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ID GUIDE TO WILD BEES
OF THE NATIONAL BUTTERFLY CENTER
Mission, Texas

Osmia subfasciata mason bee - (c) Copyright 2019 PaulaSharp

MASON BEES
Osmia

MASON BEES
Genus Osmia - Tribe Osmiini

The bee tribe Osmiini contains three of the bee genera shown in this guide: Osmia, featured here; and Heriades and Ashmeadiella, featured in the next guide sections. Bees of the tribe Osmiini belong to the family Megachilidae.

Mason Bees and Prickly Pear Cactus

The name "mason bee" derives from Osmia bees' practice of utilizing plant parts and soil to construct nests. Mason bees employ materials such as masticated leaves, resin, mud and even pebbles to engage in "masonry". The bees build partitions separating their nests' egg chambers; construct walls to seal nest entrances; and occasionally line walls with transported materials. Mason bees often nest in hollowed-out pithy stems or in pre-existing holes and cavities found in wood or, less often, in soil or rock cavities.

In the Lower Rio Grande Valley of Texas, mason-bee activity may entail the novel practice of using the succulent flesh surrounding the stickers of prickly pear cactus as building material. The bee shown at right was observed in late March, 2019, harvesting pieces of the soft, newly-emerged buds of a spring prickly pear cactus growing at the National Butterfly Center.

The bee was one of many iridescent blue mason bees engaging in this activity: over the period of a few days, this website's authors observed numerous mason bees visiting the same prickly pear and then using their jaws to carry mouthfuls of cactus to a wooded area where the bees disappeared into tunnels leading inside a rotted log.

Afterwards, we examined various prickly pear plants throughout the county of Hidalgo and encountered other groups of the same industrious blue bees using prickly pear cacti to build their nests. In all cases, the bees seemed to do no damage to the cactus: they focused on collecting soft materials gathered around the spines protruding from newly budding nopales (cactus pads).

The blue mason bee species observed on prickly pear and shown here has been identified as Osmia subfasciata by Texas bee expert Dr. Jack Neff, President of the Central Texas Melittological Institute and author of a vast compendium of publications on bees, among them "Nest biology of Osmia (Diceratosmia) subfasciata Cresson in central Texas (Hymenoptera: Megachilidae)".

According to this article, Osmia subfasciata mason bees are polylectic -- that is, they forage on a wide gamut of plants. They are solitary, like most members of their genus. Osmia subfasciata build their nests in beetle burrows; abandoned snail shells; plant stems; and abandoned wasp nests. The bees fashion mortar from chewed-up plant parts mixed with coarse sand or soil.

Osmia subfasciata mason bees transport balls of masticated plant material to a nesting area, drop each ball onto the ground and then chew and knead it while rolling it along the ground to incorporate sand or dirt into the mass. The bees use this cement mixture to construct the walls and partitions of their nests. The viscous flesh of prickly pear cactus may well provide a good glue base for a natural cement.

Although several females may collect materials from the same cactus, Osmia subfasciata mason bees are not gregarious; they do not build their nests close together to form nest aggregations like some solitary bees. Each female Osmia subfasciata bee independently constructs and provisions her own nest, arranging her egg chambers in linear series along a tunnel-hole, and laying around twelve or fewer eggs. When utilitiling snail shells as nest cavities, the female bee deposits a single egg in a shell, provisions it with food and then encloses it within protective materials.

After hatching, Osmia subfasciata bee larvae pupate for as long as ninety days. In Texas, they emerge in August and then overwinter as adults. In the Lower Rio Grande Valley, adults re-emerge in late March to recommence the species' life cycle.

Mason Bees' Importance as Pollinators

Mason bees are essential pollinators of crops, wildflowers and woodland, grassland and desert plants. There are many varieties of mason bee within the state of Texas -- among them are Osmia ribifloris, an important pollinator of blueberries; Osmia georgica and O. texana, which pollinate strawberries, caneberries and melons, as well as an array of wildflowers and garden flowers; and Osmia chalybea, a thistle specialist. In many parts of the United States, mason bees are bought and sold commercially for use as pollinators in apple, cherry and other fruit orchards.

