Specialist deterrent chemoreceptors enable Pieris caterpillars to discriminate between chemically different deterrents

Pieris butterflies (Lepidoptera: Pieridae) are specialist herbivores of cru ciferous plants. They exploit glucosinolates, secondary plant metabolites c hemotaxonomically characteristic for this plant family, as token stimuli. I n addition to particular glucosinolates, some genera of the Cruciferae cont ain cardenolides, steroidal allelochemicals that act as potent feeding and oviposition deterrents to several Pieris species. We investigated the senso ry mechanisms by which these compounds are perceived in larvae. Pieris cate rpillars and many other lepidopterous species are endowed with so-called ge neralist deterrent receptors, that respond to a broad spectrum of secondary plant substances. In Pieris caterpillars we found a second type of deterre nt chemoreceptor in maxillary styloconic taste sensilla. This neuron is ver y sensitive to cardenolides (threshold 0.1-0.3 mu M). The generalist deterr ent receptor also responds to these substances but its threshold lies at 50 -100x higher concentrations. In behavioural preference experiments Pieris b rassicae L. caterpillars preferred cardenolide-treated cabbage leaf discs w hen confronted with a choice between them and a deterrent substance that do es not occur in the Brassicaceae. The cardenolides acted as potent deterren ts when offered against untreated cabbage leaf discs. This demonstrates tha t the balance of activity elicited in the two types of deterrent chemorecep tors determines the behavioural decision.