A new study from Michigan State University shows that planting too much genetically modified corn designed to fight off a tough insect — the corn rootworm — especially in the eastern U.S. Corn Belt states may be causing more harm than good.

Bt corn was created to produce natural pesticides, through the introduction of genes from a soil bacterium called Bacillus thuringiensis, or Bt for short. Some varieties of Bt corn kill corn rootworms by making the roots poisonous to them, though the corn is safe for humans and animals. But after years of research across 10 states in the U.S. Corn Belt, scientists have found that the more this corn is planted, the less effective it becomes.

According to study author, Felicia Wu, a John A. Hannah Distinguished Professor at MSU, whose graduated doctoral student Ziwei Ye served as lead author of the study, the U.S. is the largest field corn producing nation in the world, supplying nearly 40% of global corn exports.

“Field corn in the U.S. is used for human food, animal feed and ethanol production. We eat field corn in our cornmeal, grits, corn chips, corn flakes and corn tortillas — not as corn on the cob,” Wu said. “But field corn is plagued by the corn rootworm, which is one of the toughest, most amazing pests that exists. I like to think of the corn rootworm as ‘Loki’ because it’s a trickster — it just keeps evolving resistance to everything we throw at it, not just pesticides or Bt corn, but even crop rotation. And it’s costing farmers billions of dollars.”