A female Megachile taraxia exiting the mud nest she is constructing on Wolwekraal.

It was spring on Wolwekraal Nature Reserve in the arid Karoo, and the bare patches of ground were alive with solitary bee activity. Besides the busy work of Tetraloniella brevikeraia—the short-horned long-horned bees—other solitary bees were active, too.

The :deflation hollow, its surface cemented by fine dust particles, was bordered by scattered stones and sparse vegetation. Several of the larger stones provided ideal nesting sites for another solitary bee, Megachile taraxia. Unlike the better-known mason bees (Osmia), which usually build in cavities, this species constructs exposed domes of mud on rock surfaces, hence its description as a mud-nesting or “dauber” bee. Until recently, M. taraxia was known only from the male and recorded at just two localities, one in Namaqualand and the other in Gauteng. Finding it here, and finding both males and females, seemed an unusual and important turn up for the books.

Stones on the edge of a deflation hollow at Wolwekraal, providing firm foundations for Megachile taraxia mud nests.

I first noticed a female sitting on a prominent stone. At a glance I thought she was resting; she had been circling me for a while with a deep buzz. Only when she alighted did I see a small wet patch. On closer inspection, I realised I was looking at a mud nest. Without that faint spot of moisture, it might have seemed no more than a chance splash of mud.

Female Megachile taraxia completing the outer seal of her mud nest using saliva.

Reading through the literature on how such mud nests are built, I came upon the works of Jean-Henri Fabre (1913) in his celebrated Souvenirs Entomologiques. He applied the name “mason bees” to those that build their cells with materials similar to our own building supplies. “It is masonry,” he wrote, “but made by a rustic workman, better used to dried clay than to hewn stone.” In his essays he describes the mud-dome nests of Chalicodoma bees. While Megachile taraxia belongs to the subgenus Pseudomegachile, Fabre’s evocative accounts of Chalicodoma nest building, with its mixing of clay, sand, and saliva into a stone-hard mortar, help to illuminate how enduring these dauber bees’ constructions can be. He writes that a calcareous clay is mixed with a little sand and kneaded with the mason’s own saliva (a mix of long-chain hydrocarbons, mainly hentriacontene and tritriacontene, produced by the labial glands). Damp spots that might make the work easier and spare her saliva, are disdained by this dauber bee, which refuses fresh earth for building, just as our builders refuse old plaster and lime. Such materials, when soaked with humidity, would not hold properly. What is needed is a dry powder that readily absorbs the disgorged saliva and, with its albuminous principles, forms a kind of Roman cement that hardens quickly, something like what we obtain with quicklime and egg white. The glandular mixture is used not only as mortar but also spread over the exterior of the nest to render it hydrophobic. The mud dome dries as rapidly as our hydraulic cements, becoming almost as hard as stone. A strong knife blade is needed to cut it. In its final form the nest scarcely recalls the original work, yet it lasts through the year without notable injury. The dome remains much as it was at the start, so solid is the masonry; only the round holes, corresponding to chambers inhabited by the larvae of the past generation, mark its surface. Such dwellings, needing only minor repairs, save much time and effort. Mason bees reuse them, and build new nests only when old ones fail.

Various stones on Wolwekraal showing Megachile taraxia mud nests, their circular openings indicating completed development and emergence. The original mud nest where the first M. taraxia female was discovered is shown top left.

Ten months after my first sighting of the female M. taraxia on her mud nest, and with the early onset of warm spring weather, the first holes appeared in the mud dome nests on Wolwekraal, marking the emergence of the next generation. Within weeks, one hole became four, and soon I began noticing many more nests I had previously overlooked, camouflaged until the perfect round openings revealed them.

Megachile taraxia males and females both utilise mud nests for resting. Top: Female resting in a nest cell (left) with close-up view (right). Bottom: Male resting in mud nest (left) with close-up detail (right).

In one abandoned hole I found a male asleep. On other occasions, I discovered females sleeping inside nests they were still constructing and provisioning. I spent several hours a day watching a female at work. She arrived with her crop full of nectar and her scopa bright yellow with pollen. Head first, she entered the mud cell to disgorge nectar. Once emptied, she backed out, turned, then reversed in to deposit pollen, brushing it from her abdomen with her hind legs. She made several trips to build up a paste-like mass of nectar and pollen, shaping it into a ball like playdough. When the provisioning was complete, she laid an egg and sealed the cell. Only then did she begin work on the next, each cell fully finished before starting another. The rock beneath her nest proved a firm and lasting foundation.

Megachile taraxia female provisioning behaviour sequence. The female arrives with her crop full of nectar and scopa laden with bright yellow pollen, enters the mud cell head-first to regurgitate nectar, then backs out, turns around, and reverses into the cell to deposit pollen.

At another location within Wolwekraal, I discovered more mud nest sites and watched both male and female M. taraxia visiting Blepharis mitrata flowers within a 50-meter radius of these nests. This dense, hard-leaved plant is locally called “scorpion bush” for its formidable spines, or “shooting seed” for its explosive seed pods when wet, and appears to be a preferred foraging resource. The males, who emerge from their mud cells first, use a smart strategy: they wait at the flowers where females come to collect pollen, which is the essential protein food for their developing larvae.

Female arriving at Blepharis mitrata to gather nectar and pollen.

By positioning themselves on or near the flowers, males dramatically improve their chances of meeting a potential mate. Between flower visits, I watched males sunbathing on rocks, stones, and patches of bare sand to warm up, often returning to the same favourite spots. Following these males actually helped me locate more nest sites, as they regularly checked mud nests, apparently monitoring for newly emerging females.

Megachile taraxia foraging and mate-seeking behavior. Left: Female with head deep in Blepharis mitrata flower collecting nectar and pollen. Right: Male basking for thermal energy while waiting for females.

Mating typically occurs either at flowers or near nest sites. Male solitary bees have developed remarkable features for finding females, including modified eyes, antennae, and legs that work like sophisticated detection equipment. Some species have even evolved enlarged brain regions that boost their vision or sense of smell, depending on whether they track down their partners by sight or scent.

Watching these bees at work left me with a sense of awe for their resilience and ingenuity. In the middle of the Karoo, on bare stones near a wind-swept deflation hollow and exposed to temperature extremes, a species so seldom recorded constructed homes almost as enduring as the stone they were built on and carried on its quiet cycle of life. To stumble upon Megachile taraxia here, in numbers and with nests so well hidden, felt like uncovering a treasure. It’s one that reminds us how much there is still to discover, and how much depends on noticing the small, stone-hard wonders at our feet.

Male Megachile taraxia emerging from a mud cell after resting overnight.

A glimpse inside the mud nest: the brood cells are lined with glandular secretions, though little is known about this aspect of bee nesting.

References:

Buchmann, S. (2023). What a Bee Knows: Exploring the Thoughts, Memories, and Personalities of Bees. Island Press.

Danforth, B. N., Hinckley, R. L., & Neff, J. L. (2019). The Solitary Bees: Biology, Evolution, Conservation. Princeton University Press.

Fabre, J. -H. (1913). Insect Life: Souvenirs of a Naturalist. MacMillan and Co., Ltd.