Thresholds and mechanisms of survival for Bt-susceptible 'Helicoverpa' spp. living on Bollgard II® cotton

Abstract

Cultivars of Bollgard II® cotton ('Gossypium hirsutum' L.) express the cry1Ac and cry2Ab genes from 'Bacillus thuringiensis' (Bt), which are toxic to the key Australian targets, 'Helicoverpa armigera' and 'Helicoverpa punctigera'. However, sometimes 'Helicoverpa' spp. larvae can survive on Bollgard II® cotton to levels which exceed recommended thresholds for treatment. Surviving larvae do not show physiological resistance to the Bt proteins, which raises the question 'how do susceptible larvae survive on Bollgard II® cotton?'. Irrespective of the mechanism, the presence of larvae in Bollgard II® crops concerns growers because of the potential associated damage. The existing control thresholds were designed for Bt-cotton which expresses only Cry1Ac and may not be appropriate for Bollgard II®. In the first part of my PhD project I used surveys of commercial fields, and bioassays performed in the glasshouse, laboratory and field to compare the behaviour of 'Helicoverpa' spp. larvae on Bollgard II® cotton vs. non-Bt cotton as a potential mechanism of survival. Specifically, I investigated the spatial distribution of larvae throughout fields and among individual plants, their preferences for low expressing plant parts, and movement within and between plants. To help interpret patterns in behaviour I also investigated temporal and spatial variability of Bt toxin expression in Bollgard II® cotton. In the second part of my PhD project I used field experiments to examine the relationship between boll age and susceptibility to larvae, the degree of damage due to Helicoverpa spp. infestation, and the response of cotton yield to manual simulation of 'Helicoverpa' spp. damage. The overall conclusions are that 'Helicoverpa' spp. larvae shift their behaviour when exposed to Bollgard II® plants. Frequent movement, potentially within and between plants, implies that larvae may be able to detect and avoid the Bt toxin. An apparent feeding preference for flowers supports previous work which suggests that parts of this structure may express very low levels of Bt toxin. These behaviours, coupled with the sometimes temporal and spatial variability of Bt toxin expression in Bollgard II® cotton, can result in a proportion of the population becoming established. Bollgard II® cotton is highly tolerant of 'Helicoverpa' spp. larvae and compensates well to damage from them. Current action levels appear to be conservative. However it may be important to treat Bollgard II® cotton at threshold levels because if Bt-resistance has some genetic dominance then exposure to sub-lethal doses of the toxin could select for resistance.