Osmia pilicornis

Tilcayo Cats original · British Bees · Sources and editorial assessment · Originals retention rule

Osmia pilicornis is a solitary woodland bee in the family Megachilidae. It occurs from western Europe to western Siberia, including southern Britain. Females excavate nests in fallen dead branches and collect pollen from spring flowers. Its British habitats include open deciduous woodland, where sunlight reaches the ground.

Name and distinguishing features

Frederick Smith described the species in 1846. It is placed in Melanosmia, a subgenus of Osmia.

The body is about 9–11 mm long. Females have orange-red hair on the thorax and first two abdominal segments, with a black pollen-carrying brush beneath the abdomen.

Males have a fringe of long hairs along the back edge of the antennae. The function of these hairs is unknown. Females have strong jaws with a broad base and a chisel-like cutting edge. These differ from the jaws of closely related species and are used to remove wood when excavating a nest.

Distribution and seasonal activity

British records summarised by BWARS extend across southern England and into south Wales. The bee has one generation each year, flying mainly from April to June, with occasional July records.

A 2016 study assembled records across Europe and western Siberia. It found the bee in woodland below 1,000 metres, particularly clearings, rides, edges and areas opened by coppicing or thinning. The preferred sites combine fallen branches with spring flowers and a warm, moderately moist ground-level environment.

Nests in dead wood

For many years the exact nesting method was uncertain. British accounts mentioned holes in dead wood, but descriptions did not establish whether the bee excavated its own tunnels or occupied existing ones.

Observations published in 2016 resolved this for six nests in Germany and Austria. Females gnawed tunnels into fallen branches of several tree and shrub species. The branches varied in hardness and were mostly partly concealed by vegetation. Each nest contained one to three brood cells in a row. The female separated the cells and sealed the entrance with chewed leaves; wild strawberry supplied the leaf material observed in that study.

These observations describe a small number of nests, rather than every possible nesting arrangement across the species' range. They nevertheless provide direct evidence of excavation, which earlier accounts had often inferred.

Food and flower visits

Females have specialised bristles on the proboscis, the feeding structure, which help remove pollen from the narrow flowers of lungworts (Pulmonaria). In the 2016 study they preferred young red lungwort flowers and sometimes opened unopened buds to reach pollen. Analysis of pollen carried by 52 females from continental European localities showed that lungworts were especially important, alongside members of the pea and mint families.

In Britain the bee is associated particularly with bugle (Ajuga reptans). BWARS also records visits to willow, violets, comfrey and selfheal. A flower-visit record alone does not establish that a plant supplies pollen rather than nectar.

Parasites and conservation

The 2016 investigation recorded the wasps Chrysura hirsuta and Hoplocryptus confector as brood parasites associated with the nests. The authors suggested that parasitism might contribute to the bee's low local abundance, but treated that explanation as a hypothesis.

Loss of open woodland conditions is a conservation concern. When the canopy closes, the sunny patches and spring flowers used by the bee can disappear. The 2016 authors recommended retaining semi-open woodland, dead branches and suitable flowering plants, with monitoring of existing populations. Their observations also documented losses from some previously occupied continental sites; these do not by themselves measure the rate of decline in Britain.

Falk's 1991 British assessment listed the species as Nationally Scarce A, under the older name Notable A.