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Maintaining Susceptibility to Insecticidal Traits in Populations of Corn Pests
Maintaining Susceptibility to Insecticidal Traits in Populations of Corn Pests
By John P. Roche
Insecticidal traits can be engineered into corn and other crop plants to reduce losses to crop pests such as the Western corn rootworm (Diabrotica virgifera virgifera), the European corn borer (Ostrinia nubilalis), and the corn earworm (Helicoverpa zea). Such engineered crops have been a tremendous success in reducing crop damage. For example, DNA from the bacterium Bacillus thuringiensis (Bt) can be inserted into corn DNA, allowing corn to produce Bt proteins that are toxic to corn insect pests. Before the use of Bt corn began in 1996, losses to Western corn rootworm were over $1 billion per year; losses to the pest are far lower today.
Insecticidal crops have reduced losses, but pest populations can develop resistance to insecticidal traits in crop plants, reducing their effectiveness. David Onstad, Ph.D., and colleagues at DuPont Pioneer published a comprehensive review this month in Environmental Entomology that investigates the two leading strategies for reducing development of resistance to insecticidal traits.
The goal of both strategies is to maintain susceptibility to insecticidal traits in pest populations—in evolutionary terms, to maintain genetic alleles for susceptibility in the pest population. In one strategy, farmers plant crops that do not express insecticidal traits in “refuge” crop fields. This allows genetic alleles for sensitivity to the insecticidal traits to be maintained in pest populations living in the refuges. In the second strategy, farmers plant seed that is a blended mixture of seeds that produce plants expressing insecticidal traits as well as seeds that produce plants that do not express insecticidal traits. In this blended seed strategy, genetic alleles for sensitivity to the insecticidal traits are maintained in pest populations that eat the plants from the non-insecticidal seed.
To investigate the relative effectiveness of these strategies and the factors that affect maintenance of pest susceptibility, Onstad and colleagues performed an extensive analysis of research on refuge and blended seed strategies.
Determining the effectiveness of different resistance-management strategies is complex because many factors influence outcomes, including larval movement, the behavior of adult insects, and farming practices. For example, if adult insects mate only once in the small area where they emerge, there could be matings between rare resistant insects within a Bt cornfield. This would increase the probability of evolution occurring faster.
In light of these complexities, Onstad and colleagues concluded that the use of blended seed should be considered on a case-by-case basis. There are situations, however, in which clear predictions can be made about the relative success of the two strategies. For example, blended seed is likely to be ineffective for species that graze on leaves rather than bore within stems when there is a lot of overlap of vegetation from adjacent plants, because pests would probably feed on both insecticidal plants and non-insecticidal plants. On the other hand, blended seed is a good option when farmers choose not to plant refuge plots due to cultural or monetary considerations.
In their analysis, the researchers note that there are many factors influencing the effects of seed blends on susceptibility, with different explanations being supported in different situations. And they note that different explanations are not necessarily mutually exclusive, so multiple factors could determine the effects of seed blends on susceptibility in a given situation. Results also vary by type of crop. For example, seed blends tend to be more effective in reducing resistance in pests of corn than in pests of cotton.
One complication faced by farmers wishing to reduce resistance to insecticidal traits is cross pollination. In cross pollination, corn is pollinated by a different strain of corn. This could produce a mosaic of Bt and non-Bt grain within a plant, with some seed containing insecticidal traits and some not containing insecticidal traits. Onstad and colleagues reviewed 14 field experiments testing the influence of cross-pollination on corn earworm survival in locations where Bt corn grew next to refuge corn. 
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