The Egg Parasitoid Trissolcus basalis uses n-nonadecane, a Cuticular Hydrocarbon from its Stink Bug Host Nezara viridula, to Discriminate Between Female and Male Hosts

The Cuticular Hydrocarbons of Dasineura Oleae Show Differences Between Sex, Adult Age and Mating Status

In insects, cuticular lipids prevent water loss and act as semiochemicals. Because of their ecological function, the profile change across the insects' sex and development offers insight into insect biology and possible tools for pest management. Here, the first work on cecidomyiid cuticular extracts is proposed considering Dasineura oleae (Diptera: Cecidomyiidae) males and females at different adult ages (0-12 h, 12-24 h, 24-36 h) and distinct sexual conditions (virgin and mated). A set of 49 compounds were recorded (12 alkanes, 1 monomethyl alkane, 11 fatty acids, 4 esters, 1 aldehyde, 1 allylbenzene, 1 amine, 1 flavonoid, 1 ketone, 1 phenol, 1 steradiene, 1 sterol, 1 terpene, 1 triterpene and 11 unknown compounds), and 18 of them showed significant differences between groups. Among alkanes, hexacosane (nC26) exhibited a decreasing trend from the youngest to the oldest females, while pentacosane (nC25) and nonacosane (nC29) showed a decreasing trend from 0 to 12 h to 12-24 h virgin females. In addition, nonadecane (nC19) was significantly more abundant in the youngest males compared to older males and females. The alkanes nC25, nC26 and nC29 have been reported to be age-related also in other dipterans, while nC19 has been described as gender-specific chemical cue for platygastrid parasitoids. Further behavioural trials and analyses are required to assign the specific ecological roles to the characterized compounds. Our results may contribute to develop new low-impact control strategies relying on the manipulation of D. oleae's chemical communication (e.g. disruption of mating or species recognition). HIGHLIGHTS: • Cuticular hydrocarbons are often involved in dipteran intraspecific communication. • We explored the cuticular profile of D. oleae at different age, sex, mating condition. • Five alkanes and one mono-methyl alkane showed differences among groups. • Linoleic acid is the most abundant compound in virgins, absent in mated insects. • Eleven compounds disappear in mated insects, but were present in all virgins.

Cuticular hydrocarbons of alpine bumble bees (Hymenoptera: Bombus) are species-specific, but show little evidence of elevation-related climate adaptation

Alpine bumble bees are the most important pollinators in temperate mountain ecosystems. Although they are used to encounter small-scale successions of very different climates in the mountains, many species respond sensitively to climatic changes, reflected in spatial range shifts and declining populations worldwide. Cuticular hydrocarbons (CHCs) mediate climate adaptation in some insects. However, whether they predict the elevational niche of bumble bees or their responses to climatic changes remains poorly understood. Here, we used three different approaches to study the role of bumble bees’ CHCs in the context of climate adaptation: using a 1,300 m elevational gradient, we first investigated whether the overall composition of CHCs, and two potentially climate-associated chemical traits (proportion of saturated components, mean chain length) on the cuticle of six bumble bee species were linked to the species’ elevational niches. We then analyzed intraspecific variation in CHCs of Bombus pascuorum along the elevational gradient and tested whether these traits respond to temperature. Finally, we used a field translocation experiment to test whether CHCs of Bombus lucorum workers change, when translocated from the foothill of a cool and wet mountain region to (a) higher elevations, and (b) a warm and dry region. Overall, the six species showed distinctive, species-specific CHC profiles. We found inter- and intraspecific variation in the composition of CHCs and in chemical traits along the elevational gradient, but no link to the elevational distribution of species and individuals. According to our expectations, bumble bees translocated to a warm and dry region tended to express longer CHC chains than bumble bees translocated to cool and wet foothills, which could reflect an acclimatization to regional climate. However, chain lengths did not further decrease systematically along the elevational gradient, suggesting that other factors than temperature also shape chain lengths in CHC profiles. We conclude that in alpine bumble bees, CHC profiles and traits respond at best secondarily to the climate conditions tested in this study. While the functional role of species-specific CHC profiles in bumble bees remains elusive, limited plasticity in this trait could restrict species’ ability to adapt to climatic changes.

Evaluating Chemical Cues Associated with Halyomorpha halys Toward Enhanced Sensitivity of Surveillance for Trissolcus japonicus

In Asia, Trissolcus japonicus (Ashmead) (Hymenoptera: Scelionidae) is the predominant egg parasitoid of the brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae). Detections of adventive T. japonicus populations in North America since 2014, where invasive H. halys populations have impacted various specialty crops, spurred surveillance efforts to track T. japonicus, and yellow sticky cards (YSC) deployed in H. halys host trees have proven effective for this purpose. While T. japonicus exhibits positive behavioral responses to several olfactory stimuli associated with H. halys under laboratory conditions, these have not been evaluated for their potential utility to enhance surveillance of T. japonicus in the field. In northwestern Virginia, where both H. halys and T. japonicus are well-established, we examined the effect of baiting tree of heaven, Ailanthus altissima (Mill.) Swingle (Sapindales: Simaroubaceae), with lures containing the H. halys aggregation pheromone and pheromone synergist on the abundance of H. halys egg masses and captures of T. japonicus in YSC. We also assessed the effect of baiting YSC with newly-laid H. halys egg masses or n-tridecane, a component of H. halys tarsal prints, on T. japonicus captures. Destructive sampling of pheromone-baited and nonbaited trees revealed no significant differences in H. halys egg mass abundance on foliage. Similarly, YSC deployed in pheromone-baited and nonbaited trees showed no significant differences in T. japonicus captures. Moreover, YSC augmented with H. halys egg masses or n-tridecane showed no increase in T. japonicus captures compared with nonbaited controls. The implications for surveillance of adventive T. japonicus are discussed.

Parallel evolutionary paths of rove beetle myrmecophiles: replaying a deep-time tape of life

The rise of ants over the past ~100 million years reshaped the biosphere, presenting ecological challenges for many organisms, but also opportunities. No insect group has been so adept at exploiting niches inside ant colonies as the rove beetles (Staphylinidae) - a global clade of>64,000 predominantly free-living predators from which numerous socially parasitic 'myrmecophile' lineages have emerged. Myrmecophilous staphylinids are specialized for colony life through changes in behavior, chemistry, anatomy, and life history that are often strikingly convergent, and hence potentially adaptive for this symbiotic way of life. Here, we examine how the interplay between ecological pressures and molecular, cellular, and neurobiological mechanisms shape the evolutionary trajectories of symbiotic lineages in this ancient, convergent system.

