As Kevin Steffey reported in last week's Bulletin (issue no. 5, April 27), southern corn leaf beetles were observed in St. Clair County the week of April 16. Other observations from the field this spring suggest that this insect pest is here to stay. On April 30, Randy McElroy, agronomic research manager, observed southern corn leaf beetle leaf-feeding injury to seedling corn in a no-till field located near Louisville, Clay County. Dale Burmester, Gateway FS, Red Bud (Randolph County), also reported that southern corn leaf beetles are present and feeding in some fields. So far, injury has not been too heavy. On May 1, Matt Montomery, crop systems unit assistant, Sangamon/Menard counties, and Jerry Harbour, Lincoln Land FS, examined a cornfield that was in need of a rescue treatment for southern corn leaf beetles. Many other nearby cornfields were also infested. The message seems clear--don't hesitate to begin scouting for these insects. They are very difficult to find. However, with a little bit of practice, you can begin to "calibrate" your eyes. Producers are strongly encouraged to keep their eyes open for southern corn leaf beetles in any cornfield where seedlings are beginning to emerge. |
The following questions and answers are intended to shed some light on many of the questions we've received concerning southern corn leaf beetles. The answers provided to these questions are derived primarily from an article ("The Southern Corn Leaf-Beetle") written by E.O.G. Kelly, Bulletin No. 221, USDA, and published on June 16, 1915. Kelly first observed southern corn leaf beetle injury in 1905 while examining injured corn plants in southern Illinois.
Why are southern corn leaf beetles so difficult to find?
Adult southern corn leaf beetles (Figure 1) are small, only reaching 3/16 of an inch in length. In addition, they are experts at camouflage, covering their dark brown bodies with small bits of soil. After you find one of these small beetles, a more careful examination should reveal three "teeth" on each side of a plate located just behind the head.
How do southern corn leaf beetles injure plants?
The adults feed on stem and leaf tissues of seedling corn plants. Injured corn plants are ragged because of the notched out leaves (Figure 2). If sufficient numbers of southern corn leaf beetle adults are present, seedling corn plants can be killed. Some entomologists have suggested that southern corn leaf beetle injury has often been misdiagnosed as cutworm injury. This may be especially true with respect to stem-feeding injury.
Are some cornfields at greater risk to injury by southern corn leaf beetles?
Adults emerge early in the spring and begin feeding on weed hosts such as cocklebur. Early-planted fields are at greater risk for potential problems. Corn planted into fields that have been devoted to pasture also may be more susceptible to southern corn leaf beetles. No-till cornfields also may improve the survival of this pest.
Where do southern corn leaf beetles overwinter?
Adults overwinter beneath soil and plant debris and in clumps of some species of weeds. In the spring, the adults emerge and begin to feed on plants such as cocklebur and early-planted corn.
What other information is available concerning the life cycle of southern corn leaf beetles?
Following mating, females lay eggs in clusters of 10 to 50 in weed debris or in the soil at the bases of corn plants. In a week to 10 days, the larvae hatch and begin to feed on corn roots. The larval period lasts for about 10 weeks and occurs from early May until mid-July in the central portion of the Corn Belt. Adults emerge from the soil beginning in mid-July and after a limited feeding period, begin to secure their overwintering site. The adults are strong fliers and movement from field to field is made with relative ease.
Is there an established economic threshold for southern corn leaf beetles?
No. However, consider the following thresholds for other insect pests as a starting point. We suggest that a rescue treatment should be considered when true armyworms have injured 25% of the seedling corn plants within a field. True armyworm larvae may consume only leaf margin tissue (similar to southern corn leaf beetles) on seedling plants, or they can remove foliage to the midribs. If southern corn leaf beetle feeding on stem tissue results in cutting of seedling corn plants, you may wish to consider the cutworm threshold we've suggested previouslyconsider a rescue treatment when 3% or more of the plants are cut.
Are there any products labeled for use as rescue treatments against southern corn leaf beetle injury?
Yes. Capture 2EC is labeled as a postemergence rescue treatment at a rate of 2.1 to 6.4 ounces of product per acre. The label indicates that *Capture 2EC should be applied in a minimum of 10 gallons of finished spray per acre with ground equipment. Observations from the field suggest that the use of 15 gallons per acre improves efficacy. Dow AgroSciences also has issued a supplemental label that adds southern corn leaf beetle to its list of corn insects controlled by *Lorsban 4E as a postemergence treatment (1 to 2 pt/acre). *Capture 2EC and Lorsban 4E are restricted-use insecticides and can be applied only by certified applicators. Please read and follow all product labels for more specific application instructions.--Mike Gray and Kevin Steffey
Southern corn leaf beetle. (Courtesy of Marlin E. Rice, Iowa State University.)