University of Illinois

No. 6/May 2, 1997

First-Year Corn Rootworm Injury: Prospects for 1997

In 1996, producers throughout east-central Illinois suffered less corn rootworm larval injury in their first-year cornfields than in the preceding year.

Why was root injury less of a problem in 1996 compared with 1995? Some have argued that fewer complaints occurred last year because so many growers treated their first-year cornfields with a soil insecticide at planting. This explanation has some merit; however, larval injury was less significant last year even in untreated check strips (no soil insecticide used) of our on-farm research cooperators.

As part of our continuing research efforts to establish an economic threshold for western corn rootworm adults in soybean fields, we rated roots from 14 producers' fields across five counties last year. Root injury exceeded a rating of 3.0 (some pruning, but never an entire node) in only three of the producers' fields (two in Vermilion County, one in Iroquois County). For the remaining 11 fields, root ratings on the Iowa State 1-to-6 scale, ranged from 1.26 to 2.67, indicative of very minor root injury.

What happened to the corn rootworms? Although we can only speculate, the saturated soils at the time of egg hatch last year may have contributed to poor larval establishment. In essence, corn rootworm larvae may have failed to find a root system to begin feeding upon in water-soaked soil.

Is there any indication of what the densities of egg-laying beetles were last season in east-central Illinois soybean fields? Beetle counts in our trapping studies suggest that densities of western corn rootworm adults were very impressive from July through early September in 1996. Figure 3 reveals the number of western corn rootworm beetles captured in vial traps last season for a pair of adjacent corn and soybean fields in Champaign, Iroquois, and Livingston counties. Beetle totals (vertical axis) represent the number of beetles caught for a given sampling date in five vial traps (five traps in each of the corn and soybean fields). For Champaign and Livingston counties, beetle densities peaked on August 20; whereas, in Iroquois County, the density of beetles was still climbing in the soybean field on September 10. Vial traps were made from 60-ml amber-colored plastic vials with snap caps. Ten holes, each 5 mm in size, were drilled into a container to allow for beetle entry. An acetate transparency film strip that contained a one-to-one mixture (by volume) of Sevin XLR Plus and water was placed in each vial. Powdered squash was also applied to each acetate strip. The traps were secured to corn plants at ear height and just above the soybean canopy on metal stakes. Vial traps are not attractive to beetles, instead, beetles find the traps in a more random manner and eventually wander in through the openings. Once in a vial, beetles begin to feed compulsively on the powdered squash, receiving a lethal dose of Sevin XLR during the process.

The information in Figure 3 suggests that more western corn rootworm beetles were caught in soybean fields than cornfields at these three sites last season. Also, densities of beetles in soybean fields remained impressive late into the season, suggesting that eggs were probably being laid in soybeans. Bottom line--western corn rootworm larval injury in 1997 could return to economically threatening levels if first-year cornfields are left untreated in east-central Illinois.

What were beetle densities like in soybean fields outside of eastern Illinois last year? Sweep net samples (mid-August 1996) of soybean fields made by Eli Levine, Illinois Natural History Survey, indicated very few western corn rootworm beetles were present in more western counties such as Knox, Peoria, Warren, and Woodford. Presently, this problem does not appear to have spread significantly in a western direction. For producers who live in areas other than east-central Illinois, the present recommendation not to treat first-year cornfields with a soil insecticide for corn rootworm control remains in effect.

Finally, this spring has brought us cool temperatures and less precipitation than last year. In an upcoming issue of this Bulletin, we will provide a prediction regarding an expected egg-hatch date for this season's crop of corn rootworm larvae.

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