Photos by Tom Eickhoff, UNL

Turf issues that are “out of sight, out of mind” are the most difficult to deal with. Damage from sod webworms most certainly fits into this category.

Damage symptoms

When looking at damage from sod webworms, there aren’t a lot of easily recognizable symptoms. In fact, the injury mimics damage from summer patch disease, soil compaction, billbugs, and sometimes leaf infecting diseases, such as bipolaris leaf spot. They feed on most turfgrasses including tall fescue, fine fescue, zoysiagrass, bentgrass and Kentucky bluegrass.

Webworms are an aboveground feeder, but do so in a rather discreet manner. As with most caterpillars, adult sod webworms do not damage turf directly. Damage results from larvae feeding at night on grass leaves and stems near the soil surface, hidden from view. During the day, they hide within small tunnels constructed with silken webbing, which extend through the thatch layer and into the upper levels of soil.

In most cases, the first sign of infestation is small, ragged, tan to brown spots in the turf. Upon closer inspection, these areas have a scalped appearance, as if they have been mowed closely with a dull mower or roughed with a paint scraper or sander. As feeding continues, affected areas enlarge and may converge with nearby infestations, taking on the appearance of one large mass of damaged sod.

When webworm numbers are high, and the turf is under stress, turf status can change from damaged to dead. Because most stressors occur in midsummer, the most serious turf injury occurs then, even though webworms are active from early spring through fall.

Sod webworm larvae. An adult sod webworm.

Inspection for webworms

Adult sod webworm moths often exhibit a zigzag-esque pattern of flying close to the sod at dusk. Egg laying occurs during these flights. If you observe .5-inch, buff-colored moths fluttering about just before twilight, it’s wise to look closely for the presence of larvae. They usually hatch within one to two weeks of egg laying.

Because many other damaging agents can cause similar injury, it’s important to be sure that webworms are present before applying an insecticide.

The two most effective ways to inspect for and confirm an infestation of sod webworms are:

1. Using a lemon-scented soap flush—To verify whether sod webworms are responsible, mix up a “detecting” solution. Combine one tablespoon of 1 percent pyrethrins or .25-cup of lemon-scented household detergent or dishwashing liquid with 2 gallons of water. Choose the most fragrant product over brand names or flashy looking labels. Mark off several 1-square-yard sections of the turf that you suspect are infested with webworms and apply 1 gallon of the solution to each section. Wait about 10 minutes and look closely for worms. The solution works by irritating the caterpillars, causing them to move to the surface where they can be seen and counted.

2. Probing with a pocketknife—Carefully cut through the sod and examine the top inch of soil and thatch layer for the presence of larvae, silken tubes and webbing.

A close-up view of sod webworm injury.

Life cycle

Sod webworms overwinter as partially grown larvae in silken tunnels in the upper soil layers and lower sections of thatch. These larvae resume activity and begin feeding in midspring. After six to eight weeks, they finish feeding and pupation (a brief resting stage) and emerge as adults. Adults rest in shrubbery and on turf blades during the day and lay eggs at dusk. Small, first-generation larvae hatch from the eggs after a week or two, and then feed until midsummer. A second and third generation takes place during the remainder of the season. Depending on the region of the country, it’s quite common for the generations of webworms to overlap, creating a scenario where all life stages are present by late summer or early fall.

Threshold for treatment

In many turf areas, sound cultural practices, such as efficient irrigation, proper nutrition, compaction relief and mowing practices, will usually allow turf to outgrow light to moderate webworm injury. However, if 15 or more webworm larvae are present per square yard, an insecticide application may be justified.

Control steps

If an insecticide application is necessary, several steps can be taken to enhance performance. First, the turf should be mowed and the clippings removed, which will reduce the amount of vegetation that might interfere with the insecticide-to-insect contact. Second, irrigate to drive the worms upward in the soil profile. A thorough (.5 to .75-inch) application will decrease oxygen levels and make the silken tunnels less habitable, encouraging upward movement into a more vulnerable location. Next, apply the insecticide in late afternoon or early evening when webworms are generally more active. Finally, lightly irrigate (1/8 inch) to wash insecticide residue off turf blades. This is especially important for granular applications. After the light irrigation, avoid heavy watering for 24 hours to prevent dilution of the insecticide solution.

Many insecticide products are available for controlling webworms, including bifenthrin, cyfluthrin, deltamethrin, fipronil, permethrin, carbaryl and acephate. Ease of application, price, need for additives, residual and overall effectiveness are some of the features to consider. Local, state and regional turfgrass conferences are key events for the comparison of various products, as well as a good opportunity to receive research updates and briefings from university researchers and extension professionals.

John Fech is an extension educator specializing in turf and ornamentals at the University of Nebraska-Lincoln.