Safe Solutions – Bt for Sod Webworm

We get a lot of calls at this time of year from customers who are concerned about the brown patches of dead grass in their lawn, and the culprit is almost always sod webworm (Herpetogramma phaeopteralis). It can be a problem for centipede lawns, but sod webworm especially likes St. Augustine and in Tallahassee that’s where the problem most often occurs. This pest is present earlier in the year, but it takes time for the populations to build to the point where we begin to get worried calls from homeowners.

If you have not had this problem yet, be on the lookout for small brown moths with a 3/4 to one inch wingspan that scatter as you walk across your lawn or disturb nearby shrubbery. These moths rest there during the day and are active at dusk when the female will lay clusters of 10 to 35 eggs on the upper surface of grass blades. She produces hundreds of eggs in her lifetime (up to two weeks). The eggs are creamy-white, maturing to brownish-red, and usually hatch in three to four days.

The caterpillars are cream-colored (sometimes darker) with brown spots on each segment and a dark, yellowish brown head. The green in the picture below is the content of their stomachs. They spend the daylight hours curled up at or near the surface of the soil and feed at night. When they first hatch (and through the first few instars) the tiny sod webworm feeds only on the surface of the grass blade (called window feeding) so the damage is not always noticed. As the larvae grow they begin to consume entire sections of the leaf blade and finally the whole thing.

If you suspect your lawn has a sod webworm infestation, here are a couple ways you can determine if they are present. One method is to dig down through the grass (on the edge of the damaged area) and inspect the surface of the soil. You may very well find sod webworms, silken webs or frass (worm poop—it’s surprisingly large). Another method is to use a soap flush or drench. Mix liquid soap and water at a rate of one tablespoon of soap to one gallon of water and drench the soil at the edge of the damaged area (if damage is already apparent) or wherever you suspect a problem. You can use a large can. Cut both ends out of the can, creating a cylinder. Press one end into the ground and pour enough soap/water solution into the can to flood the grass within with an inch or two of soapy water. If they’re present it will not be long before you see the sod webworm larvae crawling up out of the solution. If you cannot find a large can, simply make a large amount of the solution (two or three gallons) and pour it over the area in question.

Here are three things you can do to protect your lawn from sod webworm:

1.       Do not use chemical pesticides on your lawn. In fact, do not use any broad-spectrum pesticides on your lawn, chemical, natural or certified organic, if at all possible. A broad-spectrum pesticide kills pretty much everything. When you feel it’s absolutely necessary, try to choose a product that will target the pest in question. There are beneficial insects and spiders that feed on sod webworm (spiders, lady bugs, ground beetles, wasps and more). They can consume a lot of sod webworms if you do not kill them before they have a chance to do so.

2.       A healthy lawn is less susceptible to sod webworm. You can improve the health of your lawn by watering adequately (but only when it needs it), by not over fertilizing, not over using pesticides or mowing too short. When you over fertilize, you might as well put out a neon sign, ‘Sod Webworm Buffet’. Ask for a copy of our handout, ‘How to Have a Great Lawn and Protect Your Lakes and Aquifer at the Same Time’ next time you come by the nursery, or click here.


3.       If you already have a sod webworm infestation, treat it with Bt (Bacillus thurigiensis). Bt is a biological control that will not harm you, your children, your pets, beneficial insects or the planet. It is however deadly for caterpillars. It comes in a granular form that’s easy to apply, but for sod webworm we recommend the liquid concentrate (called Thuricide). You’ll get better coverage with the liquid form, and that’s important with a pest that can cause as much damage as fast as sod webworm does. You can spot treat if you catch the problem early on or use a hose-end sprayer to treat your entire lawn if you do not. Because this pest feeds at night, an evening application is most effective. Sunlight breaks the product down, so do not expect it to continue to work for more than a few days. You will need to treat more than once. Although it takes a day or two for the larva to die after ingesting Bt, it loses its appetite right away and stops feeding on your lawn.


Last of all, don’t panic. Just keep an eye out for moths and early signs of damage. Start doing so around August. Assuming you do not let the problem go without treatment for too long, your lawn will usually recover from a sod webworm infestation.

Some information for this blog post came from the following sources –