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The two closely related moth species, <i>Helicoverpa armigera</i> and <i>H. assulta</i> 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 <i>H. assulta</i> and 4th or 5th instars of <i>H. armigera</i> not exposed to strychnine before were subjected to strychnine-containing diet for 24 h, 36 h, 48 h, or 72 h. Whereas <i>H. assulta</i> displayed habituation to strychnine after 48 h, it took until 72 h for <i>H. armigera</i> 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 <i>H. assulta</i> habituated faster to strychnine than the generalist <i>H. armigera</i> and hypothesis that desensitization of deterrent-sensitive neurons contributed to habituation.