Resistance evolution is an important consideration for the target pests of transgenic Bt cotton products like Bollgard and Bollgard II because of the consistent exposure to insecticidal proteins that they provide. However, the heliothine target pests of Bt cotton in the US, tobacco budworm and cotton bollworm, have been observed to infest a wide variety of crop and non-crop hosts other than cotton. This high degree of polyphagy has implications for resistance management of these pests in Bt cotton. Some portion of the population of these pests will develop on hosts other than cotton and therefore will not be exposed to the control methods used in cotton, meaning that this alternative hosts can act as a form of natural refuge for Bt cotton. This natural refuge will slow the rate of resistance evolution to Bollgard and Bollgard II.

This paper reports on a two-year, multi-state study of tobacco budworm that quantifies the proportion of the population developing on cotton versus other hosts in different parts of the US Cotton Belt. Intensive pheromone trapping was carried throughout the cotton growing season at multiple locations of each of at least six states in 2004 and 2005. Novel methods were used to estimate the proportion of the trapped adults that had developed on cotton versus other hosts at different time points at each location. The size of the natural refuge provided by alternative hosts was calculated from these estimates.