Sod Webworms in St Augustine Grass

Discover the sod webworms treatment for St. Augustine grass to bring back the flourishing beauty of your lawn.

Nelmie Jane Pardo | Updated March 6, 2023
sod webworms in st augustine grass

Are you noticing small, brown patches on your St. Augustine grass lawn? You may have an infestation of sod webworms.

These tiny, tan-colored moths invade lawns and lay their eggs on grass blades. The larvae begin feeding on the grass or other plants when they hatch, causing damage to the lawn. If left unchecked, an infestation of sod webworms can seriously harm or eventually destroy your St. Augustine grass lawn. 

To help protect your lawn, we’ll explore the lifecycle of sod webworms and how to identify an infestation.

What Are Sod Webworms? 

Sod webworms are white to brownish moths that feed on warm-season lawns, including St. Augustine grass, carpetgrass, zoysia, centipede grass, and seashore paspalums. They are also known as lawn moths.

However, they are not the problem. It’s the larvae that hungrily munch on grass leaves and damage lawns. 

The developmental growth stages of sod webworms are: 

  • Egg
  • Larvae
  • Pupae
  • Adult moth

Female moths drop up to 200 eggs, which leads to an outbreak of hungry caterpillars that can kill large parts of your St. Augustine lawn. 

Signs of a Sod Webworm Infestation in St. Augustine Grass

Sod webworm damage in St. Augustine grass lawns typically looks circular and ranges from several inches to an entire lawn. It shows a noticeable gray or brownish color. 

  • First, you’ll notice saucer-sized brown patches as the caterpillars chew grass leaves. 
  • You’ll also see lawn moths buzzing around late afternoon to early evening.  
  • The small brown damaged patches expand, and your lawn looks drought-stricken. 

Sod webworms (Herpetogramma phaeopteralis) feed on the grass leaves and cut down the grass, causing an infested lawn to look shorter in damaged areas. They create a somewhat mowed lawn appearance as they move from leaf to leaf, chewing near the stem rather than eating the whole leaf. The cut leaves then turn gray to tan, giving damaged areas a brownish color. 

Looking closely at damaged areas, you’ll see cut marks on leaves where sod webworms feed. You’ll also see tiny, green caterpillars on the soil surface. 

How Do I Confirm a Sod Webworm Infestation? 

If you notice the signs of sod webworm damage discussed above, you must check for an infestation immediately. 

A homemade drench test is an excellent method to flush out and determine the severity of larval pest infestation in your lawn. Perform the following steps to identify larval and adult insects feeding below the soil surface: 

Step 1: Create a soap solution. 

Add 2 to 4 tablespoons of liquid dish soap to 1 gallon of water. You may have to use 2 gallons of water for dry soils. You can use a rope to mark 1 square yard for even application. 

Step 2: Sprinkle the solution. 

Use a sprinkling can to distribute the solution evenly over 1 square yard of the potentially infested lawn area. When irritated by the soap solution, the caterpillars wiggle out and move to nearby dry surfaces. 

Step 3: Count the moth larvae. 

Count all caterpillars within 10 minutes of doing the drench test. If the test flushes out at least 15 moth larvae in a square yard, it’s time to treat your St. Augustine lawn.  

Effective Treatment Options for a Sod Webworm Infestation 

While a few sod webworms are not harmful, lawn areas with a high population of these pests will be severely damaged and eventually destroyed. Fortunately, proper treatment will help your St. Augustine lawn to recover fully. 

Best insecticide for lawn moths

Most lawn insecticides are effective in controlling a sod webworm infestation. Sprayable insecticides, like Bonide Pyrethrin Outdoor Insecticide and Ortho’s Bug B Gon Insect Killer for Lawns, can eliminate sod webworms and other surface-feeding insects. 

Effective lawn moth insecticides contain Chlorantraniliprole, Bifenthrin, Trichlorfon, and Lambda-Cyhalothrin. You can use a sprayable insecticide or a granular option if you prefer a fertilizer spreader. 

  • Apply the insecticide one to two days after watering your lawn. 
  • Lightly water the insecticide into the ground, allowing the active ingredient to reach the caterpillars in the thatch layer. 

Biological Treatment 

Bacillus thuringiensis (Bt) is a soil-borne bacterium that produces a natural toxin. It eliminates moth larvae without causing harm to beneficial insects, earthworms, animals, people, and the environment. Caterpillars that consume Bt-treated grass typically die within three days. 

Using a pesticide with Bt is an effective way to eradicate sod webworms in St. Augustine grass and decrease future possible lawn moth populations. 

  • Add around ½ to 4 teaspoons of a Bt-based product to a gallon of water. 
  • Spray the pesticide thoroughly over your lawn using a handheld garden sprayer. 
  • Wear a facemask, protective eyewear, long pants and sleeves, and shoes to prevent irritation. 
  • Do the treatment within 7 to 10 days to eliminate all caterpillars. 

Check any particular directions on the product label, whether you’re using insect control products or biological treatment. 

When Should I Apply the Sod Webworm Treatment? 

Treat your St. Augustine grass lawn only when the weather is warm and dry throughout the treatment period. The ideal time to spray your lawn is early in the evening when moth larvae come out to feed on the grass leaves. It’s best not to water your grass for two days after treatment application. 

Final Thoughts 

After eradicating sod webworms in your St. Augustine grass lawn, you must ensure they never return. We recommend treating preemptively for these lawn pests if you’re planning to get new grass.

Proper lawn maintenance is also essential, from fertilizing and watering to aerating and mowing your lovely lawn. Your mowing height should remain within a 3-to-4-inch range. Your lawn will likely dry out fast if you cut it too short, making it more vulnerable to pests. Keep your flood lights turned off at night to avoid attracting moths. 

With these tips in mind, you can restore the healthy look of your St. Augustine grass lawn and keep it pest-free! 

Written by Nelmie Jane Pardo

Nelmie Jane Pardo

Nelmie Jane Pardo is a senior contributing writer who lends insight into gardening trends and solutions for common lawn problems.