Attraction of Egg Parasitoids Trissolcus mitsukurii and Trissolcus japonicus to the Chemical Cues of Halyomorpha halys and Nezara viridula

Simple Summary

When an alien species reaches a new environment, the natural enemies present in that habitat might fail to regulate its population as they might not be host-adapted. Hence, the best solution might be the use of alien biological control agents that are co-evolved with the exotic pest in question. This is the case of Halyomorpha halys, which is native to Asia and has recently invaded Europe and the Americas. Trissolcus japonicus and Trissolcus mitsukurii are among its main parasitoids. Adventive populations of the latter were found in Northern Italy, suggesting its employment within augmentative biological control. Homologous programs with T. japonicus are already ongoing in Italy. This procedure implies releasing the parasitoid to increase its population and spread to new areas invaded by H. halys. However, a fundamental aspect that must be investigated is the risk-assessment beforehand, i.e., the systematic process of identifying the hazard associated with such a procedure. In this context, the preference of T. japonicus and T. mitsukurii between two stinkbugs was evaluated in this study. We found that T. japonicus preferred the naturally released traces of H. halys while T. mitsukurii exhibited a higher preference for the natural and synthetic chemical cues of N. viridula.

Abstract

Trissolcus mitsukurii and Trissolcus japonicus are two Asian egg parasitoids associated with different pentatomids such as Halyomorpha halys. Adventive populations of T. mitsukurii were found in Northern Italy, suggesting its employment as a biological control agent (BCA) against H. halys. Nevertheless, to reduce the latter’s population, T. japonicus was released in Italy. Releasing an exotic parasitoid requires investigating the interaction between the BCA and the environment to avoid negative impacts on the entomofauna of the new habitat. Trissolcus mitsukurii is mainly associated with Nezara viridula in its native area. Therefore, we investigated and compared the ability of female T. mitsukurii and T. japonicus to distinguish between naturally released cues of H. halys and N. viridula. A single parasitoid was exposed to contact kairomones of both pests to evaluate its modifications in orthokinetic and locomotory behaviour. The behaviour of female T. mitsukurii was also tested on synthetic compounds simulating the cues of the two pentatomids. When naturally released cues were used, T. japonicus preferred the traces of H. halys, while T. mitsukurii preferred N. viridula’s cues. Moreover, the attraction of T. mitsukurii to N. viridula’s cues was confirmed with synthetic cues. Additional studies are needed to judge if this parasitoid can be used as a BCA.

A Study of Cuticular Hydrocarbons of All Life Stages in Sarcophaga peregrina (Diptera: Sarcophagidae)

Sarcophaga peregrina (Robineau-Desvoidy, 1830), a synanthropic flesh fly species found in different parts of the world, is of medical and forensic importance. Traditional methods of inferring developmental age rely on the life stage of insects and morphological changes. However, once the larvae reach the pupal and adult stage, morphological changes would become barely visible, so that the classic method would be invalid. Here, we studied the cuticular hydrocarbon profile of S. peregrina of the whole life cycle from larval stage to adult stage by GC-MS. Sixty-three compounds with carbon chain length ranging from 8 to 36 were detected, which could be categorized into four classes: n-alkanes, branched alkanes, alkenes, and unknowns. As developmental increased, branched alkanes dominant, and the content of high-molecular-weight hydrocarbons is variable, especially for 2-methyl C19, DiMethyl C21, docosane (C22), and tricosane (C23). This study shows that the composition of CHC could be used to determine the developmental age of S. peregrina and aid in postmortem interval estimations in forensic science.

Stink Bug Communication and Signal Detection in a Plant Environment

Plants influenced the evolution of plant-dwelling stink bugs' systems underlying communication with chemical and substrate-borne vibratory signals. Plant volatiles provides cues that increase attractiveness or interfere with the probability of finding a mate in the field. Mechanical properties of herbaceous hosts and associated plants alter the frequency, amplitude, and temporal characteristics of stink bug species and sex-specific vibratory signals. The specificity of pheromone odor tuning has evolved through highly specific odorant receptors located within the receptor membrane. The narrow-band low-frequency characteristics of the signals produced by abdomen vibration and the frequency tuning of the highly sensitive subgenual organ vibration receptors match with filtering properties of the plants enabling optimized communication. A range of less sensitive mechanoreceptors, tuned to lower vibration frequencies, detect signals produced by other mechanisms used at less species-specific levels of communication in a plant environment. Whereas the encoding of frequency-intensity and temporal parameters of stink bug vibratory signals is relatively well investigated at low levels of processing in the ventral nerve cord, processing of this information and its integration with other modalities at higher neuronal levels still needs research attention.

Cuticular Hydrocarbon Trails Released by Host Larvae Lose their Kairomonal Activity for Parasitoids by Solidification

Successful host search by parasitic wasps is often mediated by host-associated chemical cues. The ectoparasitoid Holepyris sylvanidis is known to follow chemical trails released by host larvae of the confused flour beetle, Tribolium confusum, for short-range host location. Although the hexane-extractable trails consist of stable, long-chain cuticular hydrocarbons (CHCs) with low volatility, the kairomonal activity of a trail is lost two days after release. Here, we studied whether this loss of kairomonal activity is due to changes in the chemical trail composition induced by microbial activity. We chemically analyzed trails consisting of hexane extracts of T. confusum larvae after different time intervals past deposition under sterile and non-sterile conditions. GC-MS analyses revealed that the qualitative and quantitative pattern of the long-chain CHCs of larval trails did not significantly change over time, neither under non-sterile nor sterile conditions. Hence, our results show that the loss of kairomonal activity of host trails is not due to microbially induced changes of the CHC pattern of a trail. Interestingly, the kairomonal activity of trails consisting of host larval CHC extracts was recoverable after two days by applying hexane to them. After hexane evaporation, the parasitoids followed the reactivated host trails as they followed freshly laid ones. Cryo-scanning electron microscopy showed that the trails gradually formed filament-shaped microstructures within two days. This self-assemblage of CHCs was reversible by hexane application. Our study suggests that the long-chain CHCs of a host trail slowly undergo solidification by a self-assembling process, which reduces the accessibility of CHCs to the parasitoid’s receptors as such that the trail is no longer eliciting trail-following behavior.

