How to Get Rid of Sod Webworms | Stop Lawn Damage Fast

Patchy, brown grass that pulls up like a loose carpet often points to sod webworms chewing through the roots. These small green or brown caterpillars hide in silken tunnels near the soil surface, feeding at night. The reliable way to get rid of sod webworms is to confirm the larvae are present before choosing a treatment, then hit them at the right time with the right product.

Confirm Sod Webworms Before You Treat

Adult moths flying over the lawn in the evening are a poor reason to spray. The University of New Hampshire Extension notes that adult presence is not a reliable sign of damaging larval numbers. Instead, inspect the thatch and damaged areas for green or brown caterpillars and the silken tubes they spin. A soap flush is the quickest way to see what’s actually living in your turf.

Mix 1 tablespoon of liquid dishwashing detergent or 1/4 cup of powdered detergent with 1 gallon of water. Pour it over 1 square yard of suspect turf. Most caterpillars will surface within 2 to 5 minutes, wriggling to escape the soap. Count what appears. When you find 10 to 30 or more larvae per square yard, treatment is justified. For highly managed lawns like golf greens or manicured turf, the lower end of that range is the trigger point.

Use Nematodes or Bt for Biological Control

If the count justifies action, beneficial nematodes are the cleanest first option. The species Steinernema carpocapsae hunts down webworm larvae in the soil and is commercially available. The University of Minnesota Extension recommends applying nematodes at a rate of 25 million per 1000 square feet, ideally in the early morning or evening when the sun won’t dry them out.

Nematodes are living organisms, and they die quickly in heat and dry conditions. Water the lawn lightly before application, apply the nematodes, then water again with a light irrigation to wash them into the thatch where the larvae feed. A second biological option is Bacillus thuringiensis var. kurstaki (Bt), a bacteria that kills young caterpillars when they ingest it. Bt works best on small, actively feeding larvae and breaks down quickly in sunlight.

Time Insecticide Applications for Maximum Effect

When biological options aren’t enough, a registered insecticide gets the job done, but timing decides success. Sod webworms feed at night and are most active in the late afternoon and early evening. Insecticides penetrate best when applied then, hitting the larvae as they come up to feed.

Prepare the lawn first. The University of New Hampshire Extension suggests mowing to about 1 1/2 inches to improve spray penetration. If thatch is thicker than 1/2 inch, light aerification opens the surface so the product reaches the soil. Apply 1/2 inch of water 48 hours before treatment to hydrate the turf, then a light 1/2 to 3/4 inch after treatment to move the product into the root zone. Avoid heavy irrigation after granular applications; a drenching wash can push the product past the larvae and reduce effectiveness. Heavy rain within 24 hours of application may also weaken results.

Preventative insecticides like imidacloprid or clothianidin work best on newly planted lawns where webworm damage has been a recurring problem. For established turf showing active damage, a curative product aimed at the current larvae is the better play. Always check the product label and confirm the insecticide is registered for use in your state, since registered products vary by location.

If you’re weighing product choices and want to see what tested options are worth your money, our roundup of proven sod webworm control products breaks down the top picks.

Stop Webworms From Coming Back

Prevention starts with grass that doesn’t attract egg-laying moths. Endophyte-infected perennial ryegrasses and fescues are naturally resistant to webworm feeding and are worth planting where they’re adapted. Beyond grass choice, avoid over-fertilizing and overwatering. Both encourage lush, tender growth that draws egg-laying females and builds the thick thatch layer where larvae hide.

Cultural practices and treatment work together: keep thatch under 1/2 inch, water deeply but less often, and mow at the recommended height for your grass type. Regular monitoring with a soap flush through late summer catches problems early, when a single targeted treatment still solves the issue.

FAQs

How long does a soap flush take to reveal webworms?

Most sod webworm larvae surface within 2 to 5 minutes after you pour the soapy water over the turf. The soap irritates the caterpillars and drives them up out of the thatch so you can count them accurately before deciding whether treatment is necessary.

What time of day should I apply insecticide for webworms?

Apply insecticides in the late afternoon or early evening. Sod webworm larvae feed at night and are most active near the soil surface then, making evening applications more effective at reaching the caterpillars before they retreat into their silken tunnels.

Can nematodes survive hot summer weather?

Nematodes are highly sensitive to heat and drying, and they die quickly under hot, dry conditions. Apply them in the early morning or evening, water lightly before and after application, and choose an overcast day if possible to give the nematodes the best chance of surviving to reach the larvae.