Stop The Western Bean Cutworm

The key to controlling this corn pest is to spot an infestation early.

by Rebeca Kuropatwa
PHOTO: Tracey Baute

Despite its name, the western bean cutworm is a pest of all kinds of corn—field, sweet and seed—though it also appears on edible beans (except soybeans) and, to a lesser extent, tomatoes and other nightshades. Unlike other cutworms that are true to their name, cutting the plant when young, western bean cutworms feed on the fruit of the plant, such as the corn ears and bean pods.

Tracey Baute, a field crop entomologist and the program lead with the Ontario Ministry of Agriculture, Food, and Rural Affairs, is likely Canada’s best expert on this pest. In 2008, for the first time ever in Ontario, trap captures found western bean cutworm moths present, though no damage was documented.

“WBC has established itself, successfully overwintering and impacting yield and quality of corn in some regions of Ontario,” says Baute, noting that damage has been found in dry beans too over the last few years.

The pest overwinters as larvae in soil chambers, and Baute says the adult moths are easy to identify: A white band runs along the edge of each wing with signature moon and boomarang-like marks. They start to appear in early to mid-June in Canada’s cool climate, when corn is in the whorl to pre-tassel stage. They lay their eggs on the upper surface of the leaves, preferring the hybrid varieties with upright leaves.

Jocelyn Smith
Jocelyn Smith

“Once the corn crop is in tassel or beyond, they prefer to lay their eggs on dry bean crops,” Baute says. “Unfortunately, the larvae are very mobile and can disperse from the original egg site to other plants in the vicinity, both up and across corn rows. High risk fields include no-till fields and fields with sandy soil.

The young tan-to-pink larvae munch on the tassels and silks, and once large enough, they tunnel to the ear’s kernels, where they feed. If you look closely, you can see their entry holes on the outside of the husk, though they may also enter through the silk channels, in which case, their damage won’t be visible. Because western bean cutworms are not cannibals, multiple larvae can feed on the same ear, leaving little left to harvest.

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“Estimates on yield loss from other jurisdictions indicate a field infestation of one larva per ear can cause a 3.7 bu/ac loss,” Baute says. “Additional impact can be expected from ear rots and secondary pests that may enter and feed on damaged ears.

Controlling Western Bean Cutworms

Several natural enemies feed on western bean cutworm egg masses and young larvae, including:

Older larvae may be eaten by birds and some vertebrates, such as, raccoons and skunks, but once in the ears of the corn or pods of the beans, they’re protected from predation.

Scouting For Western Bean Cutworm In Your Fields

“In corn, scout 20 plants in five areas of the field from the end of July to the end of August,” Baute advises for northern areas, though you may want to check with your extension agent to find out when your area is most at risk. “Focus on fields in the pre-tassel stages and inspect the top three to four upper leaves of the plant. Look for egg masses and young larvae.”

If you find white egg masses, then that means they are newly laid. Purple means they’ll be ready to hatch in one to two days. Flag plants with white masses, and return in a few days so you can learn when young larvae will be active. If you choose to use pesticides to treat the infestation, this is the stage when it will be most effective.

“Newly hatched larvae will initially climb to the top of the plant to feed on the tassel, but only for a short time before migrating down the plant to feed on the ear,” Baute says. If your application is timed correctly, you may be able to curb the problem.

For more information on WBC, visit Baute’s blog.

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