile is typically species specific, many species additionally show differentiation between the sexes by sex specific compounds and/or compound ratios. It is often assumed that a clear sexual dimorphism of cuticular lipid profiles is a prerequisite for a potential function as sex pheromones. Both, sex specific single compounds or the profiles as a whole have been shown to serve as sex pheromones in parasitoid wasps. Here, we studied parasitoid wasps of the species Tachinaephagus zealandicus (Encyrtidae, Hymenoptera). Chemical analyses revealed that this species presents a case where males and females produce the same set of CHCs in similar relative amounts. To test whether these wasps nonetheless can use the cuticular lipids for close range mate recognition, we tested the reaction of males toward freeze-killed conspecifics. Males showed copulation behavior exclusively toward dead females, but not toward dead males. Dead females from which the cuticular lipids had been removed did not elicit copulation behavior by tested males. Reapplication of female whole body extracts restored bioactivity, and males reacted with copulation attempts as often as toward the freeze-killed females. Bioassays with lipid fractions revealed that only the CHC fraction was bioactive on its own. Here, again, males reacted to female, but not to male CHCs. Our results indicate that these wasps are capable of using CHCs for close range sex recognition despite the similarity of male and female profiles

Cumulative effects of sex pheromone components in mate recognition of Muscidifurax raptorellus

In insects, chemical information is often crucial for mate recognition. The chemical signal may be perceived already from a distance or only at close range, depending on the volatility of the sex pheromone compounds. In many species, close-range mate recognition is mediated by compounds of low volatility that originate from the insects’ cuticular lipid layer. The most prominent constituents are usually non-polar hydrocarbons, but other, more polar compounds can be present as well. Upon detection, these lipids can elicit courtship behaviour. We studied mate recognition in the parasitoid wasp Muscidifurax raptorellus Kogan & Legner (Hymenoptera: Pteromalidae). In this species, courtship behaviour starts with bouts of wing fanning (high frequency wing beats while standing or walking). Once the male has mounted the female, he shows stereotypical antennal movements (antennal courtship) that lead to female receptivity signalling and copulation. We investigated the role of chemical compounds for mate recognition by evaluating the reaction of male M. raptorellus towards live conspecifics and dummies treated with whole-body extracts or with extract fractions (non-polar, intermediately polar, and polar compounds) in bioassays. Our results indicate that despite the sex specificity of cuticular hydrocarbon profiles, these compounds alone are not sufficient for mate recognition. Rather, we found that cuticular lipids of all three fractions seem to constitute the mate recognition signal, with a ternary mixture eliciting wing fanning significantly more often than single fractions. Overall, our results suggest that the three lipid fractions contribute cumulatively to the mate recognition signal

Male sex pheromone in the parasitoid wasp Nasonia longicornis: Chemical and behavioral analyses

The use of sex pheromones for the attraction of potential mating partners is widespread in insects. Species-specificity of these chemical signals is essential, particularly in closely related species with overlapping habitats. In parasitoid wasps of the genus Nasonia, it is the males that produce sex pheromones in their rectal vesicles. The genus consists of four species: N. vitripennis (Nv), N. giraulti (Ng), N. oneida (No), and N. longicornis (Nl). The cosmopolitan species Nv is sympatric with Ng and No in eastern North America and with Nl in the west. Interspecific courtship is common in Nasonia although hybridization is prevented in most combinations by Wolbachia-mediated cytoplasmic incompatibility. The pheromone of Nv males differs from all Nasonia congeners by an additional component that is believed to ensure precopulatory isolation from the sympatric congeners. Detailed investigations on these interactions, however, exist only for the species combination Nv/Ng. Here, we report the results of chemical and behavioral investigations on the sex pheromone of Nl males. The pheromone consists of (4R,5S)-5-hydroxy-4-decanolide (RS) and 4-methylquinazoline (MQ) as a minor component which are produced only after eclosion. Pheromone titers peaked 2 days after eclosion and remained constant on day three. The pheromone is deposited by abdomen dipping which males increasingly exhibited after mating or brief contact with a female. The presence of hosts containing females about to emerge did not increase marking behavior. Site fidelity of males at their own pheromone markings is mediated by MQ. Both natural and synthetic pheromone attracted virgin but not mated females and both RS and MQ are required for female attraction. Females collected during emergence from the host responded likewise to the pheromone and 84% of them produced mixed-sex offspring showing that most of them emerge unmated. Nl females preferred pheromone extract from conspecific males to extract from Nv males, and the addition of the Nv-specific component (4R,5R)-5-hydroxy-4-decanolide (RR) to the Nl pheromone made them unresponsive. The present study demonstrates that Nl uses the male sex pheromone in a similar way as previously shown for Nv and Ng. Furthermore, it shows that Nl females use the Nv-specific pheromone component RR to avoid costly sexual interactions with Nv males in regions of sympatry

The chemical basis of mate recognition in two parasitoid wasp species of the genus Nasonia

