The introduction of insecticidal transgenic varieties into the Australian cotton market in the mid-1990’s allowed the industry to substantially reduce its pesticide use but resistance continues to threaten its efficacy. Indeed, CSIRO has isolated resistance in the key targets H. armigera and H. punctigera to all three toxins (Cry1Ac, Cry2Ab, Vip3A) in the current Bollgard 3 varieties. This is set in the context of an emerging global pesticide crisis that could see novel resistant variants of these pests selected elsewhere arrive into Australia.

The industry relies on a pre-emptive strategy to slow the development of Bt resistance. This is underpinned by independent monitoring of background resistance frequencies to enable the industry to autonomously respond to emerging issues, as well as research on other high priority stewardship issues related to Bt resistance in Helicoverpa species. The project was conducted in the following three parts.

PART 1: Does multiple resistance to Bt toxins in Helicoverpa spp. pose a threat to 3 gene cotton?

There is a high chance of an insect being resistant to Cry2Ab and Vip3A. Cry1Ac declines as plants age which creates selection opportunities. Our laboratory bioassays demonstrated that it is possible to select for Cry1Ac resistance in a Cry2Ab / Vip3A background.

Experiments with multi-resistant colonies (created from resistant field colonies) challenged with field grown 3-toxin cotton suggest that they carry a fitness cost but can nevertheless survive well from the neonate to 3rd instar stage. As the larvae mature they are likely to die on 3-toxin cotton but a small proportion can survive.

PART 2: Are the frequencies of resistance to 3 gene cotton increasing?

During 2017/18, we used F1 screens to sample populations of H. armigera and H. punctigera throughout the industry and did not find evidence of increases over time in the frequencies of resistance to Cry1Ac, Cry2Ab and Vip3A. We also performed F2 screens and did not isolate any dominant forms of Bt resistance but we did isolate a new recessive Vip3A resistance in H. armigera. Our continued survey of CCA members since 2008 supports frequency estimates which suggest that Bt resistance in Helicoverpa species is not increasing.

To assist with development of the molecular tool (as part of CSE1801) we examined F2 and F1 individuals previously identified as resistant using bioassays during our monitoring program. We: (1) screened for previously identified mutations and (2) examined whole genome data for novel mutations.

PART 3: What are the characteristics of different variants of Cry2Ab resistance?

There is no indication from our characterisation work that a novel variant of Cry2Ab resistant H. armigera poses a different threat to that of the first isolated variant.

Summary: It is unclear to what degree multiple resistance to Bt toxins in Helicoverpa spp. is a threat to stacked gene cotton. Our data suggest that currently there is no reason to consider changes to the Resistance Management Plan for Bollgard 3 cotton. However, it will be important to get a more complete understanding of the characteristics of the various isolated Vip3A resistance colonies to inform future methods / tools for monitoring resistance. It is also essential to validate the molecular tools being developed with standard bioassays to translate and incorporate them into future monitoring programs. Although there currently is no evidence of increasing resistance, it is critical to pre-emptively ascertain any future changes due to, for example, incursions of novel resistances from overseas and/or changing climates driving local selection in Helicoverpa and/or other pests that may carry novel resistance genes to key technologies used in Australia (i.e., Fall Armyworm).