Chemical ecology meets conservation biological control: identifying plant volatiles as predictors of floral resource suitability for an egg parasitoid of stink bugs

Conservation biological control aims to enhance natural enemy populations in crop habitats, e.g. by providing flowering plants as food resources. Suitable flower species must enhance the survival and fecundity of natural enemies but in addition they also need to be highly attractive and thus frequently visited. To date, few examples exist that have considered both criteria. In this study, we tested the effects of the flowering plants alyssum (Lobularia maritima), buckwheat (Fagopyrum esculentum), French marigold (Tagetes patula) and sweet basil (Ocimum basilicum) on the fecundity and olfactory attractiveness of the egg parasitoid Trissolcus basalis, an important biological control agent of the stink bug Nezara viridula. Our results showed that access to buckwheat and basil flowers increased the parasitoid offspring. However, in olfactometer experiments where T. basalis was allowed to choose between flowering and non-flowering plants, only buckwheat floral scent was attractive. Headspace analyses of the odour emitted by the four plant species revealed very distinct profiles with little overlap in compounds. Buckwheat floral scent was characterized by an unpleasant smell for the human nose due to the presence of short-chain carboxylic acids. Headspace extracts of buckwheat flowers and a blend of six buckwheat plant volatiles consisting of butanoic, 2-methylbutanoic, 3-methylbutanoic and pentanoic acids, (Z)-3-hexenyl acetate and α-farnesene were significantly attractive in olfactometer bioassays. Furthermore, electrophysiological experiments showed most of these compounds elicited significant responses in T. basalis antennae. Integrating chemo-ecological methods into conservation biological control allowed us to identify a potential resource plant and attractive compounds for field studies.