Armies of these fuzzy critters eat SC yard plants in the fall. Here’s how to tell, stop them

South Carolinians might notice some fuzzy new visitors munching on their lawns this month. The spodotera frugiperda is actually a moth caterpillar, but its propensity to feed in large numbers in the autumn has earned it the common name fall armyworm.

While the moths feed on nectar, the hungry caterpillars have 80 recorded host plants and are especially partial to corns and grasses, according to the University of Florida.

“The problem with them is they move so quickly that a lot of times they’ve moved through a yard within a matter of hours or even or a couple days, and then they’ve done their damage and gone before people even realize what’s happened,” said Clemson urban horticulture agent and master gardener coordinator at the Horticulture Program Team in Horry County Brad Fowler.

When an “army” of the caterpillars settle in to feed on a yard, they munch on plants and leave behind brown grass. Here’s what homeowners should know this autumn.

What are fall armyworms?

Fall armyworms start in clusters of tiny, fuzzy whitish eggs which Fowler says are about the size of the end of a ballpoint pen.

They eventually grow into grayish moths with frosted areas around the tips of the wings, but as caterpillars fall armyworms come in an array of grayish and brownish colors with a distinctive yellow or white marking shaped like an upside-down letter Y on their heads.

As caterpillars, they’re a food source for birds in the area and as moths they play an important ecological role as pollinators.

What fall armyworms do to SC yards

“Here, they primarily feed on grasses, but they also feed on kind of everything you might have in your garden,” said Eric LoPresti, professor of biology at University of South Carolina.

Fall armyworms can be a major nuisance to yards in the autumn season. As the hungry caterpillars move through yards, they consume foliage and leave patches or entire lawns brown.

“It looks like the yard has essentially gone dormant, almost where it kind of turns brown,” Fowler said. “Sometimes people might mistake it for not enough water, like it’s getting too dry.”

Fortunately, fall armyworms rarely inflict lasting damage. The caterpillars feed on yard plants and lawns, but unlike species like the invasive Japanese beetle, they leave grass stems intact.

Lawns that are already very unhealthy due to issues like fungus or overexposure to water may be permanently damaged, but according to Fowler most healthy lawns recover from fall armyworms within two to three weeks.

“Unless you’re really, like, a farmer, they’re not going to be a real problem. Grasses are really well evolved to get eaten by herbivores … They’re not likely to do any serious long-term damage to your lawn, or even most of your garden plants,” said LoPresti.

Yard owners who suspect fall armyworms could be causing unsightly brown patches can test their theory with a simple soap trick.

The Clemson University Home and Garden Information Center (HGIC) recommends mixing 1 to 2 tablespoons of lemon-scented detergent in 1 to 2 gallons of water until suds appear, then slowly pouring the liquid over a square yard of healthy turf next to the brown area. If army worms are present, they should emerge within 10 minutes.

When to use insecticides on fall armyworms

Fall armyworms can be cleared from yards with insecticides, but LoPresti cautions against using pesticide treatments unnecessarily.

“To some extent, I would hesitate to recommend that, because the more times we dump pesticides on them, the more opportunities they have to evolve pesticide resistance,” LoPresti said. “I am not actually against pesticides whatsoever, but it’s more effective to use them for important things like crops than it is to potentially prime the caterpillars to survive pesticides to save a little bit of lawn.”

For gardeners settled on using chemical treatments, HGIC says insecticides with bifenthrin, Bacillus thuringiensis (B.t), cyfluthrin, cypermethrin, lambda-cyhalothin, permethrin or spinosad are all effective in clearing the insects. When using insecticides, it’s a good idea to check the forecast and turn off sprinklers for 24 to prevent rainfall or irrigation from washing away the treatment and creating chemical run off.

“It’s just something that we have to contend with most years, and there’s no rhyme or reason to it,” said Fowler. “There’s no need to get too out of shape about it. Normally, the grass will recover.”