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Corn+Soybean Digest

Bt Refuges: The Closer The Better

Refuge areas of non-Bt corn may need to be close to Bt corn in order to ward off European corn borer resistance to Bt hybrids. That's the conclusion from University of Nebraska research on corn borer moth movement within and near cornfields.

Current rules call for a 20% refuge of non-Bt corn on Corn Belt farms. Refuges must be within one-half mile of Bt fields, preferably closer. Each refuge can be a single block or strips across a field. If strips, there should be a minimum of six rows per strip. Most farmers prefer blocks.

The present refuge strategy is based on the assumption that borer moths move around quite a bit to find a mate. Thus, in theory, Bt-susceptible moths from the refuge area will randomly fan out and mate with any Bt-resistant moths that might emerge in a given cornfield.

A big question: How far do moths, particularly females, move before they mate?

University of Nebraska entomologist Tom Hunt conducted a three-year study of moth movement. He found that premating movement is quite variable.

He also found that, under some conditions, moths apparently stay closer to home than has generally been believed. Moth gender, stage of corn growth and moisture conditions all affect moth movement.

“We found that many of the female moths we recaptured had apparently mated within just a quarter mile of the release sites,” Hunt points out. “Sometimes they were in the same field where released.”

Male moths travel farther. The wider dispersal of males offsets the limited movement of unmated females.

Hunt also learned that corn growth stage is a factor affecting moth movement. As corn becomes larger early in the season, it's attractive for egg-laying. Later in the season, pollinating corn is attractive. Females travel farther to find that attractive corn. This travel is more of a factor in June (first-generation borers) when there's greater variation in corn development and attractiveness than in August (second generation).

As part of his study, Hunt looked at irrigated vs. non-irrigated corn. Moths moved less of a distance in irrigated fields. “They found it more attractive for mating due most likely to relatively high humidity, heavier plant populations and reduced light penetration,” he explains.

Previous studies in non-irrigated Iowa fields showed that moths often move completely out of corn to the more favorable environment of nearby, damper grassy areas for mating. But conditions in the higher humidity of the eastern Corn Belt, even without irrigation, might be somewhat similar in attractiveness to irrigated fields.

“Our results indicate there is no single appropriate set of moth-flight parameters,” Hunt notes. “Refuge plans must accommodate the variability we find in moth movement. When field conditions are favorable for mating, refuges should be placed as close as possible to their respective cornfields.”

The current refuge strategy addresses moth variability with its distance requirements and recommendations, Hunt concludes.

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