Mark Fromme with Western FS has found southern corn leaf beetles injuring 2-leaf-stage corn in the Illinois River bottoms in Scott County. He indicated that 50% of the plants had signs of leaf-feeding injury, and he found at least one beetle per injured plant. In the past the field had been infested with quackgrass and possibly cocklebur. |
This very early observation of an insect pest that has been prevalent in western Illinois only in recent years may be an indication of what we might expect during the next few weeks. The southern corn leaf beetle overwinters as an adult under debris and in clumps of some weed species. It is logical to speculate that the mild winter favored this beetle's survival. So this is a "heads-up" for all corn growers in western and southwestern counties. Be on the lookout for this pest because it can cause significant damage rather quickly.
Let's review some information about southern corn leaf beetles.
What does the southern corn leaf beetle look like?
Adult southern corn leaf beetles (Figure 1) are 3/16 inch long, dark brown, and often covered with bits of soil, rendering them difficult to find in the field. The shield just behind the head has three "teeth" on each lateral edge.
What does the damage caused by southern corn leaf beetles look like?
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 notches on the edges of leaves of corn seedlings (Figure 2); injured plants appear ragged. Sometimes the beetles feed in such large numbers that injured plants die.
What types of fields are most susceptible to damage caused by southern corn leaf beetles?
The southern corn leaf beetle appears most frequently in fields previously devoted to pasture or in fields that have not been cultivated for several years. However, the beetle also is prevalent in fields infested with cocklebur, another host. Because nothing new about this insect has been published since 1915, we know very little about other potential host plants for this species. Therefore other species of weeds might be hosts for this insect.
When is control justified, and what insecticides can be used for control of southern corn leaf beetles?
Because this insect has been reported so infrequently in corn, economic thresholds have not been established. The economic thresholds established for black cutworms could be used as management guidelines. However, no insecticide is labeled for control of southern corn leaf beetles, and information about insecticide efficacy is nonexistent. Therefore we invite anyone who does battle with this pest to let us know what products are being used and what products seem to be effective.
As more corn emerges throughout the state, we would appreciate receiving any reports about this insect. I am willing to visit some damaged fields just to get a handle on its occurrence and the severity of injury it causes.--Kevin Steffey