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Lawn Insects

Sod Webworm

Sod webworms can be highly destructive pests. The damaging stage is the caterpillar or larval stage. Typical sod webworm caterpillars have a dark head and light brown or gray bodies with dark spotting. Webworm-infested thatch has silken tunnels produced by the caterpillars. During the day, they hide within these webbed tubes, and at night they move out a short distance to feed. Sod webworms are typically about 1inch long when fully grown.

The moths are about half an 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 general, sod webworms have 2 to 3 life cycles a year. The webworms overwinter as young larvae a few centimeters below the soil line. During early Spring, the larvae feed on upper root systems, stems, and blades of grass. They build protective silken webs, usually on steep slopes and in sunny areas, where they feed and develop. In early May, they pupate in underground cocoons made of silk, bits of plants, and soil. About 2 weeks later, adults emerge. Larvae feed nocturnally for 3 to 4 weeks. Beginning in late May or June, moth flights may occur until October. The moths are erratic and weak flyers, and only live for a few days, feeding solely on dew. They are active at dusk, resting near the ground in the grass during the day.

The eggs, which are deposited indiscriminately over the grass, hatch in 7 to 10 days. Young larvae immediately begin to feed and construct their silken tunnels.

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Sod Webworm Closeup

The most severe damage occurs in July and August when the grass is not growing rapidly. During this hot weather, the larvae feed at night or on cloudy days. The sod webworm feeding exposes the crown of the grass to the hot 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.

To control potential damage caused by sod webworms, it is important to ensure correct mowing and watering, adequate fertility, a balanced soil pH and low soil compaction to encourage a robust lawn that will be able to tolerate some insect feeding. Overseeding with a mix of entophytic perennial ryegrass is another excellent strategy to prevent future sod webworm damage, as it contains a naturally occurring fungi that deters insect feeding on the leaves.

There are a variety of control strategies to deal with sod webworms, including the use of predatory nematodes, as well as conventional insect controls. If you think you have a sod webworm problem, talk to your local Nutri-Lawn and we will visit you to diagnose the problem and recommend the best control strategy.

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