Planting window requirements for Bt cotton in Australia: do they limit the exposure of Helicoverpa spp. (Lepidoptera: Noctuidae) to Bt toxins?

The threat of Bt resistance in Helicoverpa armigera (Hubner 1808) and H.punctigera (Wallengren 1860) (Heliothinae) is a major impediment to the sustainability of transgenic (Bt) cotton crops in Australia. A resistance management plan has, therefore, been instigated that in part requires farmers to sow their Bt cotton within specified dates to limit the number of generations of Helicoverpa spp. that are exposed to Bt toxins during the growing season. This study evaluated this strategy by utilising industry-wide databases of planting dates and other cropping/pest management events throughout 11 growing seasons. The analysis included model predictions of when Helicoverpa is likely to enter winter (pupal) diapause and emerge as moths in spring, and the long-term pheromone trapping records of moths. In New South Wales and southern Queensland (Qld), most first-generation moths were caught at the same time, or before, cotton was planted (mostly in October). This generation was, therefore, unlikely to have been exposed significantly to Bt toxins as feeding larvae in spring, but some individuals (especially H.armigera) may have been exposed to Bt prior to pupating in the previous autumn. Three subsequent generations, which could have been exposed to Bt during summer-autumn, were recognisable in trap catches before moths became very rare in early April. By then, the majority of Helicoverpa had entered winter diapause as pupae and cotton harvest was well underway. We conclude that the current planting window (pre-mid-November in these regions) achieves little in limiting the exposure of Helicoverpa to Bt. However, insufficient is known of the population dynamics of Helicoverpa in central Qld to be as definitive about the need for planting windows there. Importantly, Helicoverpa are active all year round in central Qld and cotton can be grown over a longer period, thus increasing the risks of greater exposure to Bt.