Patches of brown grass in your lawn can be a sign of fall armyworms (Spodoptera frugiperda), who got their name from the way they move as a group from one field to another to feed, leaving devastation in their path like an army.
Armyworms are native to the tropical regions of the western hemisphere from the United States to Argentina. They only overwinter in the warm climate of the Gulf states from where they move north every spring. The onset of cool autumn weather usually puts an end to the spread of the pest. But, several factors including a warming climate, weather changes, and the lack of natural predators are causing heavier infestations of fall armyworms in many parts of the country like the Mid-Atlantic region by mid-summer and the upper Midwest and northeast in the late summer.
Of the different species of armyworms, fall armyworms pose the most serious threat to homeowners. If undetected and untreated, they can wreak havoc in a lawn within a few days. Warm-season grasses such as Bermudagrass may regenerate from an armyworm infestation, whereas cool-season grasses such as fescue are often permanently destroyed. Whichever type of grass you have, early identification of the pest is key. To get rid of armyworms or prevent them in the first place, it is also important to know the armyworm’s life cycle because it is the worm-like larvae that causes the serious damage, not the adult moths.
What Do Armyworms Look Like?
Armyworms go through several generations in a year, and they look differently in each life stage. The fall armyworm has four life stages: egg, larva, pupa, and adult.
The egg masses of the fall armyworm are laid in clusters of 100 to 200 eggs and are often covered with fuzzy scales. As they get closer to hatching, the eggs turn from light-colored to dark brown.
After the larvae first hatch, they are a light green or tan color and will gradually turn dark green, brown, or black as they grow. The larvae are hairless and 1 ½ to 2 inches long. The two characteristics that help distinguish the armyworm from other caterpillars is the line between the eyes that looks like an inverted “Y”, which becomes more pronounced with the age of the larvae and three whitish narrow stripes down the body.
Adult moths have a 1 ½-inch wingspan and white underwings. The front wings of the female are dark grey mottled with some light flecks; the front wings of the males have both light and dark areas.
2 Ways to Get Rid of Armyworms
Getting rid of armyworms is all about the right timing—you need to get the population under control by the time they reach the larval stage in which they feed voraciously. As always, closely monitoring your lawn is your first line of defense and applying chemicals should be the last resort.
Hand-pick Them Off
If there are just a few armyworms, pick them off the grass blade or plant and drop them in a bucket with soapy water.
Use an Insecticide
There are different insecticides available against heavy armyworm infestations. Products with pyrethroids as the active ingredient are common. Always carefully check the label to make sure that it is intended for armyworm control, and follow the instructions for application rates, methods, and timing.
The Life Cycle of Armyworms
Armyworm populations grow rapidly in warm weather because a single female can lay eggs in batches of up to several hundred eggs, which hatch after a few days. The next two to three weeks is the stage to watch out for because that’s when the larvae feed voraciously on any grass blades they can get to. After that, the larvae burrow into the soil and pupate before emerging as adult moths about 10 to 14 days later, upon which the life cycle starts again.
The total life cycle takes approximately four weeks. It will repeat itself indefinitely unless freezing temperatures in the fall put an end to it. That’s why in warm climates, armyworm populations are present year-round.
Signs of Armyworm Infestation
In the early stages of their development, armyworm larvae feed very little. It is during the last two stages that they consume more than 90 percent of the food they eat during their entire lifetime. Pinpointing the life stages of the armyworm larva development is difficult for home gardeners—you must be on the lookout for the first signs of damage in your lawn. While Bermudagrass is a favorite food of armyworms, cool-season grasses are also susceptible.
Larvae and Feeding Damage
A subtle indicator of an armyworm presence is the thin silky thread the newly emerged larvae produce to lower themselves down onto the grass to feed. The larvae start their feeding on the tips of the grass blades and then chew their way through the entire blades. At first, the surface of the blade looks transparent like a windowpane. Later, the grass will look like it is under drought stress and entire patches will turn brown if untreated. The severity of the damage increases when the weather is hot and dry, and the grass is already under drought stress.
To check if you have armyworm caterpillars in your lawn, you can pour a bucket filled with cold, mildly soapy water onto the damaged spot to bring them to the surface. A distinct line between a damaged area and intact lawn might also be an indicator for armyworm damage, as armyworm larvae start in one location and move towards the next feeding area.
You can also check the grass for actively feeding caterpillars or their frass (droppings). In their more mature larvae state, they are easier to spot than earlier. Do this in the early morning or late evening because that’s when they armyworm larvae are the most active. During the daytime they are seeking shelter from the heat under clods of sod, grass clipping, or leaf litter.
A secondary sign are birds suddenly hanging out on specific spots on your lawn because they are attracted by the caterpillars as a new food source.
Adult Armyworm Moths
Also check any places near the lawn where the female adult moths like to place their eggs, such as the undersides of tree leaves, fences, and other structures. Armyworms fly and mate at night, and since moths are attracted by light, check any light posts in your yard for egg masses. Remove the egg masses promptly by scraping or brushing them off with soapy water.
What Causes Armyworms?
Fall armyworms can be a yearly recurrence since armyworm moths work their way up north every spring with weather fronts and tropical storms, flying long distances to look for new feeding grounds. They go through several generations during this journey and the further north you live, the longer it will take for them to reach your location.
Other than armyworms landing in your yard naturally, they can also be brought in from infested sod. If you install sod in your yard, make sure it comes from a reliable source, and inspect it carefully for leaves with windowpane damage. While you might not be able to detect larvae in their small early stages, you can spot larger armyworms and control any infestation right away.
How to Prevent Armyworms
As armyworms can cause significant damage in a short time, keeping an eye on your yard for any signs of them is important. If someone in your neighborhood has armyworms, they will likely be moving into your yard as well so be extra vigilant.
Dethatching your lawn makes it less inviting for the larvae who like to rest there during the day. Improve your lawn’s drainage if it has any wet spots as armyworms like to lay their eggs in damp patches of grass.
Inspect sod before laying it down to be sure that there are no armyworms present.
Armyworms vs. Cutworms
The caterpillars might seem similar but there are several ways to keep them apart. Cutworms are solitary feeders whereas armyworms come as a large group and lay their eggs in masses. The damage caused by cutworms is different, they chew off the stems of young plants at the base. Read more about cutworm identification and control here.
How long will armyworms last?
If you live in an area with freezing temperatures, armyworms will die when temperatures drop in the fall, but you might get new infestation the next year. If you live in a warm climate, armyworms can be a yearlong occurrence and getting them under control is crucial. Global warming is expanding the northern reach of armyworms.
Will grass come back after armyworms?
It depends. If you live in the south, vigorous and fast-growing warm-season grasses have a better chance of recovering than the cool-season grasses typically grown in northern climates, where the damage is usually more serious and permanent.
Do birds eat armyworms?
More than 40 bird species, including native birds, eat armyworm larvae. These include crow blackbirds, yellow-headed blackbird, chipping sparrow, bluebird, prairie hen, and European starling all eat armyworm larvae.