Many of the assumptions that the use of GMO crops are based upon are not true, researchers at the University of Arizona have found. Specifically, the widely-used new strategy of planting GMO crops that produce two or more toxins to provide “redundant” killing of insect pests is based on assumptions that simply aren’t always true. These multi-toxin crops are “necessary” because of the increasing emergence of pests resistant to single toxins.
These findings are helping to provide an explanation for why some major pests are developing resistance to GMO produced toxins much faster than was previously predicted.
According to the new research, this strategy, which is widely-employed as an attempt to slow the rate of adaption amongst pests, is likely to fail in many cases unless better preventive actions are taken.
“Corn and cotton have been genetically modified to produce pest-killing proteins from the bacterium Bacillus thuringiensis, or Bt for short. Bt crops were first grown widely in 1996, and several pests have already become resistant to plants that produce a single Bt toxin. To thwart further evolution of pest resistance to Bt crops, farmers have recently shifted to the ‘pyramid’ strategy: each plant produces two or more toxins that kill the same pest. The pyramid strategy has been adopted extensively, with two-toxin Bt cotton completely replacing one-toxin Bt cotton since 2011 in the U.S.”
“Most scientists agree that two-toxin plants will be more durable than one-toxin plants. The extent of the advantage of the pyramid strategy, however, rests on assumptions that are not always met, the study reports. Using lab experiments, computer simulations and analysis of published experimental data, the new results help explain why one major pest has started to become resistant faster than anticipated.”
“The pyramid strategy has been touted mostly on the basis of simulation models,” said Yves Carrière, a professor of entomology in the UA College of Agriculture and Life Sciences who led the study. “We tested the underlying assumptions of the models in lab experiments with a major pest of corn and cotton. The results provide empirical data that can help to improve the models and make the crops more durable.”
The primary assumption of the multi-toxin strategy is that it provides redundant killing, Carrière explained. “Redundant killing can be achieved by plants producing two toxins that act in different ways to kill the same pest,” he said, “so, if an individual pest has resistance to one toxin, the other toxin will kill it.”
But reality is considerably more complicated than this.
The University of Arizona press release continues:
“We obviously can’t release resistant insects into the field, so we breed them in the lab and bring in the crop plants to do feeding experiments,” Carrière said. “For their experiments, the group collected cotton bollworm — also known as corn earworm or Helicoverpa zea -, 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.”
“If the assumption of redundant killing is correct, caterpillars resistant to the first toxin should survive on one-toxin plants, but not on two-toxin plants, because the second toxin should kill them,” Carrière explained.
“But on the two-toxin plants, the caterpillars selected for resistance to one toxin survived significantly better than caterpillars from a susceptible strain.”
“These findings show that the crucial assumption of redundant killing does not apply in this case and may also explain the reports indicating some field populations of cotton bollworm rapidly evolved resistance to both toxins.”
“Moreover, the team’s analysis of published data from eight species of pests reveals that some degree of cross-resistance between Cry1 and Cry2 toxins occurred in 19 of 21 experiments. Contradicting the concept of redundant killing, cross-resistance means that selection with one toxin increases resistance to the other toxin.”
The new research was just published in the Proceedings of the National Academy of Sciences.
Image Credits: Jose Roberto Peruca.