The presence of worms in a yard often signals a healthy ecosystem, as earthworms are beneficial for soil aeration and nutrient cycling. However, when their population becomes excessive, certain species can create unsightly mounds of excrement called castings on the surface of the lawn. The goal for homeowners is generally not total eradication, which would harm soil quality, but rather reduction and management to mitigate the surface disruptions they cause. Achieving this balance requires understanding the specific problem and implementing adjustments that make the yard less attractive to large populations of surface-dwelling worms.
Identifying the Source of the Problem
The first step in yard management is accurately diagnosing whether the issue is nuisance earthworm activity or a more destructive pest. True earthworm activity is characterized by the presence of surface castings, which are small, muddy, coiled piles of soil and feces deposited by the worms, primarily during the cool, wet periods of autumn and spring. These fine, muddy mounds can dull mower blades and create patchy areas when smeared across the turf.
In contrast, the term “worm” is often mistakenly used for destructive lawn pests like white grubs, which are the larvae of beetles such as the Japanese beetle or June bug. Grub damage appears as irregular brown, dry patches of grass that can often be easily lifted like a rug because the pests have eaten the roots. A true grub is typically a pale white, C-shaped larva with a dark head and legs near its body, unlike the cylindrical, segmented body of an earthworm. Identifying the correct culprit is essential, as the control methods for nuisance earthworms and for destructive grubs are entirely different.
Cultural Adjustments to Deter Worm Activity
Long-term, non-chemical control focuses on modifying the yard environment to make it less hospitable for the specific earthworm species that create surface castings. These worms thrive in moist soil rich in organic matter, so management practices should target these conditions. Improving the yard’s drainage is a significant step, as excessive moisture is conducive to high earthworm populations and increased casting activity.
Managing the surface food supply is another effective strategy, since earthworms feed on dead and decaying plant material on the soil surface. Removing grass clippings after mowing, instead of mulching them back into the lawn, directly reduces this food source. Dethatching the lawn regularly also decreases the amount of organic material available to the worms, encouraging them to seek food deeper in the soil or reducing their overall numbers.
One highly specific method involves topdressing the lawn with sand, which can physically disrupt casting formation. Earthworms reportedly do not prefer to ingest large amounts of angular sand, and a sustained program of topdressing can suppress casting activity over time. This practice also helps the casts that are produced to dry out more quickly, making them easier to disperse and less likely to smear into the turf. Regular, light applications can gradually alter the soil profile to make the surface less appealing to casting species.
Active Intervention and Direct Reduction Methods
When cultural controls are insufficient, or when immediate action is needed, other intervention methods can temporarily reduce earthworm populations or address pest worms directly. One temporary technique involves manipulating the soil’s pH, as many casting-prone earthworm species prefer neutral to slightly alkaline soil. Applying acidifying fertilizers such as ammonium sulfate or ferrous sulfate can temporarily lower the pH, creating a short-term unfavorable environment that may reduce earthworm activity. This must be done cautiously and after proper soil testing, as excessive application of acidifying agents can harm the turf.
Manual removal can be achieved in localized areas using a technique known as “soap flushing.” This involves applying a solution of water mixed with a non-detergent soap or saponin-containing product, which irritates the earthworms and causes them to rise to the surface where they can be collected and relocated. This method is temporary and labor-intensive but provides a non-chemical means of clearing a specific area.
Addressing actual destructive pests, such as grubs, often requires chemical intervention. When dealing with these pests, specific insecticides are used, and the selection must be targeted. For instance, certain broad-spectrum insecticides, including some neonicotinoids, are toxic to earthworms and other beneficial soil organisms, even when applied for pest control. It is important to avoid the use of these broad-spectrum chemicals whenever possible to preserve beneficial soil life.
Molluscicides, often used for slugs and snails, are another category of chemical that can affect non-target earthworms. Products containing iron phosphate that is chelated with agents like EDTA can be harmful to earthworms, particularly at high concentrations. When considering chemical control for any pest, it is imperative to use products specifically labeled for the target organism, carefully following all application instructions, and recognizing the risk of collateral damage to non-target soil inhabitants.