University of Illinois

No. 9/May 23, 1997

Soil Heat Units and Corn Rootworm Egg Hatch: What Should We Expect?

In 1995 and 1996, corn rootworm egg hatch was delayed considerably. Instead of rootworm eggs hatching in late May through early June, larvae were not commonly observed until the middle of June. In fact, egg hatch was so significantly delayed in 1996 that we experienced the joy of digging our rootworm experiments twice. Normally, by waiting until mid-July, we can begin to rate roots for larval injury in our insecticide evaluation plots. Last season, corn rootworm larvae were still present in late July and early August. At our Urbana site last year, root ratings (Iowa State 1-to-6 scale) in the check plots (no soil insecticide used) increased from 4.13 (one node of roots destroyed) to 5.15 (two nodes of roots destroyed) from July 15 to August 2. In DeKalb (1996), root injury in the check plots increased from an average rating of 3.77 (nearly one node of roots destroyed) on July 31 to a rating of 4.15 on August 19.

Will 1997 bring another late hatch of corn rootworm eggs and late-season larval injury? First of all, I believe we can throw out the old predictive tools of calendar date and the first firefly (lightningbug) sightings as reliable aids in predicting accurately the hatch of corn rootworm eggs. The last two seasons indicated clearly that these techniques didn't work very well. Based upon soil heat-unit accumulations from January 1 to May 12 (4-inch soil level, base 52 degrees F), this year is beginning to resemble last season. Provided in Figures 4 and 5 are actual heat-unit accumulations (base 52 degrees F) at the 4-inch soil level from January 1 to May 12 for 1996 and 1997, respectively. These figures indicate that soil heat-unit accumulations for the upper two-thirds of the state are virtually identical for 1996 and 1997. In 1997, heat-unit accumulations for the southern one-third of Illinois actually lagged behind those for 1996 through May 12. Bottom line: Expect another delayed egg hatch for corn rootworms this season. This will likely put more pressure on the performance of soil insecticides, as the larval feeding period persists through late July and early August. At the end of May, we will provide an update on where we stand with regard to actual heat-unit accumulations.

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

Figure 4. Actual heat-unit accumulation (4-inch, base 52 degrees F) from January 1 to May 12, 1996.Figure 5. Actual heat-unit accumulation (4-inch, base 52 degrees F) from January 1 to May 12, 1997.