Abstract
To delay resistance development to Bacillus thuringiensis (Bt) plants expressing their own insecticide, the application of the Insect Resistance Management strategy called “High Dose/Refuge Strategy” (HD/R) is recommended by the US Environmental Protection Agency (US EPA). This strategy was developed for Bt plants expressing one toxin. Presently, however, new Bt plants that simultaneously express two toxins are on the market. We used a mathematical model to evaluate the efficiency of the HD/R strategy for both these Bt toxins. As the current two-toxin Bt plants do not express two new Cry toxins but reuse one toxin already in use with a one-toxin plant, we estimated the spread of resistance when the resistance alleles are not rare. This study assesses: (i) whether the two toxins have to be present in high concentration, and (ii) the impact of the relative size of the refuge zone on the evolution of resistance and population density. We concluded that for Bt plants expressing one toxin, a high concentration is an essential condition for resistance management. For the pyramided Bt plants, one toxin could be expressed at a low titer if the two toxins are used for the first time, and a small refuge zone is acceptable.
Highlights
Bacillus thuringiensis (Bt) was discovered in 1901 in Japan in diseased silkworms and has been produced on an industrial scale for its insecticidal properties since 1938; only in 1953 was it shown that its insecticidal properties were the result of the parasporal inclusion of a crystal protein called Cry toxin [1]
The spread of resistance in the population is efficiently delayed by Bt plants producing one toxin with a high concentration level
The critical threshold of 50% resistance allele frequency in the population (GF50) is reached at the 48th generation. This strategy is more effective with Bt plants expressing simultaneously two toxins because resistance is reversed in the population
Summary
Bacillus thuringiensis (Bt) was discovered in 1901 in Japan in diseased silkworms and has been produced on an industrial scale for its insecticidal properties since 1938; only in 1953 was it shown that its insecticidal properties were the result of the parasporal inclusion of a crystal protein called Cry toxin [1]. The death of the insect follows within hours or days [3]. These insecticidal proteins are active ingredients of some environmentally friendly insecticides because they are harmless to natural enemies and other nontarget organisms due to their narrow host specificity and short life in the environment [4]. Website online: http://www.epa.gov/oppbppd1/biopesticides/regtools/biotech-reg-prod.htm#crops (accessed on 27 April 2012).
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