Resistance is one of the greatest threats to effective pest control in the Australian Cotton Industry, both against insecticides as well as transgenic cotton. For the primary pests of cotton, the cotton bollwonn Helicove1pa armigera and to a lesser extent H. punctigera, this threat could in the worst case result in loss of an important insecticide or loss in effectiveness of one or more Bt genes. It is important therefore that resistance monitoring, and associated mechanism research, is continued to detect the development of resistance and determine the mechanisms involved, in order that appropriate strategies are formulated accounting for this information and implemented before resistance is observed in the field in the form of control failures.

The 2004/05 cotton season saw the introduction of large scale Bollgard II plantings with minor restrictions on the total area that could be grown. Insecticides however have continued to be in demand for use against Helico1·erpa spp. on conventional cotton, and have been used on Bollgard II crops either targeting other insects such as mirids, using a chemical that also kills Helicoverpa spp. (eg endosulfan), or to control Helicoverpa spp. under conditions of high insect pressure and/or growing conditions that adversely affect Bt expression. Sprayed conventional cotton (non Bt) is still a popular cropping option as well as an important refuge option for Bollgard IL While Bollgard II may dominate total plantings, conventional cotton plantings represent a significant area that requires insecticidal control and protection against insecticide resistance.

The effects of significant plantings of Bollgard II and planting trends for the future are uncertain for the foreseeable future given such factors as economic constraints (eg low cotton prices and increased Bollgard II fee) and resistance issues associated with Bt toxins. Findings of Dr Robin Gunning of NSW DP!, as part of the CRDC funded project DAN I 72C (Gunning et al., 2005) identified an esterase mediated cross resistance in H. armigera between pyrethroids and the Cry1 Ac gene, one of the Bt genes expressed in Bollgard IL In addition, while accurate estimates of the frequency of resistance to Cry2Ab, the other Bt gene in Bollgard II, continue to be established, the CSIRO Bt resistance monitoring project has identified resistance associated genes in the field, with the data suggesting an increase in frequency in 2007/08 (Sharon Downes, pers comm.). Both these findings have serious implications for the control of I-I armigera in the field using both conventional chemistry and transgenic cotton, and emphasise the need to continue insecticide resistance monitoring and associated resistance mechanism research, both An insecticide resistance management strategy (IRMS) is implemented in the Australian Cotton Industry to protect insecticides. This strategy relies on resistance monitoring data and mechanism research as part of assessing the success of the strategy as well as formulating changes to account for resistance development that may be detected, and for occtmenee of cross resistance between different insecticides. This project aims to provide such data and any additional useful data from the monitoring program in the development of an effective strategy and guidelines for minimising the development of insecticide resistance.

In addition to resistance monitoring and mechanism research for chemicals currently registered for use on cotton, it is essential that new chemistries entering the industry have accurate dose-response data measured prior to their introduction. This accumulation of the baseline response allows for measurement of future changes and the detection of resistance development. Without this baseline data there is no means with which to detect resistance development until it is too late and field control problems or failures occur.