Exploring the Kairomone-Based Foraging Behaviour of Natural Enemies to Enhance Biological Control: A Review

Kairomones are chemical signals that mediate interspecific interactions beneficial to organisms that detect the cues. These attractants can be individual compounds or mixtures of herbivore-induced plant volatiles (HIPVs) or herbivore chemicals such as pheromones, i.e., chemicals mediating intraspecific communication between herbivores. Natural enemies eavesdrop on kairomones during their foraging behaviour, i.e., location of oviposition sites and feeding resources in nature. Kairomone mixtures are likely to elicit stronger olfactory responses in natural enemies than single kairomones. Kairomone-based lures are used to enhance biological control strategies via the attraction and retention of natural enemies to reduce insect pest populations and crop damage in an environmentally friendly way. In this review, we focus on ways to improve the efficiency of kairomone use in crop fields. First, we highlight kairomone sources in tri-trophic systems and discuss how these attractants are used by natural enemies searching for hosts or prey. Then we summarise examples of field application of kairomones (pheromones vs. HIPVs) in recruiting natural enemies. We highlight the need for future field studies to focus on the application of kairomone blends rather than single kairomones which currently dominate the literature on field attractants for natural enemies. We further discuss ways for improving kairomone use through attract and reward technique, olfactory associative learning, and optimisation of kairomone lure formulations. Finally, we discuss why the effectiveness of kairomone use for enhancing biological control strategies should move from demonstration of increase in the number of attracted natural enemies, to reducing pest populations and crop damage below economic threshold levels and increasing crop yield.

Only Females Oviposit: Chemical Discrimination of Adult Stink Bug Sex by the Egg Parasitoid Trissolcus japonicus

Egg parasitoids foraging for suitable hosts scattered in the environment rely mainly on chemical cues. Elucidating the chemical ecology of natural enemies is important in the development of effective and successful strategies for conservation biological control. In this context, the host cuticular hydrocarbons, which are exploited by several species of egg parasitoids as contact kairomones, could be used to retain them by providing information about the presence and the sex of adults of the target species: sex is important because only females of the host species lay the eggs that can be subsequently utilized for parasitoid reproduction. However, the chemical basis of host sex discrimination in egg parasitoids is not well understood. We carried out behavioral and chemical bioassays to investigate the role played by contact chemical cues left by adults of the brown marmorated stink bug, Halyomorpha halys Stål, in host egg searching behavior and adult host sex discrimination by the egg parasitoid Trissolcus japonicus (Ashmead). A first set of bioassays showed that parasitoids spent more time exploring patches contaminated with chemicals associated with adult H. halys females compared with adult males. Similar responses were displayed by T. japonicus when hexane extracts of H. halys were tested suggesting that non-polar chemical compounds are involved in host sex discrimination. GC-MS analysis of hexane extracts revealed quantitative differences in the cuticular compounds of the two sexes, with 1-hexadecene (more abundant in males) being the most important component in determining these differences. Hexane extracts of H. halys females blended with synthetic 1-hexadecene significantly reduced the wasps’ arrestment responses compared to crude extracts.

Chemical Ecology, Biochemistry, and Molecular Biology of Insect Hydrocarbons

Insect cuticular hydrocarbons (CHCs) consist of complex mixtures of straight-chain alkanes and alkenes, and methyl-branched hydrocarbons. In addition to restricting water loss through the cuticle and preventing desiccation, they have secondarily evolved to serve a variety of functions in chemical communication and play critical roles as signals mediating the life histories of insects. In this review, we describe the physical properties of CHCs that allow for both waterproofing and signaling functions, summarize their roles as inter- and intraspecific chemical signals, and discuss the influences of diet and environment on CHC profiles. We also present advances in our understanding of hydrocarbon biosynthesis. Hydrocarbons are biosynthesized in oenocytes and transported to the cuticle by lipophorin proteins. Recent work on the synthesis of fatty acids and their ultimate reductive decarbonylation to hydrocarbons has taken advantage of powerful new tools of molecular biology, including genomics and RNA interference knockdown of specific genes, to provide new insights into the biosynthesis of hydrocarbons.

Identification and expression of chemosensory receptor genes in the egg parasitoid Trissolcus basalis

Parasitic wasps largely rely on chemosenses to locate resources. Understanding the evolution of their chemoreceptors can help elucidate the mechanisms underlying host adaptation and speciation. Trissolcus basalis is a biological control agent of the southern green stink bug, a pantropical pest, and is ideal for investigating the evolution of chemoreceptors. We identified 34 gustatory receptors, 170 odorant receptors, one odorant co-receptor, and 23 ionotropic receptors. Comparison with other Hymenoptera revealed species-specific expansions of 21 Grs and 53 Ors. Most of these Or expansions have 9 exons. Gender- and tissue-specific analyses showed that 5 Grs and 54 Ors are expressed only in antennae in both sexes, 66 Ors in female antennae only, and 4 Ors in male antennae alone. The identification and expression profile of chemosensory receptor genes in T. basalis helps in understanding the link between the evolution of chemoreceptors and speciation in parasitic wasps.

