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revised June 24, 2010

Least-Toxic Control of Insects

Least toxic control of Sod Webworms


Sod Webworm

Source: D. Bree,
Moth Photographers Group
Sod Webworm

Source: D. Bree,
Moth Photographers Group


Adult webworms:

  • Are buff-coloured moths.
  • Are 12-19 mm (1/2 to 3/4 inches) long.
  • Sometimes have a small, dark line on the top of each wing cover.
  • Have two small finger-like projections that are visible at the front of the head and look like a snout (hence the name 'snoutmoth').
  • Have a tubular shape when at rest and the wings wrap around the body.
  • Will fly upward when disturbed by movement.
  • Will move a short distance before darting back into the grass.


Webworms actually damage grass at the caterpillar stage.

Webworm caterpillars:

  • Are only 3 mm (1/8 inch) long at the beginning of their development.
  • Reach up to 25 mm (1 inch) in length when fully-grown.
  • Vary in colour depending on species and plant source.
  • Are mostly greenish, grey, or brown and usually have dark spots scattered along the body.
  • Have light brown with some dark markings on the head capsule in later stages.

Behaviour & effect

Several species of webworms attack grass. All of them spin threads of silk as they move, webbing leaves and soil particles together to form horizontal silk tubes in the thatch.


Most species spend the winter as large caterpillars in 'hibernacula': tent-like structures that larvae weave to provide extra protection a few inches below the surface of the lawn.

When the caterpillars become active again in the spring, they feed for a short period before pupating and emerging as moths. Female moths hover just above the surface, laying individual eggs as they fly.

The eggs hatch small caterpillars about a week later. The caterpillars begin feeding almost immediately on the leaf tissue above the thatch. Caterpillars go through six to ten moults as they feed and grow.

Since there are several webworm species and weather conditions vary year to year, webworms can appear in different stages of development. Eggs, small caterpillars, large caterpillars, pupae, and adults can all be found at the same time in the grass in July and August.


Sod webworms feed on a variety of grasses. They often thrive on lush, healthy grass, even though this type of grass is most able to withstand insect infestations.

Webworms commonly attack Kentucky bluegrass, perennial ryegrass, fine fescue, and bentgrass maintained at a variety of mowing heights.


Webworm damage starts as small yellow or brown patches in the leaf blades in lawn areas. As the caterpillars feed and grow, the patches gradually increase in size.

Caterpillars are nocturnal, so you won't notice them on the surface during the day. However, if you gently pull the grass apart, you'll notice tunnels or burrows in the thatch. Often these burrows will be lined with green, pellet-like matter. This is caterpillar excrement, also known as 'frass.'

By midsummer, large sections of a lawn might be destroyed by the sod webworm caterpillars, which prefer sunny areas. Webworm damage is particularly severe in dry conditions, partly because the grass is less able to recover from caterpillar feeding.

Damage is usually most apparent in July and August, when temperatures are highest and cool season grasses are not as robust. Affected areas recover slowly from webworm feeding and often are overrun by weeds.

Detecting Sod Webworms

Think you might have webworm caterpillars in your lawn? Follow these steps to know for sure:

  • Step 1 - Drop one or two tablespoons of lemon-scented dish detergent in one or two gallons of water
  • Step 2 - Pour the soapy solution over an area which is about two feet on each side. (The caterpillars will be irritated by the solution and will wriggle out of the thatch and up to the surface, where you can count them.)

Most caterpillars will respond to the soapy solution within two to five minutes. Inspect the edge of the affected area because this is where the caterpillar will be found.

This technique is particularly helpful for determining what stages (sizes) of caterpillars you have. Most of the caterpillars can be found on the edges of the affected patches, so be sure to pour the soapy flush and inspect there.


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Revised: June 24, 2010


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