Chemosensory basis of behavioural plasticity in response to deterrent plant chemicals in the larva of the Small Cabbage White butterfly Pieris rapae

Behavioural and electrophysiological responsiveness to three chemically different secondary plant substances was studied in larvae of Pieris rapae L. (Lepidoptera: Pieridae). Three groups of caterpillars were studied that during their larval development were exposed to different rearing diets: an artificial diet or one of two host-plants, cabbage, Brassica oleracea, or nasturtium, Tropaeolum majus. In dual-choice leaf disc assays, caterpillars reared on cabbage were strongly deterred by the phenolic chlorogenic acid, the flavonol glycoside naringin and the alkaloid strychnine. However, behavioural plasticity was found in caterpillars reared on nasturtium or artificial diet in that these did not discriminate against chlorogenic acid. Caterpillars reared on the artificial diet were also significantly less sensitive to naringin and strychnine in the behavioural assay. Electrophysiological studies of the maxillary sensilla styloconica revealed that the deterrent neuron in the medial sensillum, but not in the lateral sensillum, of cabbage-reared caterpillars was more sensitive than the same neuron type of caterpillars reared on nasturtium or artificial diet. We conclude that (1) the diet-induced behavioural habituation to deterrents can at least partly be explained by chemosensory desensitisation of a generalist type of maxillary deterrent neuron; (2) behavioural cross-habituation to the three structurally diverse deterrent compounds can be traced back to cross-sensitivity for these compounds in the same gustatory neuron.