Habituation to a Deterrent Plant Alkaloid Develops Faster in the Specialist Herbivore Than in Its Generalist Congener and Coincides with Taste Neuron Desensitisation.

The two closely related moth species, and differ strongly in their degree of host-plant specialism. In dual-choice leaf disk assays, caterpillars of the two species that had been reared on standard artificial diet were strongly deterred by the plant-derived alkaloid strychnine. However, caterpillars of both species reared on artificial diet containing strychnine from neonate to the 5th instar were insensitive to this compound. Fifth instar caterpillars of and 4th or 5th instars of not exposed to strychnine before were subjected to strychnine-containing diet for 24 h, 36 h, 48 h, or 72 h. Whereas displayed habituation to strychnine after 48 h, it took until 72 h for to become habituated. Electrophysiological tests revealed that a deterrent-sensitive neuron in the medial sensillum styloconicum of both species displayed significantly reduced sensitivity to strychnine that correlated with the onset of habituation. We conclude that the specialist habituated faster to strychnine than the generalist and hypothesis that desensitization of deterrent-sensitive neurons contributed to habituation.