From
http://www.spab.org.uk/advice/technical ... onry-bees/
Technical Q&A 17 : Masonry Bees
They’re small, furry and firmly part of traditional British fauna. Douglas Kent, SPAB Technical Secretary, reveals all about the masonry bee, an opportunist loner with a taste for old brickwork.
Q. What are masonry bees?
A. "Masonry" (or "mortar") bees are one of those solitary types that do not nest in a colony but within individual holes in the ground and occasionally in walls in mortar joints, soft bricks and stones themselves, or cob. In Britain, there are nearly 20 species, the most common being Osmia rufa (the Red Mason Bee). Masonry bees favour sunny, south-facing elevations, as these enhance the germination of their eggs. Nests are established in spring or summer and contain six to 12 eggs, each in a cell provisioned with pollen and nectar and sealed, usually with mud. New adults emerge the following year to repeat the cycle. Masonry bees are honeybee-like in appearance. The female has a sting, but will not use it unless squeezed between your fingers!
Q. Do masonry bees seriously damage buildings?
A. Not necessarily. A few holes will not cause significant decay to building fabric. Often, masonry bees are opportunist, utilising pre-existing crevices etc, such as old nail holes, irregular cavities in stone, beetle borings in timber, as well as spaces under roof tiles, behind ventilation grilles and even inside locks. However, though solitary, females seem attracted to sites with other females present, and some may also be able to burrow into soft material. They use their mandibles to excavate or enlarge holes, kicking out spoil behind them. The annual burrowing activities (especially of Colletes daviesanus) may create an extensive system of galleries. This can disrupt the bearing capacity of masonry, and tends to fill with water that expands on freezing and causes deterioration. Birds can also attack walls to get at the bee larvae.
Q. Should I control masonry bees?
A. Action will clearly be required where a dense aggregation of bees threatens the fabric of a building. The sight of numerous nesting holes and large numbers of bees milling around a wall are the most obvious signs that action is required, but excavated mortar and small flakes of newly broken off pieces of brick or stone on the ground below is a further indication. Fortunately, though, mortar bees are not generally as building-unfriendly as frequently perceived, and in many instances intervention will be unnecessary. These insects, like wildlife generally, may not only contribute to a building’s setting but are efficient pollinators.
