Alone in a crowded house

By Athayde Tonhasca

A hole in a stone wall on a busy street does not sound like a great housing choice, but it is a haven for the chocolate mining bee (Andrena scotica). You can find this bee in streets, gardens, parks and countryside across Britain at this time of year, even during this never-ending wintery spring. Like approximately 70% of the 20,000 or so known bees in the world, this species is fossorial (from the Latin fossor for ‘digger’), that is, it excavates a nest in the soil and spends most of its life underground. And like about 80% of all bees, it is solitary: each female builds her own nest chambers in which she stocks pollen and lays her eggs.

Although solitary, chocolate mining bees tend to nest near each other. Many other solitary bees do the same, sometimes forming nest aggregations hundreds strong. As far as we know, they don’t interact with each other. But chocolate mining bees have one quirk:  they often share an entrance to their burrows. Behind a communal gate and out of sight, each of several female bees constructs and provisions her nest. 

Chocolate mining bees’ communal nest entrance along a pavement © Athayde Tonhasca

These concentrations of bees frighten some people. Beekeepers and council officers have been summoned to deal with ‘swarms,’ only to witness the comings and goings of chocolate mining bees. Worse, people afraid of being stung or concerned about damage to structures have wiped out whole colonies with insecticides. But these citizens are mistaken on both counts. Like any other solitary bee, the chocolate mining bee is docile and never goes out of its way to attack people or animals; besides, her sting is too weak to penetrate human skin. And their nesting galleries are temporary and too small to cause any damage to a wall.

A group of chocolate mining bees hanging around a communal nest entrance © Athayde Tonhasca

We don’t know why some bees chose to gather together for nesting. It’s a risky strategy because these concentrations may lure parasites and predators, and this seems to be the case for the chocolate mining bee. Bees may aggregate out of necessity; a sturdy, long-lasting nest requires a specific set of conditions (soil texture, moisture level, temperature, inclination), so bees may have to share suitable but limited real estate. Aggregations may be a manifestation of natal philopatry, which is an animal’s tendency to return to its place of birth. Or it could be that bees are attracted to each other out of self-interest: by building on a good spot already discovered by a trail blazer, a bee saves time and energy that would be spent searching for nesting sites on her own.

Whatever the reasons for its communal living, the chocolate mining bee should be welcomed. This species is highly polylectic – it collects pollen from a variety of unrelated plants – so it helps pollinate a wide range of spring-flowering shrubs, trees and wild flowers. They are not put off by nosy humans, so if you find them going in and out of a crack in an old wall or behind a stone in your local park, stop and watch for a moment. Soon you may spot a bee sprinkled with pollen grains, which is a sure sign she has been hard at work providing for her next generation and assuring the reproduction of many of our plants.  

A chocolate mining bee with her ‘pantaloons’ (pollen brushes) covered with pollen © Dave Smith