Chad Patterson, Webster City area, recently reported finding corn flea beetles feeding on seedling corn in central Iowa. Corn flea beetles are small, shiny and black (Fig. 1), and about 1/16 inch long. The adults feed by scraping parallel to the veins along upper or lower leaf surfaces. The scratches create a “windowpane” effect where the tissue looks bleached or silvery (Fig. 2). Sometimes the feeding scars resemble foliar diseases such as gray leaf spot or Northern corn leaf blight.
The feeding rarely causes economic damage to established corn, but can sometimes kill corn seedlings with high beetle populations. Growers should be aware that corn leaf beetle can vector a bacterium called Pantoea stewartii (Erwinia stewartii), that causes Stewart’s wilt bacterial disease. Susceptible varieties of sweet corn hybrids and seed corn inbreds may be infected during the seedling stage but not show any symptoms until later in the summer. Infected plants will be covered in leaf lesions, not produce an ear, or die.
In an ICM News article earlier this year, Forrest Nutter made predictions about Stewart’s wilt disease in Iowa. He summarized the statewide risk level as “negligible to low” based on the Stevens-Boewe Index and Iowa State Stewart’s disease model. Although the predicted risk is low, scouting for corn flea beetle as corn emerges is recommended. Special attention to scouting and feeding damage should be made for seed corn and susceptible varieties. Adults move into fields from grassy overwintering areas, and infestations typically start at field edges. Begin looking at 20 plants at five locations in each field and determine the number of adults per plant. Use the following thresholds for rescue treatments in corn:
- Field corn - prior to stage V5, 50 percent of plants with severe feeding injury and five or more beetles per plant.
- Seed corn - on susceptible inbreds, 10 percent of the plants with severe feeding injury and two or more beetles per plant.
Currently labeled products include pyrethroids (Asana XL, Mustang Max, Warrior II), organophosphates (Lorsban 4E, Nufos 4E), and carbamates (Lannate LV, Sevin 4F). See manufacturer's labels for use rates and restrictions. Corn flea beetle management considerations include seed selection and cultural practices. Incorporating host plant resistant hybrids will help prevent feeding and disease transmission. Systemic seed treatments provide early season control of corn flea beetle and Stewart’s wilt in corn. Keep fields and surrounding areas weed-free to minimize overwintering habitat and food sources for larvae and adults. For susceptible varieties, plant later to avoid the spring migration of adults.