What are Solitary Bees ?
As well as Bumblebees 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. Mason Bees and Leafcutter Bees are well-known examples and are common in gardens.
Solitary 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.