Perception of oviposition-deterring pheromone by cabbage seed weevil (Ceutorhynchus assimilis).
Following oviposition into a pod of oilseed rape ([rape], Brassica napus), the female cabbage seed weevil (Ceutorhynchus assimilis) marked the pod with oviposition deterring pheromone (ODP) by brushing it with her eighth abdominal tergite. On an unmarked pod, oviposition site selection was always accompanied by intensive antennation of the pod. Females approaching a freshly ODP-marked pod brought their antennae within 1 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 petioles as oviposition sites but showed no behavioural response to ODP. Extracts of volatiles air-entrained from ovipositing weevils failed to inhibit oviposition. Air passed over a behaviourally active extract of ODP did not elicit a detectable electroantennogram response. By contrast, when presented as a gustatory stimulus to the sensilla chaetica of the antennal club, a behaviourally 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 period of 10 seconds. Extract from prediapause (and hence sexually immature) females elicited neither behavioural nor electrophysiological (contact) responses. Thus the ODP of the cabbage seed weevil is sensed primarily by contact chemoreception at the sensilla chaetica of the antennae, and the electrophysiological 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.