Heat, disease, and bone-dry soil aren’t the only reasons a lawn might brown in summer. Often overlooked is the damage caused by colonies of a grass-eating caterpillar called the sod webworm.
This common and widespread insect is sometimes called a “lawn moth” for its skinny, cream-and-tan adult stage that can be seen flitting around lawns in late afternoon to early evening. These adults lay eggs in the soil, which within a week hatch into spotted, brown-gray caterpillars that grow to nearly an inch long.
The adult moths don’t cause any damage. But the ground-dwelling caterpillars chew off grass blades at the base, causing a ragged brown appearance that can kill patches when enough of them are feeding. Sod webworms are often the culprits when a summer lawn is browning despite adequate rain or when the lawn doesn’t quickly “green up” after a good soaking that breaks a dry spell.
Do you have webworms?
Seeing those half-inch moths flying over the lawn surface is a good first clue. Detecting the caterpillars is less obvious because they live in little tubes in the soil surface or thatch layer. They also feed at night.
If you look closely, you might be able to see the silky webs the webworm caterpillars make, or you may see tiny green pellets between blades of grass (webworm feces). You can also do a “flotation test” that involves mixing two tablespoons of dishwashing liquid into two gallons of water and pouring it over a square foot of ground. Within a few minutes, any soap-doused sod webworms in the soil will come wiggling up.
Important webworm notes
- Entomologists say that more than 12 to 15 sod webworms per square foot is enough to cause noticeable damage.
- Two generations of sod webworms are produced each year in cooler regions of the U.S., while warmer regions can run into three generations a year.
- Sod webworms feed on almost all types of turfgrass varieties and favor sunny areas over shadier ones.
For all of these reasons, it is important to eradicate sod webworms as soon as possible to avoid any further damage to your lawn.
What to do about sod webworms
Dense, healthy lawns usually grow through minor to moderate sod-webworm attacks. Encouraging growth through using high-quality grass seed varieties and regularly fertilizing is a good front-line defense that avoids pesticides.
If damage is bad enough to warrant action, Bt (Bacillus thuringiensis) is a soil-dwelling bacteria product that kills caterpillars without harming bees, birds, pets, or people. Products with azadirachtin (neem oil) and spinosad are two other natural webworm controls, while beneficial nematodes (Steinernema) are microscopic parasites that feed on caterpillars and other lawn-dwelling pests, including grubs. Nematodes are available in packets at some garden centers and catalog companies.