Down to Earth

By Athayde Tonhasca

Of the more than 20,000 known species of bee in the world, most (~80%) are solitary, that is, each female constructs and provisions a nest by herself. And most (~70%) nest underground; these are fossorial (from the Latin fossor for ‘digger’), a term applied to animals adapted to digging and living underground. In the UK, about half of the 250 or so bee species are fossorial. They are better known as ‘mining bees’ or ‘miners’.

Common furrow bee (Lasioglossum calceatum) and Gwynne’s mining bee (Andrena bicolor), two widespread fossorial species © gailhampshire, Wikipedia Creative Commons

There are many types of mining bee nests, but a typical design comprises an entrance surrounded by a small ‘volcano’ of excavated soil, known as a tumulus. This entrance leads to a tunnel, which may branch into cells. The female bee lines these chambers with a waterproof waxy or paper material, or sometimes with mud, pebbles or pieces of leaves and petals. She will then stock each cell with a ball of pollen and lay an egg on it. The larva will feed on the pollen until it is ready to emerge as an adult. 

A tawny mining bee (Andrena fulva), a regular occupier of garden lawns, and its nest entrance © entomart (L), Sarah (R) Wikipedia Creative Commons
Multiple life stages of the alkali bee (Nomia melanderi) © Art Cushman, USDA Systematics Entomology Laboratory,

Bees and their pollination services have been the subject of numerous initiatives around the world, largely focused on safeguarding pollen and nectar. These plans are laudable and helpful, but fall short. All the food in the world is no good to bees if they cannot build nests and raise their young.

Underground nesting does not sound like a tricky housing specification – after all, we are surrounded by solid ground. But the properties of a patch of earth can determine whether a mining bee can move in. Many fossorial bees need bare ground at a certain angle, so that nests are adequately warmed by the sun and protected from flooding. The physical properties of the soil must be just right: too hard, and the bee can’t dig; too soft, and the nest may collapse. Even soil acidity and organic matter content are important for some species.

This fussiness about nesting spots may help explain why females of some species nest near each other despite the risk of attracting parasites. These nest aggregations may be thousands strong, all sharing an area of suitable habitat – which could be a section of a well-trodden footpath or dirt road: these bees don’t mind sharing their space with us.

An aggregation of tawny mining bee nests © Africa Gomez, BugBlog

Mining bees and other solitary bees pollinate wild plants and crops, especially fruit trees such as apples. Despite the importance of these bees, conservation programmes have paid scant attention to their dietary needs, and even less to their nesting requirements. This omission is partially understandable because we know little about bees’ biology and behaviour. But broadly speaking – and this is certainly not applicable for all species – bare ground creation is a promising management option. This can be done by digging and compacting the soil on field margins, or by scraping the ground surface with farmyard machinery, ideally on south-facing slopes. The vegetation should be kept low by mowing or trimming.

Bee nests in a surface scraped with a digger © Nichols et al. (2020) Conservation Evidence 17: 15-18

In some situations, all it takes is to leave bees alone. Land managers may be tempted to ‘tidy up’ exposed ground by planting or landscaping it. By doing that, a well-intentioned practitioner may destroy whole colonies of mining bees.

Eroded ground, prime real estate for mining bees

In other cases, mining bees need protection. The Northern colletes (Colletes floralis) is one of the rarest British bees. It is present in low densities in continental Europe, but the bulk of its population is confined to coastal areas of Ireland and Scotland, with a couple of locations in England. In Britain, the greatest threat to this bee is the destruction of its nests, which are built in bare or sparsely vegetated coastal sand dunes. Housing, golf courses, motor racing tracks, marinas, wind farms, sand quarries and sewage treatment plants are all wearing away Northern colletes habitat.

Bees and people can often coexist, but sometimes we need to give them a break. 

The Northern colletes © Arnstein Staverløkk, Wikipedia Creative Commons