Behavioural observations of Pieris brassicae larvae indicate multiple mechanisms of action of analogous drimane antifeedants

We tested 11 analogous synthetic drimane antifeedant compounds for their fe eding inhibiting effects on larvae of the large white butterfly Pieris bras sicae L. (Lepidoptera: Pieridae) in no-choice tests on the host plant Brass ica oleracea L. Furthermore, we observed larval feeding behaviour in no-cho ice tests to analyze temporal effects of five drimanes. The results show th at the five analogous antifeedants differentially influence feeding behavio ur and locomotion activity. Warburganal and polygodial are most likely sens ory mediated antifeedants. Habituation to these compounds occurs soon after the onset of the tests (i.e., within 0.5-1.5 h). Compound 5 and confertifo lin are probably not direct, sensory mediated antifeedants. After 0.5-1.5 h of exposure, these compounds inhibit not only feeding, but also locomotion behaviour, indicating postingestive, toxic effects. Isodrimenin inhibits f eeding from the onset of the test and is probably a sensory mediated antife edant. No habituation occurs to this compound, indicating that isodrimenin is either a very strong antifeedant or that it additionally has postingesti ve, toxic effects. Topical application of the drimanes on the larval cuticl e revealed feeding inhibiting effects, but these could not be related to th e occurrence of postingestive feeding inhibiting effects, indicating that t his method is inappropriate to show possible postingestive effects of drima nes in P. brassicae. In conclusion, the behavioural observations performed in this research indicate that analogous drimanes inhibit feeding by P. bra ssicae larvae through multiple mechanisms of action. The results show that, when developing a structure activity relationship (SAR) for a series of an tifeedants, it is important to distinguish the mode of action which underli es inhibition of feeding.