Bacterial volatiles elicit differential olfactory responses in insect species from the same and different trophic levels

Insect communities consist of species from several trophic levels that have to forage for suitable resources among and within larger patches of nonresources.To locate their resources,insects use diverse stimuli,including olfactory,visual,acoustic,tactile and gustatory cues.While most research has focused on cues derived from plants and other insects,there is mounting evidence that insects also respond to volatile organic compounds(VOCs)emitted by microorganisms.However,to date little is known about how the olfactory response of insects within and across different trophic levels is affected by bacterial VOCs.In this study,we used Y-tube bioassays and chemical analysis of VOCs to assess how VOCs emitted by bacteria affect the olfactory response of insects of the same and different trophic levels.Experiments were performed using two aphid species(Amphorophora idaei Börner and Myzus persicae var.nicotianae Blackman),three primary parasitoid species(Aphidius colemani Viereck,A.ervi Haliday,and A.matricariae Viereck),and two hyperparasitoid species(Asaphes suspensus Nees and Dendrocerus aphidum Rondani).Olfactory responses were evaluated for three bacterial strains(Bacillus pumilus ST18.16/133,Curtobacterium sp.ST18.16/085,and Staphylococcus saprophyticus ST18.16/160)that were isolated from the habitat of the insects.Results revealed that insects from all trophic levels responded to bacterial volatiles,but olfactory responses varied between and within trophic levels.All bacteria produced the same set of volatile compounds,but often in different relative concentrations.For 11 of these volatiles we found contrasting correlations between their concentration and the behavior of the primary parasitoids and hyperparasitoids.Furthermore,olfactometer experiments on three of these compounds confirmed the contrasting olfactory responses of primary parasitoids and hyperparasitoids.The potential of these findings for the development of novel semiochemical-based strategies to improve biological aphid control has been discussed.

The effect of host plants on genotype variability in fitness and honeydew composition of Aphis fabae

Aphid species can be polyphagous, feeding on multiple host plants across genera. As host plant species can have large variation in their phloem composition, this can affect aphid fitness and honeydew composition. Previous research showed significant intraspecific genotype variation in the composition of the honeydew carbohydrates of the black bean aphid Aphis fabae, with the ant attractant trisaccharide melezitose showing especially large variation across different genotypes. In this study, we test if variation in melezitose and carbohydrate composition of aphid honeydew could be linked to the adap- tation of specific aphid genotypes to particular host plants. To this end, 4 high and 5 low melezitose secreting genotypes of the black bean aphid Aphisfabae were reared on 4 com- mon host plants： broad bean, goosefoot, beet, and poppy. The carbohydrate composition, and in particular melezitose secretion, showed important aphid genotype and host plant in- teractions, with some genotypes being high melezitose secreting on 1 host plant but not on another. However, the interaction effects were not paralleled in the fitness measurements, even though there were significant differences in the average fitness across the different host plants. On the whole, this study demonstrates that aphid honeydew composition is influenced by complex herbivore-plant interactions. We discuss the relevance of these findings in the context of ant-aphid mutualisms and adaptive specialization in aphids.

Hormonal modulation of reproduction and fertility signaling in polistine wasps

In social insects,it has been suggested that reproduction and the production of particular fertilitylinked cuticular hydrocarbons(CHC)may be under shared juvenile hormone(JH)control,and this could have been key in predisposing such cues to later evolve into full-fledged queen pheromone signals.However,to date,only few studies have experimentally tested this“hormonal pleiotropy”hypothesis.Here,we formally test this hypothesis using data from four species of Polistine wasps,Polistes dominula,Polistes satan,Mischocyttarus metathoracicus,and Mischocyttarus cassununga,and experimental treatments with JH using the JH analogue methoprene and the anti-JH precocene.In line with reproduction being under JH control,our results show that across these four species,precocene significantly decreased ovary development when compared with both the acetone solvent-only control and the methoprene treatment.Consistent with the hormonal pleiotropy hypothesis,these effects on reproduction were further matched by subtle shifts in the CHC profiles,with univariate analyses showing that in P.dominula and P.satan the abundance of particular linear alkanes and mono-methylated alkanes were affected by ovary development and our hormonal treatments.The results indicate that in primitively eusocial wasps,and particularly in Polistes,reproduction and the production of some CHC cues are under joint JH control.We suggest that pleiotropic links between reproduction and the production of such hydrocarbon cues have been key enablers for the origin of true fertility and queen signals in more derived,advanced eusocial insects.