Insects are well known for their ability to rapidly develop resistance to many different insecticides. More than 500 different species of insects and mites have been shown to develop resistance, particularly for insecticides that are used repeatedly and at high concentrations. Many factors contribute to resistance development. In any population of insects, a few individuals will carry genes that confer resistance. However, these genes are likely to be extremely rare and remain at low frequencies without insecticide exposure. If an insecticide is used to control these populations, the resistance genes provide increased survival and reproductive success for individuals that carry the resistance genes. The remaining insects that carry genes for resistance mate with other insects carrying the same genes for resistance. As a result, the frequency of individuals carrying the genes increases and the overall population of resistant individuals increases with each generation. At some point, control failures occur with resistant individuals reaching infestation levels comparable to untreated fields.
The behavior and ecology of insects including the number of generations in a growing season, the ability to migrate, and whether or not the insect has a natural refuge from exposure are particularly important to predicting whether resistance is likely to develop. Additionally, the genetics and inheritance of resistance (i.e., how the resistance is passed from generation to generation) can play an important role in the rate of resistance evolution. Finally, factors related to insecticide application such as the number, persistence and timing of applications are also critically important.
It is considered likely that insect pests that are targets of Bt transgenic crops have the potential to develop resistance. These crops produce a protein from the bacterium, Bacillus thuringiensis (Bt) to provide protection from certain insect pests. Some strains of the bacteria that produce these insecticidal proteins are also formulated into sprayable pesticides, which have been in use for over 50 years. Some insects have been shown to develop resistance to these Bt formulations further supporting the contention that pest species can develop resistance to Bt transgenic crops. Additionally, because these plants produce the insecticidal protein throughout the growing season and at high concentrations, the selective pressures on target pests is likely to be high.
As a safeguard, the Environmental Protection Agency (EPA) has requested that companies seeking registration of Bt crops develop and implement science-based plans to manage pest resistance to plant-expressed Bt proteins. The goal of these plans, also known as Insect Resistance Management (IRM), is to ensure continued effectiveness of both the plant expressed and the sprayable formulations of this family of pesticidal proteins.
Currently, a coordinated scientific approach is being used to establish management practices that will minimize the risk of resistance and sustain the performance of Bt pesticidal proteins. These strategies have been developed through collaborative efforts between leading academic, government, and industry scientists. One management practice that is being implemented couples ěhigh doseî Bt plants with a structured ěrefugeî of non-Bt plants. High dose refers to the amount of Bt protein in the plant necessary to kill the vast majority of pests, including insects that may be partially resistant because they inherited a single resistance gene. Refuge refers to a portion of the crop that does not contain the Bt protein. A typical refuge plan in corn requires that 20% of the growerís corn acreage be planted to non-Bt corn, and the refuge acreage be in close proximity to the Bt corn (within ‡ mile). The purpose of the refuge is to maintain a population of insect pests that are susceptible to the Bt protein. Those susceptible insects then can mate with any rare resistant insects that may emerge from the Bt crop, decreasing 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 Bt protein.
