University of Illinois

No. 14/June 25, 1998

Reports of Rootworm Larval Damage Are Numerous

During the weeks of June 15 and June 22, we received several reports of severe damage caused by corn rootworm larvae. Unfortunately, many of these instances of corn rootworm damage had occurred in corn planted after soybeans, and, more recently, in fields that had been treated with a soil insecticide at planting time. Some reports of rootworm larval damage in corn planted after soybeans were from areas of Illinois not previously considered to be part of the "problem area" of about 12 to 15 counties in east-central Illinois. However, these reports have not been verified. Within the next few days, we will visit some of the fields to determine if the problem with western corn rootworms laying eggs in soybeans has "spread" into additional counties.

Western and northern corn rootworm adults

Are we in for a bad rootworm season? When we examined our data regarding densities of western corn rootworm adults caught on sticky traps in soybeans in east-central Illinois in 1997, we stated a collective "Uh oh." Densities of rootworm beetles in 1997 were much greater than in 1996, and the severity and frequency of rootworm larval problems werefairly high last year. Consequently, we have anticipated that densities of rootworm larvae this year would be very large in many fields. Despite excessive rainfall in some areas, rootworm larvae apparently survived reasonably well. As more people begin to look for rootworm larval damage as corn fields dry out, they may find severely pruned roots, the most obvious symptom of corn rootworm damage.

How long will rootworm larvae continue to feed on corn roots? As we have indicated previously, egg hatch of rootworms this year was "back to normal," or maybe slightly early in some locations. Eggs hatch for 2 to 3 weeks beginning in late May, and peak hatch occurs in early June in much of the Corn Belt. Therefore, larvae have been feeding for weeks in most fields.

Rate of development of the larvae is temperature dependent. Development occurs fastest between 70 and 75 degrees F. Within the soil-temperature range of 64 to 81 degrees F, the time from egg hatch to adult emergence is 46 and 23 days, respectively. When larval development is complete, feeding ceases and a prepupa creates an earthen cell before it pupates; the pupal stage lasts 6 to 13 days. Entomologists at Purdue University began finding corn rootworm pupae in some of their fields last week. John Shaw, research entomologist with the Illinois Natural History Survey, observed the first western corn rootworm adult in Illinois this season in a field trial in Champaign County on June 24. We suspect that emergence of adults is under way in southern counties, and folks should begin finding plenty of adult corn rootworms in fields in central and northern Illinois very soon.

Should I be looking for rootworm larval damage right now? Although rootworm larvae will continue to feed on corn roots well into July, peak larval feeding is likely occurring in most fields right now. If the field is dry enough to walk into, check roots from several areas of the fields to determine if rootworm larvae are feeding and causing damage. In issue no. 12 (June 12, 1998) of this Bulletin, we offered a detailed explanation about assessment of rootworm larval damage and determination of effectiveness of insecticides applied at planting time. We recommend that if you are concerned about rootworm larval damage in any field of corn, whether it's corn planted after corn or corn planted after soybeans, now is the time to take a look. The only way to verify that rootworm larvae have caused damage in corn planted after soybeans is to dig up roots and examine them for damage. Remember, aboveground symptoms like lodging, or maybe even stunting, are not verification that rootworms caused the problem.

As the adults begin to emerge, people's attention will turn to scouting for rootworm beetles in both corn and soybeans. We have prepared a fact sheet that offers guidelines for people who want to use sticky traps to scout for rootworm adults in soybeans. The fact sheet is in press and should be available by early July. We will include copies of this fact sheet when we mail the Bulletin, hopefully during the week of June 29 or July 6 at the latest. However, if you have access to the World Wide Web, you can check out a version of this fact sheet at We'll offer more information about adult corn rootworms and scouting in a future issue of the Bulletin.

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