Genetically modified crops that produce insect-killing proteins from the bacterium Bacillus thuringiensis (Bt) have reduced reliance on insecticide sprays since 1996. These proteins are lethal to some devastating crop pests, but do not harm most other creatures including humans.

Yet, just as insects become resistant to conventional insecticides, they also can evolve resistance to the Bt proteins in transgenic crops. 

To delay pest resistance to Bt proteins, the U.S. Environmental Protection Agency, or EPA, has required farmers to plant "refuges" of crops that do not produce Bt proteins near Bt crops. Refuges are planted with standard, non-Bt crops that pests can eat without ingesting Bt toxins. 

Planting refuges promotes survival of susceptible pests. If susceptible pests greatly outnumber resistant pests, resistant individuals are unlikely to mate with each other and produce resistant offspring.

But how much refuge acreage is enough?

In an article appearing in the June 2012 issue of the Journal of Economic Entomology, authors Bruce Tabashnik from the University of Arizona and Fred Gould from 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, or IPM, and promote more sustainable crop protection.

To slow resistance in the western corn rootworm (Diabrotica virgifera virgifera), a beetle that is one of the most economically important crop pests in the U.S., the EPA currently requires 20 percent of the total acreage being set aside as refuges for corn producing one Bt protein (Cry3Bb1), and a 5 percent refuge portion for corn that simultaneously produces two different Bt proteins.

However, the authors note that this adaptable 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 percent for corn producing one Bt protein and 20 percent 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 Tabashnik, who heads the department of entomology in the UA College of Agriculture and Life Sciences.

"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."