Corn Flea Beetle: Expectations for Injury in 2000

April 7, 2000
Corn flea beetles are very small insects that cause some concern in the seed-production industry each spring, especially in years following mild winters. The "flea" beetle name is well deserved because of their size (1/16 inch, 1.8 mm) and impressive leaping ability when they are disturbed. Flea beetles spend the winter as adults and are most apt to cause damage when corn plants are slowed in their development by cool spring conditions. Adult flea beetles overwinter in clumps of grass near cornfields. Following mating in the spring, females lay their eggs in the soil of cornfields. Larvae hatch from eggs in approximately 1 week and complete the larval stage and pupate in about 2 weeks. After emergence, adults feed and mate for the remainder of the summer. Of primary concern to those in the seed-production business is the potential for the transmission of Stewart's bacterial wilt to susceptible inbreds or sweet corn varieties. Although corn is the preferred host, corn flea beetles are known to feed on other plants such as orchard grass, crabgrass, fall panicum, redtop, witch grass, Kentucky bluegrass, Sudan grass, yellow foxtail, giant foxtail, barley, and wheat. Foxtail, oats, and wheat are known to sustain corn flea beetle populations until their preferred host, corn, begins to emerge.

Corn flea beetle adults on corn seedling.

Corn flea beetle injury to the epidermis of corn leaves, in and of itself, rarely results in economic losses, due to the relatively small amount of tissue consumed. Small streaks of absent epidermal tissue serve as evidence of flea beetle feeding. The bacterium, Erwinia stewartii, transmitted by the feeding of flea beetles, is able to overwinter in the soil and plant debris, as well as within the vectors of the disease. As many as 20 percent of emerging corn flea beetle adults in the spring may be infected with bacteria responsible for Stewart's wilt. By midsummer, 75 percent of the corn flea beetle population may serve as vectors of this disease. Seedling plants that are infected may become stunted, wilt, and exhibit linear lesions. As the infection increases in severity, overall yellowing of leaves intensifies and moves upward on plants. These disease symptoms may be displayed in some sweet corn varieties at any stage of plant development. Most dent corn hybrids are resistant to the wilt phase of Stewart's disease following the 5-leaf stage of development. However, many hybrids are susceptible to the leaf-blight phase of this disease.

Entomologists have long reported that mild winters favor the survival of flea beetles and increase the potential that Stewart's disease may be a problem. In an effort to quantify the effect of winter conditions on beetle survival, it is commonly suggested that if the average monthly temperatures (°F) for December, January, and February sum to more than 90, flea beetle survival through the winter may be good. In Figure 1, Bob Scott, Illinois State Water Survey, has provided a map for Illinois that clearly depicts the potential for severe corn flea beetle problems this spring for the entire state. Even in northern counties, winter survival can be rated as very good for flea beetles.

We encourage vigilant scouting for corn flea beetles this spring (even in northern Illinois), especially where sensitive inbreds (or sweet corn varieties) to Stewart's disease will be grown. If susceptible inbreds are infested before the 5-leaf stage and two to three adults per plant are found, and 10 percent of the plants are silver or white because of flea beetle injury, a rescue treatment may be warranted. In certain sweet corn IPM programs in the northeastern United States, consultants recommend rescue treatments when 6 adults are found per 100 plants. Finally, early planting dates can exacerbate the severity of Stewart's disease in susceptible inbreds or varieties. Insecticides labeled as rescue treatments for corn flea beetle control include *Ambush, *Asana XL, Lorsban 4E, *Penncap-M, *Pounce 3.2 EC, Sevin XLR Plus, and *Warrior T or 1E (*use restricted to certified applicators only). Gaucho 600, by Gustafson, also is labeled as a commercial seed treatment (6.4 to 12.8 fluid ounces per hundredweight of seed) for field corn to provide "protection" against flea beetle injury.--Mike Gray

Author: Mike Gray