The Importance of Methyl-Branched Cuticular Hydrocarbons for Successful Host Recognition by the Larval Ectoparasitoid Holepyris sylvanidis

Cuticular hydrocarbons (CHCs) of host insects are used by many parasitic wasps as contact kairomones for host location and recognition. As the chemical composition of CHCs varies from species to species, the CHC pattern represents a reliable indicator for parasitoids to discriminate host from non-host species. Holepyris sylvanidis is an ectoparasitoid of beetle larvae infesting stored products. Previous studies demonstrated that the larval CHC profile of the confused flour beetle, Tribolium confusum, comprises long chain linear and methyl-branched alkanes (methyl alkanes), which elicit trail following and host recognition in H. sylvanidis. Here we addressed the question, whether different behavioral responses of this parasitoid species to larvae of other beetle species are due to differences in the larval CHC pattern. Our study revealed that H. sylvanidis recognizes and accepts larvae of T. confusum, T. castaneum and T. destructor as hosts, whereas larvae of Oryzaephilus surinamensis were rejected. However, the latter species became attractive after applying a sample of T. confusum larval CHCs to solvent extracted larvae. Chemical analyses of the larval extracts revealed that CHC profiles of the Tribolium species were similar in their composition, while that of O. surinamensis differed qualitatively and quantitatively, i.e. methyl alkanes were present as minor components on the cuticle of all Tribolium larvae, but were absent in the O. surinamensis CHC profile. Furthermore, the parasitoid successfully recognized solvent extracted T. confusum larvae as hosts after they had been treated with a fraction of methyl alkanes. Our results show that methyl alkanes are needed for host recognition by H. sylvanidis.

Kairomones from Euschistus heros egg masses and their potential use for Telenomus podisi parasitism improvement

Telenomus podisi Ashmead (Hymenoptera: Scelionidae) is the most important egg parasitoid of Euschistus heros (Fabricius) (Hemiptera: Pentatomidae), and its successful parasitism is related to their searching ability to find suitable hosts under a complex chemical environment using host-reliable cues. Thus, the objective of this study was to elucidate chemical substances on the external layer of E. heros eggs and report its potential kairomonal activity on T. podisi. We tested female wasps in olfactometer system to synthetic compounds obtained from a chemical identification of E. heros egg masses. The synthetic blend was also evaluated in parasitism tests under laboratory and semi-field conditions. We identified 31 substances from egg surface extracts, including monoterpenes, aldehydes and alkanes. Among those compounds, a synthetic solution including camphene, β-pinene, limonene and benzaldehyde-induced chemotaxic behaviour on the wasps and increased the parasitism on E. heros eggs, either in laboratory or semi-field test, suggesting its potential use to T. podisi manipulation and parasitism improvement.

Isolation and identification of attractants from the pupae of three lepidopteran species for the parasitoid Chouioia cunea Yang

BACKGROUND

Chouioia cunea Yang (Hymenoptera: Eulophidae) is a parasitic wasp and natural enemy of several lepidopteran pests during their pupal stage. The volatiles from pupae of three hosts, Hyphantria cunea (Arctiidae), Antheraea pernyi (Saturniidae) and Lymantria dispar (Erebidae), were analyzed and compared to elucidate the chemical cues used by C. cunea to locate its hosts.

RESULTS

The attraction of C. cunea to H. cunea pupae has no obvious association with the types of plant leaves consumed by H. cunea before pupation. C. cunea exhibited the strongest attraction to the pupae of H. cunea, followed by those of A. pernyi and L. dispar based on behavioral experiments. Gas chromatography-mass spectrometry and GC-electroantennography (GC-EAD) analyses showed that these three host pupae consisted of essentially the same active volatile components but at different relative amounts. Active components derived from these pupae by GC-EAD were alkanes from C12 to C27, and C. cunea showed different levels of attraction to different single compounds.

CONCLUSION

Host location by C. cunea primarily depends on common compounds emanating from the pupae of several host species. The relative amount of each component varies across host species, guiding host preferences by C. cunea. Optimal blends of several components were identified. Understanding the chemical cues used by C. cunea to locate its host could increase the possibility of developing attractants for parasitic wasps and subsequently increasing the parasitism rate of C. cunea on various hosts. © 2019 Society of Chemical Industry.

Foraging behavior of two egg parasitoids exploiting chemical cues from the stink bug Piezodorus guildinii (Hemiptera: Pentatomidae)

Several parasitoids attacking the same host may lead to competition. Adult parasitoids' abilities to find, parasitize and defend hosts determine resource's retention potential. In soybean, two egg parasitoid species, Telenomus podisi and Trissolcus urichi (Hymenoptera: Platygastridae), compete on the egg masses of Piezodorus guildinii (Hemiptera: Pentatomidae) one of the major pest of this crop. We evaluated parasitoid's abilities to exploit hosts' footprints; and parasitoid's behavior when competing for the same host. Both arena residence time and retention time were similar for T. podisi and T. urichi on male or female host footprints. In its turn, T. urichi reentered the area contaminated with P. guildinii more times and staid longer in it than T. podisi. Furthermore, when competing for the same egg mass, each parasitoid species won (was in possession of the host by the end of the experiment) half of the replicates, and the number of times each wasp species contacted host in the first place was similar, without affecting replicate outcome (who ultimately won). Both species started agonistic and non-agonistic encounters. This study provides information about the potential interspecific competition between these parasitoids, which contributes to evaluate the compatibility of multiple natural enemies' biological control programs for stink bugs.

Hydrocarbons catalysed by TmCYP4G122 and TmCYP4G123 in Tenebrio molitor modulate the olfactory response of the parasitoid Scleroderma guani

Hydrocarbons (HCs) present on the epicuticle of terrestrial insects are not only used to reduce water loss but are also used as chemical signals. The cytochrome p450 CYP4G gene is essential for HC biosynthesis in some insects. However, its function in Tenebrio molitor is unknown. Moreover, it is not yet known whether CYP4G of a host can modulate the searching behaviours of its parasitoid. Here, we explore the function of the TmCYP4G122 and CYP4G123 genes in T. molitor. The TmCYP4G122 and CYP4G123 transcripts could be detected in all developmental stages. Their expression was higher in the fat body and abdominal cuticle than in the gut. Their transcript levels in mature larvae under desiccation stress [relative humidity (RH) < 5%] was significantly higher than that in the control (RH = 70%). Injection of dsCYP4G122 and dsCYP4G123 caused a reduction in HC biosynthesis and was associated with increased susceptibility to desiccation. Individuals of the parasitoid Scleroderma guani that emerged from mealworm pupae showed host preference for normal pupae whereas S. guani that emerged from pupae lacking CYP4G122 or/and CYP4G123 lost this searching preference. The current results confirm that CYP4G122 and CYP4G123 regulate the biosynthesis of HCs and modulate the olfactory response of its parasitoid S. guani.

