Monsanto‘s (NYSE:MON) genetically engineered corn is supposed to resist insects, raising harvest yields for farmers, and reaping profits for the biotech company. However, it appears that Mother Nature has found a way around the genetic modifications.

The U.S. Environmental Protection Agency (EPA) has commented on a report issued in August that rootworms in Illinois were developing a resistance to the insecticide produced by YieldGard, the genetically modified brand of corn, Bloomberg notes.

Rootworm resistance to the corn was originally observed last year, when a number of Midwestern corn fields fell victim to the bugs. Prior to the development of the genetically enhanced corn, which hit the market in 2003, rootworms caused $1 billion in annual damage to corn crops.

Responding to the study, the EPA said there is “mounting evidence” that insects are adapting to the genetic modifications.  Monsanto disputed the finding, noting that the studies did not confirm resistance and called for more research.

Shares of Monstanto rose about 1% in Wednesday mid-day trading.

Corn yields are down this year as the Midwest experiences the longest drought since 1936. Production is expected to fall 13% from last year, despite more planting, sending corn prices soaring.

Enhanced corn products have been a profit center for Monsanto, generating a gross profit of $2.86 billion last year.

If confirmed, the rootworm resistance to YieldGard won’t be the first time pests have adapted to counter Monsanto’s products. Crops modified to ignore the effects of its Roundup herbicide have become infested with weeds that have adapted to Roundup.