To recognize one's mate is essential for all sexually reproducing animals. In insects, mate recognition is often based on chemical cues such as hydrocarbons which are distributed over the insect's cuticle. In the parasitoid wasp genus Nasonia (Hymenoptera: Pteromalidae), interspecific mating possibly occurs in microsympatry between Nasonia vitripennis Walker and Nasonia giraulti Darling despite post-zygotic isolation mechanisms preventing hybridization. Males of N. vitripennis are known to equally court con- and heterospecific females, which they recognize by means of cuticular hydrocarbons. A recent study surprisingly showed that this might not be the case in N. giraulti, leaving open how males of this species achieve the recognition of mating partners. In this study, we investigated chemical mate recognition in N. giraulti in more detail and compared observed behaviors with behaviors of N. vitripennis by conducting experiments with both species concurrently and under the same experimental conditions. We disentangled the role of female-derived non-polar cuticular lipids – i.e., cuticular hydrocarbons – and more polar cuticular lipids in the ability of males to recognize con- and heterospecific females. In addition, we tested whether females of the two species discriminate similarly between con- and heterospecific males. We demonstrate that, in contrast to N. vitripennis, males of N. giraulti prefer live conspecific females over heterospecific ones. Furthermore, in contrast to N. vitripennis, mate recognition in N. giraulti males is not based on cuticular hydrocarbons, but rather involves other chemical messengers, presumably more polar cuticular lipids. In both species, discrimination against heterospecific males decreases with female age

Varying importance of cuticular hydrocarbons and iridoids in the species-specific mate recognition pheromones of three closely related Leptopilina species

Finding a suitable mate for reproduction is one of the most important tasks for almost all animals. In insects this task is often facilitated by pheromone-mediated communication. While insect pheromones in general show enormous chemical diversity, closely related species often use structurally similar compounds in their pheromones. Despite this similarity, pheromones of congeneric species living in sympatry need to be species specific. We investigated the pheromone-mediated mate recognition by males of three closely related species of Leptopilina, a genus of parasitoid wasps that utilize the larvae of Drosophila as hosts. The study species, L. heterotoma, L. boulardi, and L. victoriae, occur sympatrically and have a similar ecology and life history. We have found that mate recognition is species specific in all three species. This species specificity is achieved by a differing importance of cuticular hydrocarbons (CHCs) and iridoids in the female mate recognition pheromones. In L. heterotoma the iridoids are of major importance while CHCs play a negligible role. In L. boulardi, however, the CHCs are as important as the iridoids, while in L. victoriae, the CHCs alone elicit a full behavioral response of males. Our results provide novel insights into pheromone evolution in insects by showing that selection on two completely different classes of chemical compounds may generate conditions where compounds from both classes contribute to a varying degree to the chemical communication of closely related species and that this variation also generates the species specificity of the signals

Sublethal doses of imidacloprid disrupt sexual communication and host finding in a parasitoid wasp

Neonicotinoids are widely used insecticides, but their use is subject of debate because of their detrimental effects on pollinators. Little is known about the effect of neonicotinoids on other beneficial insects such as parasitoid wasps, which serve as natural enemies and are crucial for ecosystem functioning. Here we show that sublethal doses of the neonicotinoid imidacloprid impair sexual communication and host finding in the parasitoid wasp Nasonia vitripennis. Depending on the dose, treated females were less responsive to the male sex pheromone or unable to use it as a cue at all. Courtship behaviour of treated couples was also impeded resulting in a reduction of mating rates by up to 80%. Moreover, treated females were no longer able to locate hosts by using olfactory cues. Olfaction is crucial for the reproductive success of parasitoid wasps. Hence, sublethal doses of neonicotinoids might compromise the function of parasitoid wasps as natural enemies with potentially dire consequences for ecosystem services

Pheromone Diversification and Age-Dependent Behavioural Plasticity Decrease Interspecific Mating Costs in Nasonia

Interspecific mating can cause severe fitness costs due to the fact that hybrids are often non-viable or less fit. Thus, theory predicts the selection of traits that lessen reproductive interactions between closely related sympatric species. Males of the parasitic wasp Nasonia vitripennis differ from all other Nasonia species by an additional sex pheromone component, but the ecological selective forces underlying this pheromone diversification are unknown. Here we present data from lab experiments suggesting that costly interspecific sexual interactions with the sympatric species N. giraulti might have been responsible for the pheromone evolution and some courtship-related behavioural adaptations in N. vitripennis. Most N. giraulti females are inseminated already within the host, but N. giraulti males still invest in costly sex pheromones after emergence. Furthermore, they do not discriminate between N. vitripennis females and conspecifics during courtship. Therefore, N. vitripennis females, most of which emerge as virgins, face the risk of mating with N. giraulti resulting in costly all-male broods due to Wolbachia-induced cytoplasmic incompatibility. As a counter adaptation, young N. vitripennis females discriminate against N. giraulti males using the more complex conspecific sex pheromone and reject most of them during courtship. With increasing age, however, N. vitripennis females become less choosy, but often compensate mating errors by re-mating with a conspecific. By doing so, they can principally avoid suboptimal offspring sex ratios, but a microcosm experiment suggests that under more natural conditions N. vitripennis females cannot completely avoid fitness costs due to heterospecific mating. Our study provides support for the hypothesis that communication interference of closely related sympatric species using similar sexual signals can generate selective pressures that lead to their divergence

Nitric oxide radicals are emitted by wasp eggs to kill mold fungi

Detrimental microbes caused the evolution of a great diversity of antimicrobial defenses in plants and animals. Insects developing underground seem particularly threatened. Here we show that the eggs of a solitary digger wasp, the European beewolf Philanthus triangulum, emit large amounts of gaseous nitric oxide (NO center dot) to protect themselves and their provisions, paralyzed honeybees, against mold fungi. We provide evidence that a NO-synthase (NOS) is involved in the generation of the extraordinary concentrations of nitrogen radicals in brood cells (similar to 1500 ppm NO center dot and its oxidation product NO2 center dot). Sequencing of the beewolf NOS gene revealed no conspicuous differences to related species. However, due to alternative splicing, the NOS-mRNA in beewolf eggs lacks an exon near the regulatory domain. This preventive external application of high doses of NO center dot by wasp eggs represents an evolutionary key innovation that adds a remarkable novel facet to the array of functions of the important biological effector NO center dot