Reports of large densities of western corn rootworm adults are common across much of the northern half of Illinois. We've discussed the topic of corn rootworm management in several previous issues of the Bulletin; however, the severity of injury in some producers' fields warrants further discussion. In addition to root pruning and lodging, many farmers are scouting fields and finding both western corn rootworm and Japanese beetle adults clipping silks. Shawn Jones, a field sales agronomist with Pioneer Hi-Bred Inc., indicated that some cornfields in Piatt County have as many as 5 to 10 western corn rootworm adults per plant. Shawn also reported that many cornfields in Christian, DeWitt, Macon, and Moultrie counties have been treated to prevent excessive silk clipping caused by corn rootworm and Japanese beetle adults. Because of the late planting in many areas of east-central Illinois, growers are justifiably concerned about the combination of dry weather and insect injury during this critical period of corn development. John Fulton, an Extension unit leader in Logan County, indicated on July 15 that some seed production fields were being treated for western corn rootworm silk clipping with densities averaging about three adults per plant.|
In northeastern Illinois, reports of corn rootworm larval injury in first-year cornfields are common in some of the local coffee shops. Gary Bretthauer, an Extension unit educator in Kendall County, reported on July 11 that he examined a first-year cornfield that had severe larval injury. Gary observed some regrowth on the injured root systems, but he anticipates reports of lodging to accelerate if any significant storm activity moves through northeastern Illinois. Perhaps the most impressive corn rootworm larval injury to rotated corn was reported by Dale Baird, an Extension educator in the Rockford Extension Center. Dale observed a first-year cornfield in northern LaSalle County that had severe lodging. During the evening of July 8, a storm accompanied by high winds swept through an area about 10 miles north of Utica. The winds toppled many plants that had severely pruned root systems. Kevin Black, Growmark Company, also visited the field and noted that the rootworm injury was among the worst that he had ever seen.
Why are we experiencing another summer of rootworm problems in central, east-central, and northern Illinois?
Recall that in 2001, rootworm densities were very high across the northern one-half of the state. Egg laying was undoubtedly impressive in many of these counties. Because of the very mild winter, the overwintering success of the eggs was more than likely well above average. Although we had above-average precipitation this spring, by the time larvae hatched in many fields, the soils were no longer saturated. Larval establishment was quite good and occurred in fields with small root systems from the delayed planting. For much of July, precipitation has been sparse. Dry soils may have compromised the performance of soil insecticides, a group of products with overall very low water solubility. Soil insecticide performance cannot be linked exclusively to water-solubility properties of insecticides or rainfall amounts; however, an examination of product performance data since 1988 reveals that timely precipitation does improve the level of control afforded by many rootworm products. Unfortunately, since late June, many areas of east-central Illinois have not received any rainfall.
Next week we will evaluate the performance of many soil insecticide treatments that have been targeted at corn rootworms in our experimental trials located in DeKalb, Monmouth, and Urbana. We look forward to reporting the results of these trials in future issues of the Bulletin. In addition to these efficacy trials, we intend to conduct some on-farm "root digs" during the first 2 weeks of August. We hope to survey for root injury from 300 first-year cornfields. Five to 10 roots will be evaluated per field from 100 fields in each of eastern, central, and western regions of Illinois. Results from this survey effort may reveal some differences in the level of first-year injury in each of these areas of the state. Testimonials from growers, dealers, and consultants suggest that the western corn rootworm variant continues to spread each year to the north and west from its epicenter in east-central Illinois. We hope to shed some light on this discussion later this summer.--Mike Gray
Severe pruning on roots dug from first-year cornfield in LaSalle County. (Photo courtesy of Dale Baird, Extension Educator, Rockford Extension Center.)
Lodged plants in first-year cornfield located in LaSalle County. (Photo courtesy of Dale Baird, Extension Educator, Rockford Extension Center.)
Brian Robinson, LaSalle County FS Dealer, in first-year cornfield with severe root pruning and lodged plants. (Photo courtesy of Kevin Black, Growmark Company.)