Behavioral Response of Halyomorpha halys (Hemiptera: Pentatomidae) and Its Egg Parasitoid Trissolcus japonicus (Hymenoptera: Scelionidae) to Host Plant Odors

Insects use a range of cues to help them interact with each other and their host plants. Among these cues, olfaction plays a major role in host selection. The present study investigated the behavioral response of the brown marmorated stink bug, Halyomorpha halys (Stål), and its egg parasitoid, Trissolcus japonicus (Ashmead), to host plant-related odors. We used H. halys nymphs since their response to host odors is relatively unknown. In a Y-tube, we first evaluated the behavioral response of H. halys nymphs to whole-fruit odors of apple [Malus domestica (Borkh.)] and peach [Prunus persica (L.) Batsch)]. Subsequently, we tested the behavioral response of H. halys and T. japonicus to 18 selected synthetic volatiles previously identified from H. halys and its common host plants. In the greenhouse, we further tested H. halys attraction to the most promising of these volatiles individually and as blends. In single-choice tests, H. halys nymphs preferred odors from apple and peach over the control (no odor). In dual-choice tests, H. halys did not show any preference between apple and peach odors. Among the 18 volatiles tested, H. halys nymphs were attracted to ethyl salicylate (ES), undecane (UN), and ethyl acetate (EA) compared to the control. In the greenhouse, H. halys nymphs were similarly attracted to blends of 1:1 ratio of ES and EA but not to single compounds. Also in the Y-tube, female T. japonicus preferred the arm that had ES, β-caryophyllene, and decanal and a blend of these three compounds at a 1:1:1 ratio. Trissolcus japonicus was more attracted to the control arm than to the arm containing tridecane or α-pinene. These results indicate the potential of developing H. halys and T. japonicus attractants or/and repellents based on host plant volatiles and suggest possible adaptive responses of this pest and its egg parasitoid to similar host plant odors.

Differences in the Olfactory Sensitivity of Ceratitis capitata to Headspace of Some Host Plants in Relation to Sex, Mating Condition and Population

The Mediterranean fruit fly, Ceratitis capitata Wied., is among the most serious pests in horticulture worldwide, due to its high reproductive potential, difficulty of control and broad polyphagy. The aim of this study was to measure—by means of the electroantennogram recordings— the antennal olfactory sensitivity of virgin, mated, male, female, lab-reared and wild C. capitata following stimulation with fruit and leaf headspace of some host-plants: clementine, orange, prickly pear, lemon and apple. The results show that: (a) lab-reared mated males are more sensitive to host-plant fruit and leaf headspace than females, while the opposite was true for wild insects; (b) antennae of wild virgin males were more sensitive than the mated ones, while no difference was observed among lab-reared medflies; (c) lab-reared virgin females were more sensitive than mated ones, while few differences were found within wild medflies; (d) in mated insects, lab-reared males were more sensitive to both host-plant fruits and leaves than the wild ones, while the opposite was found for females. Taken together, these results show that the olfactory sensitivity to host-plant odors differs between virgin and mated and lab-reared and wild flies.

Olfactory receptors tuned to volatile mustard oils in drosophilid flies

Plant toxins are effective defenses because they are aversive to enemies. The same molecules, however, are co-opted as host-finding cues by specialist herbivores. Although such behavioral shifts are central to our understanding of herbivorous insect diversification, it is not well understood how they evolve. We addressed this in Scaptomyza flava, a herbivorous drosophilid fly within a lineage that shifted to feeding on toxic mustard plants (Brassicales) <10 million years ago. S flava lost the ancestral attraction to yeast volatiles and the attendant chemoreceptors that detect these odors. Here we report that S. flava, but not its close microbe-feeding relatives Drosophila melanogaster and S. pallida, is attracted to mustard host-plant odors, including volatile mustard oils (isothiocyanates or ITCs). Our genomic analysis uncovered three S. flava paralogs of an olfactory receptor gene (Or67b) that likely experienced positive selection. We then tested whether these chemoreceptors could underlie the observed attraction to volatile ITCs. Our in vivo recordings revealed that two of the S. flava Or67b proteins (Or67b1 and Or67b3) – but not the homologous Ors from microbe-feeding relatives – responded selectively and sensitively to volatile ITCs. These Ors are the only ITC chemoreceptors other than TRP channel family members (e.g., the TrpA1 ‘wasabi’ receptor) known from animals. Remarkably, S. flava Or67b3 was sufficient to drive olfactory attraction toward butyl ITC when expressed in an attractive olfactory circuit. Our study illuminates that ancestrally aversive chemicals can be co-opted as attractants through gene duplication, leading to the origin of hedonic valence shifts in herbivorous insects.Significance StatementPlant toxins trigger aversive olfactory (volatile-mediated) and gustatory (contact-mediated) responses in animals. Paradoxically, toxic plants are colonized by specialist insects that co-opt toxins as host-plant finding cues. The mechanisms underlying these behavioral shifts, from indifference or repulsion, to attraction, remain unclear. To address this, we used a fly lineage, Scaptomyza flava, that switched from yeast-feeding to feeding on mustard plants less than 10 million years ago. We found that S. flava is attracted to mustard-plant odors and volatile mustard oils (isothiocyanates or ITCs) such as ‘wasabi’, a behavior enabled by the evolution of genes encoding odorant receptors highly sensitive to ITCs. Our study illuminates how insects colonize toxic host plants through duplication and ecological repurposing of genes encoding pre-existing chemoreceptors.

