#brood cells

Above: the photo at the top shows a mason bee building its nest, while the illustration at the bottom shows the nest's internal structure

Bees of the family Megachilidae typically build their nests in the gaps and crevices in tree stumps, rocks, plant stems, and wooden structures, but there are a few species that prefer to nest in discarded snail shells. Most of these shell-nesting bees are found in Europe, North Africa, and the Near East; a few can also be found in North America, South Africa, and Japan.

Above: just some cozy little mason bees, Hoplitis zandeni and Osmia rufohirta

These are solitary bees, meaning that they don't form colonies or live together in hives, so each female builds her own individual nest. The nest is constructed as a series of brood cells, and each cell contains a single egg with enough pollen and nectar to sustain the larva until it reaches adulthood.

Shell-nesting mason bees are known to exist in at least five different genera, including Osmia, Hoplitis, Rhodanthidium, Wainia, and Protosmia.

Above: Osmia spinulosa and Rhodanthidium septemdentatum

When the female is ready to nest, she carefully selects a shell and then drags it into a shaded or well-hidden spot. Moving the shell is no easy feat, but she clings to it with her hind legs and pulls herself along by grabbing objects with her mandibles. A single bee may travel like that for several meters before finally settling on the right spot to prepare her nest.

Above: Osmia rufohirta dragging her shell into position

This article describes how the nest is then constructed:

The bee begins to build its nest, mainly within the “whorl” or spire of the shell. A typical nest consists of a few chambers (about two or four in number, depending on the size of the shell) known as cells, the walls of which consist of masticated leaf pulp known as leaf mastic. When fresh, the colour of this material is bright green, but with time, it assumes a brownish or black colour. Each cell is provisioned with a mixture of pollen and nectar, an egg is laid on this, and the cell sealed with further leaf mastic.

Above: two nests with their internal structures partially exposed, revealing the brood cells, larvae, pollen, nectar, and several layers of debris

The layout of the nest varies depending on the species, with some bees producing brood cells that are arranged into rows, while others create clustered or overlapping cells.

Above: the nest of a gold-fringed mason bee, Osmia aurulenta

The female must make dozens of trips just to gather the provisions for a single brood cell, and completing the entire nest can take days.

Once the brood chambers have been constructed and provisioned, the entrance to the shell is "bricked up" with several layers of plant pulp, soil, pebbles, and shell fragments. In some cases, the female will also apply patches of plant pulp to the outer surface of the shell as a way to provide camouflage.

Above: Oamia bicolor and Osmia aurulenta sealing their nests with plant pulp

The completed nest is then carefully maneuvered so that the entrance faces the ground. Some females will conceal the nest beneath a pile of twigs, pine needles, and plant stems, weaving moss and blades of grass throughout the pile. All of the debris is carefully selected, positioned, and then "glued" together with saliva, forming a tangled, tent-like structure over the nest.

Above: Osmia bicolor, commonly known as the red-tailed mason bee. constructing a protective thatch over her nest

In other cases, the female will conceal the nest by creating a small hole in the sand and then dragging the shell into it, ensuring that the nest is partially buried.

Above: Osmia aurulenta and Osmia rufohirta

This is just one of the many peculiar nesting habits that can be found among solitary bees. Several other examples have been featured in my previous posts, which describe the nest-building strategies of woolcarder bees, resin-pot bees, and a ground-dwelling species known as Osmia avosetta.

