Rootworm evolving to eat GMO corn, say scientists

The insects were unable to survive exposure to Bacillus thuringiensis-infected corn, but resistance has been growing.

According to a recent paper in the Proceedings of the National Academy of Scientists, corn rootworms may be evolving rapidly to develop a tolerance to GMO corn. Aaron Gassmann, an Iowa State University entomologist, warns that management practices have left the nation’s corn supply vulnerable to the pests, which are supposed to be unable to feed on the three varieties of GMO corn currently in use.

So-called BT corn was introduced in 1996 and contains genes of the Bacillus thuringiensis bacteria, making it poisonous to rootworms and corn borers. USDA says BT corn constitutes 75 percent of the nation’s corn crop. Consuming Bacillus thuringiensis should cause the cell walls in rootworms’ bodies to break down, resulting in death from septicemia.

However, concerns over effective management began to spring up almost immediately. In 2002, EPA recommended a 50 percent refuge requirement for non-BT corn to prevent BT resistance. Rootworms in these refuges would remain susceptible to the Bt toxin, and would prevent resistance from building up by mating with any Bt-resistant worms that evolved in neighboring fields.

However, pressure from industry led EPA to reduce the voluntary guidelines at a 20 percent refuge requirement. According to a 2003 report from Gregory Jaffe of the Biotechnology Project, 19 percent of farms did not comply with the refuge recommendation maximum, and 13 percent don’t plant refuges at all.

Then in 2009, Gassmann was alerted to extensive rootworm infestations in Iowa which suggested rootworms had become resistant to one of the three brands of BT corn on the market. In 2011, rootworm outbreaks were found in Illinois, Minnesota, Nebraska and South Dakota, states that are contiguous with Iowa. Gassman found evidence that resistance to one variety of BT corn heightened the chances of resistance to another, meaning corn engineered to contain multiple BT toxins won’t be as effective as had been hoped.

There is no evidence that BT corn has become ineffective against other pests, so farmers will likely continue to use it. However, Gassman says, farmers will likely turn to pesticides to rid their crops of rootworms, which increases costs and mitigates the ecological benefits of BT corn.