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The sod webworm complex (Family Pyralidae) is a large group of insects consisting of over 20 species that infest turfgrasses in the United States. Among the more important turfgrass-infesting species are those belonging to the genus Crambus.
Adult sod webworms, sometimes referred to as "lawn moths" are buff-colored, about 1/2 to 3/4 inches long with snout-like projections extending forward from the head. At rest, they fold their wings around the body, giving them a cylindrical or cigar-shaped appearance. Sod webworm larvae (caterpillars) are gray to tan with small dark spots on the body and brown heads. They reach 3/4 to 1 inch when fully grown.
Sod webworms overwinter as partially grown larvae in silk-lined tunnels in the thatch and soil. In most years, larval activity resumes in April or early May. Webworms complete development, pupate and emerge as adults from mid-May to mid-June. Adult sod webworms rest in the turf and on shrubbery during the day and randomly scatter their eggs into the grass in the late afternoon and early evening while flying in a zigzag fashion just above the turf surface. Eggs hatch in about one week, and first-generation larvae feed until mid-summer. A second and partial third generation occurs during the remainder of the season. Frequently, generations overlap with all stages present by late summer.
Sod webworm moths do not damage turf. Larvae feed at night on grass leaves and stems near the soil surface, and hide during the day within burrows lined with silk webbing (hence the name "webworms") which penetrate through the thatch layer and into the soil. Sod webworms feed on most turfgrasses including bluegrass, bentgrass, tall and fine-leafed fescues, zoysiagrass and buffalograss.
One of the first signs of webworm infestation is small, ragged brown spots in the turf. Upon closer inspection, these areas will have a grazed or scalped appearance. As webworms continue growth and feed, the injured areas enlarge and coalesce. Under heavy sod webworm pressure, large areas of turf can be defoliated and even killed during periods of summer heat and drought. While sod webworm larvae are active from early spring through fall, the most serious turfgrass injury usually occurs in mid to late summer.
An early sign of potential infestation is sod webworm moths zig-zagging over the turf at dusk. If a sod webworm infestation is suspected, closely examine the turf for evidence of insect activity. Small patches of grass will be chewed off at ground level. Fresh clippings and green fecal pellets are also usually present. Examine the thatch layer and top inch of soil for larvae, silken tubes and webbing.
To confirm the presence of webworms, mix 1 tablespoon of 1% pyrethrins or 1/4 cup of household detergent with 2 gallons of water. Mark off several one square yard sections of the turf suspected of having a webworm infestation, and apply one gallon of the solution to each section. The solution irritates the caterpillars, causing them to move to the surface within 5 to 10 minutes where they can be counted and a management decision made.
Sound cultural practices, especially proper irrigation, will usually allow turf to outgrow light to moderate webworm injury. However, if 15 or more webworm larvae are present per square yard in reasonably healthy turf, an insecticide application may be justified. Fewer larvae can cause significant damage to stressed or less vigorous turf.
If insecticides are used, the turf should be mowed and the clippings removed before treatment to enhance insecticide movement into the turf canopy. A thorough irrigation (1/2 to 3/4 inch) prior to application will move webworms closer to the surface. For best results, apply insecticides in the late afternoon or early evening when larvae are active. Following application, the treated area should be lightly irrigated (1/8 inch), but delay heavy watering for 24 to 48 hours unless irrigation is indicated on the insecticide label. Granule applications also should be lightly irrigated immediately after application to wash granules off grass blades and activate the insecticide.