The emergence of multiple resistance to insecticides in Helicoverpa populations has had a significant impact on the production of most major field crops in Australia. This, coupled with industry's increasing awareness of the need to reduce environmental impacts from pesticide use, has led to the demand for effective alternatives to traditional pesticides. Current alternative control options for Helicoverpa available to cotton growers include the use of 'Gemstar', a nucleopolyhedrovirus (NPV) specific to Heliothine species. Gemstar has been widely used in cotton and grains crops throughout growing areas. It is often used in combination with other pesticides and, in Queensland, in repeated and low-rate applications.

This wide-scale and repeated use has led to industry concerns about the potential for emergence of resistance to Gemstar. There have been several examples in other insect species where resistance of between 5 and 800 times normal susceptibility to insect viruses has been generated in the laboratory, although there have been no reports of resistance to baculoviruses in the field. There is, therefore, a need to establish the baseline level of susceptibility against which future resistance can be assessed, to develop some understanding of the mechanisms of resistance to insect viruses, and to develop genetic markers that might lead to rapid identification of resistant populations.