Chelostoma

Distinguished from other osmiine bees by combination of slender elongated body, specialized pollen-collecting behavior on specific host plants, and nesting in narrow preexisting cavities. Subgenera and species distinguished by morphological details of mouthpart length, head shape, and host plant associations. Molecular and detailed morphological keys available for some subgenera (e.g., Chelostoma s.s., Gyrodromella).

Slender, elongated body form with narrow abdomen; body proportions adapted for colonization of narrow linear cavities. Some species possess elongated heads and very long mouthparts, representing adaptations for accessing nectar from long-tubed zygomorphic flowers such as those in Lamiaceae. Sexual dimorphism present: females carry pollen on scopal hairs beneath the abdomen (typical of Megachilidae), while males lack pollen-carrying structures.

Preexisting linear cavities in dead wood or hollow plant stems; requires warm, sheltered locations with nearby floral resources. Some species associated with specific host plant communities (e.g., Campanula-rich meadows, Ornithogalum habitats).

Primarily Palaearctic region; 19 of 21 species in subgenus Chelostoma s.s. occur in Western Palaearctic. Documented from Europe, Turkey, Middle East, and parts of Central Asia. Specific species have restricted ranges (e.g., new species described from southeastern Turkey, northern Iraq, Iran, Levant, Israel). North American records limited to introduced or vagrant species.

Active from early spring through late summer depending on species and host plant flowering phenology; specific timing tied to bloom periods of specialized host plants.

Most species are strict pollen specialists (oligoleges) collecting from actinomorphic flowers of single plant taxa: Campanula and related Campanulaceae (subgenus Gyrodromella and C. rapunculi), Ornithogalum (Asparagaceae), Ranunculus (Ranunculaceae), Dipsacoideae (Caprifoliaceae), Asteraceae, or Brassicaceae. Two pollen generalists known in subgenus Chelostoma s.s. Three related species with elongated heads and long mouthparts use Lamiaceae. Nectar feeding from host and non-host flowers documented.

• Campanula - pollen hostPrimary host for subgenus Gyrodromella and C. rapunculi
• Ornithogalum - pollen hostAsparagaceae; host for some Chelostoma s.s. species
• Ranunculus - pollen hostRanunculaceae
• Dipsacoideae - pollen hostCaprifoliaceae subfamily
• Asteraceae - pollen hostFamily-level specialization
• Brassicaceae - pollen hostFamily-level specialization
• Lamiaceae - pollen hostUsed by three species with elongated heads and long mouthparts adapted for long-tubed zygomorphic flowers

Solitary nesting: each female constructs individual nests comprising series of brood cells. Egg cells provisioned with pollen-nectar mixture, then sealed. Development through egg, larva, pupa, and adult stages within sealed cells. No social colony structure. Some species may nest in aggregations at favorable sites.

Nesting females use mud to construct brood cell partitions and nest plugs, often reinforcing outer surface with embedded tiny pebbles and sand grains. In C. florisomne, females construct empty cells in front of brood cells as anti-parasite defense; empty cells found in front of 64-74% of brood cells, reducing parasitism from 28.9% to 5.4%. Females detect and remove parasite eggs deposited prior to cell closure. Rapid cell closure construction (5 flights, 6-13 minutes) minimizes vulnerable provisioning period. Some species occasionally visit non-host flowers (e.g., C. rapunculi visiting Malva moschata, Geranium sanguineum), attracted by olfactory cues in naïve bees and visual cues in experienced foragers.

Important pollinators of specialized host plant taxa due to strict oligolecty; pollen transfer efficiency enhanced by dry pollen carriage on body hairs without nectar mixing. Contributes to maintenance of plant communities dominated by Campanula, Ornithogalum, and other specialized hosts. Serves as host for cleptoparasitic bees and wasps (e.g., Sapyga clavicornis).

Potential pollinators in gardens and urban parks where host plants (especially Campanula species) are cultivated. Some species documented in bee hotel nesting aggregations. No direct economic importance for honey production (solitary, non-honey-producing).

• OsmiaBoth osmiine megachilid bees, but Osmia typically broader-bodied, often uses chewed leaf material or mud for nesting (not pebble-reinforced mud), and shows different patterns of pollen specialization
• HoplitisRelated osmiine genus with different nesting substrates (often soil, plant stems, or cavities with different closure materials) and generally different host plant associations
• MegachileLeafcutter bees in same family but cut leaf pieces for nest cell construction rather than using mud with pebbles; broader body form; different pollen carriage

• solitary bee
• oligolectic
• pollen specialist
• cavity nester
• mud nest
• Palaearctic
• Campanula pollinator
• anti-parasite behavior

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Gardening for Solitary Bees - Buglife Blog - Buglife
• Western Palaearctic Chelostoma bees of the subgenus Chelostoma (Megachilidae, Osmiini): biology, taxonomy and key to species
• Floral Cues of Non-host Plants Attract Oligolectic Chelostoma rapunculi Bees
• Palaearctic Chelostoma bees of the subgenus Gyrodromella (Megachilidae,    Osmiini): biology, taxonomy and key to species
• Interaction between the solitary bee Chelostoma florisomne and its nest parasite Sapyga clavicornis  − empty cells reduce the impact of parasites
• PATTERNS OF HOST-PLANT CHOICE IN BEES OF THE GENUSCHELOSTOMA: THE CONSTRAINT HYPOTHESIS OF HOST-RANGE EVOLUTION IN BEES
• Descriptions of the Egg and Mature Larva of the BeeChelostoma(Prochelostoma)philadelphiwith Additional Notes on Nesting Biology (Hymenoptera: Megachilidae: Megachilinae: Osmiini)