Mating Status of an Herbivorous Stink Bug Female Affects the Emission of Oviposition-Induced Plant Volatiles Exploited by an Egg Parasitoid

Insect parasitoids are under selection pressure to optimize their host location strategy in order to maximize fitness. In parasitoid species that develop on host eggs, one of these strategies consists in the exploitation of oviposition-induced plant volatiles (OIPVs), specific blends of volatile organic compounds released by plants in response to egg deposition by herbivorous insects. Plants can recognize insect oviposition via elicitors that trigger OIPVs, but very few elicitors have been characterized so far. In particular, the source and the nature of the elicitor responsible of egg parasitoid recruitment in the case of plants induced with oviposition by stink bugs are still unknown. In this paper, we conducted behavioral and molecular investigations to localize the source of the elicitor that attracts egg parasitoids and elucidate the role of host mating in elicitation of plant responses. We used as organism study model a tritrophic system consisting of the egg parasitoid Trissolcus basalis, the stink bug host Nezara viridula and the plant Vicia faba. We found that egg parasitoid attraction to plant volatiles is triggered by extracts coming from the dilated portion of the stink bug spermathecal complex. However, attraction only occurs if extracts are obtained from mated females but not from virgin ones. Egg parasitoid attraction was not observed when extracts coming from the accessory glands (mesadene and ectadene) of male hosts were applied, either alone or in combination to plants. SDS-PAGE electrophoresis correlated with olfactometer observations as the protein profile of the dilated portion of the spermathecal complex was affected by the stink bug mating status suggesting post-copulatory physiological changes in this reproductive structure. This study contributed to better understanding the host location process by egg parasitoids and laid the basis for the chemical characterization of the elicitor responsible for OIPV emission.

The relationship between epicuticular long-chained hydrocarbons and surface area - volume ratios in insects (Diptera, Hymenoptera, Lepidoptera)

Long-chain cuticular hydrocarbons (CHCs) are common components of the epicuticle of terrestrial arthropods. CHC serve as a protective barrier against environmental influences but also act as semiochemicals in animal communication. Regarding the latter aspect, species- or intra-functional group specific CHCs composition and variation are relatively well studied. However, comparative knowledge about the relationship of CHC quantity and their relation to surface area—volume ratios in the context of water loss and protection is fragmentary. Hence, we aim to study the taxon-specific relationship of the CHC amount and surface-area to volume ratio related to their functional role (e.g. in water loss). We focused on flower visiting insects and analyzed the CHC amounts of three insect orders (Hymenoptera, Lepidoptera and Diptera) using gas chromatography—mass spectrometry (GC-MS). We included 113 species from two grassland plots, quantified their CHCs, and measured their body mass and surface area. We found differences in the surface area, CHCs per body mass and the CHC density (= amount of CHCs per surface area) across the three insect taxa. Especially the Hymenoptera had a higher CHC density compared to Diptera and Lepidoptera. CHC density could be explained by surface area-volume ratios in Hymenoptera but not in Diptera and Lepidoptera. Unexpectedly, CHC density decreased with increasing surface area—volume ratios.

The role of contact chemoreception in the host location process of an egg parasitoid

Taste allows insects to detect palatable or toxic foods, identify a mate, and select appropriate oviposition sites. The gustatory system strongly contributes to the survival and reproductive success of many species, yet it is rarely studied in insect parasitoids. In order to locate and assess a host in which they will lay their eggs, female wasps actively search for chemical cues using their sensory organs present mainly on the antennae. In this paper, we studied the role of antennal taste sensilla chaetica in the perception of contact semiochemicals in Trissolcus brochymenae (Hymenoptera: Platygastridae), an egg parasitoid of the brassicaceae pest Murgantia histrionica (Heteroptera: Pentatomidae). Methanolic extracts obtained from male and female hosts elicited action potentials in taste neurons housed in antennal sensilla chaetica, indicating that these sensilla are involved in the perception of non volatile host kairomones. In behavioural assays, wasp females displayed an intense searching behaviour in open arenas treated with host extracts, thus confirming that these kairomones are soluble in polar solvents. We further investigated the extracts by Gas Chromatography-Mass Spectrometry (GC-MS) and found that they contain several compounds which are good candidates for these contact kairomones. This study contributes to better understanding contact chemoreception in egg parasitoids and identifying gustatory receptor neurons involved in the host location process.

The response of an egg parasitoid to substrate-borne semiochemicals is affected by previous experience

Animals can adjust their behaviour according to previous experience gained during foraging. In parasitoids, experience plays a key role in host location, a hierarchical process in which air-borne and substrate-borne semiochemicals are used to find hosts. In nature, chemical traces deposited by herbivore hosts when walking on the plant are adsorbed by leaf surfaces and perceived as substrate-borne semiochemicals by parasitoids. Chemical traces left on cabbage leaves by adults of the harlequin bug (Murgantia histrionica) induce an innate arrestment response in the egg parasitoid Trissolcus brochymenae characterized by an intense searching behaviour on host-contaminated areas. Here we investigated whether the T. brochymenae response to host walking traces left on leaf surfaces is affected by previous experience in the context of parasitoid foraging behaviour. We found that: 1) an unrewarded experience (successive encounters with host-contaminated areas without successful oviposition) decreased the intensity of the parasitoid response; 2) a rewarded experience (successful oviposition) acted as a reinforcing stimulus; 3) the elapsed time between two consecutive unrewarded events affected the parasitoid response in a host-gender specific manner. The ecological role of these results to the host location process of egg parasitoids is discussed.

