Confirming Sod Webworm Activity
The surest way to tell if you have sod webworms is to find a suspected area of infestation (brown patches). Get down on your hands and knees, take your two index fingers and part the grass blades in the area between dead and live grass and look for an area with small green pellets. The pellets, called frass, are the excrement of the larvae and indicate that a larva is close by. Sod webworm adults are about 3/4-inch long, cigar-shaped and buff-colored moths. They typically roll their wings around the body when resting on a grass blade. Two small snout-like projections are visible at the front of the head.
The adult moths fly mainly in late June and again in mid-August though some species may be present any time during the summer. Seeing these moths fly up while mowing or walking around the lawn does not confirm that damage is, or will be done by the larvae. The adult moths can fly considerable distances and may be coming from other infested areas.
If you still suspect sod webworm activity but are unable to find the larvae or their frass, use a soap disclosing drench. Simply mix up two gallons of tap water with two tablespoons of liquid dishwashing detergent. Sprinkle this mix over a one square yard of the affected turf. Within a couple of minutes, the flesh-colored, spotted larvae will wriggle to the surface. If you get 10 to 15 larvae in a one square yard of turf, treatment is warranted.