During the middle of May, numerous fields in the southern tier of counties were attacked by an uncommon Iowa insect, the southern corn leaf beetle. Tracy Cameron, agronomist with Crestland Cooperative in Creston, reported several fields were heavily defoliated by this insect and sprayed with insecticides. By the time most of you read this article, there will probably no longer be a problem. However, the southern corn leaf beetle has been a fairly persistent pest during the 1990s in Illinois according to Kevin Steffey, extension entomologist, University of Illinois. This is an insect that may demand more of our attention in Iowa in future years.
Very little is known about the southern corn leaf beetle. The following information was taken from the Corn Insect Handbook published by the Entomological Society of America.
Adult southern corn leaf beetles are 3/16 inch (5 mm) long, dark brown, and often covered with bits of soil, rendering them difficult to find in the field. The prothoracic shield (=neck) just behind the head has three "teeth" on each lateral edge. The larva can be found in the soil and somewhat resembles a grape colaspis larva. It is creamy white and approximately 1/4-5/16 inch (6-8 mm) long. The undersides of the second through eighth abdominal segments each bear a pair of ambulatory processes.
Although reports about this beetle injuring corn date almost entirely from the early 1900s, the southern corn leaf beetle was observed causing injury to corn in Illinois in the 1990s. In some instances, the injury was severe enough that insecticides were applied to prevent additional stand loss, or the plant population was reduced so significantly that replanting was necessary. However, throughout; the United States, this beetle has been reported infrequently. The southern corn leaf beetle occurs most frequently in fields previously devoted to pasture or in fields that have not been cultivated for several years. The beetle also is prevalent in fields infested with cocklebur, apparently another host.
The adults feed on the stems and chew on the edges of leaves of corn seedlings; injured plants appear ragged. Sometimes the beetles feed in such large numbers that injured plants die. The extent of injury to corn roots caused by the larvae has not been determined.
The southern corn leaf beetle overwinters as an adult under debris and in clumps of some weed species. Adults emerge early in the spring to feed on young hosts, especially cocklebur, and early planted corn. After mating, the female deposits eggs in clusters of 10-50 in debris or in soil near corn plants. Larvae hatch in 6-10 days and apparently feed on corn roots for about 10 weeks, from early April until mid-June in southern states from early May until mid-July in the central Corn Belt. The larvae pupate in the soil, and adults emerge from mid-July into August, depending upon latitude. After feeding for a short time, the adults seek overwintering shelter in summer or early fall.
Because this insect has been reported so infrequently in corn, economic thresholds have not established. Preventive insecticides applied at planting time are not recommended. The economic thresholds established for black cutworms could be used as management guidelines. Control of weed hosts, particularly cocklebur, could reduce densities of this insect.
This article originally appeared on page 93 of the IC-484(12) -- June 5, 2000 issue.