Monsanto Co. (MON)’s insect-killing corn is toppling over in northwestern Illinois fields, a sign that rootworms outside of Iowa may have developed resistance to the genetically modified crop, according to one scientist.

Michael Gray, an agricultural entomologist at the University of Illinois in Urbana, said he’s studying whether western corn rootworms collected last month in Henry and Whiteside counties are resistant to an insect-killing protein derived from Bacillus thuringiensis, or Bt, a natural insecticide engineered into Monsanto corn.

The insects were collected in two fields where corn had toppled after roots were eaten by rootworms, Gray said today. Planting Bt corn year after year increases the odds that the bugs will develop resistance to the insecticide, he said. While the symptoms parallel bug resistance that’s been confirmed in Iowa, analysis of the Illinois insects won’t be complete until next year, he said.

‘Whatever is the cause, it is generating a lot of concern.