Feeding behaviour and survival of Bacillus thuringiensis‐resistant and Bacillus thuringiensis‐susceptible larvae of Helicoverpa armigera (Lepidoptera: Noctuidae) exposed to a diet with Bacillus thuringiensis toxin

Abstract

Although higher than expected numbers of surviving larvae of Helicoverpa spp. are reported in some Bollgard II cotton fields from time to time, there is no convincing evidence that field resistance has developed. A behavioural component, specifically the avoidance of the toxin, was considered to contribute to larvae surviving on Bacillus thuringiensis (Bt) toxin-expressing plants. Experiments were conducted with Bt-susceptible and Bt-resistant larvae of Helicoverpa armigera to investigate (1) how larval responses to food deprivation might lead to differences in survival and therefore how far neonates could move to find a suitable feeding site and (2) whether larvae that are physiologically susceptible vs. resistant behave differently when feeding on diet with Bt and without Bt toxins. Bt-susceptible neonates of H. armigera, which were significantly heavier, could survive without food for longer and recover better than Bt-resistant neonates. Although H. armigera neonates did not shift their behaviour with respect to Bt toxin on artificial diet before their first feeding event, the tendency of Bt-susceptible larvae to remain on non-Bt diet and move off Bt diet may allow them to survive in a Bt environment. This behaviour resulted in a higher percentage of survival in situations where a diet choice was offered.

Life history traits of a key agricultural pest, Helicoverpa armigera (Lepidoptera: Noctuidae): are laboratory settings appropriate?

“…In other cases, it is reported that larvae were reared in incubators (Luong et al . ) or under fluorescent light (Tessnow et al . ), although fluorescent light sources can differ from manufacturer to manufacturer in the peaks of their wavelength emissions, which are sharp and narrow compared to natural light (Longcore et al .…”

“…; Luong et al . ). Potential pest control and mitigation strategies have been proposed on the basis of these experiments (Reddy & Manjunatha ).…”

Stay or move: how Bt‐susceptibleHelicoverpa armigeraneonates behave on Bt cotton plants

“…In the field it is not uncommon to find small groups of eggs on individual plant structures on cotton (Titmarsh, ) and the first hatched neonates could encounter their non‐hatched conspecific eggs. Such behavior could be adaptive as it would increase the likelihood of surviving the vulnerable first instar stage on plants that are poor hosts (Kyi et al, ), including Bt and non‐Bt cotton, but should not be interpreted as behavioral resistance without more detailed experiments (Zhao et al, ; Zalucki & Furlong, ; Luong et al, ).…”

“…They have been able to survive off the plant for at least 1 h, and travel up to 80 cm from where they dropped to the ground (for a similar finding see Terry et al, ). Under laboratory conditions larvae can go without food for 24 h and recover if they find suitable food (Luong et al, ). Larvae can also ‘fight’ by cannibalizing conspecific eggs or neonates.…”

“…Bioassays that form part of the cotton industry’s resistance monitoring program, and a separate study comparing the susceptibility of larvae from Bollgard II fields vs. non‐Bt crops, show that survival on Bollgard II cotton is not due to widespread physiological resistance (Lu et al, ; Sharon Downes, CSIRO, 2015, unpubl.). Various explanations for larval survival on Bt cotton have been proffered (Luong et al, , ). The pattern and degree of larval survival is not easily predicted but depends on their ability to establish on host plants (Zalucki et al, ).…”