Mason Bees
Osmia lignaria
pollinator · pollinator · both
Last updated
Quick Reference
Identification
Adult Description
Small, stout, metallic blue-black bee, 11-13 mm (females) and 9-11 mm (males). Fuzzy, dark body hair. Females carry pollen on a ventral abdominal brush (scopa) rather than leg pollen baskets. Males have a white tuft of facial hair and longer antennae. Both sexes have two pairs of membranous wings (WSU Kittitas Extension; Utah State ENT-162-12).
Larval Description
White, C-shaped grub developing inside a sealed mud cell within a cavity nest. Larvae feed on a pollen-nectar provision placed by the mother, progressing through five instars before spinning a cocoon (Utah State ENT-162-12).
Size Range
9-13 mm (adults)
Key Features
Metallic blue-black coloration; ventral scopa (abdominal pollen brush) distinguishes from leg-collecting bees; solitary, non-aggressive; no hive or colony structure
Lifecycle & Phenology
Univoltine (one generation per year). Adults overwinter as fully developed bees inside cocoons within nest cavities. Males emerge first in spring when daytime temperatures reach 50-55°F (10-13°C) for several consecutive days, typically late March in the Puget Sound lowlands. Females emerge 1-3 days later. After mating, males die within approximately two weeks. Females begin nesting in pre-existing cavities (woodpecker holes, hollow stems, beetle galleries, or artificial bee houses), provisioning each cell with a pollen-nectar loaf, laying a single egg, and sealing with a mud partition. Females place female eggs at the back of the tube (larger cells) and male eggs at the front (smaller cells). A female fills 2-4 tubes of approximately 6 inches each during her 4-8 week lifespan, laying 20-35 eggs total. After the last egg chamber is filled, she seals the tube entrance with a thick mud plug. Eggs hatch within about one week. Larvae feed on stored pollen provisions through five instars during May-July. The fifth-instar larva spins a cocoon and enters a 2-6 week period of prepupal dormancy in July-August. By late summer, larvae pupate and develop into adults within their cocoons, remaining dormant through fall and winter until the following spring (Utah State ENT-162-12; OSU Extension EM 9130; Bosch & Kemp, 2001; WSU Extension).
| Generations per Year | 1 (strictly univoltine) |
| Overwintering Stage | adult |
| Egg Capacity | 20-35 eggs per female lifetime; up to ~34 eggs documented; average 1.9 provisioned brood cells per day under favorable conditions (WSU Kittitas Extension; Flower World USA; Pitts-Singer et al., 2022 Apidologie) |
| Larval Duration | 60-90 days through five instars, May through July (Utah State ENT-162-12) |
| Development Duration | Egg to adult approximately 5-6 months (spring through late summer), but most of this period is spent in prepupal dormancy and pupal development within the cocoon (Utah State ENT-162-12) |
| Adult Lifespan | 4-8 weeks for nesting females (mid-March through May/early June); males live approximately 2 weeks post-emergence (WSU Kittitas Extension; Flower World USA; OSU Extension EM 9130) |
| Active Months | Late March through early June (adult flight); larvae develop inside sealed cells May through August |
Habitat & Conservation
Three essential elements: (1) nesting cavities — pre-existing holes 5/16 inch (8 mm) diameter, approximately 6 inches (15 cm) deep, in untreated wood, hollow plant stems, or artificial bee houses; (2) early-blooming floral resources within 100 yards providing pollen and nectar continuously through the 4-8 week nesting period; (3) a source of moist clayey mud within foraging range for cell partition construction. Nests should face south to southeast for morning sun exposure and be protected from wind and rain by an overhang. Cavities must be sealed at the back; open-ended tubes are not used (Utah State ENT-162-12; WSU Extension; Flower World USA).
Nectar & Pollen Sources
Conservation Practices
- Artificial nesting cavities (bee houses with 5/16-inch diameter tubes, 6 inches deep, sealed at back) compensate for declining natural cavity availabilityTiming: Installed before emergence in late February to early MarchCompensates for habitat loss; natural nesting cavities (beetle galleries, woodpecker holes) are increasingly scarce in managed landscapes
- Cocoon harvest and sanitation removes parasitic mites and parasitoid wasps that accumulate in reused nesting materialTiming: October through December, after adults have fully developed inside cocoons but before spring emergenceParasitic mites (Chaetodactylus spp.) and parasitoid wasps (Monodontomerus spp.) accumulate in reused nesting material; sanitation reduces population loss to 10-20% annually versus 50%+ in unmanaged nests
- Exposed clayey mud source within 100 yards of nesting site is essential for nest constructionTiming: Required March through June during active nesting periodMud is essential for cell partition construction and nest sealing; without it, nesting ceases
- Early-blooming Rosaceae and native shrubs extend the floral resource window through the full nesting periodTiming: Fall establishment provides spring benefitThe 4-8 week adult flight period requires continuous bloom; gaps in floral resources reduce nesting success and population retention
Pesticide Sensitivity
| Product Class | Impact | Notes |
|---|---|---|
| Neonicotinoids imidacloprid, clothianidin, thiamethoxam | lethal | Systemic presence in pollen and nectar is lethal to foraging adults; sublethal doses impair navigation and nesting behavior (Xerces Society, 2016) |
| Pyrethroids bifenthrin, permethrin, lambda-cyhalothrin | lethal | Broad-spectrum contact toxicity; residual activity on treated surfaces persists 2-4 weeks, overlapping with the brief adult flight period (PNW Insect Management Handbook) |
| Organophosphates malathion | lethal | Highly toxic to all bee species on contact (PNW Insect Management Handbook) |
| Spinosad spinosad | moderate | Toxic to bees on wet contact; reduced risk after spray has dried (Xerces Society, 2016) |
| Horticultural oils mineral oil | low | Direct contact only; no residual. Dormant-season application timing typically precedes adult flight period (UC IPM) |
| Insecticidal soaps potassium salts of fatty acids | low | Contact only; no residual. Low risk to bees after spray has dried (UC IPM) |
| Fungicides (copper-based) copper hydroxide, Bordeaux mixture | moderate | Copper residues on flowers can reduce larval survival when consumed with contaminated pollen provisions; bloom-period applications pose highest risk to foraging adults (Xerces Society) |
Commercial Availability
Approximately 250 females per acre for commercial orchard pollination; 6-12 cocoons (mixed sex) for a typical home garden (OSU Extension EM 9130; Flower World USA)
Release when daytime temperatures reach 50-55°F (10-13°C) for several consecutive days and early bloom is present; place cocoons on top of or behind nesting tubes (Flower World USA; OSU Extension)
Commercially supplied populations establish readily when nesting cavities and floral resources are present within 100 yards; populations can double annually with proper cocoon management and adequate habitat (Flower World USA)
Refrigerate dormant cocoons at 35-40°F (2-4°C) from fall harvest through spring release; do not freeze; maintain slight humidity to prevent desiccation (Utah State ENT-162-12; Flower World USA)