Corn Illustrated Weekly: Where Did All the Beetles Go?

Entomologists say rootworm beetles spiked, then left quickly.

One surprise touring the Corn Illustrated plots near Edinburgh, Ind., last week was that the later planted corn still looked green and healthy. What's more, pollination appeared to either be complete or just finishing in the corn planted June 1, with minimal, if any interference from silk-clipping insects. Usually a small amount of late-planted corn will act as a trap crop, bringing in extra beetles. Prepared to spray by hand with an insecticide to mitigate that effect and give the later-planted corn a fair chance, it proved unnecessary. It doesn't appear that there ever was serious silk feeding in the plots.

Rootworm beetles have been spotted on various trips to the plot this summer, but never in large numbers, except in a few isolated fields. They certainly weren't numerous on the last trip to the Corn Illustrated plots. Now Kevin Steffey, long-time entomologist at the University of Illinois, says that's probably no accident. He spoke before a training group at the Purdue University Crop Diagnostic Training Center last week.

"Rootworm hatch came early, and their entire growth cycle seemed to be compressed by the rapid growing season," Steffey says. In fact, rootworm adults seemed to peak in many fields during the last of July, then drop to very low levels.

"Overall, the number of rootworm problems and pressure as a whole this year ahs been relatively low," he says. That doesn't mean there haven't been problems in individual fields. And some university plants unearthed considerable rootworm feeding damage on roots where they purposely attracted rootworm beetles to lay eggs last summer. Bu8t overall, it's been a down year for rootworm beetles.

Christian Krupke, Purdue Extension entomologist, confirmed the same observation in Indiana. Both are referring to the western corn rootworm beetle, the dominating species in Indiana and Illinois. This species is not known to have a diapause phase like the northern corn rootworm beetle, but there is a variant that lays eggs in soybean fields. That's why some farmers, especially in east-central Illinois and northern Indiana, have been forced to rootworm protection on first-year corn after soybeans for the past decade or so. The problem first broke in Illinois in the early '90s.

While the entomologists aren't sure why the beetle numbers dropped so quickly this summer, there are theories. Studies form Nebraska several years ago indicate that natural pests of the beetle can cause a blood-poison like sickness amongst adult beetles in dry years. Their sharp reduction in numbers may also be linked just to the very hot, dry weather.

This doesn't mean that rootworms didn't lay eggs in soybean fields in areas where they worry about first-year corn due to the western corn rootworm variant, entomologists stress. It does mean it's too late to expect to learn much by placing sticky traps into soybean fields now.

Recommendations for protecting first year corn in '08 and all continuous corn fields are likely to remain the same.

Steffey notes that there also appears to be a cycle extending back some three decades at least whereby corn rootworm is more of a problem in the middle part of each decade, say years 3-6, then less of a problem in intervening years. However, there is no proof of this theory, nor explanation for it. But it's an observation hat has held during his career as an entomologist.

TAGS: USDA Soybean
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