A hole in the ground often sparks curiosity, raising the question of whether it is the work of a pest or a natural phenomenon. Holes on a property can be attributed to sources ranging from tiny insects to large burrowing mammals and geological processes. Understanding the differences in size, shape, and surrounding debris is the first step in identifying the responsible agent. This knowledge is useful for managing potential pests and recognizing when a hole signals a larger, non-biological issue.
Burrowing Mammals and Their Signatures
Small mammals create distinct subterranean structures identified by their entry points and associated spoil mounds. Moles, which are insectivores, spend nearly their entire lives underground, creating deep, permanent runways and shallow, temporary foraging tunnels that appear as raised ridges in the lawn. The most visible sign of a mole is the volcano-shaped molehill, a circular mound of finely pulverized soil. This mound lacks a clear, open entrance because the mole pushes the dirt up from below and plugs the center hole.
In contrast, pocket gophers are herbivores that dig extensive tunnel systems, resulting in larger, crescent- or fan-shaped mounds of dirt. The opening to a gopher burrow is typically found off to one side of the mound and is usually plugged with soil. The soil in a gopher mound is often fine and loose, distinguishing it from the clumpier soil found in mole mounds.
Voles, sometimes called meadow mice, are herbivores that use both above-ground runways and small, open burrow entrances. Their holes are typically small, only one to two inches in diameter, and lack the large spoil mounds seen with moles and gophers. Voles are identified by their multiple small, open entrances and the presence of clipped, narrow runways through the grass.
Other common surface diggers, such as skunks and raccoons, create shallow, temporary holes while foraging for insects. Skunks tend to leave small, conical holes, usually one to three inches wide, as they root for grubs, often making the lawn look aerated. Raccoons often cause more disruptive damage by using their paws to pull back and flip over sections of sod to expose insects, leaving behind larger, torn patches of turf.
Insect and Invertebrate Excavations
The smallest holes are frequently the work of solitary insects or colony-forming invertebrates using the soil for nesting or shelter. Cicada killer wasps, one of the largest wasp species in North America, dig burrows in sandy soil, leaving a distinctive, perfectly round entrance hole. This opening, about a half-inch to an inch wide, is often surrounded by a small, U-shaped pile of excavated dirt, distinguishing it from mammal tunnels.
Ants create holes leading to complex, subterranean colonies, but the entrance appearance varies by species. Common garden ants often create an anthill by depositing fine, granular soil around a central entrance hole. Fire ants, however, construct large, irregular, dome-shaped mounds of loose soil that typically lack a central, visible entrance, as they use underground tunnels for access.
Smaller holes can be the result of beetle larvae or spiders. Larval beetles, or grubs, leave small exit holes when they emerge from the soil after pupation. Certain spider species, such as trapdoor spiders, construct silk-lined burrows with a hinged lid. These small holes are generally less than half an inch in diameter and are often singular, unlike the clusters created by colonial insects or voles.
Natural Forces and Geological Formations
Not all ground holes are caused by living organisms; natural forces create voids through the movement of water and the decay of organic material. Erosion from heavy rainfall or subsurface water flow can wash away fine soil particles, creating irregular channels, washouts, or small, uneven depressions, especially on slopes. This process leads to subsurface voids that may eventually collapse to form a hole.
A more dramatic natural hole is the sinkhole, which occurs in areas with soluble bedrock, such as limestone or gypsum. Slightly acidic groundwater slowly dissolves this rock, creating subterranean cavities. A sinkhole forms when the overlying material collapses into this underground void, resulting in a circular depression that can range from a few feet to hundreds of feet in diameter.
The decomposition of large tree roots after removal is another common cause of ground holes over time. As the organic material decays, it leaves behind a void in the soil that the surrounding earth settles into. This subsidence results in a small, gradual depression or a hole that can appear several years after the tree removal, often requiring repeated filling.
Identifying the Culprit: Key Clues for Investigation
Determining the source of a hole requires examining three main characteristics: the dimensions of the opening, the type of debris surrounding it, and the hole’s location. The diameter is a strong indicator; openings less than one inch suggest insects or small voles, while holes exceeding two inches are likely entrances for larger mammals or geological features. The shape also provides clues: a perfectly round opening suggests a purpose-built insect burrow, while an irregular opening may indicate a shallow foraging hole made by a skunk or raccoon.
The presence and form of a spoil mound are equally informative. A conical mound of loose dirt indicates a mole, while a fan-shaped mound with a plugged entrance is the signature of a gopher. The complete absence of a spoil mound, especially with a small, open hole, often points to a vole or natural settlement from root decay. Insect holes, such as those of the cicada killer wasp, are often accompanied by a small, crescent-shaped pile of soil next to the entrance.
Observing the hole’s context and surroundings can narrow the possibilities significantly. Holes located near foundations, under dense cover, or along walkways are common vole habitats. Conversely, open, sunny areas of sandy soil are preferred by cicada killer wasps. Large, wide depressions or irregular sinkages, especially in areas with known limestone geology, suggest an underlying natural or erosional process rather than a biological excavation.