Issue No. 9, Article 4/May 25, 2007
Activity of Multiple Soil-Inhabiting Insect Pests
Having written several articles regarding reports of black cutworm injury in some areas of the state, we have little more to say but that we continue to receive reports and that all corn fields should be scouted now. Obviously, the threat from black cutworms diminishes as the corn plants grow and the larvae develop to seventh instars and finish feeding.
Although we have not said much about our entirely subterranean friends (grape colaspis, white grubs, wireworms), the time seems right for an update. Several people have reported significant injury caused by wireworms this year, although there's no way to associate the reports with any given geography. Plant populations have been reduced significantly in some fields, with replanting being considered in heavily infested fields. Unfortunately, some of these reports of badly damaged fields included indications that the seed had been treated with a nicotinoid insecticide. We continue to question the purported control of wireworms (and black cutworms, too) provided by Poncho and Cruiser when economic infestations occur.
A few people have reported feeding activity by white grubs, but most fields infested with Japanese beetle grubs, at least thus far, have not shown symptoms of white grub injury (i.e., stunted, wilted plants with purple stems). However, it's important to point out that symptoms of that injury may begin to show up in fields with dry soils. We have learned from past observations that even Japanese beetle grubs can cause noticeable injury to corn when soils are dry, even though Japanese beetle grubs usually cause little economic injury under "normal" circumstances. If a lack of moisture persists, symptoms of white grub injury might appear.
Over the past week, a few people have reported observing injury caused by grape colaspis larvae. Lisa Coorts, Maxi-Yield Consultant Service in Carlinville, has observed quite a bit of injury in west-southwestern Illinois. Corn plants injured by these "tiny white grubs" have symptoms similar to injury caused by "big white grubs." Grape colaspis larvae denude the roots of root hairs, and injured plants do not take up water or phosphorous very well. Consequently, the leaves of injured plants appear "scorched" and the stems are purple.
Injury to corn plants caused by grape colaspis larvae (node roots without root hairs) (University of Illinois).
Corn plants injured by grape colaspis larvae in Cass County, 2003 (note stunting and "scorched" leaves) (University of Illinois).
Technically, grape colaspis larvae are grubs because the larvae of all species of beetles (including rootworms) are referred to as grubs. However, as many people know, we usually use the term white grubs to refer primarily to Phyllophaga grubs or Japanese beetle grubs, even though grape colaspis larvae resemble these white grubs. A grape colaspis larva is small (1/8 to 1/6 inch), 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.
Grape colaspis larva (photo courtesy of Benjamin Kaeb, Iowa State University).
Except for black cutworms, application of insecticides after discovery of injury caused by grape colaspis, white grubs, or wireworms will not provide effective control. Hopefully the injury caused by any of these pests is not so bad that replanting is necessary. However, replanting may be the only alternative if the plant population is reduced significantly. Refer to replanting decisions, regardless of cause, that are published annually by agronomists.Kevin Steffey