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Species guide - species detail

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Hylaeus mesillae (Masked bee, yellow faced bee)



Bees in the genus Hylaeus are different than many bees we find nesting in wood or stems; they belong to the family Colletidae, also known as “plasterer bees". Colletidae use a special secretion from their mouths, unique in their family, to line their nests. This family is considered a more ancient family of bees. Bees in the genus Hylaeus are sometimes called “yellow-faced bees” or “masked bees” because of the distinctive yellow markings on their faces.


Hylaeus are very small, black, nearly hairless bees, frequently with yellow or white markings on their faces, collars and legs. Males have more yellow on the face, frequently filling the lower face between the eyes and looking more like a “mask.” Females frequently have yellow face markings restricted to two distinct triangles right near the eyes, sometimes described as “devil horns.” Hylaeus do not have pollen collecting hairs. Instead, they carry pollen and nectar home inside their bodies, in their crop. These are the smallest bees we find in stem or wood nests, and they can be easily mistaken for small sphecid wasps because they share similar size, body shape and color, and lack of hair. Sphecid wasps can be distinguished by the wider, more square “block-head” look to their faces. They also often have silvery hairs on their faces, which Hylaeus bees do not have. There are some behavioral cues that can help too, if you see a live one at your block. When at rest inside their tunnels, both Hylaeus bees and most wasps I have seen will face out, while most other bees face in. This can be helpful since it can give the observer a good look at the face shape and markings. Also, I frequently observe wasps moving their antenna very rapidly up and down, while Hylaeus bees do not show this behavior. Remember, sphecid wasps are shy and solitary like our bees, and they stock their nests with hundreds of aphids.

Nest Structure

Unlike Megachilid bees, Hylaeus generally do not bring in outside materials to build their nests. Instead, they use a special secretion from their mouths, sometimes called “bee spit”, to line their nests and make their nest plugs. They have specially shaped tongues with broad ends that aid in “painting” the secretions inside the nests. The secretion dries to a thin, clear, water proof coating for the cells. Hylaeus nests have been known to be submerged in water and still emerge, and many species are associated with wetlands. Hylaeus provision their cells with a semi-liquid combination of nectar and pollen, instead of the more doughy “bee bread” of many solitary bee species. The cell lining is extremely delicate; a cellophane nest plug moves and crinkles easily with the lightest touch of a blade of grass. Plug appearance can vary from an opaque white color to an almost colorless, transparent film, like plastic wrap.

Hole Sizes

Hylaeus bees nest in column 3 holes (1/8-3/16") in Bee Atlas blocks.


We are looking into whether these bees have more than one generation per year and are interested to hear from anyone who has them in their blocks.

Activity Period

No information at this time.


No information at this time.

Minnesota Record Map

These data are from the Minnesota Bee Atlas project.

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