Garden Tips: Armyworms may be on the march

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 say that armyworms, cutworms and fruitworms are larvae of closely related night-flying moths in the Noctuid family. These moths are sometimes called millers by nonentomologists.

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

With three possible generations throughout the summer, the numbers can grow over the spring and summer months. They also do not overwinter well because cold temperatures kill the eggs, larvae and adults. However, dry and mild winter conditions allow more of them to survive, leading to a buildup of the population. 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 resident has 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 crops and weeds, but may also feed on other plant parts. Usually, the damage they cause with their feeding in yards and gardens is insignificant, but the large numbers can be alarming.

When faced with an armyworm invasion, apply a nonorganic 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 the armyworms’ natural enemies, like “bigeyed bugs, spiders, minute pirate bugs, damsel bugs, lacewings and at least a dozen species of parasitic wasps.”

Walsh said 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 family and pets.

There are a number of home garden products containing spinosad, including Bonide’s Captain Jacks Deadbug Brew, Green Light Lawn and Garden Spray, and Monterey Garden Insect Spray. Of course, if you only find one or two of these caterpillars in your house or around the yard, 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.

This story was originally published August 23, 2015 at 6:00 AM with the headline "Garden Tips: Armyworms may be on the march."