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Problem: Bluegrass Sod Webworm - Parapediasia teterrella
Description: The Bluegrass Sod Webworm is one of the most destructive pests of turf in Kansas. Infestations often damage bluegrass or mixtures of bluegrass and tall fescue. This webworm also attacks bentgrass greens on golf courses, can sometimes be of concern in plantings of pure tall fescue, and has been reported occasionally on bermudagrass. In most instances, bermudagrass develops vigorously enough that serious problems are rare.
Sod webworms are the caterpillars of lawn moths. The moths are small (1/2 inch long) and whitish-gray. They clasp or roll their wings close to their bodies when resting and have mouthparts projecting forward from the head like a snout. The moths are usually noticed when flushed out by a lawn mower or people walking. When disturbed, they fly in a jerky zig zag manner and quickly return to the grass to hide. Around dusk, they may be seen flying a few feet above the grass and dropping their eggs. In a few days, these eggs and others laid on the lower parts of the grass stems hatch into small caterpillars. It is the caterpillar that damages the grass.
The caterpillars generally have dark heads and rows of light-brown spots arranged in rings around the greenish-gray bodies. They live near the soil surface in silken shelters covered with bits of grass, essentially webbing the thatch into a mat. The larvae clip off grass blades close to the round and pull them back into their silken 'runways'. After several weeks of feeding, they reach maturity (now about 3/4 inch long), change into pupae (the resting stage), and soon emerge as moths. Two generations occur each year. The first generation adults appear in June and the second-generation adults in late July and August. Historically, the second, generation larvae have caused the greatest amount of damage in Kansas.
The close clipping of grass blades by the sod webworm larvae does not kill the grass directly but exposes the crown to the hot, beating sun; thus, the injury is much worse during hot, dry weather. As the caterpillar grows, it can damage an area of lawn about the size of a softball. If the infestation is severe, the spots may develop into much larger areas.
If lawn moths are abundant, watch for signs of caterpillars feeding during the next 7 to 14 days. Realize though, that an abundance of moths does not automatically mean that damaging larval populations will necessarily develop. Also indicative of a sod-webworm infestation are large numbers of birds, particularly starlings, pecking holes in the lawn looking for caterpillars.
Recommendations: One method used to drive the webworm larvae (and many other above, ground feeders) to the surface for counting involves the application of detergent or pyrethrum solution to the lawn. Mix 1 quart of water and one tablespoon of powdered detergent (Tide or a similar product), then sprinkle over 4 square feet of turf. One teaspoon of pyrethrum (if available) may be mixed with the water instead of I tablespoon of detergent if you desire. Within 10 minutes, the aboveground insects should crawl to the surface of the grass as they try to escape the irritating substance. They then can be collected and counted. Use this technique in several places to get an average count of the insects present.
Control measures should be considered when you find 2 to 4 webworms per square foot of sod. A healthy lawn may tolerate more webworms than a poor lawn because the grass has a much better chance of recovering from feeding activity. If possible, mowing the lawn before applying the insecticide will minimize the amount of post-spray contact necessary. Using a grass catcher to catch the clippings will also reduce the amount of insecticide that is intercepted.
One method for timing spray treatments is to apply the insecticide 7 to 10 days after moth numbers peak. Occasionally, treatment is necessary to control the first generation (sometime in June). Often, however, treatment for the first generation larvae can be avoided altogether. Realize that high moth numbers do not necessarily indicate a serious larval problem must develop. Be sure to use enough water to thoroughly moisten grass and thatch, but not so much as to encourage runoff. If treatment is attempted and a post-spray irrigation becomes necessary, only use a light sprinkling. The idea is to only wash the insecticide down around the crown of the plants. Do not resprinkle the lawn for at least 2 more days.
Even if treatments were made for the first generation, another application will often be necessary to control the second generation. This late July or August treatment can be needed because many of the chemicals applied for the first generation will work from 1 to 4 weeks. Also, it is likely that moths from untreated lawns in the area will reinfest the lawn by laying eggs in it for the second generation. A post-treatment evaluation should always be conducted to determine whether the insecticide eliminated the problem. Typically, aboveground insects like webworms should be affected within 2 to 4 days of treatment.
Children and pets must be prevented from entering the treated areas, at least until all
sprays have dried (see labels for other cautions). Clean out all equipment after use and
return all insecticides to areas where children cannot get to them.
References:
1. Lawn and Turf Insect Management Part II. Webworm, Cutworm, Armyworm, Chinch Bug, and Ant, K-State Research and Extension Entomology Publication 480
2. Turfgrass Insects of the United States and Canada, Cornell University Press, pg. 70 - 90
Last Update: 03/28/2003
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