May 7, 2007

Landscaping Tips:

Army cutworm the unwanted garden ‘soldier’

By Scott Schell, Assistant Entomologist
University of Wyoming Cooperative Extension Service

The army cutworm is a “soldier” you don’t want guarding your garden.

The caterpillars of this native moth, Euxoa auxiliaries, make use of suitable habitats in urban, suburban, agricultural and natural areas of Wyoming and surrounding states.

The common name of this insect is very descriptive as the caterpillars can, at times, form “armies” when they are very abundant. They feed by cutting plants off at ground level at night. They aren’t very popular as adult insects either, as they often comprise a large proportion of the migratory swarms of “miller moths.” The “millers” become pesky to people in the late spring and early summer when they are attracted to lights during their migration to the mountains.

A complete explanation of the entire life-cycle of this pest will help a gardener understand how to better manage them.

An army cutworm begins its life as an egg laid in the late summer to early fall. The adult female moths prefer areas of tall, dense vegetation for this activity. The eggs rapidly hatch, and the tiny caterpillars start feeding and growing. The caterpillars do little damage in the fall as plants are big but they are tiny.

During the winter they quit feeding and hide in soil cracks, under rocks and in leaf litter. As winter ends and plants start to grow in early spring, the caterpillars resume feeding and grow rapidly. They can feed on a wide variety of plants but generally prefer broad-leaved plants over grasses.

New seedlings and transplants are very vulnerable to cutting by the nocturnal feeding caterpillars.

After completing growth, the caterpillars dig a small chamber underground and pupate to undergo metamorphosis to the winged adult stage. Adult moths emerge by late June and begin feeding on flowers. As the moths follow the flowering plants into the mountains, they build up fat and energy reserves to later migrate back and reproduce in the lower elevations in late summer and fall.

You may first notice damage caused by the army cutworm in the spring when early flowering plants are cut down during the night. Check under nearby plant debris, landscaping rocks and landscape timbers for the culprit. You will probably find a small, smooth-skinned, drab brown and olive-colored caterpillar.

Many creatures prey on the army cutworms, and the cutworms are susceptible to fungal disease outbreaks in wet years; however, it doesn’t take very many of them to damage flower and vegetable gardens and wreak havoc on new seedlings. If you aren’t squeamish, rake up plant debris, flip over landscaping rocks and timbers and remove caterpillars by hand.

Tilling vegetable gardens early in the spring can drastically reduce the population of army cutworms. This can kill them mechanically or bury them too deep for them to survive. Tilling is best done a week or two before planting. The caterpillars, with nothing to eat, will starve or be found by predators on the bare tilled soil.

There are also many insecticide products that can be used to eliminate army cutworms. The label of any insecticide product chosen must be followed carefully and exactly. Local lawn and garden stores commonly carry products labeled for this pest, and Colorado State University’s home use insecticide database Web site lists most commonly available products. The address is http://homeusepesticide.org/.
Prevention is always the best “medicine” so practicing good weed control in yards and alleys can help reduce army cutworm problems as this denies the adult moths a place to shelter and lay eggs in the late summer and early fall.

More information on the army cutworm is available in two University of Wyoming publications. B-1013.8, Army Cutworm, and MP-24, Pests of the Home Vegetable Garden, are available for free download at http://ces.uwyo.edu/Entomology/insectpublications.htm. The publications are also available at UW CES offices around the state. Contact information is at http://ces.uwyo.edu/Counties.asp.

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Contact: Robert Waggener, Editor
Phone: (307) 766-3571
E-mail: robertw@uwyo.edu

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