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Southern Corn Leaf Beetles Are Active in Western Illinois

May 8, 2003
Southern Corn Leaf Beetles Are Active in Western Illinois

Our favorite early-season insect pest--the southern corn leaf beetle--has made its appearance once again in several fields in Illinois. (Cut us some slack on the word "favorite"; we're entomologists.) This small pest has aggravated corn growers consistently in western, west-central, and southwestern Illinois for about 8 years. This year we have received specific reports of injury caused by southern corn leaf beetles in Adams, Calhoun, Fulton, and Hancock counties and more general reports of widespread injury in western and west-central counties. In the past, this insect also has caused fairly widespread problems in Kansas and Missouri.

Following is some information you should know about southern corn leaf beetles.

Identification. Adult southern corn leaf beetles (Figure 1) are 3/16 inch long, dark brown, and often covered with bits of soil, making them difficult to find in the field. The shield just behind the head has three "teeth" on each lateral edge. When disturbed, these beetles drop from the plants to the ground and hide. The adults feed mostly early in the morning, late in the evening, at night, or on cloudy days.

Southern corn leaf beetle adult. (Photograph courtesy of Mike Roegge.)

Injury to corn seedlings. Adults emerge early in the spring to feed on young weed hosts, especially cocklebur, and early-planted corn. The adults feed on the stems and chew out characteristic notches on the edges of leaves of corn seedlings (Figure 2); injured plants appear ragged. If seedlings are small, the notches the beetles chew in the stems may cut the plant off, resembling cutworm injury. Sometimes the beetles feed in such large numbers that injured plants die.

Injury caused by southern corn leaf beetles. (Photo courtesy of Mike Roegge.)

Observations over the past few years suggest that fields with reduced or no-tillage are more prone to attack by southern corn leaf beetles. The beetle also is prevalent in fields infested with cocklebur, another host. Although other species of weeds might be hosts for this insect, we still know very little about the insect's ecology, including its host range.

Life cycle. Southern corn leaf beetles overwinter as adults beneath soil and plant debris and in clumps of some species of weeds. In the spring, the adults emerge and begin to feed on weeds such as cocklebur, smartweed, and crabgrass. They fly from weed hosts into cornfields, where host plants are more plentiful, shortly after corn emerges.

After they finish feeding, the adults mate and females lay eggs in clusters of 10 to 50 in weed debris or in the soil within a field. In a week to 10 days, the larvae hatch and begin to feed on corn roots. The larvae develop over 10 weeks, from May until mid-July in the central portion of the Corn Belt. Adults begin emerging from the soil in mid-July; after a limited feeding period, they begin to seek overwintering sites. The adults are strong fliers, so movement from field to field is common.

Control. Obviously the first key to determining whether southern corn leaf beetles need to be controlled is assessment of the amount of injury and verification that the beetles are present. The latter is not always easy. Because of their small size and cryptic coloration, and their proclivity to "play possum," southern corn leaf beetles are not easy to find in the field. Some diligent searching, with a dose of patience (I strained my patience a few years ago), should enable you to find the critters if they are present.

Two southern corn leaf beetle adults (circled) on the soil surface in a cornfield.

Economic thresholds for southern corn leaf beetles have not been established. The economic thresholds established for black cutworms could be used as management guidelines, but these thresholds don't accommodate foliage-feeding injury. Until research is conducted to address this question, suggested thresholds are guesswork. Nevertheless, good guesswork is better than nothing at all.

Insecticides suggested for control of southern corn leaf beetles are *Baythroid 2 at 1.6 to 2.8 oz per acre; *Capture 2EC at 2.1 to 6.4 oz per acre; *Lorsban 4E at 1 to 2 pt per acre; *Mustang Max at 2.72 to 4 oz per acre; and *Warrior at 3.84 oz per acre. Use of products preceded with an asterisk is restricted to certified applicators. As a rule of thumb, higher volumes of water improve the coverage, and therefore the efficacy, of most products.

I am interested in learning whether any of the systemic insecticidal seed treatments (Cruiser, Gaucho, and Prescribe) have any impact on this pest. Let me know if you encounter some comparisons that would help address this question.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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