Refuge crop performance as part of the Bt resistance management strategy forHelicoverpaspp. (Lepidoptera: Noctuidae) in Australian cotton production systems

Abstract

The emergence of Bt resistance among key insect pests (especially Noctuidae) is a major threat to the deployment of transgenic (Bt) cotton crops, used to combat these pests, worldwide. Refuge crops are, therefore, commonly grown in association with Bt cotton to generate large numbers of Bt-susceptible insects, thus reducing the risk of Bt resistance. In Australian Bt cotton production systems, where the primary pests are Helicoverpa moths, refuge crops are mandatory, and pigeon pea and non-Bt cotton are the refuge options available to growers. When Bt cotton was first deployed (in the mid 1990s), pigeon pea was assessed as having twice the capacity to produce Helicoverpa moths as unsprayed, non-Bt cotton. Only half the amount (in area) of pigeon pea was, therefore, required to be grown as a refuge compared with unsprayed, non-Bt cotton. The agronomy, insecticide use, varieties of Bt cotton (now based on two Bt genes (Bollgard II®), compared with the original single gene (Ingard®) varieties) and farmer profit margins have, however, changed radically since then. This has led to questioning of the continued relativities and best management practices involving these refuge crops. This paper assesses the performance records of pigeon pea and non-Bt cotton refuges in the cotton production systems of eastern Australia since 1996. Pupae abundance is used as a surrogate for moth production. Pigeon pea has maintained its substantial superiority over unsprayed, non-Bt cotton as a refuge generator of Helicoverpa throughout both the Ingard and Bollgard II eras. This is supported by field counts of both live pupae and empty pupal cases left behind in the soil when moths have emerged. There was some evidence that Helicoverpa production has decreased in time in both pigeon pea and cotton refuges. The incidence of parasitism of pupae has increased from the Ingard to the Bollgard II era. This latter change may, at least in part, explain a reduction in Helicoverpa productivity from refuges in recent years. There was no evidence to support a difference in parasitism of Helicoverpa between the two refuge crop types, but a temporal shift in parasite community (Tachinidae becoming more common) was apparent.