Sod webworms are the larvae or caterpillar stage of a moth. The worms grow from 3/4 to 1 inch in length.They have dark, shiny brown heads but the colour of their bodies vary from green to brown, to light brown and gray. They have parrallel rows of spots along their bodies where fuzzy hairs protrude.
The adult moths are a sandy colour and can be seen flying at low level altitudes, just above the grass, in a zig zag pattern. They can often be seen dodging for cover or fleeing as you mow the grass. You'll see them at night hovering around porch lights.
The adults themselves are relatively harmless. However, they lay eggs in the grass that hatch after 6 to 10 days and it's those caterpillars that do the damage. They feed on grass blades and build silk tunnels in your thatch near the surface of the soil.
They are mostly found in the United States and Southern Canada and are particularlt fond of Kentucky bluegrass, bentgrass, tall and fine fescue and zoysia grass. In southern and western states where weather is warmer you can find up to three generations in one growing season. This means that at any one time you can find eggs, larvae and adult moths.
Late in the spring you can find small brown patches (1-2 inches) in your lawn. By mid-summer these patches become large dead patches. Highly infested areas gets wrose by July and August.
The webworm cuts the grass blade off just above the surface of ther thatch and drags the blade to its silk tunnel to feed. It doesn't necessarily kill the grass, just chews it off.
Early detection is key. Break apart thatch in those damages areas of your lawn and look for small silken tunnels or tubes. In mid spring you can typically see moths in the early morning or late evening doing their low level flying in those zig zag patterns. When you see the moths you can expect a caterpillar problem in a few weeks.
Controlling them is much easier when you have a bird friendly environment. Most robins or a few other birds will feed on the larvae.
For heavily infested lawns you may need to intervene.
Take a teaspoon of liquid detergent, dish or laundry, and mix it with a quart of water. Then remove both ends of a tin can to give you a tin tube. Push one end of the tin tube into the ground (in a damaged area), about 2-3 inches. Then pour about 1 cup of the solution into the can. Wait about 5-10 minutes and watch for anything climbing or floating to the surface. You may be surprised at what you see coming up.
If none appear then your problem is not webworms. If less than ten per square foot appear, there's not too much to worry about. Natural predators can solve most of your problems along with proper Mowing Techniques.
Insecticidal soap drench- This type of soap is specifically designed for lawn pests. It is easily and safely sprayed on infested lawns.
Bacillus Thuringiensis or Bt is a bacteria that preys exclusively on caterpillars. They sicken then die.
Predatory nematodes are a biological control. These tiny micro-organisms feed on caterpillars and grubs. You spray nematodes in a water solution directly on the grass and into the thatch where they seek out the webworms. This is the most simplest and safe method of control.
Broad-spectrum insecticides may be necessary with a heavy infestation. This is only recommended as a last resort as this procedure will kill all beneficial insects as well. It is best applied with the help of a professional.
Long tem prevention is the best approach to prevention. Proper lawn maitenance, mowing height and techniques, watering and thatch control all collectively will prevent any major problems from these pests.