Issue No. 6, Article 2/April 30, 2004
Insects Feeding on Early-Emerging Corn
Note: This article was posted on the Web site on April 23, 2004. It is repeated in its entirety for this issue of the Bulletin so that it also appears in the printed version.
For issue no. 1 (March 18, 2004) of the Bulletin, Kelly Cook wrote an article titled "Warmer Winter for Corn Flea Beetles: Early-Season Stewart's Wilt Predictions." Based on the average temperature of December, January, and February, she forecast the potential for survival of flea beetles, the primary vector of the bacterium Erwinia stewartii, which causes Stewart's wilt. The forecast for flea beetles and Stewart's wilt in 2004 ranged from moderate to severe in southern and western counties.
We've just received a report from Matt Duncan, technical service agronomist for Golden Harvest in western Illinois, that flea beetle infestations are fairly heavy in some of the earliest-planted cornfields in Adams and Pike counties. Matt found one to six beetles per plant, with an average of three to four, in some fields in the Mississippi River bottoms (where the earliest-emerging corn in his area is located). He also indicated that feeding injury was significant in some areas of some fields.
And, as if flea beetles weren't enough, Mike Roegge, Extension unit educator in crop systems in Quincy, has reported an infestation of southern corn leaf beetles in a field in Adams County planted on March 23. The seedlings in the field are 3 to 4 inches tall (one collar), and the beetles are cutting through the small stems as they feed. The injury was severe enough that an insecticide application is warranted.
We strongly encourage corn producers and agricultural advisers to check early-emerging cornfields for the presence of flea beetles, southern corn leaf beetles, and other early-season insects as well as signs of their feeding injury. The corn flea beetle is a small (~1/16- inch-long), oval, shiny black beetle, with enlarged hind legs that enable it to jump when disturbed. The southern corn leaf beetle also is small (~3/16- inch-long) and is dark brown, often covered with bits of soil. The prothoracic shield just behind the head has three "teeth" on each lateral edge.
Corn flea beetle (photograph by Marlin E. Rice, Iowa State University)
Southern corn leaf beetle (photograph by Mike Roegge, University of Illinois)
Flea beetles injure corn by feeding on the epidermis of leaves, resulting in small feeding streaks or windowpane feeding patches. This injury infrequently results in economic losses, with one obvious exception. A few years ago, when soil moisture was less than plentiful and corn flea beetles were abundant, the injury caused by the flea beetles killed corn seedlings in western Illinois.
Corn flea beetles causing severe injury to a corn seedling.
The other concern with flea beetles is the potential for their vectoring the bacterium that causes Stewart's wilt. Most dent corn hybrids are resistant to the wilt phase after they develop beyond the five-leaf stage. However, many hybrids remain somewhat susceptible to the leaf blight phase of this disease.
Southern corn leaf beetle adults feed on the stems and chew out notches on the edges of leaves of corn seedlings; injured plants appear ragged. Occasionally, the adults occur in such large numbers that injured plants die, especially if the beetles chew through the stems.
Injury to corn seedling caused by southern corn leaf beetles (photograph by Mike Roegge, University of Illinois)
Treatment for corn flea beetles on seedling corn may be warranted if you find five or more beetles per plant before the V5 stage of growth, especially if the plants are suffering from environmental stress (e.g., lack of sufficient moisture, slow growth due to cool temperatures). Don't overreact to the presence of flea beetles, but don't sit on your hands, either. You may have to make a judgment call, depending on the environmental conditions in your area. Insecticides suggested for control of flea beetles in corn are presented in Table 1.
Treatment for southern corn leaf beetles may be warranted if enough corn seedlings are being killed that the plant population could be reduced significantly. There currently are no thresholds based on numbers of southern corn leaf beetles or extent of injury. Again, environmental stresses (including cool temperatures that slow corn growth) will play a role in treatment decisions. Insecticides suggested for control of southern corn leaf beetles in corn are presented in Table 1.
On another note, because nicotinoid seed treatments (i.e., Cruiser, Poncho) are labeled for control of flea beetles and southern corn leaf beetles, you might want to assess the performance of one or the other of these products this spring. We are interested in learning more about the efficacy of Cruiser and Poncho in growers' fields.--Kevin Steffey, Mike Gray, and Kelly Cook