Advances in managing pest resistance to bt crops: Pyramids and seed mixtures

Abstract

Transgenic crops producing toxins from the soil bacterium Bacillus thuringiensis (Bt) have been widely used for the control of insect pests during the last 20 years. Although Bt crops have provided significant environmental and economic benefits, sustainable use of these crops is threatened by the rapid evolution of resistance. The primary strategy for delaying pest adaptation to Bt crops has been to ensure that sufficient refuges of non-Bt host plants occur near Bt crops. Two relatively new approaches used with refuges are “pyramids�?, which are plants that produce two or more Bt toxins effective against the same pest, and planting random mixtures of Bt seeds and non-Bt seeds of the same crop within fields. Here we review theory and data about conditions favoring success of pyramids and seed mixtures for delaying evolution of pest resistance to Bt crops. Pyramids of structurally distinct toxins can be exceptionally effective under optimal conditions, particularly when pest populations are highly susceptible to all toxins in the pyramid. Seed mixtures eliminate the problem of farmers who fail to plant separate refuges of non-Bt plants, but may accelerate evolution of resistance when larval movement between plants or pollen-mediated gene flow between plants is extensive. In the many cases where pests are not highly susceptible to the toxins in Bt crops or other conditions are not optimal, we suggest that an effective refuge percentage of at least 20% is required to substantially delay pest resistance, even when pyramids, seed mixtures, or both are used. We also recommend integrating Bt crops with other management tactics to delay resistance in pests with low susceptibility to Bt toxins.