Why sod webworms are one unpredictable pest

Sod webworms morph into moths, which can lay 200 or more eggs per night, turning a small problem into a much larger one. (Photo: Weed Man)

There are several telltale signs that sod webworms have invaded a lawn. For example, these insects chew off blades of grass just above the crown of the plant, leaving behind circular brown or yellow patches of turf.

They carry the blades of grass into visible tunnels where they feed and leave behind tiny green pellets of waste. Despite all this evidence, Jeff Kolbe says lawn care operators (LCOs) often misdiagnose these pests.

“Sod webworms get misdiagnosed more often than not because technicians don’t get down close enough to see what is causing the damage,” says Kolbe, business consultant and manager of technical education for Spring-Green Lawn Care in Chicago. “You can’t just drive by a lawn and say, ‘That is sod webworm damage.’ You have to get down on your hands and knees and comb through the turf, looking for the signs that they have been feeding.”

Sod webworms are small caterpillars measuring 1/2 inch to 3/4 inch long, with small brown spots down their bodies. There are several types of sod webworms that thrive in different areas of the country. They are most commonly found in sunny areas of healthy turf in the late summer or early fall. The caterpillars morph into brown moths that fly around at dusk “like bomber jets,” laying 200 or more eggs per night, Kolbe says. The moths do not cause damage because they don’t eat the turf, but once the eggs hatch, the caterpillars begin to feed.

Brown spots left by sod webworms are often misdiagnosed. It’s important for LCOs to take the time to confirm the cause of the damaged turf. (Photo: Weed Man)

“Once you’ve seen the moths, that’s a good indicator that you may soon have a problem,” Kolbe says. “After that, you’ll start to see what looks like burns in the lawn.”

A Case of Mistaken Identity

Chris Lemcke

LCOs often confuse these burnlike spots for other problems like drought, fungus or even pet urine. The insects themselves also can be difficult to see because both the caterpillars and the moths are most active at dusk and into the evening. Unlike other pests, sod webworms do not consistently appear every year. Chris Lemcke, technical coordinator of Weed Man, headquartered in Toronto, Ontario, Canada, says several years can go by without an infestation, making it difficult to stay in front of the problem. Lemcke says consistent scouting is the best way to prevent a sod webworm outbreak, and Weed Man trains its technicians to be on the lookout for sod webworms at certain times of the year.

“It can be hard to prepare because outbreaks can come out of nowhere,” Lemcke says, adding that these insects can be carried from place to place by storms or wind. “You don’t know from one year to the next when there will be an issue, but if we see them, we can control them pretty easily.”

Phil Klemme

Phil Klemme, owner of The Grounds Guys of Union in Union, Mo., says LCOs can determine if sod webworms are present by saturating a damaged area of turf with a mixture of 2 gallons of water and 2 tablespoons of liquid dishwashing soap. This application will force any larvae to the surface of the ground within a few minutes. Once identified, LCOs can deploy pyrethroid insecticides designed for surface-feeding insects to treat sod webworms.

“If you find more than 10 to 15 larvae within a 3-foot by 3-foot area, a treatment is definitely needed,” Klemme says. “A common mistake in treating sod webworms is misdiagnosis. You need to make sure it is sod webworms and not a fungus or pet damage, which all require completely different treatments.”