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Corn+Soybean Digest

Encroaching Western Bean Cutworm

For many corn and dry bean producers west of the Mississippi River, the western bean cutworm (WBC) has become an all too familiar insect pest that requires good scouting methods and control measures.

This pest is already well established in states like Nebraska and Colorado and has made its mark during the past few years in Iowa.

Recently WBC has also been spreading farther east — crossing the Mississippi River — into cornfields in several areas of Illinois.

So far there have been no sightings of the pest in Indiana. However, entomologists at Purdue are fully aware of its eastward movement and plan to keep close tabs on scouting efforts.

WBC can be a severe pest in corn and dry beans, according to the University of Nebraska — Lincoln (UNL). Larval feeding damages both crops by reducing yield and quality. In corn, direct feeding losses may be compounded by fungal and mold infections associated with the larval waste products.

In beans, damaged or “worm-chewed” beans are a significant quality factor for both processed and dry bagged beans. WBC infestations occur every year in western Nebraska. In some years, this pest is found in high numbers throughout the state.

“Since 2000, western bean cutworms have caused significant damage to some cornfields in Iowa,” says Marlin Rice, Iowa State University Extension entomologist.

“To assist in scouting efforts for this pest, we've placed a network of pheromone traps in areas of historical western bean cutworm activity,” he says. “These traps should reflect the emergence pattern within a county.”

Some of the highest trap catches have been in Audubon, Crawford and Shelby counties in west central Iowa, plus Butler County in north central Iowa, adds Rice.

In 2004, Rice also reported the presence of WBC in traps located in Missouri.


But like any mobile insect, many considered it to be only a matter of time before this pest started to move farther eastward and cross the Mississippi River, now identified in Illinois.

For example, based on a single night of trapping in the summer of 2004, the first WBC adults ever found in Illinois were in Warren County, IL, according to Kevin Steffey, University of Illinois Extension entomologist.

“It was a wake-up call, so in 2005 we decided to organize a network of traps in cooperation with Iowa State University,” says Steffey. “We enlisted cooperators from both university and industry and trapped moths beginning in early July and into August.”

Based on these trappings, Steffey says the results have shown that WBC has become established in northwestern Illinois, and was found in central and northeastern Illinois.

“Based on what we're finding, there's no question that this pest could potentially become significant or serious,” says Steffey. “It may not overshadow other pests like corn rootworms, but you never know about insects — only time will tell.”

Depending on climate and corn production practices in the next few years, Steffey says WBC could become firmly established and potentially “cause some real headaches.”

During this winter, Steffey and other entomologists at the University of Illinois will discuss plans on dealing with this fairly new pest, especially if it continues to spread and grow.

For now, here are a few things to consider about the western bean cutworm:


WBC has one generation per year with moth emergence, usually beginning in early July, according to the UNL.

Populations vary from year to year, but there is a tendency for higher numbers every six to eight years.

The moths are strong fliers and are known to travel several miles. Female moths emit a pheromone (scent) that attracts males for mating. After mating, eggs are usually laid on crops such as field corn, popcorn, sweet corn or dry beans.

WBC moths lay eggs on the upper surface of corn leaves, especially the flag leaf. Fields nearing tassel emergence or those planted to hybrids with an upright leaf characteristic are preferred for egg laying.

The dome-shaped eggs, which are slightly larger than the head of a common pin, are laid in flat, irregularly shaped masses, usually ranging from 15-50 eggs per mass.

When first laid, eggs are pearly white with a thin, red ring around the top. The eggs darken fairly quickly — first to tan or brown within two days and then to purple or black within five to seven days before hatching.

According to Rice, the purplish color and shape of the WBC eggs are perhaps the most distinguishing features for positive identification.

After larvae hatch, they remain near the egg mass for about 10 hours, feeding on the shells of the eggs.

But while most eggs hatch, only a small percentage of the larvae actually survive to maturity. The larvae then move to protected feeding sites, the location depending on the growth stage of the host.

Larvae feed for about 31 days and develop through five stages (instars) on the host plant. First instars are quite mobile and may infest several adjacent plants. They are dark brown with faint crosshatched markings on their backs.

In pre-tassel corn, newly hatched larvae move to the whorl where they feed on the flag leaf, the flowers of the tassel and other yellow tissue.

Once tasseling begins, larvae move to the green silks of the developing ear. In post-tassel corn, the larvae move directly to the fresh silk. Through this dispersal behavior, larvae from one egg mass may infest several plants down the row and in immediately adjacent rows in an area 6-10 ft. in diameter.

