The appearance of unexplained dirt mounds in a yard is a common sign of subterranean activity, indicating that an organism is excavating soil from an underground network. These visible soil piles are typically the byproduct of tunneling, feeding, or nesting behaviors carried out by animals or insects just beneath the surface. Identifying the precise source of the disturbance requires a close inspection of the mound’s size, shape, and composition. The characteristics of the soil pile provide direct clues about the creature responsible and the purpose of its underground work, which is the first step toward managing the issue.
Mammals That Tunnel
The largest and most disruptive mounds are often created by subterranean mammals, primarily moles and gophers, each leaving a distinct signature in the soil. Moles are insectivores that create two types of tunnels: shallow, temporary feeding runways that appear as raised, spongy ridges on the lawn surface, and deeper, permanent burrows. The dirt mounds, known as molehills, are volcano-shaped piles of finely sifted soil pushed up through a central hole that is difficult to spot from above. A molehill generally has a symmetrical, conical form because the animal pushes the soil upward from a vertical shaft as it digs its network in search of earthworms and grubs.
Pocket gophers, which are rodents and herbivores, build different mounds as they excavate deeper tunnels, usually 6 to 18 inches below ground, to forage for roots and tubers. Gopher mounds are typically crescent-shaped or fan-shaped, with the entrance hole plugged with soil and located off-center on one side of the mound. These mounds are generally larger than molehills, often reaching 10 to 20 inches in diameter. Gophers push soil out of a lateral tunnel leading to the surface. Unlike moles, gophers do not create raised surface ridges, focusing instead on deeper, more extensive burrow systems.
Insects and Invertebrates
Mounds that are smaller and composed of fine material are typically the result of insect or invertebrate activity. Earthworms create small, dark, granular piles of soil known as castings, which are essentially their excreted waste. These castings appear as small, coiled or granular pellets, often resembling dark coffee grounds, and are concentrated in areas with moist soil rich in organic matter. Earthworm activity is beneficial for soil health, as their castings are rich in nutrients and improve soil structure.
Ants also construct mounds, known as anthills, which serve as entrances and temperature regulators for their expansive underground nests. These mounds are usually composed of fine, loose, or sandy soil particles and often appear near sidewalks, driveways, or other paved areas that absorb heat. The appearance of small, dime-sized holes or mud towers signals the temporary emergence of periodical cicadas. These mud chimneys, or turrets, are built by the nymphs in the spring, especially in wet soil, and can stand two to six inches tall before the insects emerge. Cicada emergence holes are temporary and do not cause long-term damage to the turf.
How to Identify the Source
To accurately identify the source of the mounds, a systematic inspection of the pile’s features provides the most reliable clues. The initial step involves examining the shape of the mound: a symmetrical, cone-like shape with a central, often hidden plug points to a mole. In contrast, an asymmetrical, crescent-shaped mound with a distinct, off-center plug indicates a gopher. The texture of the soil is equally telling; molehills contain finely pulverized soil, while gopher mounds may contain larger clods or debris.
The size of the excavation is another strong indicator, as gopher mounds are significantly larger, typically 10 to 20 inches in diameter, while molehills are usually smaller and more compact. If the yard features raised, squishy ridges of soil that connect the mounds, the culprit is almost certainly a mole, as these are their shallow foraging tunnels. Conversely, if the disturbance consists of tiny, dark, granular piles, it is earthworm casting, a sign of healthy soil. Small, temporary mud towers or dime-sized holes that appear seasonally are the unmistakable exit points of emerging cicada nymphs.
Practical Management Solutions
Effective management begins after correctly identifying the animal responsible, as a solution for one pest is often ineffective for another.
Gopher Management
For gophers, the most practical control method for small, localized infestations is trapping, which involves setting specialized traps directly into the main, active tunnel, often requiring two traps placed in opposite directions. Exclusion barriers, such as hardware cloth or wire mesh buried at least 18 to 24 inches deep, can protect high-value areas like garden beds from gopher entry. For extensive gopher problems, toxic baits or professional fumigation may be necessary.
Mole Management
Mole control focuses on either elimination or habitat modification, since their primary food source is underground invertebrates. Trapping is considered the most successful elimination method, especially when placed in active surface runways during the spring and fall when moles are foraging closer to the surface. Homeowners can also use synthetic worm or gel baits containing bromethalin, which are designed to mimic a mole’s natural food source. Reducing the food supply by controlling grubs and avoiding overwatering the lawn can also make the area less appealing to moles.
Insect Management
If the mounds are small anthills, the most effective strategy is the use of slow-acting insecticidal baits. Foraging workers carry these baits back to the colony to eliminate the queen and the entire nest. These baits come in granular or liquid forms and should be placed near the mound or along active trails, not directly on top, to ensure the ants take the poison back to the colony. Earthworm castings are a sign of a healthy ecosystem and require no management, though improving soil drainage can naturally reduce their surface activity if the appearance is a concern.