Impact of the Egg Parasitoid, Gryon pennsylvanicum (Hymenoptera: Scelionidae), on Sentinel and Wild Egg Masses of the Squash Bug (Hemiptera: Coreidae) in Maryland

Seasonal changes in egg parasitism and predation rates on sentinel (laboratory-reared) and wild (naturally occurring) egg masses of the squash bug, Anasa tristis (De Geer), were evaluated in squash fields in Maryland from June through September in 2013 and 2014. Rates of egg parasitism were significantly higher on wild egg masses than on sentinel egg masses. Squash bug nymphal emergence was significantly higher on sentinel egg masses than on wild egg masses. Between the first week of July and the first week of September of both survey years, squash bug nymphs emerged from 24.2% of wild eggs compared with 46.2% of sentinel eggs and parasitoids emerged from 55.7% of wild eggs compared with only 21.8% of sentinel eggs. Sentinel egg masses significantly underestimated the rate of natural egg parasitism. The egg parasitoid, Gryon pennsylvanicum (Ashmead), was responsible for over 99% of parasitism of squash bug eggs. There was a significant negative correlation between parasitoid emergence and nymphal emergence, suggesting that parasitoids were able to suppress squash bug populations. The average rate of parasitoid emergence peaked on wild egg masses on the fifth week of July at 72.8%, whereas the average rate of nymphal emergence from wild egg masses was <20% from the fifth week of July until the first week of September. These results demonstrate that G. pennsylvanicum was able to efficiently track wild squash bug eggs throughout the season and that it has the potential to be an effective biological control agent of the squash bug in Maryland.

Development of high throughput 96-blade solid phase microextraction-liquid chromatrography-mass spectrometry protocol for metabolomics

In metabolomics, the workflow for quantitative and comprehensive metabolic mapping of cellular metabolites can be a very challenging undertaking. Sampling and sample preparation play a significant role in untargeted analysis, as they may affect the composition of the analyzed metabolome. In the current work, different solid phase microextraction (SPME) coating chemistries were developed and applied to provide simultaneous extraction of a wide range of both hydrophobic and hydrophilic cellular metabolites produced by a model organism, Escherichia coli. Three different LC-MS methods were also evaluated for analysis of extracted metabolites. Finally, over 200 cellular metabolites were separated and detected with widely varying hydrophobicities ranging within -7 < log P < 15, including amino acids, peptides, nucleotides, carbohydrates, polycarboxylic acids, vitamins, phosphorylated compounds, and lipids such as hydrophobic phospholipids, prenol lipids, and fatty acids at the stationary phase of the E. coli life cycle using the developed 96-blade SPME-LC-MS method.

Host chemical footprints induce host sex discrimination ability in egg parasitoids

Trissolcus egg parasitoids, when perceiving the chemical footprints left on a substrate by pentatomid host bugs, adopt a motivated searching behaviour characterized by longer searching time on patches were signals are present. Once in contact with host chemical footprints, Trissolcus wasps search longer on traces left by associated hosts rather than non-associated species, and, in the former case, they search longer on traces left by females than males. Based on these evidences, we hypothesized that only associated hosts induce the ability to discriminate host sex in wasps. To test this hypothesis we investigated the ability of Trissolcus basalis, T. brochymenae, and Trissolcus sp. to distinguish female from male Nezara viridula, Murgantia histrionica, and Graphosoma semipunctatum footprints. These three pentatomid bugs were selected according to variable association levels. Bioassays were conducted on filter paper sheets, and on Brassica oleracea (broccoli) leaves. The results confirmed our hypothesis showing that wasps spent significantly more time on female rather than male traces left by associated hosts on both substrates. No differences were observed in the presence of traces left by non-associated hosts. The ecological consequences for parasitoid host location behaviour are discussed.

Zingiberenol, (1R,4R,1′S)‑4-(1′,5′-Dimethylhex-4′-enyl)-1-methylcyclohex-2-en-1-ol, identified as the sex pheromone produced by males of the rice stink bug Oebalus poecilus (heteroptera: pentatomidae)

Bioassays using an olfactometer showed that Oebalus poecilus males produce the sexual pheromone, and the chemical analysis demonstrated that this compound is zingiberenol. Two groups of isomers, each containing four diastereoisomers, (1RS,4RS,1'S)- and (1RS,4RS,1'R)-zingiberenol, were prepared. These diastereoisomers were not separated on a chiral GC column. Therefore, to determine the absolute configuration of the carbon 1, 4, and 1' of zingiberenol produced by males, the following strategies were conducted. The extract containing males volatiles was submitted to dehydration microchemistry to produce zingiberene, in which the isomers are separated by chiral GC analysis, and by comparison with the natural zingiberene from ginger oil, the absolute stereochemistry of the carbons 4 and 1' was determined to be R and S, respectively, and the carbon 1 was determined as R from the (13)C NMR spectra of quercivorol. Finally, the bioassays showed that O. poecilus females responded to racemic mixture and to (1RS,4RS,1'S)-zingiberenol.

Major changes in the sex differences in cuticular chemical profiles of the western conifer seed bug (Leptoglossus occidentalis) after laboratory rearing

Chemical compounds covering the insect cuticle have several functions ranging from protection against water loss to inter- and intra-specific communication. Their composition is determined by several intrinsic and extrinsic factors. Among these factors, laboratory rearing has been poorly investigated even though it has a strong potential for biasing behavioral experiments. We selected an invasive species with unknown cuticular mixtures as a model. Our aim was to describe its mixtures and to determine if highly simplified laboratory rearing conditions interact with sexual signatures. We analyzed the cuticle by means of two different techniques - gas chromatography/mass spectrometry (GC/MS) and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) - to obtain data on a wide range of compounds with different molecular weight. We found that both sets of chemicals showed correlations with gender but also that cuticular waxes detected by GC/MS were highly dependent on rearing conditions, with a strong bias in sexual dimorphism. Conversely, the heavier signatures detected by MALDI-TOF showed a less clear diversification between sexes, although the discrimination power was unaffected by rearing conditions. The biological and practical implications of our findings are discussed.

Deciphering the signature of cuticular lipids with contact sex pheromone function in a parasitic wasp

The surface of insects is covered by a complex mixture of cuticular hydrocarbons (CHCs) to prevent desiccation. In many species these lipids also have communicative functions, but often it is unknown which components are crucial for the behavioural response. Furthermore, it is often ignored that polar lipids also occur on the insects' cuticle and might interact with CHCs. In the parasitic wasp Lariophagus distinguendus, CHCs function as a contact sex pheromone eliciting wing-fanning in males. Interestingly, not only females but also newly emerged males have the pheromone, resulting regularly in homosexual courtship. However, males deactivate the pheromone within the first two days after emergence. This deactivation is accompanied by the disappearance of 3-methylheptacosane (3-MeC27) and some minor components from the CHC profile of males. Here we show that 3-MeC27 is a key component of the contact sex pheromone which, however, triggers courtship behaviour only if an olfactory background of other cuticular lipids is present. Males responded to (S)-3-MeC27 enantioselectively when applied to filter paper but on three-dimensional dummies both enantiomers were behaviourally active, suggesting that physical stimuli also play a role in sexual communication of the wasps. Finally, we report that triacylglycerides (TAGs) are also essential components of the pheromone, and present evidence that TAGs actually occur on the cuticle of L. distinguendus. Our data provide novel insights into the semiochemical function of cuticular lipids by showing that the bioactivity of CHCs may be influenced by the stereochemistry and a synergetic interaction with long time ignored TAGs.

