No. 13/June 20, 1997
Egg Counts in East-Central Illinois Soybean Fields
Are producers going to experience severe western corn rootworm larval injury in first-year cornfields across east-central Illinois this season? For many growers who used a soil insecticide at planting and failed to leave a check strip, this question can't be answered very well, unless the product of their choice fails to perform adequately. Then at least we know that rootworm pressure is a reality. In about 50 percent of continuous cornfields, rootworm densities never reach economic importance despite the fact that nearly 90 percent are treated annually. We don't have estimates such as these for first-year corn yet.
Recall that corn rootworm pressure in 1996 was considerably less than in 1995 across east-central Illinois. Many have suggested that the increase in soil insecticide use during 1996 masked what otherwise would have been a big rootworm problem all over again. Our on-farm research indicates otherwise. Producers who are cooperating in a large on-farm investigation with us had very little root injury (first-year corn) last season in their check strips (no soil insecticide used). Either egg densities of western corn rootworms were low to begin with or larval establishment was poor in saturated soils last year. Where do we stand for 1997?
Preliminary egg-sample data suggest that densities of western corn rootworm eggs in certain soybean fields last fall were lower than in adjacent cornfields in east-central Illinois (Table 3). All egg-sample data provided in Table 3 were obtained by using a golfcourse cup cutter to remove soil to a depth of 4 inches during October and November of 1996. In general, egg densities of 5 eggs per pint of soil are considered of potential economic importance. However, there is some flex associated with this estimate. The greatest density (2 eggs per pint) of western corn rootworm eggs in a soybean field was found near Loda in Iroquois County. Bottom line: Egg laying continues to occur in some soybean fields in eastern Illinois, and the potential economic threat remains. Until the larval feeding period is over (late July), we won't know for sure how this season compares with the previous 2 years. Drs. Eli Levine and Joseph Spencer, with the Illinois Natural History Survey, provided egg-sampling data for this article.
Table 3. Western corn rootworm egg samples taken in corn and soybean fields during the fall of 1996.
Mike Gray, Extension Entomologist, (217)333-6652
|Field location||Total WCR egg count||Average WCR egg count|
|Northern Champaign County, Field 1, corn||24 eggs in 12 pints of soil||2.0 eggs per pint|
|Northern Champaign County, Field, soybean||0 eggs in 12 pints of soil||0 eggs per pint|
|Northern Champaign County, Field 2, corn||9 eggs in 12 pints of soil||0.75 eggs per pint|
|Northern Champaign County, Field 2, soybean||1 egg in 12 pints of soil||0.08 eggs per pint|
|Central Champaign County, Field 3, soybean (30 years continuous soybean)||0 eggs in 12 pints of soil||0 eggs per pint|
|Iroquois County, Loda, corn||73 eggs in 18 pints of soil||4.06 eggs per pint|
|Iroquois County, Loda, soybean||42 eggs in 21 pints of soil||2.00 eggs per pint|
|Champaign County, Dewey, corn||23 eggs in 12 pints of soil||1.92 eggs per pint|
|Champaign County, Dewey,|
Soybean Field 1
|3 eggs in 18 pints of soil||0.17 eggs per pint|
|Champaign County, Dewey,|
Soybean Field 2 (field was isolated)
|7 eggs in 12 pints of soil||0.58 eggs per pint|