Scientists in Illinois have detected something they call “alarming” in the state’s corn fields.

Samples taken from corn fields in two counties in the state show serious damage from rootworms, a pest that can devastate corn harvests. The finding is particularly disturbing because the fields were planted with corn that was supposed to be genetically modified (GMO) to resist rootworm infestations, Reuters notes.

A researcher with the the Illinois Natural History Survey said that rootworms were developing a resistance to the GMO corn. Repeated use of the biotech corn year after year gave the rootworms time to adapt. Measures designed to prevent rootworm resistance, including rotating crops among fields, do not seem to be working.

Rootworm-resistant GMO corn was first marketed by Monsanto (MON) in 2003 and quickly became popular with farmers. Reports of increasing rootworm resistance to the GMO corn surfaced last year.

A Monsanto spokesperson said the company was aware of “rootworm pressure in isolated areas of Illinois” and was developing solutions in collaboration with local farmers.

Earlier this year, the company won a U.S. Supreme Court victory over an Indiana farmer who used its GMO soybean seeds grown as feed to plant his fields.

Shares of Monsanto rose modestly in Friday morning trading.