Abstract
Transgenic crops that express insecticide genes from Bacillus thuringiensis (Bt) are used worldwide against moth and beetle pests. Because these engineered plants can kill over 95% of susceptible larvae, they can rapidly select for resistance. Here, we use a model for a pyramid two-toxin Bt crop to explore the consequences of spatio-temporal variation in the area of Bt crop and non-Bt refuge habitat. We show that variability over time in the proportion of suitable non-Bt breeding habitat, Q, or in the total area of Bt and suitable non-Bt habitat, K, can increase the overall rate of resistance evolution by causing short-term surges of intense selection. These surges can be exacerbated when temporal variation in Q and/or K cause high larval densities in refuges that increase density-dependent mortality; this will give resistant larvae in Bt fields a relative advantage over susceptible larvae that largely depend on refuges. We address the effects of spatio-temporal variation in a management setting for two bollworm pests of cotton, Helicoverpa armigera and H. punctigera, and field data on landscape crop distributions from Australia. Even a small proportion of Bt fields available to egg-laying females when refuges are sparse may result in high exposure to Bt for just a single generation per year and cause a surge in selection. Therefore, rapid resistance evolution can occur when Bt crops are rare rather than common in the landscape. These results highlight the need to understand spatio-temporal fluctuations in the landscape composition of Bt crops and non-Bt habitats in order to design effective resistance management strategies.
Highlights
Transgenic maize, potatoes, and cotton that carry the insecticidal toxins of Bacillus thuringiensis (Bt) are grown extensively throughout the world
Spatio-temporal variation in selection for Bt resistance could occur over the course of a year or among years, and could be driven by variation in the proportion of pest habitat that is planted to Bt crops or by variation in the total amount of pest breeding habitat, Bt-crops or otherwise
When there is potential density-dependent mortality in refuges, there can be an interaction between changes in the proportion of breeding habitat that is refuge and changes in the density of the pest insect that combine to increase surges in resistance evolution and diminish the durability of a Bt crop
Summary
Transgenic maize, potatoes, and cotton that carry the insecticidal toxins of Bacillus thuringiensis (Bt) are grown extensively throughout the world Many of these Bt crops are highly effective against lepidopteran and beetle pests, and their adoption has considerably reduced the use of PLOS ONE | DOI:10.1371/journal.pone.0169167. Temporal Variation May Speed Resistance to Bt Crops conventional chemical insecticides in some areas [1,2,3,4,5]. They risk losing efficacy if targeted pests evolve resistance. This work has generally assumed that spatial variation does not change through time, and little theoretical attention has been given to spatio-temporal variability in the risk of resistance evolution. We analyze how these types of spatio-temporal variation affect the risk of resistance evolution
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