A critical component of managing resistance to insecticide in crop pests like the corn earworm (Helicoverpa zea) is the use of refuge crops that lack insecticidal traits, such as expression of Bacillus thuringiensis (Bt) proteins, alongside crops that do express the insecticidal traits. But growers face a question: whether to plant a separate nearby refuge field of non-insecticidal crops or to use a blended mixture of seeds that produce both in the same field. A new review of existing research on this question suggests the answer is highly dependent on the characteristics of individual pest species. (Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org)

By John P. Roche, Ph.D.

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. They found that larval survival was at least as high when Bt corn could cross pollinate refuge corn as in locations where it could not. He said that there were two main conclusions from these 14 field studies. “First, there is variability in the survival of pests in the field. Because variability is common in field studies, most field entomologists would not be surprised by this variability. Second, the average survival of pests, which was 82 percent, was relatively high.”

The component of agricultural practice dedicated to controlling resistance to pest control is called Insect Resistance Management (IRM). When asked about the biggest challenge facing IRM, Onstad said, “It may be the need to account for the practical aspects and overall goals of farming in what has typically been a purely biological analysis of resistance evolution. Even though pest management has considered economics for 50 years, IRM strategies should move in that direction so that guidelines match reality and are more likely to be implemented.”

The main conclusion of the review, Onstad said, was that “refuge deployment depends on the specifics of every insect, environment, and farming scenario.” When asked what he envisions being the most important next step in this field of research, Onstad said, “We cannot assume that knowledge about one species can represent another species, even one in the same genus. Thus, a dedication to studying species-specific behavior and field survival will help refine refuge strategies in the future.”

This research holds tremendous economic importance: losses from crop pests such as the Western corn rootworm remain significant, and, if resistance rebounded, the costs could be immense. But the research also is important in adding to our understanding of the evolutionary and environmental biology of a complex ecological system. As such, new discoveries relating to the specifics of how refuges and blended seed mixtures affect the population biology of resistance in pests can provide valuable insights to growing corn and other crop plants, and to understanding complex evolutionary interactions in an ecological community.

John P. Roche, Ph.D., is an author, biologist, and science writer dedicated to making rigorous science clear and accessible. He has a Ph.D. in biology, has published more than 180 articles, and has written and taught extensively about science. For more information, visit http://authorjohnproche.com.