Normally, Japanese beetles feed on corn silks later in the season, but Mark Licht, ISU Field Agronomist, noticed them feeding on the corn leaves. I personally haven't seen this before, but I guess they have to feed on something until the silks come out. I did visit a few farms around Ames today and noticed adults in corn. I don't know if they were feeding or just trying to stay protected from the wind. I also noticed beetles defoliating young soybean. They often aggregate to feed and mate, so individual plants had skeletonized leaves.
Japanese beetles in V8 corn at the ISU Johnson Research Farm. Photos taken 14 June 2010 by Erin Hodgson.
Japanese beetle defoliation is often overestimated in soybean. Although some leaves were heavily skeletonized, the overall defoliation on these V2/V3 plants was <1%. Photos taken 13 June 2012 by Erin Hodgson.
Japanese beetle feeding and mating on grape leaves at a winery in eastern Iowa. Photo taken 13 June 2012 by Erin Hodgson.