Using Resistant Insects to Efficiently Screen for Novel Modes of Action for Insecticidal Toxins

Insecticidal proteins derived from the bacteria Bacillus thuringiensis (Bt) have been introduced to several agricultural crops to control insect pests. In particular, the crystalline toxin families, Cry1 and Cry2 have been widely used to control Lepidopteran pests, including in cotton. In recent years Vip3 toxins have also been deployed individually and in ‘stacked’ plant varieties. However, for lepidopteran pests, these three families of insecticidal toxins represent just three modes of action as there is significant evidence of cross resistance between toxins from the same family. Globally, the number of cases of practical resistance to Bt crops (one or more populations with >50% resistant individuals and reduced crop efficacy) increased from one in 2005 to nine in 2013. Therefore, there is a need for novel insecticidal proteins with novel mechanisms. One method of quickly identifying novel mechanisms is to test candidate proteins against resistant insect lines. Resistant insect lines enable rapid and cost effective testing for these novel mechanisms and also provide insight into how existing toxins work. Since the mid-1990’s CSIRO has been tracking resistance frequencies in Australia as part of the Cotton Industry’s plan to prolong the life of Bt cotton and has isolated Cry1A, Cry2A and Vip3 resistant insect lines in two different lepidopteran species, Helicoverpa armigera and H. punctigera. These insect lines, together with the experience with bioassay and molecular identification of resistance mechanisms that CSIRO has developed over two decades, represent a valuable resource that can be used to rapidly screen large numbers of candidate insecticidal toxins. The ability to rapidly detect novel modes of action can shorten the development time of new products and quickly screen libraries of Bt isolates for novel proteins of interest.