Corn rootworms are back!

Usually, the first corn rootworm beetles are found in corn by July 4 or shortly thereafter. One of my graduate students, Yong-Lak Park, spotted several western corn rootworm males in a Clinton County field on July 2, which is a couple of days earlier than average, but not unusual.

Female Western Corn Rootworm beetle with eggs expressed.

This year might bring significant corn rootworm larval injury complaints. I have had reports of a couple of fields treated at planting with a soil insecticide because they had heavy rootworm damage. In our Ames research plot, the infestation level is very high.

What does this mean? First, moisture stress will accentuate the yield losses, but rains during pollination and ear fill will help the plants recover. Second, some corn may still be prereproductive when significant numbers of beetles emerge. Because of their high numbers, this could pose a threat to pollination. As the beetles emerge, the later fields should be scouted. If there is easily observable leave feeding (stripping of the green material between veins), the silks should be watched for clipping because clipping prevents pollination. Finally, as of the end of June, some chemical manufacturers were offering postemergence Furadan applications to help protect roots where the planting-time treatment had failed. It is now too late to receive economic benefit from a postemergence larval treatment. Another option I have suggested is a beetle treatment to protect silks if the crop has not begun to tassel.

One of my former graduate students, Tim Nowatzki, developed a model that explained 85 percent of the variability in adult corn rootworm emergence. His model uses the appearance of the first adults to predict subsequent emergence. The model predicts that 10, 50, and 90 percent emergence will occur 63, 211, and 411 degree days, respectively, after the first adult is found. Using the July 2 appearance in Clinton County and an average accumulation of 12 degree days per day, scouting should be underway now, half the beetles will be out by July 20, and emergence should be ending by August 5. These dates will provide guidelines as to when to begin and end scouting and when beetle treatments should be applied.

This article originally appeared on page 141 of the IC-488(17) -- July 15, 2002 issue.

Updated 07/14/2002 - 1:00pm