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Theory for Lower Corn Borer Numbers May Not Apply to Rootworms

Theory for Lower Corn Borer Numbers May Not Apply to Rootworms
How much effect is Bt corn having on these insects?

European corn borer has traditionally been more of a problem in Illinois and states to the west of Illinois than in Indiana. However, throughout the decades following World War II, there were usually European corn borer problems somewhere in the state nearly every year, just not in big numbers. Occasionally, the population would grow high enough to cause problems across the state, but not often.

Corn borer was one of the insects that caused instructors at age-old GI veteran schools for farmers in the late '40s to encouraged late planting. In those days, late-planting meant mid-May instead of early May. The goal was to reduce the risk of having corn borer problems. Unfortunately, the message stuck so well that the bigger part of a whole generation and the next generation they raised were convinced that planting even in early may wasn't a wise thing. Very early planting in mid to late April and early May is a fairly recent phenomenon of the last two to three decades.

Now the population is so low that for the past two seasons, Purdue entomologists have no longer scouted for corn borers to do late fall counts. At one time it was considered useful information for predicting what early-season populations might be the next spring, since the pest over winters in residue and the soil.

It's also the reason some biotech companies developed different pricing schedules for the Bt corn borer trait, charging more in states like Illinois and Iowa where it was more of a perennial problem, but less in Indiana and other states where farmers might not be as inclined to pay as much for it, since the insect seldom caused big problems anyway.

Now the debate is whether the strong shift to Bt corn over the past 15 years has dropped the population. Several entomologists, of the record at least, say it's possible.

So what about corn rootworm? Is that why rootworm numbers were lower last season? Is that why less problems with rootworm adult beetles were detected in most areas of Indiana and Illinois in '09? Mike Gray, an entomologist at the University of Illinois, recently stated that it may be too soon to ascribe to that theory for rootworms.

"Considerable speculation has arisen whether or not the large-scale increase in Bt usage may be suppressing corn rootworm populations, similar to what happened with European corn borer densities," Gray says.

"Because Bt hybrids targeted at corn rootworms are considered low-to-moderate dose in their toxic effects, I suspect environmental conditions last season served as the major contributor to the collapse of the corn rootworm population," he continues.

He believes wet conditions last spring killed many western corn rootworm larvae soon after hatch. For Illinois at least, he expects light to moderate infestations of corn rootworm this year.

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