Issue No. 5, Article 3/April 25, 2008
Corn Flea Beetle: Expectations for Injury in 2008
One of the many spring insects that captures our attention on occasion is the corn flea beetle. Because of the cold and prolonged winter experienced most recently, it is not surprising that we've received some calls regarding these tiny insects as producers finalize their preparations for planting. The questions and answers here address many of the inquiries on the biology and management of these potentially damaging beetles.
Numerous corn flea beetles on seedling.
Why are these insects termed corn flea beetles? 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 the insects' size (1/16 inch, 1.8 mm) and impressive leaping ability when disturbed.
Where do corn flea beetles overwinter? 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.
Why are corn flea beetles of more concern to the seed production industry? Of primary concern to those in the seed production business is the potential for transmitting Stewart's bacterial wilt to susceptible inbreds or sweet corn varieties. Corn flea beetle injury to the epidermis of corn leaves rarely results, in and of itself, in economic losses because relatively little tissue is consumed. Small streaks of absent epidermal tissue serve as evidence of flea beetle feeding. The bacterium--Pantoea stewartii--transmitted by the feeding of flea beetlescan overwinter in the soil and plant debris as well as within the guts of vectors of the disease. As many as 20% of corn flea beetle adults emerging in the spring may be infected with bacteria responsible for Stewart's wilt. By midsummer, 75% of the corn flea beetle population may serve as vectors of this disease.
Stewart's wilt is vectored in the spring, when corn flea beetles feed on corn seedlings. The disease (bacteria) spreads throughout the infected plants, and two phases occur--a seedling wilt (at or before the V5 stage) and leaf blight. Infected seedling plants may become stunted, wilt, and exhibit linear lesions. As the infection grows more severe, overall yellowing of leaves intensifies and moves upward on plants. The 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, but many hybrids are susceptible to the leaf blight phase. Early planting dates can exacerbate the severity of Stewart's disease in susceptible inbreds or varieties.
Do corn flea beetles feed on any other hosts? Although corn is the preferred host, corn flea beetles are known to feed on other plants, including 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 the preferred host, corn, begins to emerge.
What is this spring's outlook for economic infestations of corn flea beetles? 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. Temperatures averaged across December 2007, January 2008, and February 2008 for Champaign, DeKalb, and Dixon Springs were 28.0°F, 21.3°F, and 35.4°F (Table 1). This suggests very low potential in Champaign and DeKalb this season for Stewart's wilt. However, in southern Illinois the likelihood is much greater (Table 2).
Can you scout for corn flea beetles and use economic thresholds to rescue infested fields? Despite the fact that winter may have taken its toll on flea beetles for much of the state, we encourage vigilant scouting for corn flea beetles this spring, especially in southern Illinois, particularly where inbreds (or sweet corn varieties) sensitive to Stewart's disease will be grown. If susceptible inbreds are infested before the 5-leaf stage, 2 to 3 adults per plant are found, and 10% 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 U.S., consultants recommend rescue treatments when 6 adults are found per 100 plants. Products labeled for corn flea beetles in commercial corn can be found in the Illinois Agricultural Pest Management Handbook. Because of the large volume of transgenic Bt hybrids that will be planted this spring, many soil insects, including corn flea beetles, will be exposed to the thianicotinoid insecticidal seed treatments (Poncho and Cruiser). Because of the widespread use of these insecticides and their effect on corn flea beetles, along with the cold winter, very low levels of Stewart's wilt are anticipated for central and northern Illinois counties this season.--Mike Gray