Mortar bees, or Masonry bees, are so called because they sometimes burrow into the mortar joints in brick walls or will occasionally use crevices in walls. There are a number of different species of bee that do this, but the most common has the scientific name of Osmia rufa.
Appearance and History
In appearance these bees are not unlike ordinary honey bees, and share the same soft brown and yellow colouration, as opposed to the bright yellow and black of the wasp, which mortar bees are often confused with.
However, all mortar bees are solitary and do not form the distinct social colonies that we associate so much with honeybees and common wasps, although they may be found close to each other since they exploit suitable nesting sites.
The natural habitat of mortar bees is earth banks and soft exposed rocks into which the female bee burrows. She builds a series of tunnels or galleries in the spring in which to lay her eggs from which the new adults emerge in the spring. Only one such brood is raised each year. They will only be seen for a few weeks in the spring.
The only effective way of preventing these bees is to repoint areas of soft and perished mortar as the bees can only burrow into comparatively weak materials. The joints should be raked out to a depth of 15mm (0.58in), and re-pointed with a mortar that is not too strong for the bricks, but hard enough to discourage the bees. Mortar bees will also lay eggs in old drilled holes in brick work and gaps around window and door frames.
This work is best done in late summer, after the bees have ceased their activities but before the possibility of frost damage. Spraying or injecting insecticides does not have any lasting effect, and is only recommended in those rare cases where bricks or stonework have been entered.
Mortar bees are not aggressive, they have a sting but are unable to pierce the human skin and are therefore harmless and more of a nuisance insect.