Mortar Bees

Advice leaflet about Mortar Bees

Mortar bees, or Masonry bees, are so called because they sometimes burrow into the mortar joints in brick walls, causing damage as they go. 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 Life 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 early summer. Only one such brood is raised each year.

Damage to Buildings

Unfortunately, the bees cannot tell the difference between soft rock and soft mortar, especially if it is old and perished. However, the gallery constructed by a single bee should not cause any significant damage.

Damage does occur when the brood raised in one year spends the winter in the galleries. When spring comes they may start to enlarge the existing galleries or build new ones in the same area of the wall. Over a period of a few years large numbers can become established in a small area causing quite severe damage. In the very worst cases some rebuilding of the wall may be necessary.

Prevention

The only effective way of preventing extensive damage 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.

This work is best done in late summer, after the bees have ceased their activities but before the possibility of frost damage. If the work can only be carried out in the spring, special insecticide treatments are needed to stop damage occurring to the new mortar before it hardens properly.

It should be emphasised that spraying or injecting insecticides do not have any lasting effect, and are only recommended in those rare cases where bricks or stonework have been entered.

Local Authority Service

The Public Health Department does not offer a treatment service for Mortar Bees.

However, Private Pest Control companies do offer this service and can be found listed in the local telephone directory.