Megachile ligniseca (c) Ryan Clark
Most people are familiar with honey bees and bumblebees, but look closely and there are smaller furry bees moving from flower to flower. Ryan Clark gives this concise guide to solitary bees...
What are solitary bees?
Most people are familiar with honey bees and bumblebees, but look closely and there are smaller furry bees moving from flower to flower. There are around 20,000 described bee species worldwide. Most of these bees are known as solitary bees with only 250 bumblebee species, 9 honey bee species and a number of social stingless bees worldwide. In Britain we have around 270 species of bee, just under 250 of which are solitary bees. These bees can be amazingly effective pollinators and as the name suggests tend not to live in colonies like bumblebees and honey bees.
Solitary bees in Britain are highly diverse, therefore so are their nesting habits. The majority of British species nest in the ground, excavating their own nest. The female builds the nest by herself. She chooses a suitable piece of ground in which to nest and uses her body to dig out a nesting chamber in the ground. She adds pollen to the chamber, which is often moistened with nectar, and lays an egg. She then seals off that section of the nest before moving onto the next chamber. Although most solitary bees nest solitarily, in suitable nest sites you often find aggregations of nests. There are also a number of species in Britain that nest in the ground but create turrets over their nests, these are often very distinctive.
A number of species also nest aerially, usually in old beetle holes often sealing the nests with a saliva like substance, mud, chewed leaves, resin or sections of leaves which they cut with their jaws. These species are the ones most likely to take to artificial nests in gardens. There is also one species of solitary bee in Britain, Ceratina cyanea, that excavates its own aerial nest, usually in bramble stems. This small metallic blue bee excavates out the pith of the bramble stem and nests in there. Unusually both the males and females also overwinter, hibernating in the stems.
Finally there are the snail shell nesting bees, of which we have three species in Britain. They use chewed up leaves to seal off the each section in the empty nest shells and often camouflage the shell in some way.
Most solitary bees collect pollen on their legs on specialised hairs called the scopa, however these hairs do not form a basket like we find in honey bees. Pollen may be moistened with nectar to allow it to stick more readily to these hairs when pollen is being actively collected by the female bee. Some other species, such as leafcutter and mason bee species, collect pollen on specialised hairs on the underneath of their abdomen. Finally some yellow faced bees don't have pollen collecting apparatus at all so swallow the pollen, regurgitating it when back at the nest.
Most solitary bees are polylectic, meaning that they collect pollen from a wide variety of plant species. However there are still a lot of bees that specialise in collecting pollen from one genus or species (or from only a few genera or species), this is known as oligolecty. Many solitary bee species specialise on a family of plants with the most common pollen specialism being for those plants in the daisy and pea families. This has implications for crops which fall into these families as solitary bees, which carry out the majority of pollination in Britain, are highly effective pollinators. Generalist solitary bees also carry out a vast amount of pollination, especially in gardens and on farms where there are a number of crops flowering all at once.
Hosts and Parasites
Not all bees collect pollen, around a quarter of British solitary bees are brood parasites or cuckoo bees. These bees have no pollen collecting apparatus, relying on the pollen of their hosts to provide for their offspring. These cuckoo bees search out the nests of their hosts and lay an egg in their nests. Either the young or the adult kills the hosts offspring and the larvae then eat the pollen in the nest. The larvae can directly kill the host larva or indirectly by eating the pollen, therefore starving the host larva to death. These cuckoo bees are often much rarer than their hosts and are highly adapted for their parasitic lifestyle. They are often brightly coloured, resembling wasps. This is because they often hang around the nests of their hosts and have this warning colouration to protect themselves from predators. These cuckoos are highly specialised and co-evolved with their hosts to require the same amount of pollen, and in some cases pollen from the same species of plant, to develop and fly at the same time.
Key Solitary Bee Groups and Some Common Species
Mining bees (Andrena species) - In Britain there are around 65 species of bee in the genus Andrena, making it the largest bee genus in Britain. These bees are quite variable in size ranging from 5-17mm long but all nest in the soil. These bees have short pointed tongues and are characterised by the grooves (facial fovea) running down the inside of their eyes which is more or less unique in Britain to this genus. These mining bees collect pollen on their hind legs and are parasitised by bees in the genera Nomada and, occasionally, Sphecodes.
