Earthworms in Turf
Earthworms are clearly beneficial in soil for aeration, water penetration, thatch control, addition of bacteria, organic matter, and other benefits. Most soils are probably lacking in worm populations and much has been written about the benefits of worms and using them for soil improvement. However, on rare occasions and in rare situations, earthworms may become undesirable. Earthworms are also a major food source for some species of moles and are the main host for certain cluster flies.
Several of the worm species found in soils are introduced species from other countries or continents. Some of our native species of earthworms may be being displaced by the introduced species. Extremely high populations may disrupt roots or create so much upturned castings on the surface as to smother low growing blades and create a bumpy surface. Heavy rains may cause worms to be stranded on sidewalks and driveways. These worms will die quickly due to ultraviolet light exposure and drying.
There is no treatment threshold for earthworms. Threshold is determined by the tolerance level of the homeowner or lawn caretaker. Power raking or rolling of the lawn when castings are soft will usually even the soil. Grass clippings should be collected, if desired, to remove some of the food source. This, however, also reduces the return of organic matter and nutrients to the soil. Also check the pH in the soil. Earthworms prefer less acidic soils. Many earthworm problems also tend to be in less well-drained soils.
There are no chemical pesticides for homeowners registered for treatment against earthworms. Of the turf pesticides labeled for other lawn pests and still commonly used, carbaryl (Sevin) and imidacloprid (Merit) are very toxic to earthworms. A 2009 Kentucky study by Potter et al. showed tea seed pellets very effective for deterring earthworms in certain turf.
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