Invasion of the Fall Armyworms

Have you started noticing an outbreak of worms or moths in your lawn? It’s the time of year to be mindful and observant of different pests and insects. Some species of worms or caterpillars can cause detrimental damage to your lawn and garden while others aren’t as harmful. It’s also important to be able to identify which types of insects are invading your yard so that you are aware of the level of damage they may cause.

The fall armyworm, or Spodoptera frugiperda, is notorious for traveling in small “armies” (hints its name) that devour lawns and gardens. Fall armyworms feed on the leaves, stems and other above-the-ground parts of numerous plant species.

Click the image below to enlarge.

What are fall armyworms?

The fall armyworm is a caterpillar that comes from a moth that lays eggs. Their eggs can be found in clusters such as those illustrated in the image below. Moths don’t lay eggs in the grass, however. Their eggs are commonly found on tree bark, the siding of a house or near floodlights that attract them. The eggs don’t usually last throughout winter as it gets too cold in some states. Moths tend to migrate to the southern part of the United States for this reason.

Armyworm moths can lay up to 2,000 eggs that hatch in two or three days. There are usually around 4–5 generations per year, so it’s important to keep this in mind during treatment and prevention—if you treat one growth stage of the fall armyworms, it doesn’t necessarily mean you’ve treated all of them.  

What does a fall armyworm look like?

The fall armyworm got its name by traveling in small “armies” and eating everything in its path. They’re either green or muddy brown in color with a wide, horizontal black stripe running down each of their sides.

They are around 1–1.5 inches in length and have a lightly colored upside-down “Y” that marks the head of the worm. They are usually found throughout the months of July–October.

Moths are usually ash-gray in color with a wingspan of about an inch and a half. However, it is very common for a fall armyworm outbreak to occur after rainfall delivered by a tropical system such as a tropical storm or hurricane.

Moth Image Credit: NC State Extension

What do fall armyworms feed on?

Fall armyworms feed on over 300 different plant species including the leaves, stems and other parts of the plant located above the ground. They love to eat bermuda grasses and can be found feeding on annual bedding plants, succulents and even crops such as your homegrown tomatoes. 

What time of year do fall armyworms come out?

The exact times of the year for fall armyworm activity differ by geographical location; however, they’re generally active between the months of July and October.

What are the signs of a fall armyworm infestation?

You’ll know if you have a fall armyworm infestation because hundreds of them will be feeding in broad daylight. There are a great number of places you can look for armyworms.

Fall armyworms prefer plants in the grass family such as coastal bermuda grasses, fescues, rye grasses and blue grasses, but small crops are also subject to infestation. Small brown lawn patches are the first clue that you may have an infestation.

If you see the chewing or “skeletonizing” of grass blades that create a transparent “windowpane” look, this could be a sign of an armyworm infestation. Sod webworms are also potential candidates for this kind of damage, so it’s important to know the difference for proper treatment. Sod webworms are usually solitary and move around individually while fall armyworms travel in “armies”. Sod webworms are usually a light green color with four parallel rows of dark spots along their abdomens.

Fall armyworm damage often first appears near woods, buildings, sheds or light-posts because the adult moths usually lay eggs in erected structures. Once eggs hatch, the fall armyworms make their way down to the ground below and start feeding.

Caterpillar frass, or moist, fresh, green fecal pellets, can be found on the leaves of the base of plants and grass as well. These appear as green or yellow cylindrical pellets around 1–2 mm long.

Lastly, if you physically spot a fall armyworm or armyworm moth, it’s obvious that you may have an infestation. It’s important to treat armyworms before the damage spreads any further. You can also look for a cluster of their eggs on the sides of your house, fence posts, flagsticks, tree leaves and bushes adjacent to your lawn.

Here are some images that compare the fall armyworm and fall armyworm moth with the sod webworm and sod webworm moth. You can learn more about sod webworms on our sod webworm blog here.

Fall Armyworm and Armyworm Moth

Moth Image Credit: AggieTurf

Sod Webworm and Sod Webworm Moth

How do I treat a fall armyworm invasion?

Cultural Control
Unlike the sod webworm, fall armyworms can be more difficult to control. It is helpful to always maintain good cultural practices such as mowing, reducing thatch and lightly irrigating your lawn. Building a healthy lawn will help it withstand minor infestations of armyworms.

Aerate it at least once a year, and add about a 1/4 inch of organic materials such as peat moss to eliminate the buildup of thatch. Thatch serves as a home for armyworms.

Armyworms also prefer turf that is dry and warm, so regularly watering your lawn in the hot summers prevents your soil from getting too dry. About 1-inch of water a week including rainfall is enough. Be careful not to overwater your lawn as this may make it prone to a fungi outbreak. A well-watered lawn makes the soil surface cooler and is less attractive to armyworms.

Cut the grass no shorter than 2-inches. Although fertilizer doesn’t treat infestations, a proper fertilizer schedule will help your lawn remain strong and endure stress a little better. Be sure to check out our recommended Lawnifi® Summer and Fall Fertilizer Box.

Treatment
There are numerous effective insecticides you can apply to treat a fall armyworm invasion. The earlier the treatment, the better. Insecticides containing bifenthrin, acephate or chlorantraniliprole will work. However, here are some other top recommended insecticide products:

If you’ve followed these steps and perhaps noticed a worm that doesn’t resemble the fall armyworm’s description, be sure to check out our blog on sod webworms. Symptoms of a sod webworm invasion are very similar to those of fall armyworms. For more information on insects or pests that can be found on your lawn, check out our Insect Identification blog.