Issue No. 8, Article 4/May 14, 2004
Corn Rootworm Larval Hatch: Just Around the Corner?
Is there a correlation between corn rootworm larval hatch and initial sightings of fireflies? There has been much speculation that the corn rootworm larval hatch will soon take place across Illinois. This anticipation has been fueled by the numerous reports of firefly observations. For quite some time, it has been suggested that the first sightings of fireflies coincided with the hatch of corn rootworm larvae. In recent memory at least, these independent biological events have not been very well correlated.
Can you spot the corn rootworm egg? (Give up? Click here.)
Can heat units be used effectively to predict corn rootworm larval hatch? Yes. Some entomologists have suggested they can simply use calendar date as a reasonably accurate estimate of corn rootworm larval hatch. In most years corn rootworm larvae begin to hatch during the last week of May. So predicting late May as a projected date for corn rootworm hatch would make you a winner in most years. However, in some cool springs, hatch has been delayed until mid-June. During very warm springs, hatch can occur as early as the beginning of the third week in May. So there is some variation in this annual biological event. Research has shown that after 380 to 426 soil heat units (base 52°F, 4-inch soil profile) have accumulated from January 1, approximately 50% of corn rootworm larvae should have hatched. We continue to use this published research to project when corn rootworm larvae are most likely to begin hatching. Please refer to the following Web site address to view maps for current degree-days and projected degree-days (1-week and 2-week) for the corn rootworm larval hatch: http://www.sws.uiuc.edu/warm/pestdata/choosemap.asp?plc=.
Current Degree-Days (Base 52°F 4 in. soil temperature) from January 1 to May 11, 2004
When is the corn rootworm larval hatch expected to occur based on the degree-day accumulations? Projected heat unit accumulations (base 52°F, 4-inch soil profile) indicate that by May 25, larval hatch should be well under way for the southern two-thirds of Illinois. This suggests that the initial larval hatch may occur as early as May 18. The summerlike temperatures we've experienced so far in May have helped to fuel this anticipated early hatch.
How does this year's projected larval hatch compare with previous years? On May 29, 2003, Larry Bledsoe, an entomologist with Purdue University, observed first-instar corn rootworm larvae in root tissue. To detect larval hatch, he had been busy dissecting 50 corn plants on a daily basis. Based on these dissections, he estimated that larval hatch most likely had occurred on May 25 across central Indiana in 2003. 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. Dissections of corn roots in 2001 and 2000 revealed that the corn rootworm larval hatch had occurred on May 16 and May 22, respectively. I suspect that this year's hatch may be most similar to the one that occurred in 2001, an early hatch. We offer our thanks to the entomologists at Purdue University for this continuing effort to pinpoint corn rootworm larval hatch each season.
How small are first-instar corn rootworm larvae? 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. 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.
Corn rootworm larva. (Note dark brown head capsule and supracaudal tail region).
Does the timing of the corn rootworm larval hatch affect soil insecticide performance? Yes. Early planting (early to mid-April) followed by a late hatch (early to mid-June) creates the most significant challenge to insecticide performance. Under this scenario, corn rootworm larvae may continue to feed on root tissue through most of July. If an insecticide has been applied in early April and a late hatch occurs, the insecticide will need to continue offering protection approximately 15 to 16 weeks after application. This may lead to product performance issues, especially if high densities of corn rootworm larvae are present. Late hatches can occur. In 1996 and 1997, larval hatch occurred on June 12 and June 13, respectively, across central Illinois. Fortunately, based on the heat unit accumulations that have occurred so far this season, I suspect that hatch is likely to be early this year. We'll let you know as soon as the larval hatch is confirmed officially.--Mike Gray