Above left: Neat semicircular holes removed from false acacia by a leaf-cutter bee, right: leaf-cutter bee nest in decaying wood, lined with the circles of leaf. Below left: Nest of the Red mason bee in a hollow stem showing the yellow food store and mud cap. Below right: close-up of a Red mason bee egg on the pollen and nectar "cake" in its cell.
Some solitary bee species behave like cuckoos. They do not make nests of their own but go into the nest of another bee species and lay their eggs there. These kleptoparasitic
cuckoo bees do not add to the food store in the nest, so the cuckoo bee larvae take the food intended for the larvae of the host bee. Coelioxys
species lay eggs in the nests of Megachile
species, which have a wasp-like appearance, lay eggs in the nests of ground-nesting bees, such as Lasioglossum
species lay eggs in the nests of Andrena, Lasioglossum
species lay eggs in the nests of Anthidium, Osmia, Hoplitis
species lay eggs in nests of Colletes
species in the nests of Anthophora
Male solitary bees generally emerge a few days before the females and fly around in search of females to mate with. The males are relatively short-lived and play no part in nest construction or provisioning the nest with food for the larvae. Once the female has selected a suitable nest site, she sets about creating and provisioning the cells in which the larvae will develop.
Solitary bees differ from honeybees and bumblebees in the way in which they provide food for their larvae. Social bees feed their larvae in open cells throughout their development period with a nutritious liquid diet supplied by the worker bees. Solitary bees stock their cells with a paste made of pollen and nectar. When enough food has been stored in a cell, the female lays a single egg on the food store and seals the cell. The mother bee has no further contact with her larvae, which feed themselves on their pellets of stored food. The mother bee does not live long enough to see her offspring emerge.
When fully fed, the larvae pupate within their cells. Most solitary bees have one generation a year but some have a spring and summer brood. Most solitary bees overwinter as pupae in the nests and emerge as adult bees in spring or summer.
Role of solitary bees in gardens
Solitary bees are of great value as pollinating insects. They are more tolerant of cold conditions than honeybees, so can be significant pollinators of fruit trees in cold or wet springs. Solitary bees that nest in ready-made tunnels can be encouraged in gardens by drilling holes in fence posts or providing bundles of hollow plant stems. The diameter of the holes should be in the range of 2-8mm. Garden centres sell a range of solitary bee nest sites. The more successful types consist of a bundle of cardboard tubes with a diameter of about 7mm. These are readily used by the red mason bee in spring and by leaf-cutting bees later in the summer. Solitary bee nest sites should be placed in sunny positions.
Some Andrena species like to nest in short turf, such as lawns. They sometimes cause consternation on warm sunny days in spring when large numbers of males are flying around and females are beginning to excavate new nests, creating a mini-volcano of soil above each nest entrance. Male bees lack ovipositers and cannot sting at all. Female solitary bees have stings but they are not at all aggressive and so there is no risk to children or pets. The nest building periods only lasts for a few weeks, so any nuisance caused by these bees soon stops.
Leaf-cutting bees remove pieces of leaf from various plants, especially roses, when the females are constructing their nests. Each cell is made up of 20 or more pieces of leaf and a nest can contain 10-20 cells. A female leaf-cutting bee often returns to the same plant when she needs more nest building material, so a favoured plant may have some depleted foliage by the time the nest is complete. Leaf-cutting bees are active in mid-late summer, so the leaf loss does not have a big impact on plant growth.
Other sources of information
Images and information by Steve Falk
Solitary Bees in your Garden. Excellent download
from Marc Carlton's website
. Several illustrated information sheets on various solitary bee species
Benton, T. (2017) Solitary Bees. Naturalists Handbooks 33. Pelagic Publishing (highly recommended)
Kirk, W. D. J. & Howes, F. N. (2012) Plants for Bees. International Bee Research Association especially Chapter 4: Plants for solitary bees by Christopher O'Toole
Wilmer, P. (1985). Bees, Ants and Wasps – A key to the genera of British Aculeates. A Field Studies Council AIDGAP key
Falk, S and Lewington, R. 2015 Field Guide to the Bees of Great Britain and Ireland. Field Studies Council AIDGAP key