Researchers Validate Resistance Management Practices for Bt-crops

Crops genetically engineered with the toxin Bacillus thuringiensis (Bt) have been grown in the United States since 1996. Since then, concerns about pest resistance have grown, mainly because of observed resistance of the cotton bollworm (Helicoverpa zea), a major pest of cotton. According to a 2009 published study in the Journal of Economic Entomology, insect resistance to Bt can be monitored and prevented.

At almost the same time that Bt-corn and Bt-cotton were released, the EPA established a policy that farmers growing these genetically modified crops had to devote 20 percent of their fields to non-Bt crops. The “refuge” of non-toxic crops would ensure a consistent susceptible pest population, available to mate with resistant individuals. Assuming that the resistance trait was recessive, the offspring of susceptible and resistant pests would be susceptible. For most pests, the theory worked. However, for H. zea, it didn’t work. H. zea was one of the few pests discovered later to have a dominant gene for resistance.

For years, growers followed the policy but complained about having to devote a percentage of their fields to a susceptible crop. Monsanto developed Bollgard II, cotton that contained two Bt genes, hypothesizing that the second gene would kill individuals resistant to the first gene. Monsanto officials proposed that the new “stacking” of Bt genes would reduce the need for a refuge field.

In the face of questions about the validity of the refuge fields and accusations that the engineered crops were creating “superbugs,” University of Arizona entomologist Bruce Tabashnik and his colleagues decided to explore the threat of resistance to Bt crops as well as the validity of the resistance management theory.

Resistance management for Bt crops involves three components: a “refuge” field of non-Bt crops to maintain a susceptible population, high-dose Bt crops and first and second-generation crops producing a “pyramid” for delaying pest resistance. In theory, resistant insects who mated with susceptible insects from the refuge would produce susceptible offspring, as long as the gene for resistance was recessive. The dose of Bt needed to be strong enough to kill all of the offspring from these resistant-susceptible pairings. If plants had two Bt toxins, the pyramid approach, insects resistant to the first toxin would be killed by the second toxin.

Tabashnik’s results verified the success of the refuge fields. However, he observed that states with higher percentages of refuge fields tended to have lower populations of resistant insects. For instance, he notes that North Carolina, with refuge percentages of 82%, had no resistant populations of H. zea, while Arkansas and Mississippi, both with refuge percentages of 39%, had detectable resistant populations of H. zea. His conclusion is that more refuge fields lead to less resistance.

His experiments also confirmed the effectiveness of the high-dose strategy, primarily by observing an area that did not follow the recommended Bt dose. In the Vaalharts area of South Africa, farmers reported resistant B. fusca (African maize stalk borer). However, less than 30 percent of corn farmers planted refuge plots, and the Bt dose in the corn plants was not high enough to kill hybrid populations, speeding up the rate of Bt resistance among B. fusca populations.

Although Tabashnik found that the stacking of Bt toxins in one plant effectively controlled populations, he cautioned against growing plants with a single Bt toxin together with plants with two toxins. His experiments showed that growing the two types together actually accelerates resistance, because each single toxin plant selects for each toxin separately, diminishing the ability of the two-toxin plant to delay resistance.

Overall, the study showed:

  • Refuge fields effectively manage Bt resistance, as long as the percentage of refuges is high enough to maintain a solid population of susceptible insects.
  • If the Bt dose is not high enough to kill hybrid populations, pest populations quickly become resistant.
  • Crops with two Bt toxins work effectively if crops with a single Bt toxin are not grown in the vicinity.

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