Soil heat unit accumulations (base 52°F) at the 4-inch level, from January 1 through May 5, 2003 (Figure 3), suggest that the corn rootworm larval hatch may be very similar to last year's hatch. After 380 to 426 soil heat units have accumulated from January 1, approximately 50% of corn rootworm larvae should have hatched. In 2002, entomologists at Purdue University observed second-instar corn rootworm larvae on June 4 in Tippecanoe County, Indiana. Based on this sighting, they indicated that hatch most likely occurred on May 31. So far this season, our heat unit accumulations appear to match those of 2002. The southern boundary of Tippecanoe County corresponds approximately with Highway 136 that bisects Illinois into northern and southern halves. The entomologists at Purdue University are to be commended for their "detective" work in pinpointing the corn rootworm larval hatch each season. Because first-instar corn rootworm larvae are so small, corn roots have to be dissected and stained in order for entomologists to make these early observations. How small are corn rootworm larvae? Head-capsule width ranges for first, second, and third instars are 0.2 to 0.23 millimeter, 0.3 to 0.35 millimeter, and 0.45 to 0.5 millimeter, respectively. Once the annual spring hatch begins, it typically lasts for 2 to 3 weeks.
Considerable variation exists from year to year regarding the onset of the corn rootworm larval hatch. By examining previous issues of the Bulletin and relying heavily on observations of Purdue University entomologists, I found that initiation of the corn rootworm larval hatch may vary from one year to the next by as much as 1 month. In 2001, the hatch occurred on May 16, the earliest since 1985. In 2000, first-instar corn rootworm larvae were observed on May 22 in Tippe-canoe County, Indiana. In 1996 and 1997, larval hatch occurred on June 12 and June 13, respectively. The performance of soil insecticides is often compromised when early planting (first half of April) is followed by a late (mid-June) corn rootworm hatch. In these years, larval injury may persist well into early August, significantly challenging the persistence of soil insecticides. Based on the heat unit accumulations that have occurred so far this season, I suspect that hatch is likely to occur in late May (similar to last year).
For many years, it has been suggested that the first sightings of fireflies coincided with the hatch of corn rootworm larvae. Over the past several years, these independent biological events have not been very well correlated. With the assistance of Bob Scott, Illinois State Water Survey, we'll continue to report the accumulations of soil heat units and pass along any observations that indicate the corn rootworm larval hatch has begun.--Mike Gray