Mason Bees: What’s the Buzz?

Despite environmental and habitat changes resulting in the decline of bee populations worldwide, many of us will still enjoy the sight of busy bees hovering around the flowers in our gardens this summer. But did you know there are several hundred different species of bee in the British Isles? These can be divided into two different groups, social bees, and solitary bees. Social bees live in groups within a hive and depend on a queen who lays eggs for all the group; whilst solitary bees live on their own, with the female making her nest within burrows formed underground, within vegetation, wood and even masonry. That’s right bees can make their nests in stone, it sounds like something from a horror movie but is an all too real problem!

Mason bees

The Mason Bee is a solitary bee, named after the female’s ability to use mud to create individual cells within the nest. She will line each cell with pollen and lay a single egg in each until the nest cavity is full.  Generally, the Mason Bee will make use of existing cavities and holes, sometimes those left by other insects, but occasionally they will use their mandibles to enlarge holes or to burrow into soft material, such as decaying mortar or aging sandstone. Although a solitary species, the females will often nest close to other females. You may even have noticed an arrangement of the small holes they create in masonry surfaces without realising what they are!  A few holes are unlikely to cause problems at the outset, but bees will often return to the same nest sites, and their annual burrowing activities can result in damage.  Nest holes that fill with rainwater, which then expands upon freezing, may eventually cause masonry deterioration that requires stone repair or replacement.

Help! I’ve got Mason Bees!

Bees are an essential part of our ecosystem, pollinating a third of the food we eat, and while nesting is to be encouraged it is perhaps best to discourage it within masonry elements already impacted by age and weathering. Filling the nest holes and repairing the damaged masonry surface will help to break the annual cycle and hopefully, the bees will relocate. Mortar pointing can be repaired by raking out the affected areas and replacing it with appropriate mortar to match the original. Nest holes in the masonry surface can also be repaired by filling with mortar, and a mortar gun will prove useful when carrying out this work. In extreme cases where individual stones have been damaged beyond repair, then replacement with matching stone may be appropriate.

If you own a traditional building constructed of sandstone, then lime mortars were likely used in the original construction. Unlike contemporary mortars made with cement, traditional lime mortars require a specific skill set to mix and use successfully. There are several categories of lime and the correct type must be selected and mixed at the correct ratio for a given application. Lime mortars are also vulnerable to environmental conditions during the setting process, and fresh work must be constantly monitored, and if necessary protected until they have reached sufficient strength.

Training in the use of lime mortars is available for the enthusiastic homeowner at the Scottish Lime Centre Trust, and the skills learned may be adequate for repairing minor damage such as that created by mason bees. However, work at height will be difficult for the homeowner to undertake safely, and repairs on a larger scale or where stone replacement is needed will require the services of a qualified stonemason. A competent stonemason will also be able to provide the correct advice in respect of the appropriate materials to use.

Mason Bees nesting in soft decaying sandstone within the upper courses of a chimney.

Encouraging Bees

Preventing bees nesting in the walls of your home does not mean you cannot enjoy watching them build their nests in the spring and summer. Why not construct your own shelter for bees or buy a ready-made bee and insect house to place in your garden? Friends of the Earth have lots of useful information on their website about what you can do to help bees, including a step-by-step guide for making a boutique bee hotel. The RSPB website also includes a useful video to accompany their instructions.

There’s no time like the present to do your bit to encourage bees, let us know if you’ve ever had mason bees in your property, and if you do make a bee hotel.