When pests developed resistance to Bt crops such as corn, potatoes and cotton, seed companies had an answer: two-toxin varieties.
But new research suggests it may be the wrong one.
Bioengineered crops with genes taken from bacterium Bacillus thuringiensis have been widely grown since 1996. The genes produce Bt toxins, reducing the need for pesticide use. When pests began developing resistance, seed companies adopted a “pyramid” strategy: each plant produces two or more toxins that kill the same pest.
The idea was simple — if the first toxin doesn’t kill the pest, the second one will.
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The pyramid strategy has been adopted extensively, with two-toxin Bt cotton completely replacing its one-toxin cousin since 2011 in the U.S.
But “in the real world, things are a bit more complicated,” states a press release from University of Arizona outlining the results of a new study conducted by a team of its entomologists.
“The pyramid strategy has been touted mostly on the basis of (computer) simulation models,” said Yves Carrière, the professor of entomology who led the study.
“We tested the underlying assumptions of the models in lab experiments with a major pest of corn and cotton.”
The researchers used cotton bollworm — also known as corn earworm — a species of moth that is a major agricultural pest, and selected it for resistance against one of the Bt toxins, Cry1Ac.
As expected, the resistant caterpillars survived after munching on cotton plants producing only that toxin. The surprise came when Carrière’s team put them on pyramided Bt cotton containing Cry2Ab in addition to Cry1Ac.
“On the two-toxin plants, the caterpillars selected for resistance to one toxin survived significantly better than caterpillars from a susceptible strain,” said Carrière.
In total, the researchers carried out 21 such experiments and used eight different pest species. In 19 of them they found some degree of cross-resistance between Cry1 and Cry2 toxins. Moreover, populations of cotton bollworm are starting to show up in fields.
So what’s the flaw in the pyramid strategy?
The authors concluded even low levels of cross-resistance can reduce redundant killing and undermine the pyramid strategy, and that cross-resistance is fairly common in cotton bollworm and some other pests not highly susceptible to Bt toxins to begin with.
The Arizona researchers also examined another technique for combating resistance — crop refuges — and found it may not work as well as hoped either.
They looked at insects that carry two forms of the same gene for resistance to Bt — one confers susceptibility and the other resistance. When resistance to a toxin is recessive, one resistance allele is not sufficient to increase survival. In other words, offspring that inherit one allele of each type will not be resistant, while offspring that inherit two resistance alleles will be resistant.
Refuges are based on this idea.
They consist of regular plants that don’t produce Bt toxins. This allows susceptible pests to survive and mate with resistant ones — and thus continually reduce the resistant population.
But if inheritance of resistance is dominant instead of recessive, as seen with cotton bollworm, matings between a resistant moth and a susceptible moth can produce resistant offspring, which hastens resistance. However, the size of the refuge area makes a difference.
“Our simulations tell us that with 10 per cent of acreage set aside for refuges, resistance evolves quite fast, but if you put 30 or 40 per cent aside, you can substantially delay it,” said Carrière.
According to study co-author Bruce Tabashnik, overly optimistic assumptions have led the U.S. Environmental Protection Agency to greatly reduce size requirements for refuges.
The new results are a wake-up call, said Carrière.
“We need more empirical data to refine our simulation models, optimize our strategies and really know how much refuge area is required,” he said. “Meanwhile, let’s not assume that the pyramid strategy is a silver bullet.”
