University of Manitoba - Faculty of Agricultural & Food Sciences - Sod Webworm
Sod Webworm

Problem type: Insect

Name of problem: Sod Webworm

Plant name(s): All cool season grasses

Symptoms / Characteristics:
Thinning of lawn is the early symptom, but usually goes unnoticed. Irregular brown patches in lawn become evident as larvae feed on the base of grass plants. Damage symptoms can be confused with heat and drought stress. Peck holes from birds feeding on the larvae may be noticed in the lawn.

Sod webworms prefer sunny, dry locations with very little shade during the day. Turf areas having thick thatch layers or located on steep sunny slopes and banks are commonly attacked. Larvae live in tunnels made in the thatch or just below the soil line and feed mostly at night. Tunnels are lined with silk, along with soil particles and plant debris that become webbed together. A sure way to identify sod webworm activity is to inspect closely around dead patches and look for green fecal pellets (droppings).

Sod webworm adults are moths that emerge from the soil around mid June to lay eggs. Eggs hatch about a week later and larvae begin feeding. Depending on the season, anywhere from 1 to 4 generations can occur in one year. Larvae are green or grayish brown in color with darker spots along their body and measure about ¾" long. Partially grown larvae overwinter in the soil and emerge in the spring as temperatures warm up. Damage may be much more evident when feeding occurs around July and August, as grass is not actively growing at this time.


Control / Preventions:
Flushing a mixture of 2 tbsp. soap with 2 gallons of water into the soil will usually bring larvae to the surface as they like dry conditions and are irritated by the mixture. With small areas, larvae can be raked up and removed, or this test can just be used to confirm their presence.

Healthy grass can usually outgrow damage, and helping it along with water and light fertilizer applications may quicken recovery. Control thatch layers before they build up by raking, aerating or verti-cutting once a year.
Pesticides are available, and usually provide effective control if applications are made in the late afternoon or evening when larvae are actively feeding.