One of the many spring insects that captures our attention each year is the corn flea beetle. Not surprisingly, we've received many calls regarding these tiny insects as producers finalize their preparations for planting. The questions that follow are designed to answer many of the inquiries concerning the biology and management of these potentially damaging beetles.|
Corn flea beetles on seedling corn plant.
Why do we refer to these insects as corn flea beetles?
Corn flea beetles are very small insects that cause special 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.
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 the transmission of Stewart's bacterial wilt to susceptible inbreds or sweet corn varieties. 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% of emerging corn flea beetle adults 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. Seedling plants that are infected may become stunted and wilt, and may 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. Early planting dates can exacerbate the severity of Stewart's disease in susceptible inbreds or varieties. Please refer to Loretta Ortiz-Ribbing's article in this issue of the Bulletin for more details on Stewart's wilt.
Are there any other hosts that corn flea beetles will feed on?
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.
What is the outlook for economic infestations of corn flea beetles this spring?
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 suggests seed production fields in the southern one-third of the state are at greatest risk this spring to Stewart's disease due to the anticipated good survival of corn flea beetles through the winter.
How does anticipated corn flea beetle survival this winter compare with last year?
For most of the state, corn flea beetle survival is expected to be much poorer this spring than last year. In 2000, we predicted that outbreaks of corn flea beetles should be anticipated even in northern counties of Illinois, and in fact, we did receive many reports of fields heavily infested with these insects last spring. With the exception of southern Illinois, we don't anticipate the level of interest in corn flea beetles this spring to match last year. However, producers even in northern Illinois should not ignore corn flea beetles. Because of the plentiful snow cover in some areas of northern Illinois, corn flea beetle survival may be better than the predictions suggest based exclusively on air temperatures.
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 sensitive inbreds (or sweet corn varieties) to Stewart's disease will be grown. If susceptible inbreds are infested before the 5-leaf stage, two to three 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 United States, consultants recommend rescue treatments when six adults are found per 100 plants. Insecticides labeled as rescue treatments for corn flea beetle control include *Ambush, *Asana XL, *Capture 2EC, *Lorsban 4E, *Penncap-M, *Pounce 3.2 EC, Sevin XLR Plus, and *Warrior (* indicates use restricted to certified applicators only).
Do seed treatments represent a good management approach for corn flea beetle management?
Yes, in some instances. Because most commercial hybrids are resistant to the wilt phase of Stewart's disease following the 5-leaf stage of development, a scouting and rescue treatment approach may work fine for some producers who are willing to invest the time and energy to monitor densities of corn flea beetles in their fields. Keep in mind that many hybrids remain susceptible to the leaf-blight phase of this disease. Because seed-production fields are apt to have inbreds that may be quite susceptible to Stewart's disease, the preventive seed treatment approach makes good sense. This is especially true in areas of the state where better-than-average overwintering survival is anticipated.
Seed-treatment products that contain the active ingredient imidacloprid offer seed production managers an attractive corn flea beetle management option because of their systemic (insecticide is taken up by the plant) activity. These products are manufactured by Gustafson and include Gaucho (0.165 milligrams of imidacloprid per seed), Gaucho Extra (0.60 milligrams per seed), and Prescribe (1.34 milligrams per seed). Gaucho is designed to offer protection against corn flea beetles through the first true-leaf stage of corn development. This product, because of its lowest cost of the three systemic seed treatments, may be of interest to commercial corn producers who are concerned about corn flea beetles and who are unlikely to scout their fields and use insecticides as needed. Gaucho Extra is designed to offer corn flea beetle protection through the 5-leaf stage of corn development and may be of most interest to seed-production managers. While the use of Prescribe would most likely provide very good control of corn flea beetles, this product is comparable in cost to some conventional soil insecticides and is marketed primarily for control of secondary soil insects as well as corn rootworms.--Mike Gray