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Above: two different species of fairy bee

Bees of the genus Perdita, also known as fairy bees, are some of the smallest bees in the world. Their tiny bodies can measure as little as 1.6mm long, which is smaller than the eyes of many bumblebees and carpenter bees.

Above: a fairy bee depicted next to a carpenter bee (genus Xylocopa)

The smallest species in this genus is the mini fairy bee, Perdita minima, which is so small that it's often mistaken for an ant.

Above: Perdita minima standing next to a quarter

As this book explains:

With almost 640 species, most restricted to the southwestern USA and adjacent parts of Mexico, this genus forms a species swarm of mostly very small ground-nesting bees. One of its species, the aptly named Perdita minima, shares the record for being the smallest bee in the world at just 1/16th of an inch (1.6 millimeters) in length. Unsurprisingly, it favors similarly tiny flowers, such as those of the whitemargin sandmat (Chamaesyce albomarginata).

Above: close-ups of Perdita perpallida and Perdita heliotropii

Fairy bees are solitary, meaning that they don't form colonies or live together in hives. Each female builds her own nest by creating a small tunnel in the ground and then stocking it with pollen.

Above: a fairy bee standing on a dime and another one standing on a quarter

This article describes the nesting process in greater detail:

Fairy Bees are “mining” bees, referring to the fact that they are ground nesting bees. The females excavate tunnels in the ground somewhere within a short distance of a food source. They then visit flowers, feeding on nectar and collecting pollen on specialized hairs on their legs known as “scopae.”

The females then deliver these pollen bundles to their subterranean nests as a food source for their larva. The larva hatch, consume the pollen bundle, develop through metamorphosis into adult bees and the cycle continues.

Above: Perdita minima on the antenna of a carpenter bee

Most fairy bees are specialist foragers with very short tongues, so they prefer shallow flowers. They typically fly during the summer and autumn, timing their emergence to coincide with their favorite host plant.

Above: Perdita leotola

Sources & More Info:

Minnesota Native Bees: Fairy Bees

Bees of the World: Genus Perdita

Honey Bee Suite: Perdita minima, the Smallest of the Small (PDF)

Field Guide to the Common Bees of California: Genus Perdita

Local News Pasadena: Photographing a Nearly Microscopic Bee You've Probably Never Noticed

iNaturalist: Fairy Bees