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INSECT RESISTANCE TO BT CROPS: Lessons from the First Seven Years

Bruce E. Tabashnika*, Yves Carrièrea, Timothy J. Dennehya, Shai Morina, Mark S. Sistersona, Richard T. Roushb, Anthony M. Sheltonc, and Jian-Zhou Zhaoc

Problems with insecticides have spurred the search for alternative means of insect control. Not only do conventional insecticides cause environmental and safety hazards, more than 500 species of pests have evolved resistance to them. Insecticidal proteins from the common bacterium Bacillus thuringiensis (Bt) are an environmentally friendly alternative to conventional insecticides.

Bt toxins kill insects by binding to and disrupting midgut membranes. Unlike broad spectrum insecticides, Bt toxins kill certain pests but cause little or no harm to most nontarget organisms including wildlife, insect natural enemies, and people. For decades, sprays containing Bt toxins have been useful in organic and mainstream pest control.

Transgenic crops that produce Bt toxins control some key pests, thus decreasing reliance on insecticide applications1. Surprisingly, after seven years of large scale planting of Bt crops, pest resistance to Bt crops in the field has not been documented2.

Large scale planting of Bt crops began in 1996 and grew quickly to more than 10 million ha per year. The cumulative area of Bt crops grown globally from 1996 to 2002 exceeded 62 million ha, enough to cover the states of California and Iowa. More than 99% of this area was planted with either Bt corn or Bt cotton producing Bt toxins Cry1Ab or Cry1Ac to kill larvae of lepidopteran pests. These Bt crops expose pests to Bt toxin throughout the growing season.

The widespread and prolonged exposure to Bt toxins represents one of the largest selections for resistance in insects the world has ever seen. Before Bt crops were grown commercially, many scientists predicted that pests would evolve resistance quickly. This view was supported by pervasive resistance to conventional insecticides, lab-selected resistance to Bt toxins in many pests, and field-evolved resistance to sprays of Bt toxins in diamondback moth (Plutella xylostella)3.

To counter the threat of resistance, the refuge strategy4 has been adopted widely. Growers provide refuges of non-transgenic host plants along with Bt crops to enhance survival of susceptible pests. Ideally, rare resistant adults emerging from Bt crops mate with more abundant susceptible adults from refuges. Modeling results suggest that resistance can be substantially delayed if the heterozygous offspring from such matings are killed by the Bt crop. The refuge strategy is based primarily on theoretical calculations and limited experimental evidence from small-scale experiments with diamondback moth. No rigorous large scale tests of the refuge strategy have been reported.

Although the refuge strategy works beautifully in theory, some scientists thought that real world deviations from its assumptions could doom Bt crops to early failure. Contrary to this expectation, researchers from the University of Arizona, the University of California, and Cornell University recently reported that field-evolved resistance to Bt crops has not been documented yet2. This conclusion is based on a review of published results of resistance monitoring efforts in the U.S. and China, which account for the vast majority of Bt crops grown worldwide. To enhance understanding of this surprising outcome, we review below the status of pest resistance to Bt crops, including responses of resistant strains in laboratory and greenhouse tests, and frequencies of resistance in field populations targeted by Bt crops.

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