On late planted corn, you may notice injury to the leaves that is unfamiliar to you. The leaf may have a "frosted" or dried appearance, particularly along the leaf edge and near the tip. Even though this may appear as some unrecognized plant pathogen, actually what you are seeing is the feeding injury from adult western corn rootworms. When the beetles emerge from the soil, often there are no tassels or silks for the insects to feed on in these late-planted fields. So the beetles will crawl down inside the whorl where they will feed on the tender new leaves. The feeding scars become evident when the leaf eventually emerges and unfurls. The scars run parallel to the leaf veins and may extend 12-15 inches or more along the leaf edge. Occasionally, the beetles will cut through the leaf, which causes long tears in the leaf.
None of this injury is considered economically damaging. Therefore, corn fields should not be sprayed when you see this injury. It does indicate, however, that many corn rootworms have finished their feeding as larvae in the field, and now would be a good time to dig a few roots and measure the amount of root protection provided by your soil insecticide, seed treatment, or transgenic corn rootworm hybrid.  
Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops.
This article originally appeared on page 225 of the IC-498(19) -- July 16, 2007 issue.