Transgenic crops that produce insect-killing proteins from the bacterium Bacillus thuringiensis (Bt) have reduced reliance on insecticide sprays since 1996. Yet, just as insects become resistant to conventional insecticides, they also can evolve resistance to the Bt proteins in transgenic crops. Thus, to delay pest resistance, the U.S. Environmental Protection Agency (EPA) has required farmers to plant "refuges" of crops that do not produce Bt proteins near Bt crops. But how much refuge acreage is enough?
In "Delaying Corn Rootworm Resistance to Bt Corn," an article appearing in the June, 2012 issue of the Journal of Economic Entomology, authors Bruce Tabashnik (University of Arizona) and Fred Gould (North Carolina State University) conclude the EPA should more than double the percentage of corn acres planted to mandated refuges to delay insect resistance, encourage integrated pest management (IPM), and promote more sustainable crop protection.
To slow resistance in the western corn rootworm (Diabrotica virgifera virgifera), one of the most economically important crop pests in the United States, the EPA currently requires a 20% refuge for corn producing one Bt protein (Cry3Bb1), and a 5% refuge for corn that simultaneously produces two different Bt proteins. However, the authors note that this devastating pest has rapidly evolved resistance to Cry3Bb1 in some areas of the U.S. corn belt. For Bt corn to remain effective against rootworms, they recommend increasing refuge requirements to 50% for corn producing one Bt protein and 20% for corn producing two Bt proteins.
"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 Dr. Tabashnik. "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 approach, called the 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 Dr. Gould. "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 increased refuge sizes, the authors write that the best way to postpone resistance is to use IPM, in which Bt corn is combined with other control tactics such as crop rotation and judicious use of insecticide sprays.
|Contact: Bruce E. Tabashnik|
Entomological Society of America