Physical Characteristics of Mason Bees

Female mason bees carry pollen on scopal hairs located on the undersides of their abdomens, a trait that helps identify them -- and which they share with other members of the Megachilidae family, such as leafcutter, resin and cactus wood-borer bees. The scopae of mason bees may be white, yellowish, dark brown, black or even electric orange. Mason bees sometimes have orange hairs on their lower faces as well, as sported by the Osmia chalybea featured on this page.

Osmia are small-to-medium-sized, with robust builds and relatively large heads. The colors and sizes of mason bees vary significantly by subgenus. Bees of the subgenera Diceratosmia and Helicosmia, for example, are usually metallic green or blue. Their bodies are partly covered with pale hairs and their abdomens may be striped with bands of pale hair as well. Mason bees of the subgenus Melanosmia tend to have more robust builds and very dark green or black coloring, while the subgenus Osmia includes two imported species with a markedly different appearance: the hornfaced bee and the bull mason bee, which are large, covered with dull brown hairs and endowed with brightly-covered scopal hairs.

Mason bees possess other distinctive characteristics of the tribe Osmiini: their forewings have only two submarginal cells (as opposed to the more usual three), and their front feet sport an extra part called an arolium. These traits, which aid greatly in identifying mason bees, are illustrated in the photo strip here.

Osmia subfasciata; Copyright 2021 Paula Sharp

A blue mason bee exploring a Texas prickly pear cactus (Opuntia engelmannii)

The tender buds of newly-sprouting pads on a prickly pear cactus

A female Osmia subfasciata mason bee investigates a prickly pear bud.

The bee fastens its jaws onto the prickly pear bud

The bee pulls off a piece of the cactus.

The bee transports the plant material to its nest.

An Osmia fasciata mason bee biting off part of a prickly pear.

Osmia chalybea mason bee - (c) Copyright 2019 Paua Sharp

A female Osmia chalybea mason bee

All female mason bees have pollen-collecting hairs called scopae, located on the sternum (underside of the abdomen). This Osmia georgica mason bee's scopal hairs are bright yellow-orange. On mason bees generally, scopal hairs may be white, pale yellow, light-brown or black.

A hornfaced mason bee of the subgenus Osmia. Mason bees have an extra part on each front foot, called an arolium. The arolium is a pad located between the foot's toe-like tarsal claws. This is a trait common to members of the tribe Osmiini.

Osmiini have only two submarginal cells on each forewing. (Most native bees of the United States have three marginal cells in their forewings.)

Traits of mason bees

REFERENCES

TAXONOMY OF OSMIA MASON BEES

Order: Hymenoptera

Family: Megachilidae

Subfamily: Megachilinae

Tribe: Osmiini

Genus: Osmia

Species shown on this page:
Osmia (Diceratosmia) subfasciata
Osmia (Helicosmia) chalybea

CITE THIS PAGE: Sharp, Paula and Ross Eatman. "Osmia." Wild Bees of the National Butterfly Center of Mission, Texas. 15 Jan. 2019, http://www.wildbeestexas.com. Accessed [day/month/year guide accessed].

Osmia Mason Bee Species of the National Butterfly Center

Associated plants:

Retama
(Parkinsonia aculeata)

Honey mesquite

(Propsopis glandulosa)
Family: Fabaceae

Creosote bush
(Larrea tridentata)
Family: Zygophylaceae

Cowpen daisy

(Verbesina encelioides)
Family: Asteraceae

Texas Prickly Pear
(Opuntia engelmannii)

Plant Family: Cactaceae

These bees use cactus for
building materials; they feed
on a wide array of flowers.

When seen:

March 2018, March 2019,
February 2020

Faintly-banded Mason Bee

Osmia (Diceratosmia) subfasciata

Family: Megachilidae

Size: 8 mm (female)

7 mm (male)

Bees of the species Osmia subfasciata are a striking iridescent blue-green, a trait shared by other members of the subgenus Diceratosmia.

A female Osmia subfasciata mason bee on a prickly pear cactus nopal (leaf-pad). As noted in this guide page's introduction, this bee was biting off materialis from a prickly pear plant to transport to its nest.

These are leaf buds on the edges of a spring prickly pear. Each bud is protected by prickles and exudes an orange frothy liquid that appears to attract O. subfasciata mason bees.