Watch For Western Bean Cutworm

Here's advice on how to scout and control the corn pest.

Scott Williams
By  Scott Williams , DTN Entomologist
Western bean cutworm females lay egg clusters on leaves of corn, Image by Scott Williams

If you’re growing corn somewhere in the upper Midwest, odds are you’ve caught sight of the latest threat to your corn yields: Western bean cutworm (WBC). Originally a minor pest of corn and dry beans in the Western plains, it started pushing out of its native range around 2000, reaching Pennsylvania by 2009. In time, it has replaced corn earworm and European corn borer as one of the most damaging pests of corn and dry bean.

Unlike its competitors, WBC caterpillars don’t eat each other, so multiple individuals will feed directly on the same ear. This increases the severity of the damage to the ear. Fields can see rates of damage as high as 80%. Widespread feeding also increases the likelihood of mold and ear rot. And be forewarned: If you’re growing transgenic corn, hybrids carrying the Cry1F gene offer little protection against WBC. Only the Vip3A trait (Viptera) is effective against WBC. The best way to check is to use the handy Bt trait table (see “For More Information”). Until new transgenic solutions hit the market, growers will have to rely on vigilance and properly timed foliar insecticide sprays to protect their corn fields.

IDENTIFY THE PEST. WBC can be found in Midwest and Plains states, where corn and dry beans are grown. Areas with sandy soil are at higher risk, because it’s easier for the larvae to dig their winter burrows.

The adult moths are darker brown and have a 1.5-inch wingspan with distinctive markings on the wings. Look for a pale, white band on the wings’ edges and two brown spots--one circle, the other kidney-shaped.

WBC females are attracted to pretassel corn for egg laying. When scouting, look for clusters of eggs sticking to the upper side of the corn plants’ leaves. The eggs are laid in tight rows and look like ridged balls that range in color from white to deep purple; the darker the color, the closer the eggs are to hatching.

Caterpillars feed on the tassel’s pollen soon after hatching. As the ears grow, the caterpillars move down the plant. The pest will likely have a brown body with tan head plus two black, rectangular markings just behind the head. Mature caterpillars are 1.5 inches long. Other signs that an ear is infested are characteristic “shot holes” chewed into the side of the husk.

MANAGEMENT ADVICE. Trapping and scouting are essential for proper WBC control. Pheromone traps (e.g., bucket traps) should be out in the field by late May and hung on the field’s edge 4 to 5 feet off the ground. Lures are commercially available and should be replaced every 4 to 6 weeks or as the manufacturer directs. Check traps weekly until the first moths are observed. Increase the frequency to every few days after that. If moths are caught consistently, begin scouting in the field.

Pick five locations, and examine the upper leaves (and tassels, if present) of 20 consecutive plants. An alternative “speed scouting” system, designed to reduce sampling time, is also available from the University of Nebraska. If more than 8% of the plants (less for higher-value crops) have egg masses or caterpillars, this is a serious infestation and should trigger action.

Julie Peterson, University of Nebraska entomologist, says that pyrethroids (e.g. Brigade, Warrior) are a common solution for controlling WBC. They should be rotated with other classes of insecticides like diamide (Prevathon) and indoxacarb (Steward) to minimize resistance development and conserve natural enemies. Apply insecticides when 90 to 95% of the plants have tasseled. Pyrethroid labels state spraying needs to be done before larvae bore into the ear.

For More Information:

> Michigan State University Bt Table

> University of Nebraska Extension WBC Bulletin

> University of Nebraska Speed Scouting Spreadsheet


Scott Williams