Western Corn Rootworm Adults "Thick" in Many Fields

July 23, 1999
Although root injury in our research plots has been unfortunately low this season (that's right, we like as much pressure as possible in our studies), western corn rootworm adult densities remain very impressive in many fields across Illinois. Chris Pierce, a graduate student in the Entomology Department, reported an upward surge in adult western corn rootworm abundance in his research plots located in producers' fields in Iroquois County. Chris also noted an increase in the number of female beetles and incidence of mating compared with the previous week. This suggests that significant egg laying may be just around the corner.

Because of the large densities of western corn rootworm adults in pollinating cornfields, don't forget to monitor fields for silk clipping. We recommend that producers consider a rescue treatment to protect the pollination process when an average of five or more beetles per plant are found, pollination is ongoing, and silk clipping is occurring. Typically, at least 1/2 inch of protruding silks beyond the ear tip is required for successful pollination to result. Products labeled as rescue treatments for corn rootworm adults to protect pollination include: Ambush 2E* (6.4 to 12.8 ounces of product per acre), Asana XL* (5.8 to 9.6 ounces of product per acre), Lorsban 4E (1 to 2 pints of product per acre), Penncap-M* (1 to 2 pints of product per acre), Pounce 3.2 EC* (4 to 8 ounces of product per acre), Sevin XLR Plus (2 pints of product per acre), and Warrior T* or 1E* (2.56 to 3.84 ounces of product per acre). (Products that are followed by an asterisk are restricted for use to certified applicators only.)

It is important not to confuse our recommendations to protect the pollination process with an adult-control program to suppress egg laying. A treatment to prevent silk clipping generally occurs too early in the season to substantially reduce egg laying by adults later in the summer. Late-planted fields are generally not good candidates for an adult-suppression program. Large numbers of adults are more likely to move into these fields, which are apt to have fresh silk and pollen relative to those fields that are drying down more rapidly. We recommend that an egg-laying suppression program be under the supervision of properly trained pest management personnel.

At some point early in August, we'll report the results of our insecticide trials. As indicated in last week's Bulletin, we remain interested in your observations from the field on insecticide performance problems this season.--Mike Gray

Author: Mike Gray