During the past few years, we have expected southern corn leaf beetles to show up in western Illinois at about this time of the season. Well, Mike Roegge, Adams/Brown Extension unit educator in crop systems, received his first report of southern corn leaf beetle injury on May 7. Approximately 10% of the field was affected by their feeding injury. This report follows on the heels of other recent reports of this insect infesting corn in other states--Kansas, Missouri, and Tennessee.|
Of primary interest is the fact that this pest has become troublesome in several states in the Midwest during the past few years, after having gone virtually unnoticed for decades. Although you may be weary of my referring to THE article, I am aware of only one fully developed scientific article about the southern corn leaf beetle. E. O. G. Kelly wrote an article titled "The Southern Corn Leaf-Beetle," as a Bulletin of the U.S. Department of Agriculture (no. 221) in 1915. Since then--nothing. So, Kelly's article has become the sole source of information about the biology of this pest and the injury it causes.
Following is information that we have gleaned from Kelly's article and from field observations during the past few years. This overview should give you a sense of what to look for and what to expect if you encounter the southern corn leaf beetle.
What do southern corn leaf beetle adults look like, and where should I look for them?
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. According to Kelly, the adults feed mostly early in the morning, late in the evening, or at night, or on cloudy days.
Southern corn leaf beetle adult. (Photo courtesy of Marlin Rice, Iowa State University.)
What does injury 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 characteristic 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. If corn seedlings are small, the beetles may eat the seedlings to the ground. Consequently, the injury caused by these beetles could be likened to a combination of feeding by armyworm and black cutworm larvae.
What types of fields are most susceptible to damage caused by southern corn leaf beetles?
Since 1915, when this type of information was generated, agriculture in the Midwest has changed more than just a little. Therefore, I don't think we have enough information to answer this question very well. However, observations have suggested 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. Other species of weeds might be hosts for this insect, too.
How did southern corn leaf beetles get into my field, and what happens after they get there?
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. However, 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 a week to 10 days, the larvae hatch and begin to feed on corn roots. The larvae develop over 10 weeks and from early 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.
As a bit of a side note, I wonder whether southern corn leaf beetle larvae cause injury to young corn plants by feeding on the roots. Kelly stated more than once in his article that larvae were found in close proximity to corn roots "which were more or less eaten." However, he never observed them feeding on the roots in the field, and his description of "more or less eaten" is a bit vague. The reason I am curious is that the larva of this species closely resembles the grape colaspis larva. In fact, the former scientific name of the southern corn leaf beetle was Colaspis denticollis (the current scientific name is Myochrous denticollis). The scientific name for the grape colaspis is Colaspis brunnea. Obviously these two species are closely related, and the larvae of both species can be present in the same fields at the same time. One wonders whether all of the reports of grape colaspis injury during the past few years were attributable solely to grape colaspis, or possibly to southern corn leaf beetles, or even a combination of the two.
When is control of southern corn leaf beetles justified?
Economic thresholds have not been established. The economic thresholds established for black cutworms could be used as management guidelines, but these thresholds don't accommodate foliage-feeding injury. Maybe the threshold for armyworm larvae in corn (25% of the plants are being injured) will suffice, but I don't have a huge amount of confidence in the threshold for armyworms, even for armyworms! Maybe a blending of the two thresholds is a compromise. However, until research is conducted to address this question, suggested thresholds are nothing more than guesswork. However, "nominal thresholds" (thresholds based solely on experience) are not all bad.
Over the past few years, several insecticides have been labeled for control of southern corn leaf beetles (Table 1). However, not much efficacy data have been generated. As a rule of thumb, higher volumes of water improve the coverage, and therefore the efficacy, of most products.
Although none of the new seed treatments (i.e., Gaucho, ProShield with Force ST, Prescribe) have included southern corn leaf beetle among the pests that are controlled, I am interested in learning if any of these has any impact on this pest. Because the active ingredient (imidacloprid) of Gaucho and Prescribe is systemic and because both include flea beetles (an aboveground pest) on the labels, one wonders whether these seed treatments will offer any protection against southern corn leaf beetles. Let me know if you encounter some comparisons that would help address this question.--Kevin Steffey