It's time for rootworm decisions

It's that time of year again! The first adult corn rootworm beetles should have emerged from the soil last week. Though it is too early to make a definite statement, it may be a pretty good year for the rootworm. Two weeks ago, we had substantial damage at one of our research locations. Several of the locations appear to offer the potential for damage. Last week, we observed substantial damage in a grower's field that had been treated at planting with a soil insecticide. So now is the time you should begin making corn rootworm decisions.

Western corn rootworm beetle.
Corn rootworm larval injury.
Northern corn rootworm adult.

These decisions include the following considerations: If I used a rootworm insecticide, am I satisfied with its performance, and will I need to manage rootworms next year? The emergence of the rootworm beetles indicates that the majority of the larval feeding has been completed. This would be a good time to check insecticide performance if you treated for larvae. To check, dig two to three plants at several sites in the field(s), wash off the soil, and look for larval injury. You should not have more than a few roots pruned back to within two inches of the plant stem. To decide whether you will need to apply rootworm controls next year, you should check for beetles beginning this week or next. If beetles are obvious in a field, count the number of beetles on the plants. You will need to continue monitoring the beetles until their numbers begin to decline. If you count beetles on two randomly selected plants at 27 sites in a field and find more than an average of 0.7 beetles per plant, there is a very good possibility there will be economic levels of larvae in the field next year. The next decision will be "Can I rotate to another crop next year to avoid injury, or will I have to apply an insecticide?"

This article originally appeared on pages 143-144 of the IC-478(18) -- July 21, 1997 issue.

Updated 07/20/1997 - 1:00pm