A corn pest that can devastate yields may be increasing in prevalence across Illinois and other states because Bt crops are reducing predators that once kept the pest at bay.

That was the word from an Iowa State University researcher who spoke during the recent Illinois Crop Protection Technology Conference, Urbana.

Western bean cutworms, a major pest in Nebraska and Colorado, was first detected in Illinois in 2004 and has spread to 49 counties, according to Marlin Rice, an Extension entomologist at Iowa State.

Rice and his colleagues attempted to learn why a pest that was rare in Iowa six years ago has spread as far east as central Ohio.

In laboratory experiments and field studies, Rice tested the bean cutworm's survival when placed together with corn earworm, which is the more aggressive of the two pests and will kill the bean cutworm. Both pests were allowed to feed on silks from Herculex and YieldGard plants [YieldGard is a Monsanto GM rootworm-resistant corn].

The bean cutworms had better survival rates when they fed on YieldGard, which is not labeled for cutworm control, compared to Herculex, which is. Both hybrids are labeled for corn earworm control.

"Our theory is that increased (use) of Bt cotton and YieldGard corn has suppressed (populations) of corn earworms, which are predators of western bean cutworms. This allows (more) bean cutworms to survive," Rice said.

"YieldGard corn may be one of the reasons for more damage from western bean cutworm," Rice said. "It may be influencing (pest) competition in the field."

Bean cutworms have become established in Illinois, "but we'll have to wait a couple of years to see if it is an economic problem," Rice said.

He recommended farmers scout their fields and time insecticide treatments for when eggs or young larvae reach economic thresholds.

If western bean cutworm becomes an economically damaging pest, farmers should consider planting Herculex hybrids, he said. - Kay Shipman

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