Insect egg deposition induces indirect defense and epicuticular wax changes in Arabidopsis thaliana

Egg deposition by the Large Cabbage White butterfly Pieris brassicae on Brussels sprouts plants induces indirect defense by changing the leaf surface, which arrests the egg parasitoid Trichogramma brassicae. Previous studies revealed that this indirect defense response is elicited by benzyl cyanide (BC), which is present in the female accessory reproductive gland (ARG) secretion and is released to the leaf during egg deposition. Here, we aimed (1) to elucidate whether P. brassicae eggs induce parasitoid-arresting leaf surface changes in another Brassicacean plant, i.e., Arabidopsis thaliana, and, if so, (2) to chemically characterize the egg-induced leaf surface changes. Egg deposition by P. brassicae on A. thaliana leaves had similar effects to egg deposition on Brussels sprouts with respect to the following: (a) Egg deposition induced leaf surface changes that arrested T. brassicae egg parasitoids. (b) Application of ARG secretion of mated female butterflies or of BC to leaves had the same inductive effects as egg deposition. Based on these results, we conducted GC-MS analysis of leaf surface compounds from egg- or ARG-induced A. thaliana leaves. We found significant quantitative differences in epicuticular waxes compared to control leaves. A discriminant analysis separated surface extracts of egg-laden, ARG-treated, untreated control and Ringer solution-treated control leaves according to their quantitative chemical composition. Quantities of the fatty acid tetratriacontanoic acid (C34) were significantly higher in extracts of leaf surfaces arresting the parasitoids (egg-laden or ARG-treated) than in respective controls. In contrast, the level of tetracosanoic acid (C24) was lower in extracts of egg-laden leaves compared to controls. Our study shows that insect egg deposition on a plant can significantly affect the quantitative leaf epicuticular wax composition. The ecological relevance of this finding is discussed with respect to its impact on the behavior of egg parasitoids.

Chemical ecology and pollinator-driven speciation in sexually deceptive orchids

Sexually deceptive orchids mimic females of their pollinator species to attract male insects for pollination. Pollination by sexual deception has independently evolved in European, Australian, South African, and South American orchid taxa. Reproductive isolation is mainly based on pre-mating isolation barriers, the specific attraction of males of a single pollinator species, mostly bees, by mimicking the female species-specific sex-pheromone. However, in rare cases post-mating barriers have been found. Sexually deceptive orchids are ideal candidates for studies of sympatric speciation, because key adaptive traits such as the pollinator-attracting scent are associated with their reproductive success and with pre-mating isolation. During the last two decades several investigations studied processes of ecological speciation in sexually deceptive orchids of Europe and Australia. Using various methods like behavioural experiments, chemical, electrophysiological, and population-genetic analyses it was shown that minor changes in floral odour bouquets might be the driving force for pollinator shifts and speciation events. New pollinators act as an isolation barrier towards other sympatrically occurring species. Hybridization occurs because of similar odour bouquets of species and the overlap of flowering periods. Hybrid speciation can also lead to the displacement of species by the hybrid population, if its reproductive success is higher than that in the parental species.

Plant surfaces of vegetable crops mediate interactions between chemical footprints of true bugs and their egg parasitoids

During the host location process, egg parasitoids can eavesdrop on chemical cues released from immature and adult hosts. These indirect host-related cues are highly detectable, but of low reliability because they lead egg parasitoid females to an area where oviposition is likely to occur rather then providing wasps with direct information on the presence of eggs and their location. In the host-parasitoid associations between true bugs and their scelionid egg parasitoids, female wasps perceive the chemical residues left by host adults walking on substrates as contact kairomones, displaying a characteristic arrestment posture. In this study, we demonstrated that epicuticular waxes of leaves of two vegetable crops, broad bean, Vicia faba and collard greens, Brassica oleracea, mediate the foraging behaviour of Trissolcus basalis (Wollaston) by adsorbing contact kairomones from adults of Nezara viridula (L.). Trissolcus basalis females showed no response when released on the adaxial leaf surface of broad bean or collard green plants with intact cuticular wax layers that had not been exposed to bugs, whereas wasps displayed the arrestment posture when intact leaves were contaminated by chemical residues from host females. Adaxial leaf surfaces that were dewaxed with an aqueous solution of gum arabic and afterwards contaminated by N. viridula females elicited no arrestment responses from wasp females. Similarly, leaves contaminated by host females and subsequently dewaxed did not elicit responses from female wasps. These findings reveal the important role of plant waxes in N. viridula-T. basalis semiochemical communication.

Behavioral and chemical investigations of contact kairomones released by the mud dauber wasp Trypoxylon politum, a host of the parasitoid Melittobia digitata

Contact kairomones from the host mud dauber wasp Trypoxylon politum Say (Hymenoptera: Crabronidae) that mediate behavioral responses of its ectoparasitoid Melittobia digitata Dahms (Hymenoptera: Eulophidae) were investigated. Chemical residues from host by-products, the cocoon, and the meconium, induced arrestment behavior of macropterous female parasitoids, while those from the host stage attacked, i.e., the prepupa, did not. Melittobia digitata response to polar and apolar extracts of host by-products indicated kairomone(s) solubility mainly in hexane. GC and GC/MS analysis of cocoon and meconium apolar extracts revealed a mixture of linear carboxylic acids from C(6) to C(18), and both extracts contained almost identical compounds. When a reconstructed blend of host by-product carboxylic acids was tested, M. digitata females showed only a weak response, thus suggesting that other unidentified compounds present in small quantities also may be involved. Melittobia digitata's response to contact kairomones was innate and not affected by previous host exposure experience. Our results provide evidence of contact kairomone exploitation in the genus Melittobia. The ecological significance of these findings in the host selection process of M. digitata is discussed.

