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Insect Resistance Management
Dow AgroSciences - July 23, 2001
Several crops derived through biotechnology produce a protein from the bacterium, Bacillus thuringiensis (Bt) to provide protection from certain damaging insect pests. Some strains of the bacteria that produce these pesticidal proteins are also used to formulate sprayable pesticides. The successful adoption of Bt crops by farmers has led some to suggest that the widespread use of Bt proteins in crops will lead to the development of insect populations that are resistant to these proteins thus rendering Bt crops and Bt sprays less effective in controlling these destructive insects. As a safeguard, the U.S. Environmental Protection Agency (EPA) requires registrants to implement science-based plans to minimize the potential development of pest resistance to plant-expressed Bt proteins. A recent report from the U.S. National Academy of Sciences recommends the development of insect resistance management (IRM) plans for both Bt crops and sprayable Bt pesticides. The goal of these plans is to ensure continued effectiveness of both the plant expressed and the sprayable formulations of this family of pesticidal proteins.
Resistance
Insect populations usually become resistant to a practice used to control them only after the majority of susceptible insects have been removed by that control practice. The remaining insects that carry genes for resistance mate with each other and the resulting resistant insect populations subsequently increase in numbers, making control difficult. Examples of control practices that insects have become resistant to include crop rotation, traditional host plant resistance, and pesticides (including sprayable Bt formulations).
Integrated resistance management plans
A coordinated scientific approach is used to establish management practices that will minimize the risk of resistance and sustain the performance of Bt pesticidal proteins. These IRM strategies have been developed through collaborative efforts of leading academic, government, and industry scientists. Recently developed IRM plans for Bt crops couple Bt plants with a structured "refuge" of non-Bt plants. Refuge refers to a portion of the crop plants in or near a field that does not contain the Bt protein. A typical IRM plan in corn requires that at least 20% of the grower's corn acreage be planted to a non-Bt corn refuge within 1/2 mile of the Bt corn fields. The purpose of the refuge is to maintain a population of target insect pests that are susceptible to the Bt protein. Those susceptible insects can mate with rare resistant insects that may emerge from the Bt crop so that the resulting offspring will be susceptible to the Bt protein. The availability of susceptible insects decreases the probability that resistant insects will mate with each other. Consequently, with continuous implementation of this IRM strategy, subsequent generations of insects will remain susceptible to the Bt protein. Using this strategy increases the probability that the effectiveness of these important insect control proteins can be sustained.
It is important to note that the only documented instances of insect resistance to Bt in the field have been in vegetable growing areas where numerous and frequent microbial Bt spray applications have been made. To date, there is no evidence that insect pests are developing resistance to Bt crops or that the use of Bt crops is affecting the continued use and effectiveness of Bt sprays. However, providers of Bt technology are committed to IRM so that Bt use in crops and sprays can be sustained, allowing farmers and society to fully realize the economic and environmental benefits of this technology.
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