Greg Grant

Armyworms, typically known for marching through hay meadows, have been everywhere lately, including home lawns and landscapes. I even had some munching on my Florida flat-leaf purslane and old-fashioned petunias planted in the cracks of the sidewalk outside of my office. There are many, many different kinds of caterpillars, but only one fall armyworm.

A fall armyworm is the caterpillar stage of a drab brown moth. It feeds primarily on grasses, though it has been reported feeding on dozens of non-grass plants and weeds. It earns the name “armyworm” from its habit, during times of major outbreaks, of marching like an army across fields, roads and yards, consuming everything in its path. This has been one of those years.

While some farmers and ranchers may be hit in the pocketbook by the outbreak, homeowners don’t have much to worry about. Armyworm damage in home lawns can be breathtaking, but it does not usually hurt the lawn in the long term. Because armyworms feed on the leaves, and not on the critical roots and stolons, our recent rains should restore lawns to their original condition within a week or two. Fall armyworms will feed on most common lawn grasses including Bermuda grass, centipede grass, St. Augustine grass, and zoysia grass. One year while I was teaching at Stephen F. Austin State University, armyworms ate the lawn and the perennial border. Both soon recovered and I considered it free mowing and shearing.

Lawn browning often appears to occur overnight, though armyworms need three to four weeks of feeding to do their damage. The last week or so of the larval stage is when most of the feeding — and damage — occurs.

If you want to treat for aesthetic reasons, standard residual insecticides like bifenthrin, cyfluthrin, permethrin, esfenvalerate or carbaryl should quickly eliminate an armyworm invasion. These products are best applied as a spray; a hose-end sprayer is a convenient applicator. It’s important to spray before widespread damage occurs so as not to waste your time and money.

Organic gardeners should either leave the infestation to run its course or treat with the natural insecticide spinosad. The organic gardener’s other favorite caterpillar spray, Bt, can kill armyworms but has a very short life on the grass and will be less effective than spinosad.

Fall armyworms differ from many other caterpillar pests in that they do not survive the winter in areas where it freezes. In Texas, they probably survive winter only in far South Texas. As a result, whether you treat or not will have no effect on whether you are likely to see this caterpillar next year.

So don’t be shocked this fall if you’ve seen caterpillars in your yard, or marching up sidewalks or exploring the sides of your house. Unusual things like fall armyworm invasions can happen, even in your yard. But this one is no home landscape disaster. For more information on armyworms in the lawn, see the publication at https://aggieturf.tamu.edu/turfgrass-insects/armyworm.

Greg Grant is the Smith County horticulturist for the Texas A&M AgriLife Extension Service. He is co-author of “Texas Home Landscaping.” You can read his “Greg’s Ramblings” blog at arborgate.com or read his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com). More science-based lawn care information from the Texas A&M AgriLife Extension Service can be found at aggieturf.tamu.edu.