As the larvae develop, they become light tan to pink in color and the crosshatch markings on their backs become more distinct.

Third instars and older larvae also can be differentiated from other cutworms and caterpillars feeding on the host plant by two broad, dark brown stripes immediately behind the head.

Second and third instars continue to feed on silks and begin feeding on the tips of developing ears. Fourth instars feed primarily on ear tips. If the ear tips are crowded, some larvae may move to the outside of the ear, chew through the husks and initiate feeding.

While one larva per ear rarely causes an economic loss, recent research has shown that infestations above this level at dent stage can potentially reduce yield by 3.7 bu./acre, according to UNL.

WBC larvae are not cannibalistic — a condition that may result in multiple larvae infesting a single ear.

During years of severe infestations, two or more larvae per ear may be common. Corn ears infested with more than 20 larvae have been recorded, but this is unusual.

However, in these crowded situations, as much as 50-60% of an ear's kernels may sustain feeding damage, according to UNL.

Larvae continue to feed through the fifth instar after which they drop to the ground, burrow 5-10 in. under the surface, and construct an earthen overwintering cell. They spend the winter inside this cell in a pre-pupal stage. Larvae pupate in late May, followed by adult emergence in early July.

Young WBC larvae are dark brown with faint diamond-shaped markings on their backs.

They lighten in color as they mature and are gray to pinkish brown when fully grown. Mature larvae are about 11/2 in. long.

The stripes behind the head can also be used to distinguish WBC larvae from corn earworm larvae, which also are found feeding in ear tips. The pre-pupae and pupae occur in the soil and are rarely seen.


WBC moths can be detected with black light or pheromone traps, according to UNL. Most of the eggs are laid during peak moth flight in mid- to late July. Light traps should be monitored regularly until after the adult population peaks.

When scouting for WBC, check 10 consecutive plants in several randomly selected locations of each field. These locations should be representative of all areas of a field.

In later damage, the late-stage WBC will eat kernels along the side or other areas of the ear, according to Steffey. “So, from that standpoint, the damage is not very distinguishable from the fall armyworm and corn earworm,” Steffey says.


Few cultural controls are effective against this insect pest, according to Rice. Plowing or disking soil is thought to reduce overwintering larval survival, but the effectiveness of this practice on a broad scale is unknown.

There are several natural enemies and predators of WBC. Spiders may reduce WBC densities. Larvae are also susceptible to the disease pathogen Nosema sp., but the impact of this pathogen is unknown.

Predation by birds such as crows and blackbirds, especially on larger larvae, has been observed.

Presently, only those transgenic corn hybrids containing the Cry 1F toxin (Herculex I Insect Protection) are labeled for control of WBC, according to the University of Illinois.

Generally, the pyrethroid-based insecticides work the best on WBC, says Rice, but scouting for the pest and timing of application are critical to successful control.

Scouting for WBC is recommended with the beginning of moth flight in mid-July. “In corn, it's good to check 20 consecutive plants at five locations,” says Rice. “The University of Nebraska recommends that if 8% of the plants have an egg mass or if young larvae are found in the tassel, consider applying an insecticide.”

Timing of the application is critical. If the tassel hasn't emerged when the larvae hatch, they will move into the whorl and feed on the developing pollen grains in the tassel. As the tassel emerges, the larvae will move down the plant to the green silks and then into the silk channel to feed on the developing ear.

“Once the larvae reach the ear tip, control is nearly impossible,” says Rice. “If an insecticide is needed, time the application so that 90-95% tassel emergence has occurred.”

If the tassels have already emerged, time the application for when 70-90% of the larvae have hatched, adds Rice.

If an insecticide application is needed, check cornfields for the presence of spider mite colonies. If you find mites, select a product that does not stimulate mite reproduction.

Insecticides Labeled For Western Bean Cutworm In Field Corn
Insecticide Rate per acre Comments
Ambush* 3.2-6.4 oz. May cause mite flare up.
Asana XL* 2.9-5.8 oz. May cause mite flare up.
Baythroid 2* 2.1-2.8 oz.
Capture 2EC* 2.1-6.4 oz.
Lorsban 4E* 1-2 pt.
Mustang Max * 1.76-4.0 oz.
Penncap M* 2-4 pt.
Pounce 3.2EC* 2-4 oz. May cause mite flare up.
Sevin XLR Plus 2 qt.
Warrior* 1.92-3.2 oz.
* restricted-use insecticide
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