The most serious injury will occur from mid-June until September. They will attack most lawn grasses. Some lawn diseases cause brown spots that appear much like Webworm injury. The worms can be found near the edges of brown spots. Grass will be chewed off at, or just above, the soil surface.
Fresh clippings are usually present.
If 15 or more larvae are found per square yard of area, control is probably necessary. If no evidence of feeding or larvae is found, the problem is due to some other cause. Mow the lawn before application and remove clippings. Water lawn thoroughly if needed. Apply an insecticide in the late afternoon or early evening.