Five of 13 major crop pests have evolved resistance to corn and cotton genetically engineered to make their own insecticide, providing lessons for extending the usefulness of such technologies, University of Arizona researchers said in a study.
The increase in resistance, from one insect species in 2005, was expected because the crops are more widely planted, pests have been exposed to the insecticides for more years and monitoring efforts have improved, according to the study published today in the journal Nature Biotechnology. Some technologies have kept resistance at bay for more than than 15 years while others succumb in as few as two years, the study said.
More than 1 billion acres worldwide have been planted with crops engineered to produce insecticidal proteins derived from Bacillus thuringiensis, or Bt, a soil bacterium, reducing use of chemical insecticides, the study said. Where data shows large numbers of resistant pests are living in fields, and where the resistant trait is not recessive, regulators need to take stronger actions, the study said.
"When risk is indicated, either take more stringent measures to delay resistance, such as requiring larger refuges, or this pest will probably evolve resistance quickly to this Bt crop," Bruce Tabashnik, a University of Arizona entomologist and the paper’s lead author, said in a statement accompanying the study.
Regulators try to delay resistance by requiring farmers who plant Bt crops to also plant a "refuge" on adjacent land using non-Bt crops. The reasoning is that bugs not exposed to the toxin will mate with any resistant insects, creating a new generation that is once again susceptible to the insecticide.
The study analyzed data from 77 studies on 13 pest species on five continents. Three of the five cases of resistance are in the U.S., where about half of the world’s Bt crop acreage is planted.
Rootworms are the most recent insect to develop resistance, overcoming Monsanto Co.’s rootworm-killing corn in at least two U.S. states, the U.S. Environmental Protection Agency said in January, confirming earlier university studies.
Other instances of field-evolved resistance in Bt corn include fall armyworms in Puerto Rico, a U.S. territory, and maize-stem borers in South Africa, the researchers said. Bt-resistant insects in cotton include pink bollworms in India and bollworms in the U.S.
"Perhaps the most compelling evidence that refuges work comes from the pink bollworm, which evolved resistance rapidly to Bt cotton in India, but not in the U.S.," Tabashnik said in the statement. "Same pest, same Bt protein, but very different outcomes."
Indian farmer compliance with planting refuges "was low," while institutional support in the U.S. helped farmers implement "an effective refuge strategy" for pink bollworms, he said.