Above: the fully-constructed nest of a gold-fringed mason bee

Sources & More Info:

Bulletin of the Amateur Entomologists' Society: Shell-Nesting Bees

The Little Book of Bees: Snail-Nesting Mason Bees

University of Hradec Králové: Bees Nesting in Empty Gastropod Shells

Bees of the World: Snail Shells

The Wildlife Trusts: Red-Tailed Mason Bees

Insects: Nesting Preferences of Osmia orientalis

Journal of Hymenoptera Research: Comparative Biology of Four Rhodanthidium Species that Nest in Snail Shells

Journal of Hymenoptera Research: Biology of Palaearctic Wainia Bees of the Subgenus Caposmia

Cambridge University Press: The Native Shell-Nesting Bee Osmia conjuncta

Zootaxa: Palaearctic Osmia Bees of the Subgenera Hemiosmia, Tergosmia and Erythrosmia

Above: this beautiful photo shows Megachile tarltoni, a type of resin-pot bee, tucked inside an aerial nesting capsule (photo taken by William Menzies)

Bees of the subgenus Austrochile are commonly known as "resin-pot bees," thanks to their unusual nesting habits. A team of entomologists recently described 71 new species from this subgenus, all of which are native to Australia.

Above: another photo of the female resin-pot bee resting in one of her brood capsules

These are solitary bees, meaning that they do not form colonies or live together in hives. Each female creates her own brood cells out of resin, often suspending the chambers from twigs and tree branches. Nesting females have been known to sleep in these aerial chambers while their brood cells are still under construction, and their fuzzy little heads are sometimes seen poking out through the opening at the top.

They also frequently enter the capsules head-first so that they can mold the inner walls of each cell, which means that their rumps are sometimes visible instead.

Above: the same bee is shown with her head tucked toward the bottom of the capsule, providing a glimpse of her rump

The female supplies each capsule with enough nectar and pollen to feed a single offspring until it reaches maturity. She then lays a single egg in each cell and seals the entrance in order to protect the tiny egg as it develops inside.

Once the larva reaches adulthood, it uses its mandibles to cut through the top of the capsule and then emerges as a fully-developed bee.

Above: each of these sealed capsules contains a single egg/larva with enough food to sustain itself until it develops into an adult bee

Resin-pot bees (subgenus Austrochile) are unique to Australia, but several similar resin bees can also be found in Europe and Asia. Anthidiellum strigatum, or the European rotund resin bee, is a similar species from Europe that uses resin to build its brood cells, but it mixes the resin with bits of vegetation, soil, and other debris.

Above: Anthidiellum strigatum, the European rotund resin bee

Many bees build freestanding nests using other methods and materials, as this article explains:

Freestanding nests may be built with mud, resins, stones mixed with resin, plant fibers, and animal fur. Resin bees collect plant-based, sappy resin and use glandular secretions to build hard, water resistant nests. They sometimes decorate the nests with stones and pebbles. An African bee, Serapista, builds nests out of plant fiber, animal fur, and bird feathers glued to plant stems.

Another unique nesting strategy was featured in my previous post about wool-carder bees, which collect the soft little fibers from certain plants, bundle them up into "cottonballs," and then use that material to build their nests. I've also written posts about bees that build their nests out of flower petals and bees that use leaf clippings instead.

Sources & More Info:

iNaturalist: Photos of Megachile tarltoni

University of Adelaide Newsroom: Australian Scientists Describe 71 New Australian Bee Species

Australian Journal of Taxonomy: The Australian Resin-Pot Bees, Megachile (Austrochile), with Descriptions of 71 New Species

Above: Anthidium manicatum, commonly known as the European woolcarder bee

Woolcarder bees build their nests in existing cavities, often finding a hole or crevice in a tree trunk, log, plant stem, or manmade structure and then lining it with plant fibers. These soft, cottony fibers are also used to form a series of brood cells.

The fibers are known as trichomes, and they're collected from the leaves and stems of many different plants, including lamb’s ear, globe thistle, mullein, and rose campion.

Above: European woolcarder bees collecting fibers

Woolcarders are solitary bees, which means that they don't form colonies or live together in hives. Each female builds her own nest, and each male aggressively controls and defends his own territory.

Above: Anthidium florentinum, commonly known as the florentine woolcarder bee, constructing its nest in an empty keyhole

As this page explains:

The female uses her toothed mandibles to scrape trichomes off of fuzzy plants and collects a ball of the material under her abdomen. She transports these soft plant fibers to her selected nest site and uses them to line a brood cell. Next, she collects and deposits a provision of pollen and nectar into the cell, enough pollen to feed a larva until it is ready to pupate. Lastly, she lays a single egg on top of the pollen and nectar supply before sealing the cell. She will repeat this process with adjoining cells until the cavity is full.

