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Western bean cutworm now a threat in Ontario

A highly destructive corn pest has made its debut in Ontario in 2008. The first western bean cutworm (WBC) moth was captured in early July in Lambton County. The discovery was made one week after pheromone traps were set up in the province.

November 4, 2008  By Blair Andrews

Pace of arrival surprising to most

The number and arrangement of western bean cutworm eggs on the upper surface of a corn leaf can help growers identify them.

A highly destructive corn pest has made its debut in Ontario in 2008. The first western bean cutworm (WBC) moth was captured in early July in Lambton County. The discovery was made one week after pheromone traps were set up in the province. And it did not take long for the pest to introduce itself to the edible bean crop as a WBC moth was captured in a bean field in Blyth, Ontario in  late July. Just days before the first capture Tracey Baute, field crop entomologist with the Ontario Ministry of Agriculture, Food and Rural Affairs, had cautioned farmers that it would only be a matter of time before WBC would be found in their fields. “We need to start monitoring for this because this is the next new species or pest that is going to come into Ontario and potentially impact both our corn and dry bean crops.”

The name, western bean cutworm, is somewhat deceiving because WBC is a pest of corn as well as dry beans. Traditionally, it has been a pest for several decades in Western Corn Belt states like Colorado and Nebraska. Since about 2000, however, it has been moving east and north into Iowa, Illinois, Ohio and Michigan.


Similar to corn earworm, western bean cutworm is a late-season pest, with the larvae feeding on the ears of corn. Crop specialists are quick to point out that the key difference between the two is that corn earworms are cannibalistic, whereas the western bean cutworms are not, meaning that several WBC larvae can be feeding at the same time. Such a destructive scenario was on display in a few fields in the northwest part of Michigan in August 2007.

“It looks a lot like corn borer feeding in the ear, except instead of one or two larvae, you can have five or six of these critters in an ear,” says Chris DiFonzo, field crops entomologist at Michigan State University. Another giveaway is that the larvae sometimes chew through the husk into the side of the ear, so the ear looks like it has been shot. “We had fields last August (2007) that were described as almost 30 percent to 100 percent of ears damaged.”

She suspects the WBC moths blew into that section of the state, possibly on storm fronts from Illinois and Wisconsin in July 2007, noting that the damage was worse in the northwest counties along Lake Michigan.  In 2008, WBC egg laying and hatching were confirmed. Furthermore, some fields in northwest Michigan were over threshold for egg masses and small larvae.

What is driving the expansion
While storm fronts may have accelerated the WBC arrival in northwestern Michigan, there are several factors that have contributed to the gradual spread across the U.S. Midwest. One theory is tied to the increasing use of certain Bt corn hybrids to ward off other pests. And while those hybrids have been an effective control for the European corn borer, they may have provided an opportunity for the western bean cutworm. “We believe the (WBC) populations were always held in check out in the Nebraska area because that is where they typically treated for corn borer prior to the discovery of Bt corn,” explains Ben Kaehler, traits licensing leader with Dow AgroSciences in Indiana. “Then in the mid-to-late ’90s Bt corn with YieldGard took off and was getting planted to more acres,” details Kaehler. “Therefore, the insecticide applications kind of ceased and, since YieldGard has no activity on WBC, we started seeing that pest move east.” Some Bt hybrids also control corn earworm, to the point of suppression only, which because of their cannibalistic nature, would be a natural enemy of western bean cutworm.  Without corn earworm, WBC can now do well in the corn ear.

Climatic factors such as milder winters and drier spring conditions are also cited as explanations for the eastward expansion. A third theory relates to the rise in reduced tillage acres, suggesting that less soil disturbance equates to larval survivorship. 

Adult moths have a distinctive central white spot behind a white stripe on their upper wings.
Photos courtesy of Chris DiFonzo,
Michigan State University

Description and life cycle
Although western bean cutworms can be confused with other corn pests, there are some distinctive characteristics. Adult moths are dark brown with a white stripe on their upper wings. There is a central white spot behind this stripe. And further down the body of the wing, there is a marking that is in the shape of a crescent moon. Adult moths emerge and are actively flying in late June. The moths are strong fliers and are known to travel several kilometres. After mating, eggs are usually laid on field corn, popcorn, sweet corn or dry beans. Tomatoes and fruits of nightshade are acceptable but are non-preferred hosts.

The cutworm larvae are tan to pink in colour and do not have wart spots or tubercles on them like European corn borer. The only distinguishing marking is located on the pronotum, the shield-like structure just behind the head. The WBC’s pronotum has two wide dark brown stripes. 

In corn, Baute says the eggs are laid on the upper leaves of the plant. Fields nearing tassel emergence or those planted with hybrids that hold their leaves upright are the most preferred. After the eggs hatch, the caterpillars move down into the whorl and feed on the developing tassel in the centre of the plant. The young larvae feed on the tassels and silks until they are large enough to tunnel into the ear and feed extensively on the kernels.  As well as eating away at the yields, an impact to quality can be expected from ear rots and secondary pests that may come in and feed on the damaged ears. 

Once the corn crop is tasselling, the moths prefer to switch and lay their eggs in dry bean fields. In dry beans, Baute explains that the moths lay their eggs on the underside of the leaves, opposite of what they do on corn. Larvae hatch and can disperse to several neighbouring plants. Initially they may feed on the leaves and flowers but as they grow, they mine into the pod of the plant and feed directly on the bean seed. It is too late to control them once they reach the pod because insecticides will not reach the larvae inside.

Western bean cutworm management
Scouting for WBC begins with the moth flight in late June to early July. The regime involves looking at plant leaves for eggs and larval feeding. According to guidelines from the University of Nebraska-Lincoln, if five percent of field corn plants have eggs or newly hatched caterpillars and 95 percent of the corn has tasselled, an insecticide should be applied while caterpillars are exposed. The white, pin-head eggs are laid in masses of 20 to 100 eggs, usually on the upper third of the plant. As the eggs develop they darken to a dark purple just before hatching.

Using Bt hybrids may offer some relief. But again, the options are limited. In terms of transgenics, only Herculex I Bt corn (Cry1F) hybrids have been found to give protection from WBC.  Kaehler says Herculex I is a different Bt strain than YieldGard, and it does a good job of controlling WBC.  “In side-by-side trials with YieldGard versus Herculex, there is a night-and-day difference,” explains Kaehler. “We keep the damage under economic thresholds, so there will be a bit of feeding because they have to feed on the crop in order to ingest the Bt protein. But control is far better than any chemical application and you don’t have to worry about the timing or application costs.”

From top to bottom: Western bean cutworm caterpillars can
ravage a maturing ear, leaving it open to a variety of ear moulds and rots.

More scouting
Michigan and Ontario are now part of an extensive trapping network that was initiated by Iowa State University. The purpose is to gain a better understanding of the distribution of the western bean cutworm and to provide information on the proper timing to scout for this pest. DiFonzo and Baute agree that the next steps are to learn more about the WBC migration and to gauge potential damage in the corn crop. While significant damage may not be an issue for the crops in 2008, Baute notes the potential to experience injury now exists in Ontario. She recommends that traps be monitored regularly to observe when peak flights occur and to scout the corn and bean crops to check for activity.

“I think the key with this one is that we’re not accustomed to scouting as much corn as we used to because of Bt corn,” explains Baute. “We should be watching our corn fields anyway, but this gives us a reason to be looking for eggs and any suspicious feeding.


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