Drop-off behaviour of Bt-resistant and Bt-susceptible (Hübner) (Lepidoptera: Noctuidae) larvae on Bt-cotton and non-Bt cotton plants.

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 (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 to determine: (1) whether they move the same way or differently in response to Bt and non-Bt, and (2) could 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 neonates on artificial diets and cotton plants with and without Bt toxin during the first 12 h after hatching. Bt-resistant and Bt-susceptible 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.