Individual Scent-Marks of Nest Entrances in the Solitary Bee, Osmia cornuta (Hymenoptera: Apoidea)

SIMPLE SUMMARY: Osmia cornuta is a solitary mason bee that uses natural pre-existing cavities, such as beetle worm traces or hollow plant stalks, for nest construction. Such nesting opportunities can often be found pretty close to each other, leading to dense aggregations of many nesting bees. Therefore, it is crucial for females returning from collecting flights to localize their own nests’ entrance among many other similar-looking holes. Individual scent marks of entrances were suspected to be used by bees as olfactory cues additional to visual orientation. We used paper sleeves introduced in the nest entrances to identify the substances involved in marking and compared the composition with secretions of the Dufour’s gland and substances present on the body surface. Although the nest marks were found to be ample diverse, tags did not perfectly render individual odor bouquets nor did bees possess private chemicals. Instead, females used an individual mixture of body-derived substances enriched by external components to produce distinguishable tags that alter over time. The smell of the own nest has to be learned continuously by the resident female as a template to identify her own nest at the next arrival from a provision flight. ABSTRACT: The ability to recognize the own nest is a basic skill in nest constructing solitary bees. Osmia cornuta females use a dual mechanism of visual orientation to approach a nest and olfactory verification of the tube when entering it. Occupied tubular cavities were steadily marked by the resident female. Nest marking substances originate from Dufour’s gland and cuticle, enriched by external volatiles. Scent tags were dominated by alkanes and alkenes in a species-specific mixture enriched by small amounts of fatty acid esters, alcohols, and aldehydes. The individual nest tags are sufficiently variable but do not match perfectly with the nesting female. Furthermore, tags are not consistent over time, although females continue in marking. Besides the correct position of the entrance in space, bees have to learn also the bouquet of the used cavity and update their internal template at each visit to recognize their own nest by its actual smell. Due to the dominance of the species-specific hydrocarbon pattern, nest marks may function not only as an occupied sign but may also provide information on the species affiliation and constitution of the nest owner.