University of Illinois

No. 15/July 1, 1998

Diagnosis of Rootworm Problems: Accuracy Is the Operative Word

More than ever, people are examining corn fields for corn rootworm larvae and signs of their presence. Obviously, the issue that concerns most growers is whether the new "strain" of western corn rootworms that lays eggs in soybeans has shown up in their area. So, people are looking, and we (all of us) need to make sure that diagnoses and assessments of rootworm injury (or lack thereof) are accurate.

We have discussed how to examine and evaluate root systems for rootworm injury in previous issues of the Bulletin. However, we need to re-emphasize that the only way to determine if rootworm larvae have caused a problem in any corn field, whether it's corn after soybeans or corn after corn, is to dig up several roots, wash them clean, and look for damage. If root pruning (roots chewed off to within 1.5 inches of the plant) is not evident on several roots, a rootworm problem does not exist. Some scarring on the roots and a little bit of pruning should be expected in most fields, even fields treated at planting time with a soil insecticide. However, if at least one node of roots has been pruned off (a root rating of 4.0 or higher), rootworm damage could result in yield loss (refer to the next article).

The following "symptoms," in and of themselves, are not evidence that a rootworm problem exists:

  • stunted plants (could have been caused by a number of early season problems)

  • lodged plants (could have been caused by shallow roots, wet soil, wind)

  • "lots" of rootworm adults (Even soil insecticides don't control all rootworms, so the presence of rootworm adults in a corn field is not uncommon.)

  • lower yield (no comment)

We stress the importance of accurate assessment of rootworm damage because growers outside the "problem area" in east-central Illinois are nervous. Although we want to know if the problem with western corn rootworms' laying eggs in soybeans is "spreading" into other counties, we do not want to hasten the circulation of rumors. All of us want to base our insect management decisions on facts.

As we enter July, let's make certain our assessments of rootworm damage during this critical month are accurate. We'll all be better informed as a result.

Kevin Steffey ( and Mike Gray (, Extension Entomology, (217)333-6652