"The Good, the Bad, and the Ugly" can certainly be descriptive of earthworms (nightcrawlers); they are both beneficial and pests. First the good news. Earthworms are excellent aerators of the soil, creating up to 1,200 tunnels per square yard of soil, to depths of six feet or more. The tunnels, which are 1/8 to 1/4 inch in diameter, also serve as channels for plant roots and microorganisms, resulting in deeper-rooted plants and distribution throughout the soil profile of the beneficial oxygen-using microorganisms that are crucial to building good soil. Earthworms also mine minerals from the subsoil, carrying them upward into the topsoil in a natural tilling operation.
Earthworms also break down thatch and the other the raw materials of organic matter, and spread it evenly throughout the top 12 inches of soil, further improving the soil. Millions of beneficial bacteria that break down thatch are produced in the gut of earthworms. These bacteria, along with a superior fertilizer produced by earthworms, are put into the soil with earthworm "castings". This top quality fertilizer contains nitrogen, phosphorus, potassium and many micronutrients in a form all plants can use. Earthworm castings contain five times more nitrogen, seven times more phosphorus, 11 times more potassium, and 1,000 times more beneficial bacteria than the material contained before the earthworm ingested it. In a 10 x 20 foot garden with only five earthworms per cubic foot of soil, over 35 pounds of this superior fertilizer is produced by the earthworms. In healthy soil with nightcrawler populations of 25 per cubic foot of soil, more than 170 pounds of the highest grade fertilizer will be produced in a year in a 10 x 20 foot garden. This is more than 18.5 tons per acre per year. Gardening supply companies sell earthworm castings as fertilizer for about $12-$25 per pound. Earthworms can also help change alkaline or acid soil toward a desirable neutral pH over time.
Certain yard and garden chemicals, such as diazinon and chlorpyrifos (Dursban), and heavy applications of synthetic fertilizers with high nutrient content can kill earthworms and beneficial microorganisms, slowing decomposition of plant material, often leading to thatch buildup and other problems. On sites where earthworms are acceptable, soil can be improved and thatch buildup can be reduced by protecting the soil organisms, including earthworms. If you wish to protect earthworms and beneficial microorganisms, apply pesticides only on the areas that are actually infested by pest organisms rather than making blanket applications over the entire area. Using slow-release, urea-based or organic fertilizers, divided into several smaller applications, can also be helpful.
Now for the bad news, which you probably already know. Earthworms are also pests in that they create mounding of the soil surface, a trade-off for the effective aerating and fertilizing service. There are no labeled insecticides readily available to those homeowners who wish to eliminate earthworms. One method that can help to some extent is to water your lawn deeply and infrequently, allowing the top inch of soil to dry out before watering again. This encourages the earthworms to remain deeper in the soil. This method of watering is also more beneficial to plants because it results in deep root systems. Thatching rakes or power rakes, adjusted so the teeth will drag through the earthworm mounds but not down into the turf grass crowns, can also help level the mounds to some extent. Do this raking of mounds when the soil is slightly damp, but not wet. Then use light weight rollers to press mounds down. Don't use heavier weights that require machinery to pull them because the soil could be compacted, which leads to another entire set of problems.
References:
Handbook of Integrated Pest Management for Turf and Ornamentals. 1994. A.R. Leslie, ed. U.S. Environmental Protection Agency. Lewis Pub., Boca Raton, FL. 660 pp.
Mycorrhizae in Sustainable Agriculture. 1992. G. Bethlenfalvay and R. Linderman, Eds. ASA Special Publication No. 54. Americon Society of Agronomy, Inc., Madison, WI. 124 pp.
Soil Biology: Effects on Soil Quality. 1994. Hayfield, J. and B. Stewart, Eds. Lewis Publishers, CRC Press, Boca Raton, FL 33431. 169 pp.
Watschke, T., P. Dernoeden, and D. Shetlar. 1995. Managing Turfgrass Pests. Lewis Publishers, Boca Raton, FL 33431. 361 pp.
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997
Categories: Earthworms, Nightcrawlers, Control
Date: 11/26/2001