No. 8/May 15, 1998
Third Time a Charm? Southern Corn Leaf Beetles Are At It Again
In 1996, we published an article (issue no. 11, June 7, 1996) about an insect for which little modern information was available. At the time, we assumed the insect would be here and gone, a glitch on the "radar screen" of insect activity in the Corn Belt. Well, the southern corn leaf beetle visited us again in 1997, and apparently it's back for more in 1998. Since our first article in 1996, we have received inquiries about the southern corn leaf beetle from entomologists in Missouri, Kansas, and Iowa, so interest and possible incidences of this insect apparently have spread.
Many things about the southern corn leaf beetle are fascinating, but possibly the most important aspect is that the injury it causes can be confused easily with injury caused by black cutworms. At this time of year, the adults emerge from their overwintering quarters and feed on weeds and corn seedlings. The beetles feed on the stems and chew out notches on the edges of leaves of corn seedlings; injured plants appear ragged (Figure 1). They also may chew notches in the small stems, causing plants to fall over as if they had been cut off by cutworm larvae. Sometimes the beetles feed in such large numbers that injured plants die. Andy Welbourne with Zeneca found southern corn leaf beetles and symptoms of the injury they cause on as many as 65 percent of the plants in a couple of fields in Adams and Brown counties.
Figure 1. Seedling corn plant injured by southern corn leaf beetle adults.
Due to the reoccurrence of the southern corn leaf beetle, we thought it might be helpful to offer some information about its life cycle. 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 weed hosts (especially cocklebur) and early planted corn. After mating, the female deposits eggs in clusters of 10 to 50 in weed debris or in soil near corn plants. Eggs hatch in 6 to 10 days, and the larvae apparently feed on corn roots for about 10 weeks, from early April until mid-June in southern states and from early May until mid-July in thecentral Corn Belt. (The extent of injury to corn roots caused by the larvae has not been determined.) 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 the late summer or early fall.
As you are scouting for early season insect problems in corn, especially in western and southwestern counties in Illinois, be alert for the possibility that southern corn leaf beetles could be the culprit causing injury in some fields. Old reports suggest that 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. In addition, evidence thus far suggests that southern corn leaf beetles are more prevalent in no-till and reduced-till fields.
If you see plants that have characteristic injury as shown in Figure 1, look for the small beetles in the soil near the injured plants. Adult southern corn leaf beetles are 3/16 in (5 mm) long, dark brown, and often covered with bits of soil, rendering them difficult to find in the field. The prothoracic shield just behind the head has three "teeth" on each lateral edge. Figure 2 is an illustration of an adult southern corn leaf beetle.
Because this insect has been reported so infrequently in corn, economic thresholds have not been established. The economic thresholds established for armyworms or black cutworms could be used as management guidelines. No insecticide is labeled for control of this insect, so efficacy data are nonexistent.
Figure 2. Southern corn leaf beetle adult.
If you begin to observe southern corn leaf beetles causing problems in your area, please notify us. We are quite interested in documenting where this insect occurs in Illinois and trying to determine if a "common thread" runs through all infested fields.
Kevin Steffey (firstname.lastname@example.org), Extension Entomology, (217)333-6652