Helicoverpa punctigera, along with Helicoverpa armigera, are major pests in Australian cotton. They are currently controlled using “Bt cotton” which contain genes derived from the bacteria Bacillus thuringiensis, that produce proteins toxic to Helicoverpa. While most effort has focused on preventing Helicoverpa spp. developing genetic resistance to these toxins, laboratory studies have shown that larvae which are not resistant are able to tolerate low to medium levels of toxin. This “induced tolerance” could lead to larvae surviving on Bt cotton without being resistant, and it could provide a stepping stone to the development of resistance. While Helicoverpa are known to develop some tolerance after one generation of exposure to Bt toxins, we did not know whether exposure is required throughout the entire larval period or only during particular instars. The aim of this summer project was to test if exposing the larvae Helicoverpa punctigera at different larval stages to 2% or 5% toxin concentrations of the discriminating dose of Cry1Ac toxin (used by CSIRO’s Resistance Team) would affect larval development and lead to tolerance in their offspring.

The results confirmed that after exposing only one generation of larvae to low levels of Cry1Ac toxins their offspring were able to tolerate higher levels of Cry1Ac than the controls. In addition we found that larvae exposed to Cry1Ac in early instars overcompensated their growth once they fed on non-toxin diet, and those exposed as late instars actively tried to avoid the toxin and developed into smaller moths. While the offspring of larvae exposed to 5% toxin as late instars showed the most tolerance, those exposed to 2% toxin as late instars also produced significantly more tolerant offspring. These results indicate that the critical period for the development of tolerance is late in larval development.

These results have implications in respect to Bt cotton efficacy, and could have implications in respect to the placement of refuges. They suggest that larvae moving off other crops and completing their development in Bt cotton could produce offspring at least as tolerant as those completing their development within the Bt crop. Therefore ideally refuges need to be far enough away from Bt cotton to avoid older larvae moving into the cotton.