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Issue No. 13, Article 3/June 17, 2005

First Reports of Significant Rootworm Larval Feeding This Year

From a flurry of telephone calls and personal visits on June 10, I learned of observations of rootworm larvae causing noticeable injury to corn roots in relatively widely scattered areas of Illinois. Duane Frederking, with Pioneer Hi-Bred International, found relatively severe rootworm larval feeding injury in a field of corn planted after corn in bottomlands in Greene County. To the southeast, Brad Radamaker, with Golden Harvest, observed corn rootworm injury in a field of corn planted after corn in Clay County. Mike Hellmer, also with Pioneer Hi-Bred International, observed significant rootworm larval injury in a field of corn planted after soybeans in Vermilion County.

More recently, Will Mullenix, technical support agronomist with Channel Bio Corp., found larvae easily in several fields in McLean County on June 13. In one field of 7- to 8-leaf stage corn, Will found larvae in brace root tips and in most roots of the first and second nodes. The corn in this field was exhibiting severe stress from the combined impact of a lack of moisture and rootworm larval injury. Over many years, we have learned that dry soil conditions and heavy rootworm pressure can be a lethal combination.

It is important to note that in some of the fields in which significant rootworm larval injury was detected, soil insecticides had been applied at planting time in April. Most readers will not be surprised by this information, given the early applications and the mostly dry soil conditions since planting.

Based on research led by Dr. Eli Levine, Illinois Natural History Survey, 50% larval hatch occurs when 684 to 767 degree-days have accumulated since January 1 above a baseline soil temperature of 52°F. As of June 14, 677 degree-days had accumulated since January 1 in Champaign. The 11-year average for Champaign for this date is 742 degree-days. Following are accumulated degree-days (from January 1, base 52°F) on June 14 at selected sites: Springfield--745, Perry--639, Peoria--658, and Monmouth--506. You can determine accumulated degree days for any given date and several locations at the Web site hosted by the Illinois State Water Survey. Click on "Degree-day calculator," then your insect and location of interest. For corn rootworms, in addition to the actual total of degree-days accumulated for the most recent date, you also get projected 1-week and 2-week totals.

So it seems that an estimated 50% larval hatch has occurred or soon will occur throughout central Illinois. At most sites, 50% larval hatch in 2005 is behind the 11-year average, which is not surprising given the cool soil temperatures we experienced throughout much of May. Warmer temperatures in early June were catching us up, but the recent cooldown slowed things down again. The bottom line is that we still have many more rootworm larvae to experience before rootworm research evaluations begin in mid- to late July this year.

With more rootworm larvae to hatch, it is too early to assess the full extent of rootworm larval injury in most fields. However, if densities of rootworm larvae are large, noticeable injury at this time of year heralds tough times to come. The recent rains may have loosened soils sufficiently in some areas that lodging could be evident. In most fields, however, lodging probably will not occur until the corn plants have attained some height.

Take a deep breath and grab a shovel. We could experience significant rootworm problems this year.--Kevin Steffey

Kevin Steffey

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