This report presents the results of a successful collaboration between Rod Mahon and Karen Olsen (CSIRO Entomology) and Dr David Heckel (Max Planck Institute for Chemical Ecology, Jena, Germany). For the duration of this project David retained linkages to the University of Melbourne where a significant component of the work was performed.

The project explored resistance to Cry2Ab in the cotton pest Helicoverpa armigera. This species has a remarkable track record of evolving resistance to conventional insecticides and is thus the most likely of the two Helicoverpa species found regularly in cotton (the other is the native H. punctigera) to evolve resistance to the toxins present in transgenic cotton. In other research funded by CRDC, we have found that forms of genes, (alleles) conferring resistance to Cry2Ab toxin in H. armigera are surprisingly common. Because we found the resistance prior to the widespread deployment of transgenic cotton that express this toxin (Bollgard II®), it is clear that the presence of these ‘resistant alleles’ pre-dates man’s activities. Because these alleles are unexpectedly common, (approximately 4 in 100 alleles tested are the ‘resistant’ form) it is important to understand the characteristics of this resistance in order to assess the likelihood that it will become a threat to the long-term efficacy of cotton varieties that express the Cry2Ab toxin as well as a second toxin, Cry1Ac.

We have found that the resistance present in a colony of insects derived from field-collected H. armigera allele is due to a single gene. This fact was established by two quite distinct methods. Firstly, it was found that comparative bioassays of resistant, susceptible, F1 offspring and various backcrosses to the parental (susceptible and resistant) colonies implied that resistance was due to a single autosomal gene. This was confirmed through the study of linkage relationships between genetic variants and resistance. Importantly, the resistance was recessive in the laboratory. If extended to field conditions, this makes this form of resistance less of a threat than would be the case if it was dominant (like most forms of resistance to conventional insecticides).

Of immediate significance to the current varieties of transgenic cotton grown in Australia was the finding that insects seemingly totally resistant to Cry2Ab toxin, are fully susceptible to Cry1Ac, the second toxin in Bollgard II®. Thus it is only during the latter part of the cotton season when Cry 1Ac toxin is diminished in Bollgard II® (similar to the situation in Ingard varieties) that insects resistant to Cry2Ab possess an advantage over susceptible insects. Only under these conditions are the ‘resistant’ alleles likely to increase in frequency. Nevertheless, this window of opportunity is of concern, as if that advantage persists over time, it will lead to a loss of efficacy of this toxin. Clearly careful watch on the frequency of such resistance is important to enable the industry to enjoy the full benefit of any technology that involves Cry2Ab toxin.

CRDC funds an active program to monitor the frequency of resistance to Bt toxins. The most effective means to assess the frequency of resistance is by the use of a simple, but labour intensive genetic system, the F2 screen. Work in this project has identified a likely candidate gene ‘Bre-5’, mutations at which may result in the resistance we have studied. If this information proves to be correct, and can be exploited to develop a DNA means to detect these mutations, this would enable the detection of changes in frequency of the ‘resistant alleles’ earlier, and thus allow more opportunities to respond in a manner that would limit additional changes in frequency before field-resistance became a problem.