Federal monitors said this week they have found more evidence that Monsanto's genetically engineered corn is failing to kill the insects it is designed to repel.

The U.S. Environmental Protection Agency posted a report this week saying that corn rootworm — a major agricultural pest — is damaging Monsanto's corn.

This summer researchers said they found evidence of problems in cornfields in Iowa and Illinois. The agency said this week, they also have found evidence of corn rootworm damage in Minnesota and Nebraska, and called Monsanto's monitoring of the problem "inadequate."

Researchers, in lab settings, have found evidence that the pests are growing resistant to a protein that is genetically engineered into the plants and designed to kill the pests after they consume it.

Monsanto issued a statement saying it takes the report "seriously and remains committed to working with farmers to encourage the adoption of integrated pest management practices when managing high rootworm populations on farm."

Monsanto did not provide a company representative for an interview, but has said in a previous interview that the problem seems to be confined only to fields with high insect pressure. The company also says there is no "scientific confirmation" that the pest is developing resistance to the protein.