Preferences of field bollworm larvae for cotton plant structures: impact of Bt and history of survival on Bt crops

Cultivars of Bollgard II (R) cotton [Gossypium hirsutum L. (Malvaceae)] express the cry1Ac and cry2Ab genes from Bacillus thuringiensis Berliner (Bt) and are valuable for managing Helicoverpa spp. in Australia. However, substantial densities of Helicoverpa larvae can survive and feed on flowering Bollgard II (R) plants. Monitoring demonstrates that this larval survival is not because of physiological resistance. One hypothesis for this occurrence is that larvae have adapted their behaviour to reduce exposure to toxic compounds. The survival and distribution among cotton structures of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) larvae were examined using choice tests. Two hours after introduction to an arena, there were no differences between non-Bt cotton and Bollgard II (R) in the numbers of larvae on different plant structures. However, 24 and 48 h after their introduction, most larvae on Bollgard II (R) were on flowers (mean of 80.5%), whereas on non-Bt cotton, many larvae were also on flowers (58.5%), but there was also a considerable proportion (11%) on squares. In a subsequent experiment, we found similar numbers of larvae on the different structures of Bollgard II (R) plants regardless of whether their parents were collected as survivors on Bollgard II (R) cotton or non-Bt hosts. Cry1Ac and Cry2Ab expression varied among cotton structures, but there were no significant differences between the two popular Bollgard II (R) varieties used in this study. Bt toxin levels did not correlate well with the number of larvae among the structures from Bollgard II (R) plants. Thus, we conclude that although the distribution of Bt-susceptible larvae was somewhat different among plant structures on non-Bt and Bollgard II (R) cotton, history of survival on Bollgard II (R) cotton does not significantly impact on this behaviour.