Issue No. 10, Article 2/June 1, 2007
Grape Colaspis Larvae Are Being Found in Plenty of Fields
During the weeks of May 21 and 28, we have received several reports that people have encountered grape colaspis larvae in both corn and soybean fields. In some of the corn fields, because corn rootworm larvae are producers' primary concern, grape colaspis have been mistakenly identified as rootworm larvae. Although rootworm hatch has been confirmed (see the related article in this issue of the Bulletin), it's important that grape colaspis larvae not be confused with rootworm larvaeso a refresher on identifying these two insects is in order.
As I indicated in the Bulletin last week (No. 9, May 25, 2007), a grape colaspis larva is small (1/8 to 1/6 inch) and slightly curved (comma-shaped), with a plump, white body and a tan head and prothoracic shield, or plate, just behind the head. Its three pairs of legs are short. Bunches of hairs arise from bumps on the underside of the abdomen, which rootworm larvae lack.
Grape colaspis larva (photo courtesy of Benjamin Kaeb, Iowa State University).
By comparison, an early instar corn rootworm larva (first or second instar) is slender and creamy white, with a dark brown head and anal plate, which the grape colaspis larva lacks. Fully developed third instars (it's too early to be finding these right now) are about 1/2 inch long.
Corn rootworm larva (note the dark brown anal plate on the dorsal [top] surface of the last abdominal segment) (photo courtesy of Matt Montgomery, University of Illinois).
It's important to be able to distinguish between these two beetle species to avoid making a snap judgment about the efficacy (or lack thereof) of a rootworm control product. Judgments cannot be made about the efficacy of grape colaspis control products, because so little efficacy data have been generated. In other words, we are not certain what does and does not work against grape colaspis larvae, despite the fact that several products include control of grape colaspis on their labels. You should know, however, that transgenic rootworm Bt corn will not control grape colaspis larvae (although the Poncho 250 seed treatment on rootworm Bt corn is supposed to protect the corn against grape colaspis injury).
The rain that fell in many Illinois locations over the Memorial Day weekend alleviated some of the concerns about dry soils, but dry soil conditions still prevail in some locations. The reason I bring this up is that we have had unfortunate experiences with grape colaspis injury when corn and soybeans are trying to develop in dry soils. During the drought year of 1988, grape colaspis larvae caused significant injury to soybean plants in late May and early June. The larvae chew off soybean root hairs and secondary roots, much as they do when they feed on corn roots, but they also chew channels in the primary root. Such injury usually goes unnoticed when growing conditions are good, but the combination of dry soil and grape colaspis injury can cause plants to wilt and sometimes die.
Some more rain will alleviate many of our concerns about subterranean insects feeding on corn or soybeans. But in the meantime, be aware that feeding by subterranean insects can exacerbate the situation if crops are not growing under ideal conditions.--Kevin Steffey