Early-Season Insects Having a Heyday in Some Cornfields

May 5, 2000
I provided an update to the Web version of issue no. 5 (April 28, 2000), covering the reports of flea beetles we began to receive late last week, and also on some bird cherry oat aphids and thrips infesting seedling corn in Richland County. Now we can add to that update a whole potpourri of insects causing problems in cornfields all over the state, especially in southern and western Illinois.

Flea beetles top the lists of the most reports of injury, the most fields sprayed, and most cussed insect of the week. We have received reports of heavy infestations of flea beetles from a lot of people all over the state. The numbers reported range from 1 to 2 per plant to 15 per plant. The type of injury observed has included the typical "windowpane" damage caused by the beetles scraping off leaf tissue, badly scarred leaves, dead leaves, wilting plants, and, in the worst cases, dead plants. Corn has been growing somewhat slowly, so the flea beetles are adding stress to the seedlings.

By now, most people are aware of the situation with flea beetles. As I stated in the previous paragraph, many fields have been and are being treated with insecticides for control of flea beetles. People who call us ask whether treatment is justified, and our response typically is, "It depends." Obviously, if infestations are extremely heavy and plants are being killed, a treatment to prevent additional loss is warranted. On the other hand, if the numbers of flea beetles do not exceed five per plant, the injury is primarily aesthetic, and the seedlings are growing rapidly, the plants will recover from the injury. It's a judgment call based on your knowledge of the situation.

Refer to issue no. 2 (April 7) and no. 3 (April 14) of the Bulletin for a list of insecticides to control flea beetles, information pertaining to flea beetle biology and behavior, and more in-depth discussion of Stewart's wilt.

The bird cherry-oat aphids and thrips were found in a cornfield in Richland County by Brad Yonaka with Wabash Valley Service Co. Dave Thomas with Zeneca brought in a sample of injured seedlings with the insects still actively feeding. Much of the physical damage to the seedlings had been caused by the thrips using their rasping mouthparts to feed. In addition, the leaves of the seedlings were somewhat yellow, probably as a result of the combination of the aphids' feeding and cool temperatures. Typically we do not expect bird cherry-oat aphids to cause economic damage to corn seedlings. However, whenever a corn seedling is suffering from more than one stress, the results often are more than additive.

And then, of course, some people have all the fun. On May 2, Ria Barrido with Growmark in Bloomington visited several cornfields in Randolph and Monroe counties with Dale Bermester with Gateway FS and several other individuals. Dale had observed several early-season insects feeding in these cornfields, so they returned to assess the situation. They found flea beetles and bean leaf beetles in all of the fields, southern corn leaf beetles in most of the fields, black cutworm larvae beginning to cut plants in some of the fields, white grubs and wireworms feeding on seeds and seedlings in some of the fields, and grape colaspis larvae feeding on the roots of corn seedlings in one field. The corn seedlings were in the 2- to 3-leaf stages of growth, the injury was severe enough to treat the aboveground insects with insecticides in some of the fields, and at least one of the fields was going to be replanted.

The upshot of this litany of insect pests is that our recent focus on the so-called secondary insect pests of corn apparently was justified. The mild winter and early planting this year have created a situation in which these types of pests can flourish. Although flea beetles have captured most of the attention, look for the other pests, too. The cutting injury by black cutworms in southern Illinois is a heads-up for the rest of us. The observations of the other pests suggest that we need to be vigilant during the next few weeks. A large number of insect pests can reduce corn stands. Refer to the more detailed articles about some of these pests in this issue of the Bulletin.--Kevin Steffey

Author: Kevin Steffey