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Crop-devouring armyworms are showing increasing resistance in some U.S. farm fields to a popular type of genetically modified crop that should kill them, scientists said on Monday.

The evolution of insect resistance “is a great threat” long- term to the sustainability of the GMO crop biotechnology that has become a highly valued tool for many U.S. farmers, according to Fangneng Huang, an entomologist at Louisiana State University (LSU) and lead researcher for a three-year study.

The study was published on Monday in the PLOS One online journal (www.plosone.org) for peer-reviewed research, after being presented at the Entomological Society of America annual meeting in Portland, Oregon.

The research documents resistance by fall armyworms in the southeastern United States to the Cry1F protein found in many corn products developed Dow AgroSciences and DuPont to fight off the destructive pests.

It is the latest evidence in recent years showing that insects are developing resistance to crops that have been genetically modified to kill them.

Like the “super weeds” that have developed resistance to glyphosate-based herbicide and make it harder for farmers to keep fields from being overrun with weeds, the armyworms are starting to devour corn crops that should repel them, said Dominic Reisig, an entomologist at North Carolina State University.

Armyworms can be a problem for farmers in many U.S. states, but the resistant armyworms have been documented only in some areas of Florida and North Carolina. The range of these resistant armyworms is unknown, researchers said.

They said farmers should plant more non-GMO corn as a refuge and possibly increase the use of pesticides to control the resistance.

Dow and DuPont did not respond to requests for comment.

The GMO corn at issue contains Bacillus thuringiensis (Bt) genes. Bt corn, popular with farmers throughout the Americas, has been on the market roughly 18 years. Newer types of Bt corn with multiple modes of action are still showing effectiveness, Huang said.