Earthworms generally indicate a healthy ecosystem, naturally aerating turf and recycling nutrients. However, species like the nightcrawler (Lumbricus terrestris) can become a nuisance when overpopulated in manicured lawns and gardens. Their activity creates surface imperfections that disrupt the uniformity and playability of fine turf. Control methods are sought when the resulting damage outweighs the ecological benefits.
Why Earthworm Control Becomes Necessary
The primary reason for seeking earthworm control is the appearance of surface casts—small, mud-like piles of excreted soil and organic matter. Species like Lumbricus terrestris and Aporrectodea longa deposit these casts on the lawn surface, especially during cool, moist periods. These piles create an uneven, bumpy texture that is visually unappealing and disrupts the smooth surface of the lawn. When mowed over, the casts smear across the grass, which can smother turf blades and provide a seedbed for weeds.
The abrasive nature of the smeared soil quickly dulls mower blades, and the resulting unevenness can lead to turf scalping. In container gardening, excessive tunneling can sometimes compromise soil structure, leading to drainage issues or nutrient loss. The aesthetic damage and maintenance complications caused by high populations of casting species drive the need for control measures.
Environmental and Cultural Discouragement Techniques
Managing earthworm populations begins by modifying the environment to encourage worms to burrow deeper. Earthworms prefer moist, neutral to alkaline soil conditions and a readily available food source. Reducing surface moisture by adjusting irrigation schedules to be deep and infrequent helps limit their activity at the surface.
Removing their primary food source is another effective cultural practice. This involves routinely collecting grass clippings and fallen leaves rather than mulching them into the lawn. Reducing the use of organic fertilizers, which serve as a rich food source, also decreases the incentive for surface feeding.
Since many casting earthworm species prefer a soil pH above 6.0, applying acidifying agents can help deter them. Incorporating products like ammonium sulfate or iron sulfate into a fertilization program can slightly lower the soil pH. This acidity manipulation makes the environment less hospitable, causing them to move deeper into the soil profile. For immediate aesthetic relief, brushing or switching the casts when completely dry disperses them, preventing smearing and incorporating the nutrient-rich material back into the soil.
Direct Methods for Lethal Control
Lethal control involves using chemical or natural agents designed to kill earthworms or force them to the surface for removal. Note that in many regions, including the United States, no pesticide is currently registered specifically for earthworm control in residential lawns. Any chemical effect is often a secondary, non-target consequence of products applied for other pests.
Certain products, like the insecticide carbaryl (Sevin) or the fungicide thiophanate-methyl, suppress earthworm populations significantly when applied at rates labeled for other turf pests. These compounds are toxic to earthworms and can provide an 85 to 95% population reduction when watered into the soil. Another chemical option involves slug and snail baits containing iron phosphate chelated with EDTA (ethylenediaminetetraacetic acid). While standard iron phosphate is non-toxic, the chelated form can be acutely toxic, causing mortality and reducing surface foraging activity.
For a physical approach, natural expellants can force worms to the surface for manual collection and disposal. A solution of ground yellow mustard powder and water irritates the earthworm’s skin, compelling them to quickly emerge. Similarly, natural products containing saponins, such as tea seed meal, act as powerful surfactants that disrupt the worm’s protective mucous layer, leading to desiccation. These expellants allow for physical removal but should be used carefully, as they can sometimes irritate the turf itself.
Post-Treatment Safety and Lawn Recovery
Following the application of any control measure, especially chemical ones, immediate safety precautions are necessary. If an irritant or chemical was used, adequate irrigation afterward helps flush residues deeper into the soil and minimizes surface exposure. Chelated iron phosphate baits can be toxic to domestic animals if ingested in large quantities, so the area must be secured until the product is dispersed.
Suppressing earthworm populations can temporarily disrupt the natural aeration and nutrient cycling they provide. To aid in lawn recovery, implement a soil-testing program to ensure nutrient levels remain balanced. Overseeding and topdressing with a sand-based material can help correct any remaining surface unevenness. Promoting vigorous turf growth through proper fertilization and deep, infrequent watering is the best long-term strategy for lawn health, as a dense turf canopy naturally discourages earthworm surface activity.