Mortar bees (or masonry bees) are so called because they sometimes burrow into the soft mortar joints of old brick walls. There are a number of different species of bee that do this, but the most common has the scientific name of Osmia Rufa. In appearance it is rather like a small version of the ordinary honey bee but a little more hairy. Being small, they are often mistaken for wasps, but they are soft brown and yellow coloration rather than bright yellow and black.
Fig. 266 A mortar bee.
The natural habitat of the mortar bee is earth banks and soft exposed rocks into which the female bee burrows a small chamber (approximately 20mm deep), to lay her eggs. Some species (often referred to as miner bees) can excavate much deeper to form more complex tunnels and galleries. Soft mortar in old brickwork makes an ideal, alternative nesting site. Contrary to popular belief, mortar bees do not eat the mortar, but merely excavate into it. Tell-tale signs of activity are small piles of mortar dust at the base of the wall. Mortar bees will also lay eggs in old drilled holes in brickwork, gaps around window or door frames, holes in airbricks and even redundant key holes. When nesting in brickwork, mortar bees are most likely to make use of south-facing walls that receive sun for most of the day.
The adult bees live for a short period of time (approximately April to July), with only one brood raised each year and nesting taking place in early spring. The bees lay their eggs in their chambers, which they also stock with pollen and nectar, following which the chamber is sealed. The eggs hatch out as larvae that feed on the pollen and nectar left in the chamber. The larvae then pupate and subsequently hatch out as bees, which emerge from the chamber.
Mortar bees are solitary creatures that, unlike honey bees or wasps, do not form social colonies. They do, however, exploit the same suitable nesting sites, giving a false impression of a larger colony. They are not aggressive, and pay little or no attention to people. They have a sting that is unable to pierce the human skin and is therefore harmless and, like all bees, are beneficial as pollinators of plants.
In terms of preventive measures, the spraying of insecticide is generally ineffective as the bees are found only on sunny elevations and insecticides break down with UV light. Moreover, the eggs are sealed inside the chambers so are protected from the insecticide. The only effective way of preventing these bees is to re-point areas of soft and perished mortar, as the bees are able to burrow only into comparatively weak materials. This work is best done in late summer or early autumn, once the bees have ceased their activities but before winter sets in, which can cause more damage to the masonry.