Most sweat bees nest underground, digging burrows into bare soil that gets direct sunlight. A smaller number of species nest above ground in rotting wood. If you’ve spotted tiny bees hovering low over a patch of bare dirt in your yard, you’re likely looking at an active nesting site.
Ground Nesting in Bare Soil
The vast majority of sweat bees (family Halictidae) are ground nesters. They dig vertical or angled tunnels into exposed, sun-warmed soil, creating a small entrance hole roughly 1/4 inch in diameter. You might notice a tiny mound of excavated dirt surrounding the opening, similar to what an ant colony produces but with a single clean hole at the center.
These bees strongly prefer bare patches of ground rather than areas covered by thick grass or mulch. Sunny spots are essential. Shaded, densely vegetated areas almost never host sweat bee nests. If you have a thin patch in your lawn, a garden path edge, or a dry slope with sparse plant cover, those are prime real estate for nesting sweat bees.
What Kind of Soil They Prefer
Sandy and sandy loam soils are the most common nesting substrates. Research on one widespread species, Halictus rubicundus, found that nesting sites had a sand content between 35 and 100%, with very little silt or clay (under 5%), the remainder being gravel. A broader U.S. survey of 32 ground-nesting bee species found sand percentages ranging from 34 to 94% at nest locations.
The reason is practical: sandier soil drains well and is easier to excavate, but still holds enough structure that tunnels don’t collapse. Heavy clay soils are too hard to dig through, and pure sand can’t support stable burrows. That sweet spot of mostly sand with a small amount of finer particles gives the bees workable, well-drained ground. Moisture matters too. Bees often begin digging when soil is slightly damp and easier to move, which is why nest construction frequently picks up after rain.
What the Tunnels Look Like Inside
A sweat bee nest is more complex than a simple hole in the ground. The entrance leads to a main tunnel that descends at an angle, sometimes bending partway down. One well-preserved fossil nest (which matched the architecture of modern sweat bee nests almost exactly) showed a tunnel that entered the ground at a 35-degree angle, bent after about 17 centimeters, then continued for another 35 centimeters deeper. The tunnel was roughly 12 millimeters wide.
Off this main shaft, the bee constructs individual brood cells, each one tear-shaped and roughly horizontal. These cells are where eggs are laid, each provisioned with a ball of pollen and nectar for the developing larva. The cells tend to cluster toward the bottom of the tunnel and are mostly arranged along one plane, though some nests have cells branching off in multiple directions. A single tunnel can hold 18 or more brood cells. Some nests reach depths of over three feet, though the brood cells typically occupy the lower portion of the burrow.
Species That Nest in Rotting Wood
Not all sweat bees dig into soil. A notable exception is Augochlora pura, a brilliant metallic green sweat bee common in eastern North America. This species nests inside decaying logs, stumps, and fallen branches. Another wood-nesting species is Lasioglossum coeruleum. Both normally inhabit decomposed wood, though laboratory studies have shown they can also construct functional nests in soil when wood isn’t available.
The internal layout of wood nests is irregular, shaped by the texture and grain of the decaying material, but the basic structure (a tunnel with brood cells branching off it) resembles ground nests. Rotting wood also serves as overwintering habitat for some sweat bee species, giving them insulation through cold months even if they nested in the ground during the active season.
Solitary Nesters vs. Shared Nests
Sweat bees display a wider range of social behavior than almost any other bee family. Some species are strictly solitary: a single female digs her own tunnel, provisions her own cells, and has no contact with other bees. Others are semi-social or communal, meaning multiple females share a single nest entrance but each maintains her own brood cells inside. A few species are truly eusocial, with a queen and worker caste similar to (but much smaller-scale than) honeybees.
Even solitary species often nest in dense aggregations, with dozens or hundreds of individual nest holes clustered in the same patch of suitable soil. This can look alarming if you stumble across it, but it’s simply a case of many individual bees independently choosing the same favorable spot. These aggregations tend to return to the same location year after year as long as conditions remain suitable.
When Nesting Is Most Active
Nest construction is seasonal and closely tied to rainfall and temperature. In temperate climates, sweat bees are most active from late spring through fall. Research on one species documented active nest building from June through November, with foraging females occupying nests from June through October. The onset of the rainy season plays a key role: wet, loosened soil is much easier to excavate, so many bees begin digging when summer rains soften the ground.
In spring, overwintered females (or newly emerged queens in social species) start fresh nests. By midsummer, the first generation of offspring may emerge and, in social species, begin helping the founding female expand the nest. Activity tapers off in late fall as temperatures drop and the last generation of the year prepares to overwinter, often burrowing deeper into the existing nest or retreating into rotting wood.
Are Nesting Sweat Bees a Problem?
Sweat bees nesting in your yard are generally harmless. Female sweat bees can sting if they feel directly threatened, such as if you step on one barefoot or press against one, but the sting is mild. On the Schmidt sting pain index, sweat bees rank at the lowest level. Most people describe it as a brief, faint prick that fades quickly.
These bees are not aggressive defenders of their nests the way yellowjackets or honeybees can be. You can walk over a nesting aggregation without provoking a response. They’re also valuable pollinators of wildflowers, vegetable crops, and fruit trees. If you’d prefer they nest elsewhere, the simplest approach is to change the habitat: covering bare soil with mulch, dense plantings, or ground cover removes the conditions they need. Watering a dry patch more frequently can also discourage nesting, since they prefer well-drained soil that isn’t waterlogged.