Article - Osmia rufa

For many years I have had solitary bees nesting in holes in my shed. It wasn’t until recently that I found out that they are the Red Mason Bee – Osmia rufa. Each year they appear in May, and by late June they have gone.

This year, in May on removing the roof from one of my honey bee colonies at an out apiary I found the nest shown above. The female was present, and still constructing cells of mud.

Osmia rufa normally nests in hollow stems of bramble of hogweed. Other favourite nest sites include old garden canes, nail holes in fence posts and mortar or airbricks.

Having found a suitable nest site the female cleans out any debris before collecting mud to seal the end of the nest tunnel with a smooth layer. She then starts to forage for pollen which is carried back to the nest on the hairs on the underside of her abdomen. It takes between 8 and 15 trips to gather enough pollen to provision a cell. As the pollen mass grows it is moistened with nectar. Once sufficient pollen has been collected a single egg is laid on top of the pollen. The female then returns to the task of collecting mud to seal the cell. She then repeats the above process provisioning a series of cells in the nest tunnel. There are typically 6-8 cells in each tunnel. The first cells almost always contain females whilst the later cells usually contain males.

In her short life of around 10 weeks the female may make four or five nests each with 6 to 8 cells.

The female who built her nest between the roof and side wall of my super would have had to collect far more mud that she would have needed if she had found a hollow stem to use. Does this indicate a shortage of suitable sites at my out apiary I wonder?

The eggs hatch and the larvae feed on the pollen provide by their mother. In August/September the larvae are fully grown and all the pollen has been consumed. The larvae spin a cocoon and enter the pupal stage. Once pupation is complete the bees remain dormant within their cocoons until the following spring when they emerge.

If the oldest bee, the one in the deepest cell wakes first how does it get out? Well when it wakes it bites its way out of its cocoon and through the mud wall to get to the bee in front. She then bites into the cocoon and bites the tail of the bee in front. This wakes this bee up and she, in turn, wakes the bee in front. In this way all the bees are released from the tunnel.

It is possible to encourage these bees into your garden by providing suitable nest sites. One approach uses a bundle of canes which I have attached to a shed in my garden with the canes horizontal. (see picture above) It is also possible to use blocks of wood with holes drilled in them. The ideal hole diameter is 8mm. Smaller diameter holes result in a higher population of males being produced. It is also possible to buy ready made nests which are being marketed by the Oxford Bee Company. You can contact them on 01509-261654

Graham Royle

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