Monsanto Co. isn't doing enough to monitor for signs that widely grown corn plants that the company genetically modified to thwart a bug are falling prey to that insect again, Environmental Protection Agency scientists said.

In a Nov. 22 memo, the EPA scientists conclude that Monsanto needs to expand its monitoring program now that rootworms in portions of four Midwest states are "suspected" of having developed resistance to the plants.

The agency, which regulates the corn plants because they are genetically engineered to make a natural pesticide, stops short of declaring that the ability of the bug – a major agricultural pest -- to overcome the genetically-modified plants is "confirmed."

Nonetheless, the "suspected resistance" prompted EPA scientists to take a harder look at Monsanto's current monitoring program, which they conclude "is ineffective and likely to miss early resistance events."

As reported by The Wall Street Journal in August, university scientists in Iowa and Illinois were discovering fields planted with Monsanto's corn suffering from unexpectedly high levels of damage from rootworm, which could encourage farmers to switch to insect-proof seeds sold by competitors of the St. Louis crop biotechnology giant.

The EPA scientists, among other things, said in their memo that they want Monsanto to begin surveying fields at earlier signs of insect damage than it does now. The EPA didn't make any officials available to comment on the memo Thursday.

Monsanto, which has said that the problem involves only a tiny fraction of the fields that grew its genetically-modified corn this year, said Thursday that it takes the EPA's review "seriously," but said that its existing monitoring procedures are thorough.

Monsanto became the first company to sell rootworm-resistant biotech seed to farmers in 2003. The seed contains a gene from a soil microorganism that makes a protein called Cry3Bb1, which kills bugs like the rootworm, but doesn't harm mammals, birds and many beneficial insects. Seed industry officials estimate that roughly one-third of the corn grown in the U.S. carries Monsanto's Cry3Bb1 gene.