Efficacy of Bt Cotton Plants in Australia: What is Going On?

We report on the patterns and causes of variable efficacy of Bt cotton plants for control of Helicoverpa spp. in Australia. Initially, it was hoped that these plants would be efficacious against their target pests throughout the cotton growing cycle. Spatial and temporal changes in efficacy have been followed in field plots of cotton using whole leaf bioassays and changes in Bt concentration and efficacy have been quantified using a variety of new bioassay approaches. In the laboratory, we have measured the response of the plants to a variety of stresses, including extremes of water availability and temperature. A consistent decline in plant efficacy was observed from mid-summer and, in the field, some crops require alternative treatments to control Helicoverpa from peak flowering onwards. By late season, larvae can develop and survive to pupation on Bt cotton. Further, some plants have variable efficacy in early season, even before squaring has commenced. There are many possible causes for the seasonal decline and variability. At this stage, we have evidence for a decline in the amount of Bt protein present and also for possible interference by plant factors in the availability of the Bt protein to the insect.