No. 8 Article 5/May 19, 2006

Southern Corn Leaf Beetles Observed in Some Areas

We have received reports from more than one source that southern corn leaf beetles are making their presence known in western counties. Kevin Black, insecticide/fungicide technical specialist with Growmark, reported that a couple of FS certified crop specialists found southern corn leaf beetles in western and southwestern Illinois. Duane Frederking, field sales agronomist with Pioneer Hi-Bred International, found what he believed to be southern corn leaf beetles feeding on seedling corn in Menard County. These independent observations suggest that people in western Illinois should be watchful for these troublesome pests.

Following is an overview of information about southern corn leaf beetles extracted from previous articles published in the Bulletin. Because very little new information about this pest has been generated, what appears here is a compilation of old literature and relatively recent observations.

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.

Figure 1. The adult southern corn leaf beetle.

Adult southern corn leaf beetle (photo courtesy of Mike Roegge, University of Illinois Extension).

Injury to corn seedlings. Adults emerge early in the spring to feed on young weed hosts, especially cocklebur, and on early-planted corn. They feed on the stems and chew out characteristic notches on the edges of leaves of corn seedlings; 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.

Southern corn leaf beetle injury to corn seedling (photo courtesy of Mike Roegge, University of Illinois Extension).

Close-up of southern corn leaf beetle feeding on corn leaf (photo courtesy of Kevin Black, Growmark).

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 7 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. Decisions about whether to control southern corn leaf beetle infestations with insecticides are difficult to make. Because of the beetles' ability to escape notice (small size, cryptic coloration), correct identification and assessment of injury can be a challenge. There is no economic threshold established for control of the southern corn leaf beetle. Insecticides suggested for control of southern corn leaf beetles are Baythroid 2 (1.6 to 2.8 oz), Capture 2EC (2.1 to 6.4 oz), Lorsban 4E (1 to 2 pt), Mustang Max (2.72 to 4 oz), and Warrior (3.84 oz). These are all restricted-use insecticides. Please follow all label instructions and precautions.

Both Cruiser and Poncho seed treatments include southern corn leaf beetles on their respective labels. However, very little information about the efficacy of these seed-applied insecticides against southern corn leaf beetles has been generated. Please let us know if you observe any differences in injury caused by southern corn leaf beetles in corn treated or not treated with these insecticides.--Kevin Steffey

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