Microbial volatiles affect grapevine moth oviposition

Females of herbivorous insects use sensory cues, including olfaction, to select oviposition sites. Olfactory cues are responsive to modifications of the host-odour profile, which may reflect changes in host quality. Through a real-time assessment of odour modification, planteating insects may gain relevant information regarding the nutritional value of the host-plant. Microorganisms such as yeast, bacteria and fungi may colonize plants and modify their volatile emission. In addition, they may also change the nutrtional properties of the plant as food for insects. Here, we examined the oviposition response of grapevine moth Lobesia botrana to volatiles from grape (Vitis vinifera) infected with microorganisms commonly found in vineyards. We hypothesized that the odour of grape infected with yeast would stimulate a higher preference for oviposition compared to the odour of bacteria or fungi. To test our hypothesis, we carried out oviposition assays in which females were allowed to choose between the odour of a non-inoculated grape and that of a grape inoculated with one or more microorganisms. The odour released by a pool of grape-born yeast (Saccharomyces cerevisiae, Zygosaccharomyces rouxii, Metschnikowia pulcherrima, Hanseniaspora uvarum and Pichia anomala) showed a positive effect on oviposition. In contrast, volatiles from Gluconobacter oxydans and Acetobacter aceti, and from the phytopathogenic fungus Botrytis cinerea, deterred female oviposition. The stimulus to lay eggs could be restored when B. cinerea was inoculated on the grape along with the yeasts. We found that the nutritional value of the microorganisms (yeasts>bacteria>fungus) correlated with the oviposition preference. The profile of the odour released by the different microbes was characterized by chemical analysis. Behavioural assay were subsequently carried out employing synthetics. As a first output, a compound isolated from the B. cinerea headspace triggered a dose-deterring effect on grapevine moth oviposition at a comparable level to that obtained with the grape infected with the authentic fungus. In this study volatile compounds released by microbes acted as semiochemicals for grapevine moth oviposition. The behavioural effect of these compounds will be further validated in semi-field and field experiments.