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Sod Webworm

July 20, 2005
With the dry weather throughout the state, we have received reports of lawns damaged by sod webworm. In Illinois, sod webworm is typically controlled by a naturally occurring microsporidian, a microorganism with characteristics of both bacteria and fungi. As with other disease organisms that we are familiar with on plants, microsporidia are more likely to be effective in moist situations. As a result, irrigated turf is unlikely to have sod webworm problems. However, golf course roughs, unwatered or irregularly watered lawns, and nonirrigated park areas are all likely during dry periods to be damaged by sod webworms. Because there are three generations of sod webworms in much of Illinois and the generations tend to overlap, damage by sod webworm larvae is possible any time during the growing season.

Damaged turf areas initially appear as small, irregularly shaped areas 1 foot to a few feet across in which the turf appears thin, with more brown thatch showing through than normal. These rapidly develop into completely brown areas that coalesce to areas that may be several hundred square feet in size. Close examination will reveal that the grass blades are not brown, they are gone, with the area appearing brown from the thatch. Large numbers of starlings, brown-headed cowbirds, robins, or other insectivorous birds feeding in the turf are another sign that sod webworms may be numerous.

Sod webworm is verified by flushing 1 square foot of turf with a gallon of water containing either a teaspoon of 5% pyrethrin or a tablespoon of dishwashing detergent. This will irritate any caterpillars in the turf, bringing them up onto the turf surface for a minute or two. Sod webworm larvae are slender, 1 inch long or less, and covered with small brown spots. The base color may be whitish, gray, tan, or light green. Two to three larvae per square foot are enough to cause turf injury.

Sod webworm larvae live in a silk-lined tunnel in the thatch and emerge at night to feed on the grass. Grass blades are eaten off at the crown. When full grown, the caterpillars pupate in their tunnels. When the moths emerge from the pupae, the brown empty pupal shells commonly protrude from the turf. Several people have called recently about finding these pupal shells, which are less than 1 inch long, in damaged turf areas. The adults are slender tan moths up to 3/4 inch long. The moths appear tubelike because they hold their wings tightly against the body. They also have long palps that stick out in front of the head, appearing like a snout. They sit crosswise on grass blades, and when disturbed fly erratically in an up-and-down motion close to the turf. They settle back into the turf within about 30 feet, allowing one to sneak up to look at them. The moths are strongly attracted to light at night, so checking lighted windows at night is another good survey method. If sod webworm moths are numerous and the turf stays dry, an insecticide can be applied about 2 weeks after moth emergence to control newly hatching larvae.

Control can be achieved on older, feeding larvae or on newly hatched larvae with bifenthrin (Talstar), carbaryl (Sevin), halofenozide (Mach 2), spinosad (Conserve), Steinernema carpocapsae insecticidal nematodes (Biosafe), or trichlorfon (Dylox). The insecticidal nematodes need to be immediately watered into the turf after application. If you use a granular insecticide formulation, water with about 1/4 inch of water, enough to activate the insecticide off of the granule but not to flush it out of the thatch. Sprayed insecticides should be allowed to dry on the grass blades.

Author: Phil Nixon


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