Jja. Van Loon et Lm. Schoonhoven, Specialist deterrent chemoreceptors enable Pieris caterpillars to discriminate between chemically different deterrents, ENT EXP APP, 91(1), 1999, pp. 29-35
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.