Managing Corn Rootworms on an Areawide Basis: A Research Update

July 30, 1999
This week marked the beginning of our second year of root injury evaluations for the areawide corn rootworm research project located just south of Sheldon, Illinois. The experiment began with aerial applications of SLAM across cornfields and soybean fields during the summer of 1997. In all, approximately 45 producers have cooperated with entomologists from Purdue University and the University of Illinois to suppress densities of corn rootworms within a 16-square-mile block.

The original objective of the large experiment was to evaluate the potential effectiveness of SLAM in suppressing the population of rootworms to such an extent that applications of soil insecticides became unnecessary for most fields each spring. SLAM is an insecticidal bait that contains cucurbitacins and a very low level of carbaryl (1 ounce of active ingredient per acre). Cucurbitacins serve as feeding stimulants and movement arrestants for rootworm adults. The perception that cucurbitacins attract adults is incorrect. Instead, cucurbitacins, once ingested, cause adult rootworms to slow down and continue consuming more of the insecticidal bait. The desired effect is to kill as many females as possible before egg laying begins. In addition to the joint efforts of Purdue University and the University of Illinois, other areawide experiments are being conducted in Iowa, Kansas, South Dakota, and Texas.

Last season, the level of corn rootworm larval pressure was so low that we were unable to adequately assess the value of the insecticide applications during the summer of 1997. In addition, adult corn rootworm densities across much of Illinois, including our areawide research site, were very low. Despite the low densities of rootworm adults encountered last year, densities of rootworm adults in our 16-square-mile block this past week are impressive. In fact, we have already exceeded our anticipated use of SLAM this season. Recall that one of the primary objectives of this research endeavor was to reduce the density of rootworms to such an extent that soil insecticide applications for most fields became unnecessary. This is especially desirable for the interior fields of our 16-square-mile block. Unfortunately, we are observing densities of beetles in cornfields and soybean fields above established economic thresholds. This is not encouraging news for a program such as ours in its third year. In addition to alarmingly high densities of rootworm adults, particularly in soybean fields, we continue to be plagued by formulation problems with SLAM. On Tuesday of this week, our aerial applicator was forced to spend considerable time cleaning out the lines of his aircraft due to formulation problems. After many years of research with this product, we are not sure why these difficulties continue to occur.

During the first three days of this week, we removed roots from treated and untreated strips of 66 producers' fields. We continue to evaluate these roots for injury on the Iowa State 1-to-6 scale. Although many of the fields have more root injury than we observed last year, it is too early to assess the value of the SLAM applications that were applied during the summer of 1998. We will let you know what the final root-injury evaluations look like in forthcoming issues of the Bulletin.--Mike Gray, Kevin Steffey

Author: Kevin Steffey Mike Gray