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

Authors: CARRIERE, YVES - University Of Arizona; Fabrick, Jeffrey; TABASHNIK, BRUCE - University Of Arizona

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 6/9/2016
Publication Date: 8/27/2016
Citation: Carrière, Y., Fabrick, J.A., Tabashnik, B.E. 2016. Advances in managing pest resistance to Bt crops: Pyramids and seed mixtures. In: Horowitz, A.R. and Ishaaya, I., editors. Advances in Insect Control and Resistance Management. New York/Switzerland: Springer International Publishing. p. 263-286.

Interpretive Summary: Genetically modified or transgenic crops producing Bacillus thuringiensis (Bt) insecticidal proteins are key management tools against numerous important insect pests. Such Bt transgenic crops produce Bt toxins within the plant and selectively target insect pests while having little impact on beneficial insects or other plants or animals. Although Bt crops have provided significant environmental and economic benefits, sustainable use of these crops is threatened by the development of resistance. The simultaneous planting of non-Bt crops to produce susceptible pest insects (also known as the "refuge strategy") is currently the primary strategy used to delay resistance to Bt crops by insect pests. 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 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. We show that conditions in the field often deviate from optimal implementation of key resistance management factors for some pests and that additional management strategies to delay resistance to Bt crops are needed.

Technical 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 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 mixtures 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 when 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.