Fall Armyworm Invasion

By Mary Jo Gibson

Fall armyworms are caterpillars that feed ravenously on grasses, corn, alfalfa, soybeans, and over seventy additional plant species. Unlike many pests, this insect is native to the Americas. The fall armyworm (Spodoptera frugiperda) is a caterpillar of a tropical moth in the family Noctuidae. None of its life stages, egg, caterpillar, pupa, or adult, can survive a Pennsylvania winter. It is so sensitive to temperature that in the United States, it can only overwinter in south Florida or south Texas. How could this insect get to Pennsylvania to destroy lawns, pastures, and crops?

Traditionally, fall armyworms migrate northward with each generation. The bark-colored moths spread out to lay white fuzzy egg masses on crops and vertical surfaces. Eggs hatch in a few as three days and develop into caterpillars. As the caterpillars feed, they go through seven instar stages. They change in color from greenish to brownish markings with a light stripe. A white inverted Y marking on the front of the head capsule is the best identifier. Tiny caterpillars nibble only young, tender growing tips of plants at night, hiding in debris during the day. But as the caterpillars grow in size, so do their mandibles and appetites. Soon they will be feeding both day and night, marching army-style, into adjacent areas and consuming nearly all plants in their path. Suddenly, we notice the damage!

The fall armyworm feeds for two to four weeks and reaches a length of 1 and 1/2 inches. Then the mature caterpillar burrows into the ground to pupate for one to four weeks. When the moth emerges, it flies to new areas. Each generation can take as little as a month or as many as three months, depending on the temperature. With each generation, the insect moves northward. Fall armyworms are regular pests in the southeastern United States. It is unusual for them to be a pest in Pennsylvania or areas north of the Mason-Dixon line. The population doesn’t expand far enough from south Florida during the growing season. So, why where they here in 2021?

Just as the wind blew Dorothy Gale and Toto from Kansas to the Land of Oz, storms blew the moths north. Storms that reached Pennsylvania in mid-July carried these hitchhikers to us, probably for hundreds of miles. The bedraggled moths laid eggs, and within a week or so, caterpillars were feasting on our plants. The good news is that our cool temperatures slow down each stage of the life cycle, so there will probably not be another generation here.

What if fall armyworms devoured your lawn? First, don’t panic! A healthy, established lawn will recover. Raise the mower blade height to allow plenty of grass leaves for photosynthesis to create nutrients for the plant’s health. It is essential to irrigate the lawn to keep the turfgrass crowns well hydrated. An application of fall fertilizer that includes a slow-release form of nitrogen will support regrowth.

What if fall armyworms are in your area? Please realize that infestations are sporadic. Scout before treating! How can you scout for fall armyworms that like to hide during the day? Perform a soap flush test, also called a drench test. Combine 3 to 4 tablespoonfuls of dish detergent (preferably lemon-scented Joy) with two gallons of water. Pour evenly over a square yard of turf. Watch the area for 10 to 15 minutes. Count the caterpillars as they rise to the surface. The threshold for management is not well established, but generally, nine or more fall armyworms per square yard warrants treatment. Insecticides easily control fall armyworms when they are less than ½ inch long. Tiny caterpillars respond well to short-lived biological products like Bacillus thuringiensis kurstaki. Larger caterpillars require synthetic pyrethroids. For specific pesticide recommendations, see Fall Armyworms Causing Damage to Lawns, Golf Courses, and Turfgrass from Michigan State University Extension. Always read and follow all label directions.