CONTACT CHEMORECEPTION OF OVIPOSITION-STIMULATING GLUCOSINOLATES AND AN OVIPOSITION-DETERRENT CARDENOLIDE IN 2 SUBSPECIES OF PIERIS-NAPI

Electrophysiological responses to ten glucosinolates, oviposition stim ulants isolated from their cruciferous host plants, were recorded from tarsal taste neurones of two subspecies of Pieris napi: P. napi napi L, and P. napi oleracea (Harris) (Lepidoptera: Pieridae). In the clust er of fourteen medial B-sensilla on the fifth tarsomere, strong re spo nses to glucosinolates were recorded. Three receptors cells, identifie d on the basis of spike amplitude and shape, were typically active in these responses, with the cell producing the smallest spike predominat ing. The response intensity of the latter cell to a particular glucosi nolate at 1 mM concentration depended both on the chemical structure o f the stimulus and on the subspecies of insect. In P. napi napi, aroma tic glucosinolates were just as effective as aliphatic glucosinolates, whereas in P. napi oleracea the long-chain aliphatic glucosinolates g luconapin and progoitrin were stronger stimuli. At 10 and 100 mu g/ml concentrations, significant differences between the subspecies were fo und in the absolute response intensity to seven compounds. A second ce ll in P. napi oleracea, producing a medium-sized spike, was excited mo st by sinalbin, whereas in P. napi napi three aliphatic glucosinolates were equally effective stimulants for a similar cell. In both subspec ies the cell producing the medium-sized spikes in response to glucosin olates was also highly sensitive to erychroside, an oviposition deterr ing cardenolide occurring in a host of P. napi oleracea. The evolution ary divergence between the two geographically separated subspecies is reflected by differences in chemosensory recognition mechanisms.