By James Duncan
Learning and Engagement Officer
The Mason Bees are named after the characteristic habit of using 'masonry' products in the construction of their nest cells. They're a global group, with a few hundred species found across the Northern Hemisphere. The one you're most likely to see is the Red Mason Bee (Osmia bicornis), distributed widely but predominantly in the bottom half of the UK, though there are twelve Osmia species found here overall. Typically emerging in early spring, the Red Mason Bee is a frequent visitor to parks and gardens and it's likely to be one of the first to exploit artificial nesting opportunities in the form of a 'bee hotel.' It's a solitary species with no colonial structure and one that seems to have a strong affinity to urban environments. This is owing to a propensity for nesting in pre-existing holes, cracks and cavities, provided in abundance by us. Nest sites may include crumbling mortar, assorted timbers, window frames and even key holes, as well as naturally occurring cavities in plant stems, deadwood, snail shells and vertical soil walls.
The males are the first to emerge in spring, usually a couple of weeks prior to the larger females who are roughly the size of a Honey Bee (Apis mellifera). The males quickly feed-up, eagerly awaiting the emergence of females from the nest - they'll grapple viciously with each other in an effort to obtain the best possible vantage. As with all male bees they have no sting, so their weaponry is somewhat limited. Their lives are short-lived and once mated, perhaps a couple of times, their job is complete. The female will then begin the quest for a suitable burrow and undertake building of the nest cells entirely on her own. Within the Osmia family, species will use a variety of masonry materials during construction, ranging from mud to grit to leaf mastic. You may well spot a female Red Mason near small puddles during this stage, for her cavity wall material of choice is mud. In fact, it's one of only two UK Mason species that use mud. A feature making the female distinctive is a small pair of incurved 'horns' which are used to manipulate the mud within the nest.
The Red Mason Bee is a polylectic species, visiting a wide variety of flowers in order to gather pollen and is an extremely effective pollinator - interestingly they're often used commercially in the UK to assist in crop pollination. The female will collect pollen with what's known as a 'pollen brush' beneath her abdomen. She doesn't have 'pollen baskets' on the hind legs as is typical with many other bee species. Once she's completed an orientation flight around the nest, she'll take a mental note of surrounding features before she begins to harvest pollen. She'll never see her young, but they'll be fed on this protein-rich food source along with a little regurgitated nectar. She'll lay an egg right on top of this food pile and then seal each individual cell with a mud plug. When depositing pollen she'll reverse in to brush it from beneath her abdomen, though when depositing mud she'll enter directly carrying it in her mandibles (mouth-parts). The nest cells do a good job of protecting the larvae from predators and parasites, and whilst the adults perish within weeks the larvae have all the provisions they need. Eventually they'll spin a silken cocoon and pupate, with adults emerging next spring as they eat their way out of the cells.
Red Mason Bee - female © James Duncan