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.