Above: Anthidium manicatum

Female woolcarder bees will sometimes sting when their nests are threatened, but they're typically quite docile. The males, however, are notoriously aggressive -- they lack the ability to sting, but they will often chase, head-butt, wrestle, and/or decapitate any other insect that invades their territory. Some males have been known to defend their territory from intruders up to 70 times per hour.

The males are able to attack and fight other insects using a row of spikes located on the last segment of their abdomen. The central spike is the largest, and it's often used to decapitate other insects, because it has a wide, flat shape with a serrated edge, which allows it to function like a tiny bottle-opener; the woolcarder bee can simply slide that spike into the gap that separates the other insect's head from its thorax and then pry the head clean off. Woolcarder bees also have strong, sharp mandibles that can crush other bees.

There are many different types of woolcarder bee, but the most prolific is Anthidium manicatum, commonly known as the European woolcarder. Anthidium manicatum is a native species in many parts of Europe, Asia, and North Africa, but it has also become an established (and invasive) species throughout most of North America, South America, and New Zealand. It's the most widely distributed unmanaged bee in the world.

Above: a reticulated small-woolcarder, Pseudoanthidium reticulatum, and punctate woolcarder, Anthidium punctatum

Sources & More Info:

University of Florida: The Woolcarder Bee

University of Florida: European Woolcarder Bee, Anthidium manicatum

Oregon State University: European Woolcarder Bees

Bohart Museum of Entomology: Facts about the Woolcarder Bee (PDF)

Bumblebee Conservation Trust: Anthidium manicatum

World's Best Gardening Blog: European Woolcarder Bees: Likeable Bullies

Above: Megachile montivaga, otherwise known as the silver-tailed petalcutter

These bees are commonly known as leafcutters or petalcutters, thanks to their unusual nesting habits. The females build their nests in existing cavities that are often located in tree trunks, rocks, plant stems, fallen logs, and manmade structures. They line the walls of the cavity with leaves or flower petals, creating a long, tube-like nest with multiple brood cells.

Unlike honey bees, leafcutters do not form colonies or live together in hives, and each female creates her own individual nest.

Above: leafcutter bees collecting leaf fragments

The female uses her mandibles to snip off tiny pieces of vegetation, carefully trimming them into rounded segments and carrying them back to the nesting cavity. The pieces are then folded together along the inner walls of the chamber, and a series of brood cells are constructed within. Each cell is completely enclosed in pieces of foliage, and together they form a long, cigar-like structure.

Most of these bees use leaf fragments to build their nests, but there are some that use flower petals instead.

Above: silver-tailed petalcutter bees

This article explains how the nest is constructed:

The collected material is essentially used as a wallpaper to protect the nest cells from moisture. Once inside her nest, the bee chews the leaf’s edges until it becomes sticky enough to adhere to the cell walls. Each cell is provisioned with a mixture of pollen and nectar and partitioned with additional plant material. If you were to remove a completed nest from its cavity, it would resemble a small, leafy cigar.

Above: leafcutter bee nests

There are multiple brood cells built into the internal structure of the nest. Each cell contains a single egg, along with enough pollen and nectar to sustain the larva until it reaches maturity.

Leafcutter bees come in a wide variety of sizes, shapes, and colors.

Above: these photos depict several different species of leafcutter bee, including Megachile latimanus, Megachile octosignata, an unidentified Megachiloides species, and Megachile poeyi

Sources & More Info:

Florida Wildflower Foundation: Getting to Know your Native Pollinators: Leafcutter Bees

University of Nebraska: Facts about the Leafcutter Bee

Land for Wildlife: Leafcutter Bees

What's that Bug?: The Leafcutter Bee Life Cycle: a Fascinating Journey Revealed