When small, volcano-shaped mounds of dirt appear in a lawn or garden, they signal the presence of ground-nesting bees. Unlike the highly social honeybees or wasps that build large, centralized colonies, the bees responsible for these holes are typically solitary species. This nesting behavior is common to an estimated 70% of native bees, which make their home in the ground rather than in cavities or hives. Understanding their general demeanor can transform a homeowner’s concern into an appreciation for these beneficial pollinators.
Identifying the Main Culprits: Solitary Ground-Nesting Bees
The most frequent architects of these small burrows belong to several genera, with Mining Bees (Andrena) and Digger Bees (Anthophora) being particularly noticeable in North America. Mining bees, one of the most species-rich groups, are often medium-sized, fuzzy, and dark-colored, sometimes leading to confusion with small honeybees. Their burrow entrances are typically marked by a small, conical pile of excavated soil, known as a tumulus. Digger Bees can be mistaken for bumblebees due to their robust, fuzzy appearance. They may construct a distinctive turret or chimney at the entrance of their nest, particularly when nesting in clay soils. Another common group is the Sweat Bees (family Halictidae), which sometimes present a striking metallic green or blue sheen. While these species are solitary, they often nest in close proximity, forming large aggregations in areas with favorable soil conditions.
Anatomy of a Nest: Why Bees Dig Holes
The hole a female bee digs serves as a nursery for her offspring, beginning with a vertical main tunnel excavated into the soil. This main tunnel typically extends to a depth of 6 to 16 inches. From the main shaft, the female digs lateral tunnels, each ending in a single brood cell. Before sealing the chamber, the female provisions each brood cell with a mass of pollen and nectar, often referred to as “bee bread.” She lays a single egg on this food supply, which sustains the larva through its development. The young bee develops underground, often overwintering as an adult and emerging the following spring.
Interaction and Safety: Are Ground Bees a Threat?
Solitary ground-nesting species pose minimal threat to humans and pets, despite the number of holes and active bees. Their solitary nature means they have no large, centralized colony to defend, which removes the defensive aggression seen in social insects like yellow jackets. A female ground bee can sting, but she will only do so if she is physically handled or accidentally crushed. The bees most frequently seen flying close to the ground are the males, which lack a stinger entirely. Their active flight is purely a mating behavior and they are completely harmless. This low risk contrasts sharply with ground-nesting yellow jackets, which are social wasps that aggressively defend their substantial underground nest.
Living Alongside Ground-Nesting Bees
The activity of ground-nesting bees is typically short-lived, often peaking for just a few weeks in the early spring, after which the adults die and the tunnels are abandoned. Their presence should be welcomed, as they are highly efficient, early-season pollinators that contribute significantly to local ecosystems and the pollination of crops. Furthermore, their digging activity naturally aerates the soil, improving water and nutrient penetration. If their nesting location is problematic, simple, non-lethal methods are available to encourage relocation without the use of harsh chemicals. Since these bees prefer dry, bare, or sparsely vegetated soil, increasing the moisture content through regular watering can make the site unsuitable for nesting. Applying mulch or planting dense ground cover in bare patches also serves as a physical deterrent, making it difficult for the females to dig their tunnels.