After some relatively quiet days in May, we received several reports of grape colaspis larvae feeding in cornfields during the week of May 26. The cool weather has delayed corn growth in some areas, and grape colaspis larvae have been chewing away the fine roots, ultimately resulting in wilted plants with "scorched" edges. Also, the stems of injured plants have turned purple; however, that symptom can be caused by a number of factors (including weather), so many folks simply had not taken particular note of it.
Most of the reports of grape colaspis injury have come from western and west-central Illinois. However, Mike Hellmer, field sales agronomist with Pioneer Hi-Bred International, found a field near Royal (Champaign County) infested with grape colaspis on May 29. He observed an average of about one to two larvae per plant, and some stunting was evident. Fortunately, if good growing conditions prevail soon, the plants should grow past the injury with little effect on yield.
Grape colaspis damage in a field in Champaign County, 2003. (Photograph courtesy of Mike Hellmer, Pioneer Hi-Bred International.)
Duane Frederking, also a field sales agronomist with Pioneer Hi-Bred International, has observed grape colaspis larvae in some cornfields in west-central and western Illinois. One field in Pike County had an average of three to five larvae per plant; root damage was severe. In a field in Sangamon County, Duane observed grape colaspis working mostly in spring-applied NH3 knife tracks, apparently avoiding the tighter soil between the knives. Again, he found several larvae per plant, and the injury was prevalent.
Some University of Illinois Extension educators also have found grape colaspis in western and west-central counties. Mike Roegge (Quincy), Sean Evans (Macomb), and Matt Montgomery (Springfield) all have observed grape colaspis in fields in several counties. The injury in these fields has varied considerably, but all of the educators believed that most of the plants will grow out of the injury.
Grape colaspis injury to corn plants. (Photograph courtesy of Mike Hellmer, Pioneer Hi-Bred International.)
Kelly Cook, University of Illinois Extension IPM entomologist in the Department of Crop Sciences, examined some corn plants that had been injured by grape colaspis larvae. The plants had been submitted from near Buffalo in Sangamon County. The symptoms of the injured plants, which were about at the 4-leaf stage, were classicsome denuded roots, some purpling of the lower stem, wilted leaves, and scorched leaf edges. Again, our assessment was that the plants probably will grow out of the injury.
Grape colaspis injury to larger plants probably will not result in any yield loss, especially if warm weather returns to encourage corn growth. However, injury to smaller plants could be more problematic. As we have learned before, severe infestations of grape colaspis can result in significant stand loss. Unfortunately, there are no effective rescue treatments after the damage has been diagnosed. One only hopes that the stand is sufficient to avoid replanting. Obviously, at this late date, replanting is not an attractive option.
As additional reports of grape colaspis injury come into our offices, we will keep you apprised. In the meantime, take time to read the accompanying article ("Grape Colaspis: Some Background") about grape colaspis by Matt Montgomery, Extension crop systems educator in Springfield. Matt provides some background about the description and biology of the pests, as well as some speculation about why grape colaspis pose problems some years and not others. Interesting reading. --Kevin Steffey