A novel major output target for pheromone-sensitive projection neurons in male moths

The male-specific macroglomerular complex (MGC) in the moth antennal lobe contains circuitry dedicated to pheromone processing. Output neurons from this region project along three parallel pathways, the medial, mediolateral, and lateral tracts. The MGC-neurons of the lateral tract are least described and their functional significance is unknown. We used mass-staining, calcium imaging, and intracellular recording/staining to characterize the morphological and physiological properties of these neurons in Helicoverpa armigera. All lateral-tract MGC neurons targeted the column, a small region within the superior intermediate neuropil. We identified this region as the major converging site for lateral-tract neurons responsive to pheromones and plant-odors. The lateral-tract MGC-neurons consistently responded with a faster onset than the well-described medial-tract neurons. Different from the medial-tract MGC neurons encoding odor quality important for signal identification, those in the lateral tract seem to convey a robust and rapid, but fixed signal – potentially important for fast control of hard-wired behavior.

Do age and mating status affect olfactory response of the parasitoid, Microplitis croceipes (Hymenoptera: Braconidae) to host-related plant odors?

Background: Parasitic wasps (parasitoids) use volatile organic compounds released by herbivore-infested plants to locate their hosts. Response of parasitoids to plant odors may be plastic and dependent on their physiological state. Using Microplitis croceipes (Hymenoptera: Braconidae), a relatively specialized larval endoparasitoid of Heliothis virescens (Lepidoptera: Noctuidae), we asked whether age and mating status of parasitoids affect their olfactory response to host-related odors. Methods: Four odor stimuli of varying complexity were selected based on previous reports of parasitoid response to cotton volatiles: cis-3-hexenol (a green leaf volatile), α-pinene (a constitutive monoterpene), a 50/50 v/v binary mixture (cis-3-hexenol + α-pinene), and H. virescens-infested cotton odors. Female M. croceipes used in Y-tube olfactometer bioassays were either mated or unmated, and grouped 1–3, 4–6, and 7–9 d-old. Female parasitoids used in electroantennogram (EAG) recording were mated and grouped 1–3, 4–6, 7–9 and 10–12 d-old. Results: In Y-tube olfactometer bioassays, neither age nor mating status played a major role in the attraction of parasitoids to test odor stimuli, with two exceptions: 4–6 d-old mated parasitoids showed attraction to the binary mixture, and 1–3 d-old mated parasitoids showed attraction to H. virescens-infested cotton. Age did not affect EAG response of parasitoids to test stimuli. Conclusions: The present results suggest that age and mating status do not play a major role in modulating olfactory responses of M. croceipes to host-related plant odors. Instead, plasticity of olfactory response may be limited in M. croceipes due to strong innate sensitivity to host-related odor cues.

Do age and mating status affect olfactory response of the parasitoid, Microplitis croceipes (Hymenoptera: Braconidae) to host-related plant odors?

Background: Parasitic wasps (parasitoids) use volatile organic compounds released by herbivore-infested plants to locate their hosts. Response of parasitoids to plant odors may be plastic and dependent on their physiological state. Using Microplitis croceipes (Hymenoptera: Braconidae), a relatively specialized larval endoparasitoid of Heliothis virescens (Lepidoptera: Noctuidae), we asked whether age and mating status of parasitoids affect their olfactory response to host-related odors. Methods: Four odor stimuli of varying complexity were selected based on previous reports of parasitoid response to cotton volatiles: cis-3-hexenol (a green leaf volatile), α-pinene (a constitutive monoterpene), a 50/50 v/v binary mixture (cis-3-hexenol + α-pinene), and H. virescens-infested cotton odors. Female M. croceipes used in Y-tube olfactometer bioassays were either mated or unmated, and grouped 1–3, 4–6, and 7–9 d-old. Female parasitoids used in electroantennogram (EAG) recording were mated and grouped 1–3, 4–6, 7–9 and 10–12 d-old. Results: In Y-tube olfactometer bioassays, neither age nor mating status played a major role in the attraction of parasitoids to test odor stimuli, with two exceptions: 4–6 d-old mated parasitoids showed attraction to the binary mixture, and 1–3 d-old mated parasitoids showed attraction to H. virescens-infested cotton. Age did not affect EAG response of parasitoids to test stimuli. Conclusions: The present results suggest that age and mating status do not play a major role in modulating olfactory responses of M. croceipes to host-related plant odors. Instead, plasticity of olfactory response may be limited in M. croceipes due to strong innate sensitivity to host-related odor cues.