University of Illinois

No. 18/July 24, 1998

Results of On-Farm Corn Rootworm Digs: Overall Pressure Lower than in 1997

Last week we evaluated corn rootworm injury in first-year cornfields throughout east-central Illinois. Overall, the level of rootworm pressure was below that experienced in 1997. Provided in Table 1 is a comparison of root-injury averages in treated and untreated strips within producers' fields. For the majority of fields, 15 roots were removed from each of four treated and untreated areas (120 roots per field). In 1997, 12 of 17 fields(71 percent) had root injury above the economic injury index of 3.0 in untreated check strips. This season, only 8 of 15 fields (53 percent) had average injury ratings above 3.0 in our control strips. These data suggest that the very wet June reduced corn rootworm larval survival. In fact, preliminary root ratings from our experimental trials in DeKalb and Urbana revealed the lowest level of injury that we've seen in many years. In addition to the very-minor-to-moderate root injury we observed in the on-farm "digs," we also noted reduced densities of adults in most fields. Densities of beetles may intensify in some late-planted and replanted fields later this season, so stay tuned. Also, as many cornfields move beyond the reproductive phase of development, we anticipate greater concentrations of adults in many soybean fields throughout east-central Illinois.

Table 1. Mean root ratings of larval rootworm injury in first-year cornfields in seven east-central Illinois counties.

Ford (1)1.982.631.751.88
Ford (2)3.734.931.522.28
Ford (3)2.353.272.134.37
Ford (4)--------1.773.15
Grundy (1)1.982.271.551.70
Grundy (2)2.202.402.183.30
Iroquois (1)2.183.09--------
Iroquois (2)2.453.731.833.88
Iroquois (3)2.354.501.682.78
Iroquois (4)2.744.021.673.17
Livingston (1)
Livingston (2)2.002.622.463.10
Livingston (3)2.493.04--------
Vermilion (1)2.904.13--------
Vermilion (2)2.253.692.313.44
Vermilion (3)3.864.281.402.04

1Soil insecticide applied during planting.
2No soil insecticide applied during planting.
3Iowa State 1-to-6 root-injury scale used (1 = no visible damage or only a few minor feeding scars, 2 = some roots with feeding scars but none eaten off to within 1.5 inches of the plant, 3 = several roots eaten off to within 1.5 inches of the plant but never the equivalent of an entire node of roots destroyed, 4 = one node of roots destroyed or the equivalent, 5 = two nodes of roots destroyed or the equivalent, and 6 = three or more nodes of roots destroyed.)

For those individuals who will be monitoring soybean fields with Pherocon AM traps (yellow sticky traps), traps should be deployed next week (if you haven't done so already) and fields monitored for the first 3 weeks of August. In later issues of this Bulletin,we intend to let you know if the economic threshold requires some adjustment based upon this year's root ratings in producers' fields. Next week, we begin our root evaluations in the areawide experiment south of Sheldon, Illinois. As with our other studies, we'll let you know what the preliminary results look like as soon as possible.

Mike Gray (, Matt O'Neal (, Kevin Steffey (, Extension Entomology, (217)333-6652