Lawn & Turf Tips:  Sod Webworms

Sod Webworm

Sod webworms are caterpillars who damage lawns by feeding upon the grass blades.

The first signs of sod webworm injury are small patches of yellow or brown grass on the lawn. If the infestation worsens, the brown patches join together, producing large, irregular areas of dying grass. The patches are noticeable in sunny areas, while weeds and shady areas of lawn remain green.

Lawn damage appears in dry weather and is most severe by August. Although serious infestations are not common, there are a number of strategies you can use to lessen the injury if one occurs.

Make sure you identify the pest as sod webworm and assess how much damage it may do before

you decide to apply a control. Many of the moths and caterpillars on your property are part of a healthy ecosystem, providing food to birds and other predators.

Temporary Control Measures

Physical Control

Rake up thick thatch and destroy it. This reduces the number of webworm burrows. Try drenching infested patches with a soap solution. As webworm wriggle to the surface, collect them with a leaf rake and drop them into a bucket of soapy water.

Biological Control

The microbial product Bt (Bacillus thuringiensis) can be used to control webworm. This naturally occurring bacterium acts as a stomach poison when it is eaten by caterpillars, but does not damage other insects. As it has no effect on the moth stage of the sod webworm, do not apply Bt when you see large numbers of moths. Wait at least two weeks after observing moths. The eggs they will have laid should have hatched, and the webworms will be small and easily killed. The timing is important as Bt is not effective on larger webworms. Remember that Bt will kill all caterpillars, including the caterpillars of butterflies, so avoid spraying it on any plants where pest control is unnecessary.

Parasitic soil nematodes are effective against webworms. These microscopic nematodes are sprayed on irrigated turf and lightly watered in. There are two types of parasitic nematodes available commercially. One type, Heterorhabditis spp., is very aggressive, works in cool or warm soils, and inhabits soil down to 15 cm. The other type, Steinernema spp., is less aggressive, works best in warm soils, and stays in the top 5 cm of soil. Products containing parasitic nematodes include Biosafe and Scanmask (available locally) and Root Rescue (available from Better Yield Insects, 1302 Hwy #2, R.R.3, Belle River, Ont. N0R 1A0). Nematode control will be slower than a chemical control, as nematodes must find and kill the caterpillars. Nematodes may not overwinter, so the product must be re-applied if you want control of webworm the following season. Make sure you follow the instructions for application and storage given on the box.

Chemical Control

Chemical insecticide formulations containing carbaryl and chlorpyrifos as active ingredients can also used against sod webworm. These pesticides will kill beneficial insects as well as webworms, so they should only be used in the infested areas. Obtain more information about the proper application and storage of pesticides when purchasing them.

Adult Sod Webworm

Preventing Outbreaks in the Future

Sod webworm populations are reduced by natural enemies including certain microorganisms, parasitic wasps and flies, and predators such as beetles, ants and birds. A good long term strategy for pest control is to maintain an area where these webworm enemies can find the mixture of foods they require and shelter. Avoid repeated use of insecticides which can suppress natural enemies. Leave room on your property for nectar sources such as daisies, Queen Anne's lace, black-eyed Susan, dill, caraway and fennel.

Hot dry areas of lawn or areas with a thick accumulation of thatch provide a good habitat for sod webworm. Topdressing with organic matter, soil aeration and dethatching are techniques used to correct these problems. See the fact sheet "Managing Insects and Diseases on Home Lawns" for more information.

Summary of Management Stratageies for Sod Webworm

• Practice lawn improvement. Topdress with compost, fertilize with a moderate amount of slow release nitrogen, lime, irrigate and remove thatch when it is necessary to do so.
• Don't stress the grass. Leave it about 6-7.5 cm high in midsummer and dry spells.
• Check for webworm infestation when patches of damage appear.
• Try using parasitic soil nematodes Treat large infestations with the least toxic pesticide. Choose Bt before using the more toxic pesticides.Treat only where pests are present, and follow the instructions on the label.
• Consider overseeding or replacing the lawn with resistant varieties of turfgrass. The best time to do this is in early fall.