Caterpillar resistance to transgenic cotton: Impact of increased feeding rates

Currently, caterpillar control in transgenic cotton relies on pyramiding large molecular weight insecticidal proteins  and possibly nucleic acids in the future.  While there are many benefits to this approach for resistance management, there are also potential weaknesses. If insects can develop a mechanism to prevent all of these biopolymers from reaching the target site the benefits of pyramiding could be reduced.  We review the past and present research from our group of the potential impact of increased feeding rates on Bt susceptibility in two insects species, tobacco budworm (Heliothis virescens) and cotton bollworm (Helicoverpa zea). Increased feeding rates in these two species produced by increases in temperature result in reduced Bt toxin susceptibility and might explain variations in the effectiveness of Bt cotton in the field.  Evidence is also presented of natural variations in feeding rates for the budworm and bollworm before the wide adoption of transgenic cotton and how this is correlated with Bt susceptibility.  Finally, laboratory selected Bt resistant budworms and field collected Bt resistant fall armyworms had increased feeding rates than their respective susceptible strains.