CHEMORECEPTION OF OVIPOSITION INHIBITING TERPENOIDS IN THE DIAMONDBACK MOTH PLUTELLA-XYLOSTELLA

The effects of six terpenoids and two terpenoid containing extracts of the neem tree (Azadirachta indica A. Juss.) on oviposition by the dia mondback moth (Plutella xylostella L., Yponomeutidae: Lepidoptera) wer e tested. Two drimane terpenoids, the sesquiterpenoid polygodial and t he neem extract Margosan-O exerted significant inhibitory effects at t he dosages tested. Ablation experiments showed that both antennae and fore-tarsi contributed to mediation of the inhibition by a drimane. Lo cation of chemosensilla on prothoracic tarsi and ovipositor was examin ed by scanning electronmicroscopy. Electrophysiological recordings fro m ovipositor and tarsal taste sensilla showed that distilled water pro duced distinct responses from one neuron. In tarsal sensilla, ethanol and drimane solutions produced responses from two neurons, one of whic h might be the water cell that fired at a reduced rate. A drimane sign ificantly decreased the responses of tarsal chemoreceptors to a cabbag e leaf extract, which is a possible sensory mechanism leading to behav ioural avoidance of this compound.