No. 9/May 23, 1997
Southern Corn Leaf Beetles: Are They Back Again?
We have had several calls from individuals in southwestern Illinois counties describing injury to corn plants that fits the description of that caused by southern corn leaf beetles. A report from Jersey County on May 15 indicated that a field of no-till corn was so severely damaged (believed to be southern corn leaf beetles) that replanting was required. Robert Bellm, crop systems educator with the Edwardsville Extension Center, indicated May 19 that he had received several reports of southern corn leaf beetle injury from growers in Jersey and Monroe counties. In 1996, we reported on infestations of this insect in western Illinois cornfields. Specimens from one infested cornfield were positively identified (1996) by John Bouseman, with the Illinois Natural History Survey, as southern corn leaf beetles, Myochrous denticollis. Could these insects be back once again? Provided is an excerpt from Kevin Steffey's article in last year's Bulletin (no. 11, June 7, 1996).
"The best article published about the southern corn leaf beetle is in the Bulletin of the U.S. Department of Agriculture, No. 221, by E.O.G. Kelly, in 1915! Those old entomologists really knew how to describe an insect! The species is widely distributed over the southern half of the United States, extending from Arizona and Texas to as far north as northern Illinois and southern Iowa and as far east as Maryland. The author believed that the insect overwintered as an adult beetle, and the early season injury caused by the beetle seems to verify this bit of biology.
"The adult beetle (Fig. 1) is small (3/16-inch long), somewhat oval in shape, gray-brown in color, and covered with small, stiff hairs. They are often covered with bits of soil. When disturbed, the beetles drop from their food plant and hide, making them difficult to find. The beetles chew notches in the edges of the corn leaves (Fig. 2) and occasionally in the stem. When they occur in sufficient numbers, the injury can kill young plants. The beetles have also been observed feeding on sorghum and alfalfa, as well as (and here's the good news) cocklebur, smartweed, and crab grass."
Figure 1. Southern corn leaf beetle adult (actual size, 3/16 inch). From USDA bulletin no. 221, 1915.
Figure 2. Seedling corn plant injured by southern corn leaf beetle adult. From USDA bulletin no. 221, 1915.
Kevin was prophetic with his final statement in his article last year: "Don't expect an outbreak of this insect, but you might file this one away for future reference. You never know what you might run into."
Commonly asked questions include:
Is there an economic threshold for southern corn leaf beetles? No. You might consider using one (or a combination) of the following thresholds for armyworms, cutworms, and flea beetles.
Armyworms: For seedling corn, control may be justified when 25% of the plants are being damaged.
Cutworms: Consider a postemergence treatment when 3% or more of the plants are cut and larvae are still present.
Corn flea beetles: Treat when leaves on seedling plants are severely damaged and plants are being killed.
Is there any insecticide labeled for use against southern corn leaf beetles? No. Reports suggest that some insecticides that are legal for use on corn have been tried; however, no confirmations regarding efficacy have been provided.
Mike Gray, and Kevin Steffey, (217)333-6652