A primer on solitary bees

When you read ‘bees’, do you immediately think honeybees? I don’t blame you. Honeybees are fascinating creatures: they have a complex social system which has puzzled biologists for over a century, they are very important pollinators, and they are one of the few domesticated insects, producing honey and wax. We are familiar with them since we are kids. They are also big business, so that means they get a lot of press.

But bees do not mean just honeybees, because them and other social bees (like bumblebees) represent just a tiny fraction of the world bee species. There are 20,000 species of bees in the world, of which about 250 are found in the UK. Most of them are solitary: males and females mate, and females find a nest, lay their own eggs and collect nectar and pollen for their own offspring. Many provision the nest themselves, others, instead ‘cheat’ taking over other ready-made nests laden with pollen and nectar: the cleptoparasitic bees. Solitary bees include very efficient pollinators, like the Red Mason Bee (Osmia rufa or O. bicornis), which are also used commercially to pollinate particular crops (OSR, apples, cherries and strawberries). Other than their economic importance, the diverse morphologies, life histories and behaviour of solitary bees make them fascinating invertebrates to watch. In this post, I will list a few wonderful features of solitary bees using examples of common species often found in gardens. 

A male Hylaeus sp. on Fennel

1. British solitary bees range in size from the tiny Hylaeus (5 mm) to the male Wool Carder Bee Anthidium (14-17 mm), which is larger than many honeybee workers. 

A male Wool Carde Fee, Anthidium manicatum, note the prongs at the end of its abdomen

2. Males often emerge from their nests a few days before females and are smaller (Osmia bicornis male above). There are a few exceptions like the Wool Carder Bee, where males are bigger and armed with a three-pronged abdomen. 

Mating Wool Carde Fee, Anthidium manicatum

3. Male bees use a range of behaviours to secure access to females. Some species mate with females even before they emerge from their nests (Andrena carantonica), others hang around nest entrances waiting for females to emerge (Osmia bicornis), yet others patrol flowery places incessantly to attempt mating with feeding females (Antophora plumipes) and in the most extreme case, males aggressively defend territories containing suitable flowers from other males and even other species of bee, so that they can monopolise mating with females visiting them (Anthidium manicatum). 

Female Osmia bicornis (rufa) carrying a ball of mud to its nest

A leaf-cutter, Megachile centuncularis cutting a piece of leaf

A Tawny Mining bee female Andrena fulva, digging its nest in the soil

4. Bees lay their eggs in cells in the so-called ‘nests’, which they provision with pollen and nectar, and where the larva later develops. Nest building takes a lot of effort and female bees often have specialized structures in the head (the ‘horns’ of Osmia bicornis) or strong or scissor-shaped mandibles (like in the leaf-cutters Megachile), which facilitate nest building. Bees use a bewildering diversity of materials for their nests, and these are often species specific, so that they get from them their common names such as carpenter, mining, mason or leaf-cutter bees. In some species females excavate their nests in wood (Anthophora furcata), soil (Andrena fulva), or soft cliffs or mortar (Anthophora plumipes). Others use existing holes such as old beetle holes or hollow plant stems and line them with diverse materials: leaf fragments (Megachile sp.), mud (Osmia bicornis), downy hairs collected from plants (Anthidium manicatum), or their own cellophane-like secretions (Colletes, Hylaeus). A few British species even use empty snail shells to nest (see the fantastic photo series by Richard Comont here). 

Anthophora furcata on one of its favourite pollen-collecting flowers, Stachys sylvatica

5. Solitary bees need pollen to produce their eggs and feed their larvae. They may either use many species of flowers to obtain pollen (these are called polylectic) like Osmia bicornis, or they might be specialized, using only a few or a single type of flowers (oligolectic) like Anthophora furcata (above). 

Megachile willughbiella female with laden pollen basket under its abdomen

6. Bees use several ways to carry pollen. The specialized brushes to carry pollen are called ‘scopa’. The scopa can be located on their rear legs (Andrena, Anthophora), under their abdomen (Megachile, Anthidium, Osmia), or on both. Some bees (like Hylaeus) carry pollen in their crop, mixed with nectar.

Lasioglossum bee covered on dandelion pollen

7. Bees in general are very hairy. As the bee flies, its branched hair becomes electrically charged, so that it acts as a pollen-collecting device: as the bee contacts the pollen, it sticks to its hairs. To collect pollen, bees can roll in pollen-rich flowers (dandelions), make flowers vibrate using their wings to release the pollen, which falls on their body (poppies, comfrey), or push the flower open with their heads and bodies (irises or vetch flowers). After pollen is collected on the bee’s body hair, the bees groom it using their legs, mix it with some nectar and package it in their scopa to carry to the nest.

Anthophora plumipes collecting Lamium pollen

8. Bees have a specialized tongue to collect nectar. Some bees have a very long tongue - Anthophora plumipes tongue is as long as their body – and use it to reach flowers with deep corollas; others have short tongues (Colletes, Hylaeus, Andrena) and they can only reach shallow, open flowers. The tongue is made of several movable parts, and is tucked under the body when the bee is not using it.

Nomada marshamella, a cuckoo be resembling a wasp, near its host nest entrance

9. About a third of solitary bee species are cleptoparasites to other bees (they are also called ‘cuckoo bees’), laying their eggs on the ready provisioned nests of other species, where their larvae develop. Cuckoo bees are often closely related to their hosts. Cuckoo bees tend to be very species-specific, they parasitize just one or a few species of bee. Because of this, some cleptorarasitic bees are extinct or have become endangered in the UK due to the scarcity of their hosts.

10. Cleptoparasite bees, tend to be less hairy than other bees – as they do not collect pollen they do not need the hairs – and some can be confused with wasps (Nomada bees. They have also hardened cuticles to resist the stings of their hosts defending their nests. 

And there is much, much more: after all, there are just over two dozen species I am familiar with.

More information on bees
Bees in Britain. A superb, freely accessible introduction to British bees by BWARS members.