Perception of oviposition-deterring pheromone by cabbage seed weevil (Ceutorhynchus assimilis)

Following oviposition into a pod of oilseed rape (Brassica napus), the fema le cabbage seed weevil (Ceutorhynchus assimilis) marks the pod with oviposi tion-deterring, pheromone (ODP) by brushing it with her eighth abdominal te rgite. On an unmarked pod, oviposition site selection was always accompanie d by intensive antennation of the pod. Females approaching a freshly ODP-ma rked pod brought their antennae within I mm of the pod but usually did not antennate it before rejecting it for oviposition. Females with the clubs of their antennae amputated continued to discriminate pods from stems or peti oles as oviposition sites but showed no behavioral response to ODP. Extract s of volatiles air-entrained from ovipositing weevils failed to inhibit ovi position. Air passed over a behaviorally active extract of ODP did not elic it a detectable electroantennogram response. By contrast, when presented as a gustatory stimulus to the sensilla chaetica of the antennal club, a beha viorally active extract of ODP from postdiapause, gravid females elicited a strong electrophysiological response. This response usually involved more than one cell and displayed a phasic-tonic time course over the recording p eriod of 10 sec. Extract from prediapause land hence sexually immature) fem ales elicited neither behavioral nor electrophysiological (contact) respons es. Thus the ODP of the cabbage seed weevil is sensed primarily by contact chemoreception at the sensilla chaetica of the antennae, and the electrophy siological responses recorded from these gustatory sensilla are of value as the basis of a bioassay to assist identification of the active constituent (s) of the pheromone.