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GARDEN TIPS – written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written August 20, 2015


The last time the Tri-Cities was inundated with armyworms was two years ago. These caterpillars are cyclic, appearing only some years in late summer and early fall. They can be a problem feeding on garden crops and lawns, but they usually concern homeowners because they travel in masses, pretty much like an “army.” Entomologists note that armyworms, climbing cutworms, cutworms, and fruitworms are larvae of closely related night-flying moths in the Noctuid family. These moths are sometimes called “millers” by non-entomologists.

What sets armyworms apart from their close relatives is their timing and their feeding in large groups rather than individually. The Western yellow striped armyworm (WYSA) is common in Washington and is sometimes a pest in field crops, particularly in alfalfa. WSU entomologists note that they can “be serious pests…, especially in early fall following a hot dry summer which concentrates the larvae into “armies”.”

With three possible generations over the summer, WYSA numbers can grow over the spring and summer months. Add to this the knowledge that the WYSA generally do not overwinter well because cold winter temperatures kill their eggs, larvae, and adults. However, dry mild winter conditions allow more of them to survive the winter, leading to a buildup of their populations. Well, we have had a hot dry summer so far and several relatively mild winters, so I would not be surprised if armyworms may become a problem again this year.

One local resident has already noted these caterpillars traveling en masse onto her back patio. A mature caterpillar is about 1.5 to 2 inches long. Its body is a blackish color with two thicker yellowish longitudinal stripes lengthwise down its back and numerous narrow stripes on its sides.

While these armyworms may prefer eating alfalfa, they will get hungry when the alfalfa is cut and move out of the field and nearby weedy areas into surrounding areas in search of food, ending up in your yard, garden, or patio. They feed primarily on the leaves of various crops and weeds, but may also feed on other plant parts. Usually the damage they cause with their feeding in yards and gardens is not significant, but their large numbers can be alarming.

When faced with an armyworm invasion, the first inclination is to apply a non-organic insecticide in a blanket application, spraying large areas of the yard. Dr. Doug Walsh, WSU Entomologist, says a blanket application is also likely to kill a number of the armyworms’ natural enemies, like “bigeyed bugs, spiders, minute pirate bugs, damsel bugs, lacewings, and at least a dozen species of parasitic wasps.”

Walsh indicates that an application of spinosad, an organic pesticide, would be less toxic to these natural enemies and should help control the armyworm population. Using a spinosad product would also be safer for your family and your pets.

There a number of home garden products containing spinosad, including Bonide’s Captain Jacks Deadbug Brew, Green Light Lawn and Garden Spray, Monterey Garden Insect Spray. Of course if you only find one or two of these caterpillars in your house or around the yard, simply handpick and squish them.
With only one sighting of these creatures, this may be a false alarm but I want you to be prepared to deal with any invading armies of caterpillars.



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