Parasitoids modify their oviposition behavior according to the sexual origin of conspecific cuticular hydrocarbon traces

Hydrocarbons play a crucial role in insect behavior in general and in sexual recognition in particular. Parasitoids often modify their oviposition behavior according to hydrocarbons left by conspecifics on the reproductive patch, such as oviposition markers left by females after oviposition, or cuticular hydrocarbon (CHC) traces left by individuals by walking or rubbing. This study determined whether Eupelmus vuilleti females are able to distinguish CHCs left by male or female conspecifics on seeds. The results show that the cuticular profile of E. vuilleti differs according to its gender, and that females are able to detect the sexual origin of these CHCs. Moreover, they adjust their oviposition behavior according to the nature of these traces. Although females lay fewer eggs on hosts when confronted with female CHCs, they lay more daughters when confronted with male CHCs, thus changing the sex ratio.

Modifications of the chemical profile of hosts after parasitism allow parasitoid females to assess the time elapsed since the first attack

In solitary parasitoids, only one adult can emerge from a given host. In some of these species, when several eggs are laid on the same host, supernumerary individuals are eliminated by lethal larval fights. In the solitary parasitoid Anisopteromalus calandrae, the probability of a second larva winning the fight depends on the time elapsed since the first oviposition. The older the first egg is at the moment a second egg is laid, the less chance the second egg has of winning the competition. As a consequence, females of this species lay their eggs preferentially on recently parasitized hosts rather than on hosts parasitized by an egg about to hatch. Anisopteromalus calandrae females parasitize bruchid larvae located in cowpea seeds. In a series of choice test experiments using an artificial seed system, we demonstrated that the cue that allows parasitoid females to differentiate between hosts parasitized for different lengths of time comes from the host and not from the artificial seed or the previously laid egg. This cue is perceived at short range, indicating that the chemicals involved are probably partly volatile. Interestingly, although parasitism stops host development, cuticular profiles continue to evolve, but in a different way from those of unparasitized hosts. This difference in the host's cuticular profile after parasitism, therefore, probably underlies the parasitoid female's discrimination.

Parasitoids use chemical footprints to track down caterpillars

Parasitoid wasps in search for plant-feeding hosts display typical patterns of behavior. The braconid Cotesia marginiventris, which parasitizes young caterpillars, is guided by herbivore-induced plant volatiles to an infested plant. On the plant, the female wasp searches for further chemical residues (kairomones) originating directly from the host. We showed that caterpillars leave minute amounts of treacherous chemical footprints while walking over a plant surface. Female wasps are able to detect these residues for up to two days after their hosts have left the site. Analyses of the caterpillar footprints revealed that these consisted of linear and monomethyl-branched alkanes as well as few minor unidentified compounds. A reconstructed blend of the major footprint compounds, consisting of linear C(21)-C(32) alkanes, induced characteristic antennation behavior. However, the artificial blend was less attractive than the original one suggesting a role for additional minor compounds in recognizing former caterpillar presence. Previous investigations using wax mutants of barley showed that the physico-chemical traits of the epicuticular leaf wax can modulate the parasitoids' response to host footprints. We hypothesize that long-chain hydrocarbons of insect and plant cuticular origin are important mediators of insect-plant interactions and believe that their role in modulating trophic cascades still awaits full appreciation.

A finely tuned strategy adopted by an egg parasitoid to exploit chemical traces from host adults

Scelionid egg parasitoids can obtain reliable information on the presence of host eggs by discriminating host gender on the basis of chemical footprints of their co-evolved hosts, with a strong preference for the footprint left by host females. Based on the concept of dietary specialization and infochemical use in natural enemies, it could be predicted that host gender discrimination in specialist species belonging to the genus Trissolcus is further tuned to specific cues from distinctive chemical traces left by host females as a consequence of copulation and/or oviposition. To test this hypothesis we used the system Murgantia histrionica – Trissolcus brochymenae. Our results showed that the females of the egg parasitoid search intensely on chemical traces left on the substrate by host females that had mated but had not yet laid host eggs compared with the chemical traces left by virgin or parous host females. This preference for mated females that had not yet laid host eggs was strictly related to the transfer of sperm and associated substances from males to females during copulation. The compounds that mediated the arrestment response of T. brochymenae females are part of the host cuticle, and those that play a role as gender-specific cues seemed to be present in the legs of the host adult. This result represents an interesting new piece of information regarding the exploitation of indirect host-related cues by egg parasitoids. It reveals the existence of a finely tuned strategy that allows the parasitoid to find newly laid host eggs, as chemical traces left by mated host females that have not yet laid eggs are strongly correlated with the moment of oviposition.

Caterpillar footprints as host location kairomones for Cotesia marginiventris: persistence and chemical nature

Herbivores walking over the epicuticular wax layer of a plant may leave tracks that disclose their presence to hunting predators or parasitoids. The braconid wasp Cotesia marginiventris is a solitary parasitoid of young noctuid caterpillars. It can locate potential hosts from a distance by orienting toward the scent of herbivore-damaged plants. Upon landing on the caterpillars' food plant, the female parasitoid searches for further cues (kairomones) that confirm the presence of a suitable host. In a previous study, we showed that C. marginiventris recognizes the chemical footprints of absent Spodoptera frugiperda caterpillars on a leaf. Here, we report on the persistence and chemical nature of this host location kairomone. In a series of behavioral assays, we confirmed that caterpillars of S. frugiperda leave chemical tracks that elicit characteristic antennation behavior in C. marginiventris for up to 2 days. Both hexane extracts of caterpillar footprints and of the larvae's ventral cuticle induced antennation and contained almost identical long-chain hydrocarbons, thus suggesting the prolegs and claspers as the kairomones' main source. A series of linear C(21) to C(32) alkanes accounted for ca 90% of all identified compounds. Female wasps showed significant antennation responses on leaves treated with a reconstructed blend of these n-alkanes. However, wasp responses were relatively weak. Therefore, we presume that minor compounds, such as monomethyl-branched alkanes, which were also found, may contribute additionally to host recognition.

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.