It is these caterpillars that damage the grass. They live near the soil surface in silken shelters covered with bits of grass. The larvae clip off grass blades close to the ground and pull them back into their silken 'runways'. After several weeks of feeding, they change into pupae, soon emerging as moths. In Kansas there are two generations of sod webworms per season.
Sod webworms can damage an area of lawn about the size of a softball.
If the infestation is severe, spots may coalesce into much larger areas. After peak moth flights, wait 7 to 14 days and look for turf damage. Realize that an abundance of moths does not necessarily mean damage. Also, look for large numbers of birds, particularly starlings, pecking holes in the lawn looking for caterpillars. Consider control measures when you find 2 to 4 webworms per square foot of sod.