Sod webworms and armyworms are both caterpillars that destroy turfgrass, but they differ in size, feeding behavior, and the type of damage they leave behind. Sod webworms are smaller (about 3/4 inch long), feed at night from silk-lined tunnels in the thatch layer, and create patchy brown spots that spread slowly. Armyworms are larger (up to 1.5 inches), feed in groups that march across a lawn in waves, and can strip large sections of turf in just a day or two.
Telling them apart matters because the speed and scale of damage differ dramatically, and catching either one early makes treatment far more effective.
How to Tell the Caterpillars Apart
Sod webworms are translucent green or cream-colored caterpillars with small dark spots arranged along each body segment and a yellowish-brown head. They max out at about 3/4 inch long. When disturbed, they curl into a tight C shape. You’ll almost always find them hiding in the thatch layer during the day, since they only come to the surface to feed after dark.
Armyworms are noticeably bigger, reaching 1 to 1.5 inches at maturity. Their coloring ranges from green to dark brown, and the key identifier is a series of bold stripes running the length of the body. Fall armyworms, the species most destructive to lawns, have an inverted Y-shaped marking on the front of the head. Unlike sod webworms, armyworms can sometimes be spotted feeding during the day, especially when populations are high and food is running low.
How the Adult Moths Differ
The adult moths are easy to distinguish if you know what to look for. Sod webworm moths, often called “lawn moths,” are small, light-colored, and fold their wings tightly against their bodies when resting, giving them a narrow, tube-like shape. Their heads appear to have a long snout. You’ll notice them making short, erratic, darting flights just above the grass at dusk, and they’re drawn to porch lights.
Armyworm moths are light reddish brown with a small white spot near the center of each forewing. They’re also night flyers attracted to lights, but they’re bulkier and don’t have the distinctive snout or tight wing fold of sod webworm moths.
Damage Patterns on Your Lawn
This is where the two pests diverge most clearly. Sod webworm damage starts subtle. The youngest caterpillars feed only on the upper leaf surface, scraping away tissue in a way that’s easy to mistake for drought stress. As they mature into their fifth and sixth stages, they chew through entire leaf blades, and small yellow-brown patches appear in the lawn. These patches grow and merge together over time, especially in dry weather. The damage tends to look like the lawn is simply drying out, which is why sod webworms often go undetected until significant turf is lost. Peak injury shows up in late summer and early fall.
Armyworm damage is harder to miss. These caterpillars feed in large groups that move across a lawn like an advancing front, which is how they got their name. They can reduce a healthy section of turf to bare stems overnight. Instead of scattered patches, you’ll often see a clear line where green grass meets destroyed grass, with the damage expanding outward. Armyworms also chew entire blades down to the crown, and in heavy infestations, they’ll move to the next available food source once an area is consumed.
Seasonal Timing and Life Cycle
Sod webworms overwinter as immature larvae tucked inside silk-lined chambers in the thatch. When temperatures warm in spring, they resume feeding, eventually pupating in silken cocoons. Adults emerge in 7 to 10 days, mate, and females drop roughly 60 eggs per day over a two-week period. Eggs hatch in about a week during hot weather, and larvae feed for another 4 to 7 weeks. Most regions see two generations per year, though warmer climates can produce up to four. The heaviest lawn damage comes from the second or third generation in late summer.
Armyworms don’t typically overwinter in northern states. Instead, moths migrate north each spring and summer from southern regions. Fall armyworms arrive later in the season, usually mid to late summer, and populations can explode quickly. In southern states, multiple generations occur throughout the growing season. Because armyworms don’t establish overwintering populations in cooler climates, their appearance can be unpredictable from year to year.
How to Check Your Lawn
A soapy water flush test works for detecting both pests. Mix 1 to 2 tablespoons of lemon-scented liquid dish soap into 2 gallons of water. Pour the solution slowly over about one square yard of turf, placing it in healthy grass near the edge of a damaged area rather than in the dead zone itself. Watch closely for 5 to 10 minutes. The irritant drives caterpillars to the surface where you can count and identify them.
For sod webworms, Penn State Extension suggests a treatment threshold of about 4 to 6 larvae per square foot, or roughly 10 to 15 per square yard. Armyworm thresholds are similar in many turf management guides, but because armyworms cause damage so quickly, you may need to act faster once you confirm their presence. Also look for green fecal pellets in the thatch and, in the case of sod webworms, small bits of silk webbing binding grass debris together.
The best time to monitor is from June through early October, checking every week or two during peak season.
Treatment Options
Both pests respond to the same general categories of control, but timing is everything. Treatments are most effective against young, small caterpillars. By the time larvae are fully grown, they’re harder to kill and have already done most of their damage.
Biological options include parasitic nematodes and a naturally occurring soil bacterium that’s toxic to caterpillars (sold as Bt). Nematodes work especially well against sod webworms because these caterpillars spend so much time in contact with the thatch and soil where nematodes hunt. Apply nematodes during the coolest part of the day and keep the soil moist for two weeks afterward. Bt is effective on young larvae but breaks down quickly in sunlight and washes off leaves, so don’t irrigate for two days after applying it.
For armyworms, speed matters more. Because they feed aggressively and in large numbers, you may not have the luxury of waiting for biological controls to take effect. Spinosad, a naturally derived insecticide, is effective against both pests and works faster than Bt. Regardless of what you apply, mow and water the lawn before treatment, then avoid mowing or watering for at least 24 hours afterward (except when using nematodes, which need immediate irrigation).
Long-Term Prevention
One of the most effective long-term strategies is planting grass varieties that contain beneficial fungi called endophytes. These fungi live inside the grass plant and produce compounds that deter caterpillars from feeding or disrupt their development. Endophyte-enhanced varieties of tall fescue, perennial ryegrass, and fine fescues show strong resistance to both sod webworms and fall armyworms. Kentucky bluegrass and bentgrass don’t currently carry endophytes, so if your lawn is primarily bluegrass, this strategy won’t apply.
If you’re overseeding or establishing a new lawn, look for seed labeled as endophyte-enhanced. One important detail: the endophyte is a living organism inside the seed, so it dies if seeds are stored in hot or humid conditions. Buy from a reputable source and store unused seed at around 40°F in a dry location. These grasses are compatible with standard lawn care practices including fertilization, irrigation, and most herbicide and insecticide applications.
Beyond seed selection, a healthy, well-maintained lawn tolerates moderate caterpillar feeding without lasting damage. Adequate watering during summer stress periods helps turf recover from the kind of superficial feeding that early-stage sod webworms cause. Avoiding excessive nitrogen fertilization also helps, since lush, fast-growing turf is more attractive to egg-laying moths of both species.