1999 Outlook for Western Corn Rootworm Injury in First-Year Corn

March 22, 1999

Western corn rootworms got off to a poor start last year due to the exceptionally wet spring. Saturated soils in late May and early June coincided with egg hatch. First-instar corn rootworm larvae maneuver poorly through wet soils and have difficulty finding corn roots to begin feeding on. Egg hatches in 1995 and 1997 were delayed considerably, with the bulk of the eggs hatching in mid-June. Egg hatch in 1998 was more "typical," occurring in early June. Sweep-net surveys verified the decline in corn rootworm survival. Densities of adult western corn rootworms were reduced by approximately 90% from 1998. So what’s the outlook for the 1999 growing season?

Last year, many producers throughout east central Illinois and northern Indiana used Pherocon AM traps to monitor densities of western corn rootworm adults in their soybean fields. Based on research conducted by a graduate student, Matthew O'Neal, we have suggested that an average capture of two beetles per trap per day (in soybeans; 12 Pherocon AM traps, 4-week monitoring period) can result in an average root rating of 3.0 (Iowa State 1-to-6 root-rating scale). A root rating of 3.0 indicates that some pruning (never a complete node) has occurred on a root system. Recent long-term experiments suggest that root injury below a 3.0 can result in economic losses in certain years for some hybrids. If seven beetles per trap per day are captured with Pherocon AM traps in soybean fields, an average root-injury rating of 4.0 (one node of roots pruned severely) may occur the following season in first-year corn. A root rating of 4.0 suggests that a plant may be very susceptible to lodging. Fields that are lodged severely may suffer significant yield losses. For more complete information on western corn rootworm scouting procedures in soybean fields and calculating economic thresholds, consult the following web site: http://www.aces.uiuc.edu/ipm/field/corn/insect/wcr.html. If you would like to report your beetle capture information this season to University of Illinois Extension, please use this web site: http://www.aces.uiuc.edu/ipm/field/corn/imr/wcrscout/wcrscout.html. Thanks to the cooperation of producers last year, who reported electronically their trapping data to Sue Ratcliffe, we can provide county averages for adult western corn rootworms captured in soybean fields with Pherocon AM traps (Table 1).

Table 1. County trap-capture averages for western corn rootworm adults in 1998 (using Pherocon AM traps).



Trap Capture Average

(12 Pherocon AM traps)

Number of

Producers' Fields














































Even though adult western corn rootworm populations in 1998 were less than last year’s, the averages in Table 1 suggest that many first-year cornfields will be susceptible to economic densities of corn rootworm larvae this spring. Note the large number of cooperators in some areas, particularly Livingston (14) and McLean (41) counties. Trap averages for Livingston and McLean were 3.9 and 2.7 beetles per trap per day, respectively. These averages suggest that if first-year cornfields were left untreated in 1999, average root injury would likely be well above 3.0. Although westward expansion of this new western corn rootworm strain was not dramatic last year, note that western corn rootworm adults were captured in DeWitt, Piatt, and Woodford counties. The average for Piatt County is quite impressive; however, only three cooperators used 12 Pherocon AM traps for the purposes of our calculation.

Many cooperators last season used fewer than 12 Pherocon AM traps in counties outside the so-called "epicenter" of east central Illinois. In addition, some producers used traps other than Pherocon AM traps. Using fewer than 12 traps makes good sense in counties in which the first-year corn problem has not yet surfaced. However, using traps other than the Pherocon AM trap results in captures that we cannot interpret. Our economic threshold is based on the use of Pherocon AM traps only. Figure 1 depicts trap-capture averages for 1998. Please note that asterisks mark counties in which fewer than 12 Pherocon AM traps were used in soybean fields. This map reveals that the likelihood for severe first-year corn rootworm injury is still most acute in east central Illinois counties. However, western corn rootworm adults are beginning to "show up" in soybean fields further to the west.

For 1999 the use of a soil insecticide in first-year cornfields will continue to make good sense throughout much of east central Illinois. Densities of western corn rootworm adults vary considerably from field to field, and the importance of monitoring individual soybean fields with Pherocon AM traps cannot be overemphasized. If you did not monitor your east central Illinois soybean field last season for western corn rootworm adults, then using a soil insecticide at planting is your only responsible option. Producers in east central Illinois are encouraged to use 12 Pherocon AM traps this season in their soybean fields. Producers in Coles, Logan, Macon, Marshall, Moultrie, Peoria, Putnam, and Woodford counties are encouraged to use at least four Pherocon AM traps in their soybean fields this season.

If you are interested in monitoring your soybean fields this season, we offer the following two addresses. Note that there are probably many other sources for these traps; we encourage you to do some comparison shopping.

IPM Great Lakes, 10220 Church Road, NE, Vestaburg, MI 48891 (email: glipm@nethawk.com)

Gemplers, 100 Countryside Drive, PO Box 270, Belleville, WI 53508 (http://www.gemplers.com)

We will continue to provide updates on this continuing problem throughout the growing season. Stay tuned.–MG

Author: Mike Gray