The Hidden Lives of Solitary Bees
Beyond the honeybee — how the world's most diverse group of pollinators lives, and why their survival matters
A. When most people think of bees, they picture the honeybee: the golden, sociable insect living in large colonies, producing honey, and communicating through the elaborate waggle dance. In reality, the honeybee is the exception rather than the rule. Of the approximately 20,000 known bee species in the world, around 90 percent are solitary — they do not live in colonies, produce no honey, and rear their offspring in individual nests that each female constructs and provisions entirely by herself. These solitary bees are, collectively, more important as pollinators of wild plants than honey bees, and their diversity — in size, appearance, nesting behaviour, and dietary preferences — is extraordinary.
B. Solitary bees range in size from the tiny Perdita minima of North America, which at under two millimetres is one of the smallest bees in the world, to large carpenter bees capable of drilling through solid wood to create nesting chambers. Female solitary bees typically live for only a few weeks, during which time they must find and establish a suitable nest site, gather enough pollen and nectar to provision each brood cell, lay an egg in each cell, and seal the cell before death. The male's life is simpler and shorter: he emerges first in spring, mates with as many females as possible, and dies without contributing to nest-building or provisioning.
C. Nesting strategies among solitary bees are remarkably diverse. Most species nest in the ground, excavating burrows in bare or sparsely vegetated soil. Others nest in hollow plant stems, beetle burrows in dead wood, empty snail shells, crevices in stone walls, or pre-formed cavities of almost any kind that are the appropriate dimensions. Leaf-cutter bees cut neat circular or oval pieces from leaves and petals to line their nest cells. Mason bees use mud or chewed plant material to construct cells and to seal the nest entrance. The specialist nesting requirements of many species mean that the loss of particular habitat features — bare earth for ground-nesting species, dead wood for cavity-nesters, access to specific plant species for leaf-cutters — can eliminate local populations even where the broader landscape appears undisturbed.
D. Many solitary bee species are highly specialised in the flowers they visit. Generalist species — those willing to collect pollen from a wide range of plant families — are relatively resilient to changes in the availability of individual plant species. Specialist species, known as oligolectic bees, collect pollen from a very limited range of plants, sometimes from a single genus or even a single species. The squash bee, for example, is entirely dependent on plants of the genus Cucurbita — squashes, pumpkins, and gourds — and will not collect pollen from any other plant. When the plant on which an oligolectic species depends disappears from an area, the bee disappears with it.
E. The pollination services provided by solitary bees are of immense ecological and economic value. Many wild plant species — including some that are themselves important for biodiversity or for human food systems — have evolved in close association with particular bee species and depend on them almost exclusively for pollination. The blue orchard bee, a mason bee native to North America, is many times more efficient as a pollinator of apple and cherry orchards than the honey bee: a single blue orchard bee visits more flowers per trip, deposits more pollen per flower, and operates at lower temperatures than its social counterpart, making it particularly valuable in early spring when honey bees may be inactive.
F. Solitary bees face a range of threats that overlap with but are not identical to those facing honey bees. Loss and fragmentation of habitat — particularly the reduction of flower-rich grasslands and the removal of hedgerows, dead wood, and bare earth from agricultural landscapes — affects both generalist and specialist species. Pesticide exposure is a serious concern, particularly for ground- nesting species that may be exposed to soil-applied insecticides during their vulnerable larval stages. Pathogen spillover from commercially managed bees — disease transmission from dense managed populations to wild bee populations — is a growing concern, since commercial bee operations are increasingly common near wild habitats.
G. The simplest and most effective interventions for supporting solitary bees involve restoring or creating the habitat features they need. Leaving areas of bare or sparsely vegetated ground, maintaining dead wood and hollow plant stems, avoiding the use of pesticides during flowering periods, and growing a diverse range of flowering plants that provide pollen and nectar from early spring through to late autumn can all make a significant difference at the local level. 'Bee hotels' — artificial structures providing pre-formed cavities of various sizes — have become a popular garden feature, though their value depends on their siting, construction, and maintenance. The most important contribution any landowner can make, however, is providing a reliable and diverse food supply through the planting and maintenance of pollen- and nectar-rich plants.