No. 20/August 8, 1997
Root Ratings from On-Farm "Digs" in East-Central Illinois
During the past week, our "root-digging crew" enjoyed the opportunity to visit pollinating cornfields across east-central Illinois and to evaluate the level of root injury in producers' fields. Not surprisingly, based upon the number of reports that we've received this season, we found ample evidence of significant rootworm problems (Table 1). In each of the fields we visited, we removed 15 roots from each of 4 untreated strips (no soil insecticide used at planting) and also, from 4 treated strips. In other words, we dug, washed, and rated 120 roots for rootworm larval injury in each cooperator's field. Most fields were 1/2 mile in length, and roots were sampled along the full length of each strip. Needless to say, our crew enjoyed a diet of pollen during these evaluations.
What do these preliminary results offer? First of allŠlarval injury is more severe this season than in 1996. Several fields had root ratings in untreated strips above a rating of 4.0, which led to severe lodging and "goosenecking." Yields in many of these fields will most likely suffer due to physiological reactions of plants and harvest difficulties. Roots from treated strips had less injury; however, a few of the fields had average root ratings above 3.0. Again, this season, product performance has been a serious concern for many growers throughout the state of Illinois, not just in east-central counties.
An examination of these preliminary results suggests that we may be closer to developing a threshold for corn rootworm adults in soybean fields. We have records of beetle abundance from soybean fields last year that were devoted to our on-farm first-year corn experiments this season. The initial correlations appear promising. That is, low adult densities in soybean fields in 1996 appear to match reduced levels of larval injury this year. We believe we are well on our way to delivering a threshold to growers!
During the next several weeks, growers should pay attention to the presence of western corn rootworm adults in their soybean fields. This recommendation should be followed by growers even outside east-central Illinois. If western corn rootworm adults are found in soybean fields, chances are very good that egg laying is occurring within those fields. As we continue to make progress on this challenging problem, we will keep our readership informed.
Mike Gray and Matt O'Neal, Extension Entomology, (217)333-6652