Tawny mining bee (Andrena fulva) - This common spring flying bee flies from March to June. The females of this species are bright red and create nests in short turf, often in peoples lawns and leave behind volcano like spoil heaps.
Early mining bee (Andrena haemorrhoa) - This is another common spring bee species with females active from April to July. This species has a beautiful blood red tip to the abdomen and a covering of red hairs on its back. Females of this species are often found on fruit trees.
Ashy mining bee (Andrena cineraria) - This is another spring flying species, flying from April to early August, which is extending its range in Britain. The females are highly distinctive and are black with two broad ashy grey bands on their back.
Wool carder bee (Anthidium maculatum) - This attractive, robust, bicoloured bee collects the hairs from plants to build its nest and is unusual as the females of this species emerge before the males. The males are much larger than the females and are very territorial, defending a patch of flowers for its female to forage from. Pollen is collected by females on the underside of the abdomen. The hair collecting behaviour was first described by Gilbert White in the mid-18th century from his garden at Selbourne.
Flower bees (Anthophora species) - This genus comprises of 5 species in Britain, they all look fairly chunky like bumblebees (for which they are often mistaken). Most of these flower bees nest in the soil and have long tongues to reach into deep flowers. These bees carry pollen on the hind tibia back to nests which are often densely packed in the best nest sites.
Hairy footed flower bee (Anthophora plumipes) - This is a southerly distributed species although records do exist as far north as Yorkshire. This species has a fast hovering flight and can often be seen hovering around plants such as cowslips and Pulmonaria species. The males and females of this species look completely different. The males are a beautiful sandy colour while the females are completely black.
Leafcutter bees (Megachile species) -These bees collect sections of leaves in which they build their aerial nests in old beetle holes. There are 7 species in Britain, all of which look rather robust and have large jaws. Pollen is collected by females on the scopal hairs on the underside of the abdomen. Remarkably up to 40 pieces of leaf are needed just to build one nesting cell for one offspring. Leafcutter bees are parasitised by bees in the genus Coelioxys. This is a group of species that you can attract to your garden by providing a bee hotel.
Sweat bees (Lasioglossum and Halictus species) - There are around 40 bees in this group in Britain. They have short pointed tongues and often nest in soils, collecting pollen on the underside of the abdomen and on the legs. These species can be solitary or primitively eusocial in behaviour and are often used for behavioural studies.
Yellow faced bees (Hylaeus species) - There are 11 species in this group found in the British Isles, all of which are aerial nesters and are rather small, averaging 6-7mm long. These bees are often called yellow-faced bees as they usually are black with patches of yellow on their faces. This group doesn’t have any pollen collecting apparatus so collect pollen and nectar in their crops before regurgitating it back at the nest.
Mason bees (Osmia species) - There are 11 species in this genus, all of which collect pollen on the underside of their abdomens. Their life histories are all very interesting and vary dramatically though with some species nesting in snail shells and others nesting aerially in stems. Some of these species are generalists, collecting pollen from a variety of plant species, while others are very specific.
Red mason bee (Osmia bicornis) - This is probably the most familiar solitary bee to many people. This species flies in a single brood from March to July and abundant in lowland England and Wales. It is however uncommon in Scotland and Ireland (where it has recently been introduced). The females of this species are quite large (9-14mm long) and use mud to build their nests in a range of natural cavities. This mud is carried beneath the head and seals off the individual sections of the nest. This species takes rapidly to artificial bee hotels and due to its efficiency at pollinating fruit trees, is sometimes introduced to orchards.
Pollinators have never been so important and to protect them, we need to appreciate them and record them. In the case of bees, there is no better place to look for more info in Britain than the Bees, Wasps and Ants Recording Society’s website. Do put any records that you have on iRecord too as they can then go onto the database, if verified and make a difference to conservation.
If you have questions about solitary bees, or would like to know more, you can contact Ryan Clark on Twitter!