The highest natural mortality rate of larval Lepidoptera in field populations occurs in the first instar, but it is highly variable. The pattern and degree of survival is not easily predicted but depends on their ability to establish on host plants. Lepidopteran larval dispersal behaviour, known as ‘drop-off’, happens when the host is unsuitable for larvae to settle and begin feeding. Understanding drop-off behaviour of Helicoverpa armigera (Hübner) with and without physiological resistance to Bt toxins on Bt and non-Bt cotton plants is an important component for resistance management strategies for this insect. We examined the drop-off behaviour of H. armigera to determine: (1) whether they move the same way or differently in response to Bt and non-Bt, and (2) could H. armigera larvae detect Bt toxin levels in cotton plants or did they move independently of toxin levels? In this study, we assessed the drop-off behaviour of Bt-resistant and Bt-susceptible H. armigera neonates on artificial diets and cotton plants with and without Bt toxin during the first 12 h after hatching. Bt-resistant and Bt-susceptible H. armigera neonates behaved differently on Bt and non-Bt substrates. The percentages of Bt-resistant larvae that dropped off Bt and non-Bt cotton plants were not significantly different. In contrast, significantly more Bt-susceptible larvae dropped off Bt cotton than non-Bt cotton plants over time. Although Bt-susceptible larvae could not detect Bt toxin, they showed preference on non-Bt toxin substrates and were more likely to drop off substrates with Bt toxin.

Reasons for dissatisfaction with insecticidal control of disease vectors include their presumed harmfulness to non-target organisms, their cost and difficulties of organizing effective spraying programmes and physiological and behavioural resistance in vector populations. The validity of these objections and possible ways of resolving them are discussed. The importance of assessing the actual field impact of cases where resistance has been detected with laboratory tests is stressed. With regard to behavioural resistance, some data are presented on tests for genetic variation in the tendency of mosquitoes to rest outdoors and hence to avoid contact with sprayed walls. Most of the failures of insecticidal control of vectors can be attributed, not to problems of resistance, but to organizational difficulties. The impregnation of mosquito bed nets with pyrethroids, which householders can do for themselves, is therefore being field tested in several parts of the world. Such impregnation makes nets effective even when they are torn. This method will probably be particularly advantageous against mosquitoes which enter houses to feed, but do not rest on walls.