Issue No. 10, Article 2/May 28, 2004
Grape Colaspis Larvae May Be Causing Some Injury to Corn
As we have indicated in a number of previous articles, injury caused by subterranean insect pests of corn has not been prevalent in 2004--a few cutworms here, a few white grubs there, and a few wireworms here and there. Maybe the widespread use of insecticidal seed treatments has had an impact on secondary insect pests of corn. Or maybe this just isn't these pests' year. Whatever the case may be, people continue to stay alert for anything out of the ordinary in cornfields.
Within the past several days, we have heard about a few incidents of grape colaspis larvae causing injury in cornfields. Again, the problem does not seem to be widespread, but the presence of grape colaspis is worth noting.
Aboveground symptoms of injury caused by grape colaspis larvae resemble other types of maladies that affect corn early in the season. Affected plants usually are stunted and wilted; stems are purple (indicating a phosphorous deficiency), and leaf tips and edges become yellow, then brown, as leaf tissue dies. If grape colaspis larvae are the cause for all of these symptoms, you also will observe roots that have been denuded of root hairs. If you search the soil around the roots carefully, you also will find small, comma-shaped grubs.
Corn seedlings injured by grape colaspis larvae.
Healthy corn plants next to corn plants injured by grape colaspis larvae (Photo provided by Ed Greene).
A grape colaspis larva is only 1/8 to 1/6 inch long, is slightly curved, and has a plump, white body with a tan head and prothoracic shield (plate just behind the head). Bunches of hairs arise from bumps on the underside of the abdomen. Don't confuse grape colaspis larvae with small white grub larvae, which are strongly C-shaped and lack the hairs on bumps on the underside of the abdomen.
Grape colaspis larvae in the soil (Photo provided by Mike Hellmer, Pioneer Hi-Bred International).
Grape colaspis larvae compared with a fully grown white grub larva.
Obviously there isn't much you can do if grape colaspis is causing injury in a cornfield. There are no rescue treatments for this insect. The only decision to be made is whether the damage is extensive enough that replanting is a consideration. If replanting is considered, consult agronomists' recommendations about effects on yield when corn is replanted this late in the spring.
The grape colaspis has been particularly elusive for research efforts. Very little insecticide efficacy data exists. If you happen to be aware of strip comparisons (part of a field treated, part not treated) in which grape colaspis larvae have been observed, let us know. Anything we can add to our database would be useful.--Kevin Steffey