Masonry bees and mortar

The short answer, before anything else

• They are solitary bees. There is no colony, no queen to defend and no honey — each hole is one female working alone, however many holes there are.
• They almost never sting, and several of the species involved cannot sting through skin at all. Males cannot sting at any species.
• A scattering of holes in a wall does not cause significant decay to the building. Building-conservation guidance says this plainly.
• They are a symptom, not a cause. Bees nest in mortar that has already weathered soft; sound, well-pointed mortar is not attractive to them and mostly not workable by them.
• Where numbers are genuinely large and the mortar is genuinely failing, the answer is repointing at the right time of year — not insecticide, which building conservators describe as often ineffective long-term, liable to stain masonry, and ecologically undesirable.

What 'masonry bee' actually means

It is not a species. It is not even a group. 'Masonry bee' and 'mortar bee' are British vernacular terms for any solitary bee found nesting in a wall, and they cover several genera that are not closely related and do not behave the same way. That is why advice about them contradicts itself so often: two people can be describing genuinely different animals doing genuinely different things.

Why your wall and not the one next door

Three things make a wall attractive, and all three are properties of the wall rather than of the bees.

1. Soft mortar. Traditional lime mortar weathers over decades into something a bee can work; so does a badly mixed or sand-heavy modern mix. Hard cement-rich mortar mostly is not workable, which is why the problem clusters on older buildings and on cheap repointing.
2. Sun. South and south-west elevations warm up, and warmth speeds brood development. The same wall on the north side of the same house is usually untouched.
3. Existing holes. For the Osmia and Megachile half of this group the wall has to offer cavities already — old fixing holes, gaps at the ends of joints, weep holes, air bricks, even holes drilled for a satellite bracket ten years ago.

The practical consequence is worth stating directly, because it reframes the whole problem: bees in the mortar are usually telling you the pointing has reached the end of its life. The bees did not cause that and removing them would not fix it.

Do they actually damage buildings?

This is the question everybody arrives with, and it has a two-part answer that has to be kept together, because either half on its own is misleading.

The season, and why it decides everything

These bees are present as adults for a short window and inside the wall for the rest of the year, which is the single most useful thing to understand before touching anything.

1. 1Spring emergenceAdults chew out of the cells their mothers provisioned the previous year. Mason bees are typically out from around March or April; the plasterer bees that excavate mortar are later, broadly from early summer. Males emerge first and hang around the wall looking for females, which is the phase that produces the alarming cloud of bees and involves no stinging insects at all.
2. 2NestingEach female works alone: she provisions a cell with pollen and nectar, lays an egg, seals the cell, and repeats. A few weeks of conspicuous traffic in and out of the wall.
3. 3The adults dieUsually by midsummer for the earlier species. The visible activity stops entirely, and people frequently assume something they did worked.
4. 4The rest of the year, insideThe next generation is developing in sealed cells within the wall through autumn and winter. The wall looks empty and is not, which is why timing matters for any work.

If the wall genuinely does need work

Sometimes it does, and a page that told every reader to leave it alone would be giving building advice it is not entitled to give. Where mortar has failed, it needs repointing whether or not anything is nesting in it.

• Time it for when the wall is unoccupied — late summer or autumn is the window conservation guidance names, because it breaks the cycle: females commonly return to the wall they emerged from, and a soundly repointed joint gives the next generation nothing to re-occupy.
• Rake the joints out properly before repointing rather than skimming over the holes, or the old galleries stay in there.
• Match the mortar to the wall. New mortar must not be harder than the brick or stone around it — a cement-rich mix on a soft old brick wall transfers the weathering to the brick faces, which is a far more expensive problem than bees.
• On an old, listed or traditionally built building, get advice from someone who works on traditional buildings rather than a general builder. Lime mortar is a specialism and the wrong repair does lasting harm.
• Offer an alternative. Putting up bee hotels nearby is recommended by building conservators as a way of drawing nesting away from the wall, and it works because these bees are looking for cavities rather than for your house specifically.

Why insecticide is usually the wrong tool

Not on sentiment. On three practical grounds, before any argument about pollinators is reached.

• It does not fix the wall. The mortar is soft either way, and next spring's females — who may be arriving from elsewhere — find the same holes.
• Building conservators warn that chemical treatment alone is often not effective in the long term and can stain masonry, which converts a cosmetic problem into a visible one.
• It kills the wrong things. These bees are effective pollinators of fruit and garden plants, and a wall of solitary bees is doing measurable work in the gardens around it.

This site does not give pest-control instructions of any kind and does not name products. Where somebody has a genuine reason to act — a large excavating aggregation in structurally significant mortar, or a household with a diagnosed venom allergy — the right sequence is a competent survey of the masonry first, then a repair specification, then whoever is qualified to carry it out.

How this reads outside Britain

The biology is general. Solitary bees that occupy cavities or excavate soft substrates are found across the temperate world, Osmia and Megachile and Anthophora and Colletes all have wide distributions, and everything above about mortar softness, sun and season applies wherever those bees and masonry buildings meet.

Three things are specifically British and should not be assumed elsewhere. The vernacular — 'masonry bee' and 'mortar bee' are not standard usage in North America, where the same complaint tends to be filed under carpenter bees, which are a genuinely different animal that bores into timber. The season dates, which shift with latitude and climate. And the legal position: the protection of bats and nesting birds described above is British law, and other jurisdictions have their own, sometimes stronger.