What Are Sod Webworms and How Do I Get Rid of Them?

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Sod webworms are a common nuisance in residential lawns. Sometimes they're just pesky and can be easily ignored. But they can get ugly.

What Are Sod Webworms?

Sod webworms, also known as lawn moths, are white to tan-colored moths that appear to randomly flit over your lawn’s canopy during the summer as you walk through. These lawn pests don’t fly far before landing again.

These adult sod webworms are not the problem, however. It’s the larvae you need to watch out for. These hungry little caterpillars munch on grass leaves right down to the surface, potentially killing the grass plant.

What Are the Types of Sod Webworms?

Entomologists suggest there are up to 20 species of sod webworms. They’re a diverse bunch, occupying a broad geographic area and adapting to different climates and growing conditions. They all share similar characteristics:

  • They follow the same four developmental growth stages:
    • Egg;
    • Larvae;
    • Pupae;
    • Adult moth.
  • Most produce two and sometimes three generations each season.
  • They feed on bluegrass, fine fescue, perennial ryegrass and zoysiagrass lawns.

What Impact Do Sod Webworms Have on Lawns?

You’ll first notice their presence as small whitish moths that seem to aimlessly zigzag in flight. These moths don’t damage your lawn. However, as they fly around, the females drop eggs — up to 200 of them!

About a week after hatching, the hungry caterpillars go to town. Most times, you don’t know they’re there until you notice turf damage. A severe outbreak of caterpillars, if left unchecked, can kill large portions of your lawn.

What Are the Signs of a Sod Webworm Infestation?

  • First, you’ll see small saucer-sized brown patches in your lawn. Looking closer, these are the result of grass leaves being chewed down to the soil surface.
  • You’ll see sod webworm moths flying around in the late afternoon or early evening, prior to dusk.
  • The caterpillars themselves are nocturnal. They feed during the night, so it’s hard to see them in action.
  • As the caterpillars munch away, the smaller brown patches expand, taking on the appearance of drought-stricken turf.

Determine the Infestation With the “Floatation Method”

  • Mix one to two tablespoons of lemon-scented dish detergent in one or two gallons of water.
  • Drench an area about two feet square near the edges of the damaged area. This is where the larvae are feeding. The solution will irritate the caterpillars, so they’ll wiggle out of the thatch and come to the surface.
  • Wait two to five minutes and count the caterpillars.
  • If you see more than 15 to 20 larvae per square foot, a control remedy may be needed.

How To Get Rid of Sod Webworms

Most lawn insecticides effectively control sod webworms so there’s no need to spend a lot of money. Sprayable insecticides like Ortho’s BugClear Insect Killer For Lawns and Bonide Pyrethrin Outdoor Insecticide eradicate sod webworms as well as a host of other surface-feeding insects without hurting your wallet.

If you want to use a fertilizer spreader rather than a lawn sprayer, there are some good granular options. The Andersons DuoCide Professional Grade Lawn Insect Control is one of my favorites. As the name implies, it’s what the pros use. Spectracide Triazicide Insect Killer is another good choice for sod webworms and many other surface-feeding insects.

Water your lawn a day or two before applying conventional insecticides. Once the insecticide has been applied, lightly water the product into the ground. This washes the active ingredient off the grass blades and into the thatch layer where the caterpillars live. The best time to treat your lawn is late afternoon or evening, just before the caterpillars come to the surface to feed.

Biological options also work reasonably well for controlling sod webworms. Bacillus thuringiensis (subspecies kurstaki) is a beneficial bacterium, producing a natural toxin that paralyzes the caterpillars. Biological products take longer to work, but they’re safe for humans and pets and good for the environment.

Consider options like Monterey Bacillus Thuringiensis Worm and Caterpillar Killer Insecticide or Southern Ag Thuricide BT Caterpillar Control. You’ll find most Bacillus thuringiensis products are sold for use in gardens, but they are just as effective as a lawn treatment.

How To Keep Sod Webworms From Coming Back

Once you’ve won the battle against sod webworms, you need to prevent them from returning.

  • First and foremost, take good care of your lawn! Like most lawn pests, sod webworms always attack a weakened lawn first. By properly fertilizing, watering, core aerating and mowing properly, your grass will be healthy enough to fend off just about any challenge.
  • Keep your mowing height in the three- to four-inch range. Cutting your lawn too short can cause scalping and thin turf. This makes your lawn dry out quicker, which makes it more susceptible to pests.
  • Reseed your lawn with endophyte-enhanced grass seed varieties.

Endophytes are intercellular fungi that live within the grass plant, and they help many turf species prosper. Toxic compounds produced by the endophytes can deter insects from chewing on grass leaves and stems because they don’t taste good. Endophytes also help with overall heat stress in lawns and can suppress some lawn diseases.

Not all lawn species have endophytes, however. Kentucky bluegrass doesn’t. But some perennial ryegrasses, fine fescue and tall fescue varieties do.

How do you know if the varieties in your lawn seed mix are endophyte enhanced? That’s the hard part. If you’re lucky, the grass seed label will tell you. If not, search for that information online. University extension services are a good place to start.

Joe Churchill
Joe Churchill is a Senior Turf Specialist for Reinders, Inc. in Plymouth, MN with a passion to promote realistic and environmentally-sound turfgrass maintenance practices through responsible use of water, fertilizers, pesticides and other inputs. Joe's client base includes professional turf managers serving the lawn care, sports turf and golf course industries. His lawn is the envy of the neighborhood and, in his free time, he enjoys kicking back on the Northshore of Lake Superior.