Slowing insects’ development of resistance to corn hybrids with Bt is a top concern among many farmers and ag retailers now. In a new article in the June issue of the Journal of Economic Entomology, two university researchers offer a radical solution. They claim the best way to prevent the spread of western corn rootworm developing resistance to Bt corn is to have the Environmental Protection Agency double the percentage of corn acres planted to mandated refuges.
"Corn rootworms can cost U.S. farmers close to $1 billion each year. Bt corn has helped to reduce these costs and to decrease insecticide sprays, but evolution of resistance by the pests can diminish or even eliminate these benefits," said Bruce Tabashnik, who heads the department of entomology in the UA College of Agriculture and Life Sciences and is one of the authors of the new article.
"To delay pest resistance and sustain the benefits of Bt corn, we recommend planting more corn that does not produce Bt toxins active against rootworms. This refuge strategy allows the susceptible pests to survive and has worked to slow resistance of other pests to Bt crops."
"Most of the corn seed currently produced in the U.S. is transgenic and includes genes for insect control," said Fred Gould, North Carolina State University and co-author of the new article. "Enlarging refuges will require more seed without corn rootworm control genes. This shift in production will take time, so this process should begin immediately."
In addition to increasing corn refuge acres, the authors also encourage the increased use of integrated pest management practices to slow down insect resistance to Bt corn.
"We advocate greater use of integrated pest management, which is a common sense approach based on the best available combination of tactics," Tabashnik said. "The goals are to limit pest damage, maximize farmer profits and preserve environmental quality. Maintaining the effectiveness of Bt toxins can help us achieve these goals."
"We're seeing the early signs of rootworm resistance to Bt corn, which fit predictions from evolutionary theory and experiments in the lab and greenhouse," he added.
The paper indicates rootworm resistance to Bt corn was first detected in 2009 in Iowa; six years after sales of rootworm-killing Bt corn began in the U.S. and only one year after this type of Bt corn was first planted on more than 25 million acres.
According to Tabashnik, Cry3Bb1 is effective enough to be economically useful, but not effective enough to meet the so-called high dose standard, the ability to kill at least 99.99 percent of susceptible pests and also nearly all of the hybrid pests that are produced when resistant pests mate with susceptible pests.
Tabashnik pointed to a case in Puerto Rico, where adequate refuges were not planted. Within a few years, the pests evolved resistance and devoured the Bt corn plants. The biotechnology companies voluntary stopped selling Bt corn seed there, but five years later, the insects remain resistant to the toxin.
Although biotech companies recently starting selling some varieties of Bt corn that produce combinations of Bt toxins, Tabashnik said, the resistance to one toxin still raises concerns.
"You can think of the multi-Bt toxin approach as a pyramid: The base has to be stable. If one of your building blocks, which is susceptibility to Cry3Bb1, is crumbling, you have a problem. Resistance to any one toxin jeopardizes the effectiveness of the whole system."
"We're at a tipping point where decisive action can provide long-term benefits and avoid loss of an environmentally friendly tool for pest control."