Distant: Small (0.5 to 1.0 inch diameter) mounds of pellets of soil or flattened mounds that may cover and smother short-mowed turfgrass and dull mower blades (Image B).
Up close: When turf is growing on clay or heavy silt soil the earthworm castings (0.5 to 1.0 inch diameter) are heavy in consistency and can damage turf when they are pressed down by mowers, carts or foot traffic. The castings piles are of much lighter material if on sandy soil and will fall around the grass blades when they dry out or are hit by irrigation water.
When damage would be found: Earthworms make mounds when the soil is saturated with water. This is most likely to happen in early spring and possibly again in the fall. Earthworm mounds can appear suddenly in April or early May if the soil is wet, and may disappear quickly when the soil dries.
Host: Earthworms will inhabit any type of soil that grass is grown in but are only a problem on golf course tees, greens and fairways (cut at < 0.6 inches).
Site: Mostly a problem on golf course tees, greens and fairways when the turf is grown on clay or heavy silt soils. Earthworm mounds made of sand break apart easily and are not usually a problem.
Description of damaging stage: Earthworms of all sizes make castings.
Sampling: In short-mowed turf the castings may look like mud or clay bubbled up from the ground, making little volcano-like castings. Earthworm castings are not visible in healthy lawns mowed at a height of 3 inches or more. In closely-cropped turf like golf course tees and greens, when castings from heavy soil are stepped on or run-over they may form a flattened patch of mud about the size of a quarter. In contrast, ant mounds consist of small soil or sand particles that tend to crumble.
Earthworms are desirable in lawns because they mix and aerate the soil. Fortunately most insecticides currently on the market today have very little, or no effect on earthworms. One exception is carabaryl, which is toxic to earthworms for 3-4 weeks after an application at the rate used for grubs. Maintain a healthy turf so it will quickly heal from damage caused by earthworm castings. Damage in the spring will soon disappear as the temperatures warm in late April and May. There is no reason to apply a pesticide to control earthworms except in a few cases where playability of golf course greens are being compromised. Fungicide products containing thiophanate-methyl will reduce castings caused by earthworms.