The full impact of our warm winter temperatures will not be known, but one of the first warnings coming from researchers is to be on the lookout for corn flea beetles, which should have overwintered nicely, thank you.

Once your seedlings are up, flea beetles will descend, scratch the leaf surface, and infect the plants with bacteria that will cause a disease known as Stewart’s Wilt. If your operation has had mild temperatures this winter, read on…

There are two issues here: the insects and the bacterial wilt. Depending on whether you produce commercial hybrids, seed corn, or sweet corn, you will have a varying interest in these issues. But depending on where you farm in the Corn belt will also determine your need to beware.

Let’s first look at the geography. Examine the temperature maps that indicate the potential for a severe outbreak. That area includes all of Missouri, the southern halves of Illinois and Indiana, and nearly all of Ohio. North of that territory is probably safe from a severe outbreak, but farmers should beware of the potential for some damage.

Flea beetles—scouting

Agronomists at Ohio State recommend scouting several times per week as corn spikes emerge, when they are most susceptible to the flea beetle activity. If your corn is not tolerant to Stewart’s Wilt, Ohio State says, “The suggested action threshold for wilt-susceptible varieties is low: treatment is suggested if there is an average of 6 flea beetles per 100 plants. The suggested action threshold for wilt-tolerant varieties is much higher: treatment is suggested if there is an average of 2 beetles per plant and 25 percent of the stand is damaged by heavy leaf-feeding.”

If you don’t have the time, use yellow sticky traps as your hired labor. Mounted 2 feet above the ground, 9 feet inside the field, your threshold for action is 8.5 beetles per trap per week.

The prediction for potential problems comes from the average temperature for the past three months, known as the flea beetle index. Ohio State’s Crop Observation and Recommendation Network (CORN newsletter) says, “For those growers wishing to take preventive action against flea beetle, commercially applied insecticide seed treatments Cruiser and Poncho, or the grower applied products Concur and Latitude, are labeled for flea beetles.”

Disease—treatment

University of Illinois agronomists say, “During the feeding process, adult flea beetles disseminate the bacterium which accumulates in and clogs the vascular system of the plant, affecting water and nutrient movement. Susceptible corn plants may become infected by E. stewartii at ant time during plant growth, and two phases of Stewart’s wilt will occur on corn. Some plants are infected in the seedling stage, whereas others may not be infected until tasseling or later. Most varieties of commercial field corn are resistant to the wilt phase of this disease while many are somewhat susceptible to the leaf blight phase. However, many seed corn inbreds and sweet corn hybrids have varying susceptibility to Stewart’s wilt.”

Sweet corn can become infected early and either wilt or have stunted growth. If the infection is not until later, harvest will already have occurred and the disease is a non-factor.

Management—Seeds and insecticides

Plant resistant hybrids. If you cannot, delay your planting date to avoid the flea beetle emergence in the spring. Scout early and often, consider seed treatment and foliar insecticides. “Systemic seed treatments provide early season control against the corn flea beetle. The use of systemic seed treatments has been shown to reduce corn flea beetle feeding and Stewart’s wilt infection.”

Management—scouting

There are two phases of Stewart’s wilt: seedling wilt and leaf blight. “The seedling wilt stage occurs when seedlings become infected at or before the V5 stage. The growing point is easily infected. The vascular system becomes plugged with bacterium, causing the seedling to wilt, become stunted, and die. Infections of older corn plants usually result in the development of the leaf blight phase of Stewart’s wilt. This phase is characterized by long, yellow to chlorotic streaks with wavy margins along the leaves. The late infection phase, or “leaf blight phase,” of Stewart’s wilt occurs after tasseling and is generally not a concern in sweet corn because ears are harvested before damage occurs.”

Photographs of flea beetle damage and Stewart’s Wilt infection

Iowa State’s recommended economic thresholds for flea beetle control with a foliar insecticide are as follows: “In commercial hybrid corn prior to stage V5, 50% of plants with severe feeding injury and five or more beetles per plant; in seed corn on susceptible inbreds, 10% of the plants with severe feeding injury and two or more beetles per plant. Several insecticides are registered. The thresholds originally were based on insect feeding damage alone; therefore, somewhat lower thresholds may be appropriate for genotypes very susceptible to Stewart’s disease.”

Summary:

Warm temperatures the past three months have provided a good opportunity for problems with flea beetles in early corn. Flea beetles transmit a bacterial disease known as Stewart’s wilt. While many commercial hybrids have resistance, some hybrids do not. Also susceptible are inbred seed corn lines, and sweet corn. Scouting should be a priority, and if the flea beetles are above economic thresholds, rescue treatments are available. There are also other management options, including delaying planting dates.

Stu Ellis

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