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

Electrophysiological responses to ten glucosinolates, oviposition stimulants 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 cluster of fourteen medial B-sensilla on the fifth tarsomere, strong re sponses to glucosinolates were recorded. Three receptors cells, identified on the basis of spike amplitude and shape, were typically active in these responses, with the cell producing the smallest spike predominating. The response intensity of the latter cell to a particular glucosinolate at 1 mM concentration depended both on the chemical structure of the stimulus and on the subspecies of insect. In P. napi napi, aromatic glucosinolates were just as effective as aliphatic glucosinolates, whereas in P. napi oleracea the long-chain aliphatic glucosinolates gluconapin and progoitrin were stronger stimuli. At 10 and 100 mu g/ml concentrations, significant differences between the subspecies were found in the absolute response intensity to seven compounds. A second cell in P. napi oleracea, producing a medium-sized spike, was excited most by sinalbin, whereas in P. napi napi three aliphatic glucosinolates were equally effective stimulants for a similar cell. In both subspecies the cell producing the medium-sized spikes in response to glucosinolates was also highly sensitive to erychroside, an oviposition deterring cardenolide occurring in a host of P. napi oleracea. The evolutionary divergence between the two geographically separated subspecies is reflected by differences in chemosensory recognition mechanisms.