How to Make Homes for Solitary Bees

The Beeholder, January 2009.

As well as bumble bees and honeybees (that live collectively) there are some 200 species of wild bees in the UK that are called 'solitary bees' because they make individual nest cells for their larvae. Some species nest in small tunnels or holes in the ground or in sandy banks, piles of sand, or crumbling mortar. Others use the hollow stems of dead plants such as brambles, or tunnels previously bored into dead wood by beetles.

Nests for solitary beesSolitary bees are harmless and do not sting, they do not live in hives or build combs, and they do not swarm. If you find them (for example in old house walls) please leave them alone. Colonies are very faithful to their nest sites and may have been living there for many decades. They are part of the 'fine grain' of your local biodiversity - something to be cherished. A number of species are commonly seen in gardens, and they are very useful as they pollinate fruit crops. It is easy for gardeners to encourage them. By drilling holes in dry logs or blocks of wood it is possible to create artificial nesting sites for them.

Constructing the Homes

All you need is a series of holes in a piece of wood; a fence post will do. But make sure the wood is not treated with a preservative. A more elaborate scheme would be wooden box, open on one side, which is then fixed to a sunny fence or wall. You then fill it with blocks of wood or small logs in which you have drilled small holes. A variety of solitary bees will use these tunnels as nest sites. The box does not need to be deeper than 8ins, but must have an overhang at the top to keep rain off. You may already have a wooden box or a drawer from an old wooden chest of drawers that you can adapt for this purpose. If not, you can make one. But you don’t actually need a separate house a single drilled block can be placed anywhere where there is some shelter. Indeed if making more than one block of holes then you could experiment be placing them in different locations around you gardens. February is the best time for putting solitary bee homes in the garden.

A bee going into a nestInside the shell of the bee house you stack dry logs or sections of untreated timber, up to about 7ins in length, into which you have drilled a selection of holes of varying diameters between 2mm and 10mm, but no bigger. [Note that the diameter of the holes in some commercially sold wooden solitary bee houses is too large, and the bees cannot use them!] Make sure that holes are drilled slightly upwards into the wood. This prevents rain water from collecting in the borings. Don't make the borings too steep though The open ends of these holes should face outwards, and must be smooth and free of splinters. If necessary use a countersinking drill bit to clean and smooth the entrance to each hole, as the bees will not enter holes with rough splintered wood around them.

Carefully clean away any sawdust, as this will also put them off. If you are able to obtain extra- long drill bits and can drill deep holes into the wood you can make your bee house deeper, and stack longer sections of drilled logs and timber in it.

The bee house must be positioned in full sun, facing south east or south, at least a metre off the ground, and there must be no vegetation in front of it obscuring the entrances to the tunnels. The bees are cold- blooded and rely on the sun's heat to warm them up in the morning, hence the need for a sunny site. They do not have furry coats to keep themselves warm like bumblebees do.

Bees Take up Residence

Different species of Mason Bees (Osmia) will occupy different diameters of tunnels. They will construct a series of 'cells' in each tunnel. In each cell they leave a block of pollen that they have collected from nearby flowers, lay an egg, and wall it up with mud they have collected from the ground nearby (see image below). In dry weather make a small mud patch for them. Later in the summer, Leafcutter Bees (Megachile) may also use the tunnels, lining their cells with circles of leaf that they cut from wild rose bushes. Include some holes of very small diameter (e.g. 2mm) and you will get various other small solitary bees using them. I suggest drilling some blocks just with very small diameter holes, or having a whole separate bee house of them.

Bee activity will cease by mid-September at the latest. You can then remove the occupied logs and tubes and keep them in a cold dry place during the winter, to protect them from winter wet, replacing them in the bee house in March. This is very important – winter wet, not cold, is their enemy. Do not store in a warm place – they need to be cold and dry during the winter. If your bee house has a good overhanging roof and is waterproof you can leave the tubes there. From April onwards, young bees that have over wintered in a dormant state inside the tunnels will emerge, and start the cycle over again.

Various solitary bee nestsA whole insect community

Various other sorts of parasitic solitary wasps and parasitic bees will find your bee house once it is occupied, preying on, or taking over, the nest cells of mason bees. Don't worry about them, they are all part of the fascinating community of insects.

Beware Birds!

If you notice Woodpeckers or other birds attacking the tunnels looking for bee larvae, fix a piece of chicken wire across the front of the bee house. This does not seem to deter the bees.

Bundles of dead stems

Bundles of bamboo canes, sawn into lengths about 8ins long just below a joint may also be occupied by solitary bees, as will bundles of rigid dried stems of various herbaceous garden plants, especially raspberries, brambles, teasels, and elder. Some species of bees prefer these stems and will not use drilled holes. The stems must be kept dry. Rolls of dried reeds (sold as portable screens in garden centres) can also be cut up and placed in your bee house will be used by very small species of solitary bees. If you make a larger bee house you will have scope to include all of these nesting opportunities.

A large scale solitary bee houseAdapted from an article by Marc Carlton