Issue No. 10, Article 3/June 2, 2006
Stunted Corn Plants? Check for Grape Colaspis Larvae
At this time of year, it is common for us to begin receiving reports of stunted, discolored, and/or wilted corn plants in fields throughout Illinois. The potential causes for these rather generic symptoms are plentiful, so entomologists always suggest that corn plants with such symptoms of injury be dug up and that the roots and the soil surrounding the roots be examined for insects or other arthropods. Although the primary insect culprits for this type of injury are white grubs and wireworms, grape colaspis larvae occasionally become the focus of attention. Only time will tell whether grape colaspis injury becomes widespread in 2006.
I received a report on May 31 from a farmer in Macon County who encountered pockets of grape colaspis larvae while cultivating his corn on May 30. The corn had been planted on April 19, and the 2005 crop was soybeans. Although stunting was the primary symptom of injury, nearly dead plants also were detected. The corn seed had not been treated with a seed-applied insecticide, but a soil insecticide had been applied at planting.
It just so happens that Ron Estes (coordinator for the Insect Management Program in the Department of Crop Sciences) and his crew planted a couple of trials on May 30 in fields in which red clover had been harvested. Because grape colaspis frequently occur in corn planted after red clover, we hope to generate some data regarding efficacy of soil- and seed-applied insecticides against grape colaspis.
Adult grape colaspis, which can be found in many crops in late summer, mate and lay eggs in the soil. Larvae hatch 1 to 2 weeks later. In the fall, grape colaspis larvae move downward and outward in the soil profile and begin to feed on the roots of available plants. In September, as soil temperatures begin to decline, the larvae migrate 8 to 10 inches deep in the soil, where they overwinter.
When the soil warms up in the spring, larvae return to more shallow regions of the soil profile. They begin to strip away fine roots and gouge "channels" in the roots of available plants (typically corn, but also soybean), sometimes causing significant stand reduction in fields of commercial corn hybrids. Grape colaspis damage to corn inbreds can be disastrous in seed-production fields. As mentioned previously, corn plants injured by grape colaspis larvae are stunted, and severe injury results in wilting and discolorationstems become purple, the edges of leaves appear scorched. As injured plants die, the plant population may become severely reduced.
Corn plants injured by grape colaspis larvae. Note the stunting and discoloration compared with the adjacent healthy plants.
Grape colaspis injury to corn planted after red clover (photo courtesy of Robert Bellm, University of Illinois Extension).
Grape colaspis larvae resemble small white grubs, although they are much smaller than grubs of Japanese beetles or Phyllophaga species. A grape colaspis larvae is 1/8 to 1/6 inch long and comma shaped; it has a plump white body with 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. Larvae often occur in relatively large numbers around individual plants in infested cornfields.
Grape colaspis larvae.
Controlling grape colaspis after injury has been discovered is not effective. The only way to control grape colaspis larvae in corn is to use a soil- or seed-applied insecticide before knowledge of the pest's occurrence exists. Unfortunately, anticipating such occurrence is very difficult, so spending money to prevent injury is chancy. In addition, very little efficacy data exists for current soil- and seed-applied insecticides. Maybe we can learn something from our experiments this year. In addition, if you encounter a grape colaspis infestation, please let us know your assessment of control, or lack thereof, with modern insect control products.--Kevin Steffey