Mason Bee (Osmia rufa (red) Osmia coerulescens (blue)) - (insects we do not treat)


Several species of bees nest in crevices or holes in masonry and are known as "mason" or "mortar" bees (these names are also used for bees that construct their nests from mud). They are most often found in walls that receive sunshine for much of the day. The spaces beneath roofing tiles and behind ventilator grills in walls are also occasionally used as nesting sites by the Red Mason Bee.

Females use naturally-occurring holes in either the bricks themselves or, more usually, in the mortar joints (especially soft mortar, with a high lime or sand content). Some females may also be able to excavate burrows in intact but soft mortar. Nesting burrows are excavated or enlarged by use of the bee's mandibles, and the resulting spoil is kicked out of the burrow entrance. These bees do not eat the mortar or brickwork, as has sometimes been reported! Some of these bees also construct their nests in holes in paths, the sides of sand pits or in coastal cliffs. Others are opportunists, utilizing almost any cavity of a suitable size, for example nail holes, beetle emergence burrows in timber and even the insides of locks. None of these bees is confined entirely to masonry.

Life-cycle

Nests in mortar generally consist of a single burrow, or a series of branching burrows each terminating in one or more circular or oval chambers known as cells. Where more space is available a honeycomb-like aggregation of cells may be built by the Red Mason Bee. A completed nest usually contains about six to twelve cells, the number varying with the species and the site chosen. The cells are provisioned with a mixture of pollen and nectar, and an egg laid on this "pollen loaf". When completed and provisioned each cell is sealed, usually with soil. Depending on the species, the winter is passed either as newly developed adults which remain for this period in their unopened cells (as in most spring-flying bees), or as full-fed larvae (summer-flying bees).

Damage and control

Although these bees are solitary nesters (i.e. each female establishes and provisions her own nest independently of others), they sometimes occur in aggregations, with nests very close to one another. The activity around very dense aggregations of mason bees may resemble a small swarm of honeybees. The sight of bees milling about a wall is the most obvious sign of the presence of these insects, but excavated mortar (which collects in heaps at the base of the brickwork) presents a further clue to their presence. The most effective means of eradicating the problem is to rake out the affected mortar to a depth of about half an inch and repoint with a hard cement into which they cannot burrow. An insecticidal treatment is not recommended or necessary.

Mason bees are harmless to humans. Females have stings but are not aggressive and will never attack; to be stung you would have to squeeze a bee between your fingers!

Try CJ Wildlife's bee nests specifically designed for Solitary Bees, or make one yourself with hollowed out garden canes. All you need to do is place the bee nest box in a sunny, sheltered, south-facing position in the garden (under the eaves of a garden shed is ideal) and you will attract nest-seeking females of the Red Mason Bee in early spring. The bees are great fun to watch and do an excellent job in the garden. In late September, put the nest containing theoccupied tubes in an unheated garden shed or garage for safe overwintering. In the following March, return the nest to the garden to allow the new bees to chew their way out and start all over again.

No work is required - if the nest boxes are in the garden at the right time the bees will find them.