Sod Webworms in Home Lawns

Now is the time to be on the lookout for several different lawn pests, including the sod webworm. With several different species of caterpillars that come out as it warms up, it may be easy to confuse the sod webworm with the fall armyworm. They are both pests commonly found throughout the United States that damage turf extensively. However, they’re different types of worms that can evolve into very different looking moths. Learn more about fall armyworms here.

This Sod University installment discusses what a tropical sod webworm is, how to search for symptoms of an invasion and how to treat an infestation.

Click the image below to enlarge.

What are sod webworms?

So, let’s talk sod webworms. The tropical sod webworm (Herpetogramma Phaeopteralis Guenée) is a lawn caterpillar that invades and damages your lawn usually between the months of May–July and will reappear in the fall. In some of the southern-most areas, however, sod webworms can be active year-round with peak season occurring between September–November. They mostly reside in the southeastern regions of the United States as well as other tropical areas including Hawaii, Puerto Rico, Guam, Jamaica and Australia.

Tropical sod webworms tend to prefer newly established warm season turfgrass found in lawns, parks, commercial landscapes, sports fields and golf courses. Tropical sod webworms are not to be confused with the fall armyworm or black cutworm.

What does a sod webworm look like?

In the adult form, the webworm is a small beige moth, sometimes called a lawn moth or crambus, that has a wingspan of 3/4 of an inch. It’s the moth that begins the lawn damage process. The adult female moths fly at dusk, dropping eggs on the grass blade surfaces and thatch area of your lawn.

The eggs are yellow in appearance and turn brown as they mature. Each female moth can lay as many as 200 eggs, however, they’re usually laid in clusters of 10–35 on the surfaces of grass blades. After the eggs hatch, remarkably within 7–10 days, very small juvenile caterpillars, about 1/2 inch in size, emerge and begin to feed.

During their larval stage, full grown caterpillars are 3/4 to an inch in length and have four parallel rows of dark spots along their abdomen. They are solitary and move around individually. They also live in individual burrows in which they emerge from and feed during the night. It’s possible to have as many as five separate hatches throughout the year.

What do sod webworms feed on?

The larval form of sod webworms does the most damage to turfgrass. Sod webworms live and feed above-ground on the blades of warm season lawns including St. Augustine grass, centipede grass, zoysia grass, seashore paspalums, carpetgrass, bahia grass and bermuda grass. According to the University of Florida, they can also cause damage to the cool season grass, bentgrass.

What are the signs of a sod webworm infestation?

The first sign of an infestation of sod webworms is noticing transparent grass blades. The juvenile caterpillars do what is called “skeletonizing” a grass blade so that only the veining structure remains. They do this by eating the green out of the blade and leaving it transparent looking. You can see a visual representation of what this looks like close up and further away in the images below.

The second indication will be the notching of the grass blades. The older, mature caterpillars will feed on the blades and leave notches on the sides. The caterpillars mostly feed at night. During the day, they curl up in a fetal position in the thatch layer of turfgrass. You can occasionally spot them surrounded by their frass. Frass is moist, fresh, green fecal pellets that are found in the thatch and is another indication of the presence of sod webworms.

Another way to detect a problem is to do a soap flush. Before preparing the solution, determine whether you’ll be monitoring a small or a large section of your lawn. For small areas (typically 1 sq. yd. of grass), add 2 ounces of liquid dish detergent to 1 gallon of water, then mix. Soaps such as Lemon Joy, Ultra Dawn and Ivory Clear are excellent options as they cause the least damage to turfgrass. Go to the area in your yard where you suspect a problem and pour the mixture within a 12-inch circle.

You might consider taking a piece of rope to make a 12-inch circle so you can observe the area easier. It could take as much as five minutes, but this mixture will force the insects within the 12-inch circle to the surface where they will be visible on the blades of grass. You can use this to flush other harmful insects to the top for identification as well.

If you plan on monitoring a large section of your lawn, prepare at least 1 gallon of a solution containing two parts water to one part liquid dish detergent. Note that soap solutions with higher rates than these can cause serious damage to turf. Applying the solution to a smaller area of the lawn allows the mixture to be more concentrated in that spot.

Turfgrass usually turns a brown or straw-yellow color when damage occurs followed by more sightings of weeds as the canopy of the turfgrass weakens.

Sod Webworm Prevention and Treatment

Non-Chemical Control (Cultural Practices)
In general, healthy turfgrass is less prone to pest pressure and will recover much faster from pest damage. Here are a few cultural practices that may help control sod webworms:

• Mowing Height: Maintain a proper mowing height for the variety of turfgrass you have in your yard. St. Augustine lawns should be mowed at 2–4 inches while zoysia should be mowed at .5–2 inches. Never cut more than 1/3 of the blade height during one mowing.
• Reduce Thatch: Mowing at the proper height and frequency will eliminate the chance of a thatch buildup.
• Do Not Overwater: Your grass should only need about one inch of water a week including rainfall.
• Turn Off Flood Lights: Turn your flood lights off at night since moths fly at night and are attracted to the light.

With that being said, the University of Florida also states that over-fertilizing your lawn is the leading cause for caterpillar outbreaks. Check out our Maintenance page to look for fundamental tips for different seasons and grass types.

Chemical Control
Chemicals are usually applied to suppress larval (caterpillar) populations when they are feeding. Sample the thatch for sod webworms to determine how many are present before applying a registered insecticide. Best results are achieved by applying insecticides late in the afternoon or early evening when caterpillars are still active.

Some of the more effective insect control products either contain the active ingredient Chlorantraniliprole (Scotts GrubEx…it works on sod webworms too), Bifenthrin (Bifen L/P and Bifen XTS), Lambda-Cyhalothrin (Spectracide Immunox Fungus + Insect Control) and Trichlorfon (Dylox).

Remember to follow all specific label directions. This will ensure that you use the proper amount of chemical for your application, apply the chemical correctly and help you with properly adding any adjuvants necessary for the best efficacy. Here are some insecticides that can be used:

Although fertilizer doesn’t treat infestations, a proper fertilizer schedule will help your lawn remain strong and endure stress a little better. You might want to consider a fertility program such as Lawnifi®. Lawnifi is a fertilizer program designed to give your lawn the nutrients it needs when it needs them. Subscribe to receive seasonal Lawnifi fertilizer boxes throughout the year or purchase each seasonal box individually.

Lastly, for those who prefer traditional granular fertilizers, Lawnifi Foundation is a granular fertilizer used for residential lawns and gardens that can be used throughout the different seasons all year long. You can learn more about Lawnifi at lawnifi.com or by reading Why Use Lawnifi Fertilizer?.

For more information on insects or pests that can be found in your lawn, check out our Insect Identification blog, or browse through all of our Sod University insect and pest control blogs here.

Sources:

• https://edis.ifas.ufl.edu/publication/IN968