Abstract
Simple SummaryHabituation to deterrent plant compounds has been found in generalist and specialist insect herbivores. The rate at which plant-feeding insects habituate and at which sensitivity of taste neurons detecting deterrents changes has not been compared among closely related species. The generalist Helicoverpa armigera (Hübner) and the specialist Helicoverpa assulta (Guenée) (Lepidoptera: Noctuidae) differ strongly in degree of host-plant specialism. Both species habituated to the alkaloid strychnine after dietary exposure; however, the specialist H. assulta displayed habituation to strychnine faster (at 48 h) than the generalist H. armigera (at 72 h). Electrophysiological recordings from taste sensilla on mouthparts revealed that a deterrent-sensitive neuron in the medial sensillum styloconicum of both species displayed significantly reduced sensitivity to the deterrent that coincided in time with the onset of habituation. Our findings show that physiological changes in taste neuron sensitivity coincide with habituation to plant compounds that are initially avoided.The two closely related moth species, Helicoverpa armigera and H. assulta 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 H. assulta and 4th or 5th instars of H. armigera not exposed to strychnine before were subjected to strychnine-containing diet for 24 h, 36 h, 48 h, or 72 h. Whereas H. assulta displayed habituation to strychnine after 48 h, it took until 72 h for H. armigera 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 H. assulta habituated faster to strychnine than the generalist H. armigera and hypothesis that desensitization of deterrent-sensitive neurons contributed to habituation.
Highlights
The taste system of insects plays an important role in detecting and assessing foodrelated chemicals
Helicoverpa armigera and H. assulta larvae exposed to the standard diet from neonate until the molt to the 5th instar were strongly deterred by strychnine (FDI = 40.5 and 50, respectively; p < 0.01; Figure 2A)
H. armigera and H. assulta caterpillars exposed to diet containing strychnine from neonate until the molt to the 5th instar did not discriminate between pepper disks with strychnine and control disks (FDI = 4.7 and 7.1, respectively; p > 0.05; Figure 2A)
Summary
The taste system of insects plays an important role in detecting and assessing foodrelated chemicals. Insects fed a standard artificial diet containing a low level of a feeding deterrent show desensitization of deterrent-sensitive taste neurons [1] Such reduced sensory responsiveness has been reported in several studies [10,11,12,19,20,21,22,23,24]. Helicoverpa armigera (Hübner) and Helicoverpa assulta (Guenée) (Lepidoptera: Noctuidae) are closely related species which exhibit a clearly different degree of host-plant specialism The former is a generalist feeding on plants from more than 40 families and the latter is a specialist using several species in the family Solanaceae as host plants, such as tobacco and hot pepper [26,27]. We addressed three questions: (1) Do the two species habituate to the same deterrent? (2) If so, is the rate of habituation similar? (3) Are the onset of habituation and desensitization coinciding?
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