While below ground corn root feeding impact from corn rootworm is nearing completion, it’s time to turn scouting attention to the above ground corn plant portions. Whole plant adult counts during August are used to make recommendations for rootworm management decisions in continuous corn in 2007. Scouting and threshold recommendations for adult beetle counts in continuous corn are available in a previous article Adult Corn Rootworm Sampling and Economic Thresholds in Continuous Corn (Wisconsin Crop manager, Vol. 12, No. 22, Aug. 4, 2005) http://ipcm.wisc.edu/wcm/.

The purpose of this article is to alert readers that corn fields should be assessed for silk clipping, which could reach economic levels and compromise pollination and kernel fill if silks are clipped by beetles to ½ inch or shorter. Corn rootworm adult beetle populations are expected to be significant in areas of Wisconsin based on untreated check pressure at the UW Entomology Field Crop rootworm trials near Arlington, WI; and reports of first year corn damage from southeast Wisconsin, Rock and Jefferson counties (see related article, this issue). July 20^th, Dave West, West Agronomics, reported a field in Monroe County with a mixture of northern and western corn rootworms as high as 15-20 beetles per plant in some parts of the field, with many concentrated on the silks. Friday July 21^st, Peg Reedy UW-Extension Walworth County Agriculture Agent and Matt Hanson UW-Extension Dodge County Crops/Soils Agent reported “heavy” rootworm beetle presence in corn fields as they sampled corn roots for the Variant western corn rootworm Network described in a separate article, this issue.

Because corn rootworm beetles can reduce yield by silk pruning, it is important to scout corn fields during pollination. Begin checking for adult beetles before 70% of the plants are in the process of silking. Assess the extent of silk clipping from 10 randomly selected plants in five separate areas of the field for a total of 50 plants. Treatment is warranted when average silk clipping has reached to within 1/2-inch of the ear tip.  Silk clipping beyond this point will compromise pollen to silk contact and kernel fill. Insecticides labeled for adult rootworm beetle control are listed in UW Extension publication A3646 Pest Management in Wisconsin Field Crops available at http://cecommerce.uwex.edu/.