University of Illinois

No. 17/July 18, 1997

Corn Rootworm Adults Emerge; Concerns Over Insecticide Performance Verified

Observations of western corn rootworm adults continue to be reported throughout the state. As early as July 9, sightings of beetles were common across central Illinois counties. Emergence this season, not surprisingly, is somewhat later than "normal" and is related directly to the very delayed egg hatch that we've discussed many times this season. On July 15, a University of Illinois experimental plot located just a few miles northeast of Urbana had an abundance of western corn rootworm beetles crawling all over corn plants. In addition, it was not uncommon to find a few remaining larvae continuing to "dine" on root systems. We intend to begin our evaluations of registered and experimental rootworm soil insecticides in Urbana on July 23, by which time larval injury should be nearly complete. As soon as some of the preliminary data are available, we intend to share the rootratings results with our readers.

In an earlier issue of this Bulletin, we encouraged growers to check their soybean fields for western corn rootworm adults. The suggestion remains in effect. In addition to common observances of western corn rootworm beetles in cornfields, we have had reports of beetles in soybean fields throughout much of east-central Illinois. Even though we don't have an economic threshold for western corn rootworm adults in soybean fields, progress is being made on this front. Unlike last year, corn rootworm larval injury in first-year cornfields appears to have made a comeback in 1997. We've had numerous reports from growers in east-central Illinois that suggest larval injury is more intense this season. In some areas of eastern Illinois where storms have brought abundant precipitation and high winds, many cornfields have severely lodged plants. During the last week of July and throughout early August, we will be evaluating roots for larval injury in about 25 producers' first-year cornfields in east-central Illinois counties. During these "root digs," we will compare root injury in strips treated with a soil insecticide at planting with roots from other areas of the field in which no insecticide was used. As soon as these results are available, we will share our findings.

Reports of poor performance of soil insecticides have been common during the past 2 weeks. Complaints have arisen from growers who produce continuous corn, as well as those who rotate corn with soybeans in eastern Illinois.

What factors might contribute to poor performance this season?
Several explanations should be considered. First of all, because many fields were planted very early this season, soil insecticide persistence may be an issue for some products. Secondly, the delayed corn rootworm egg hatch has effectively extended the larval feeding period well into the third week of July. Together, these factors have likely pushed performance of some products to the edge. Add to this scenario the impressive densities of corn rootworm larvae, and you have severely lodged corn plants in many fields; you also have unhappy producers.

Are there other factors that can contribute to insecticide performance complaints? Yes. These other factors are most often attributed to application problems such as improper calibration. Bottom line--before root lodging is blamed solely on the soil insecticide, do your homework by checking application and calibration records. Based upon the numerous reports of insecticide performance problems this season, it is very unlikely that all growers' complaints are due to misapplication factors.

Thus far, this "rootworm season" has been a good reminder to those who believe that rootworm problems in continuous or first-year corn can always be dealt with effectively simply by applying a soil insecticide and remaining unconcerned about increasing our basic understanding of the biology and ecology of an insect pest. The shortcomings of this philosophy should be apparent.

Mike Gray, Extension Entomology, (217)333-6652