The adoption of transgenic (Bt) cotton varieties has markedly reduced feeding damage by noctuid pests, Helicoverpa armigera and H. punctigera, in Australian cotton crops. However, the potential for Bt resistance to evolve within these insect pests is a major concern. To reduce the likelihood of Bt resistance occurring, it is mandatory for growers of Bt cotton to also grow refuge crops which produce large numbers of Bt-susceptible moths. We present here findings from a series of field studies which sought to identify the relative Helicoverpa productivity of different refuge crop options. The abundance of Helicoverpa pupae (during the cotton season) was compared under crops of Ingard® (transgenic, single Bt gene) cotton, sprayed conventional cotton, and various unsprayed refuge crops (cotton, sorghum, maize, pigeon pea) from 1996 to 2003 in the major cotton-producing regions of northern New South Wales, plus St George and Dirranbandi in southern Queensland. Unsprayed, conventional cotton was used as the ‘control’ refuge in these studies. Productivity of adult Helicoverpa was estimated by surveying for pupae and discounting those that were parasitised and dead. Some supplementary experiments were also conducted to evaluate specific issues, in particular the value of additional refuge crops sown late in 2000–01 (due to accidental over-planting of Ingard® cotton that year) and the potential productivity of mixed plantings of various refuge crops which individually flowered at different times throughout the cotton season. Unsprayed refuges generally produced many more pupae than sprayed, conventional cotton and Ingard® cotton. Overall, pigeon pea, which has the advantage of prolonged flowering and thus on-going attractiveness to Helicoverpa, was the most reliable and productive refuge option, producing about twice as many pupae as unsprayed cotton. The seed mix refuge is perhaps a viable alternative option, although logistically more difficult for growers to adopt. Laboratory rearing of live pupae until moth emergence indicated that parasitism can be a substantial source of mortality in some refuge crops, especially sorghum. The most common parasitoid of Helicoverpa pupae was the ichneumonid wasp, Heteropelma scaposum. Such parasitism can greatly reduce productivity of adult Helicoverpa from refuges and needs to be taken into account when assessing relative refuge ‘value’, although such refuges will obviously contribute to the abundance of these beneficial species across the landscape.
POTENTIAL RESISTANCE OF OKRA LEAF Bt COTTON AGAINST INSECT PESTS
