H. armigera is one of the major pests faced by the Australian cotton industry. Widespread uptake of transgenic Bt-cotton has reduced the costs and environmental and health impacts associated with the use of chemical insecticides to control this pest. However, genes for Bt-resistance have been detected in H. armigera, and the danger of resistance to the limited array of Bt toxins available to control Lepidoptera, means that novel biological methods of pest control continue to be of interest.

Recent advances in molecular biology have opened possible new control technologies. CSIRO‟s large genomics effort is delivering not only the complete genome of H. armigera, but has also sequenced many thousands of genes expressed in the midgut of larvae. Successful gene silencing by RNAi would allow us to exploit the genomics results for pest control. RNAi involves introduction of double-stranded RNA (dsRNA) into a target species, resulting in highly specific gene silencing. By reducing the expression of any essential gene, RNAi could be used as an effective method of protecting crop plants from insect damage or of controlling the propagation of the pest insects themselves.

Using the limited resources available to this project, we asked whether any evidence of RNAi-caused knockdown could be observed upon transient expression of dsRNA in plant tissue. This approach proved to be challenging for a number of reasons, and no consistent and significant evidence of RNAi was observed in any bioassay. A small number of H. armigera genes, all expressed in the larval midgut, were targeted. The first four are known to be essential genes, and the fifth is an enzyme specifically required for gossypol detoxification. Initial experiments were conducted to test biosassays of larvae on Nicotiana benthamiana. These control bioassays on different plants showed that larval growth on control and mock-treated plants was quite variable; even when older larvae were tested, no significant effects were observed. The variability observed demanded that we have found it necessary to find a better experimental system. Results of control experiments using cowpea were encouraging, but no evidence for RNAi was obtained upon transient expression of the dsRNA constructs. This work suggests that a full-scale project to evaluate genes for their effectiveness as RNAi targets will be required, making use of transgenic plants, such as Arabidopsis or tobacco in the first instance.