Nesting female: about 20 days (USDA-ARS). Males: emerge 1–3 days before females. Generations: one a year. Flight: from about 12 °C (54 °F). Winter: adults in cocoons, 180–200 days at 4 °C (39 °F) for Utah bees.
How long do mason bees live?
How long do mason bees live? As active adults, only a few weeks. "A nesting BOB female lives an average of 20 days," according to the USDA-ARS handbook on the blue orchard bee (Osmia lignaria), the best-studied mason bee (Bosch & Kemp 2001). In that time she typically fills two to four nests; long-lived females may build up to seven. Males emerge first and die before the nesting season ends; their exact lifespan wasn't measured in the sources we found.
But counted from the egg, a mason bee lives about a year — and almost all of that is spent sealed inside its nest. Mason bees "produce only one generation a year, and adults developing from eggs laid in spring do not emerge, mate, and nest until the spring of the following year" (Bosch & Kemp 2001). The weeks you see them flying are the last chapter.
The planner above uses the handbook's guidance to turn your bloom month and spring temperature into dates: when to put cocoons out, how long the females will be nesting, when to take nests in, and when to cool cocoons for winter.
| Figure | Source | |
|---|---|---|
| Nesting female | about 20 days on average | Bosch & Kemp 2001 |
| Male | emerges first; not measured | Bosch & Kemp 2001 |
| Population nesting period | more than a month | Bosch & Kemp 2001 |
| Generations | one a year | Bosch & Kemp 2001 |
| Egg to adult | several months (spring to late summer) | Bosch & Kemp 2001 |
Life cycle of a mason bee
The life cycle of a mason bee runs on a yearly clock. Unlike honey bees, "which develop from egg to adult in 16 to 24 days, BOBs take several months to complete development" (Bosch & Kemp 2001).
| Season | Stage | What's happening |
|---|---|---|
| Spring | Emergence and nesting | males emerge first, females 1–3 days later; they mate and females build mud-walled cells in tubes, each with a pollen–nectar loaf and one egg |
| Late spring–summer | Egg, larva, cocoon | larvae eat their provisions and spin cocoons; nests are best kept in a shady barn, garage or porch |
| Late summer–autumn | Pupa, then adult | the bee becomes an adult inside its cocoon |
| Autumn–winter | Wintering adult | adults rest in their cocoons; bees cooled in September or October are ready to emerge by April or May |
Sources: Bosch & Kemp 2001 (USDA-ARS Bee Biology & Systematics Laboratory). In a nest, females are produced in the innermost cells and males in the outer ones, which is why males come out first. Late in the season females lay mostly male eggs, so nesting materials placed too late "will yield nests with mostly male progeny" (Bosch & Kemp 2001).
What an orchard mason bee looks like
The orchard mason bee (blue orchard bee) is metallic dark blue, sometimes blue-green. Males are about two-thirds the size of a honey bee, with a white tuft of hair on the face; females are about honey bee size but stouter, with tiny horn-like prongs on the face. They are gentle — "they rarely sting" — and efficient: 250–300 females can pollinate an acre of apples or almonds (Bosch & Kemp 2001).
When mason bees emerge
When do mason bees emerge? In spring, timed to local bloom. Their nesting "starts as early as February in the lower latitudes and elevations, and as late as June in colder areas" (Bosch & Kemp 2001). Temperature drives it:
- They can fly at about 12 °C (54 °F) once emerged and established — lower than most bees, and they forage under cloudy skies (Bosch & Kemp 2001).
- At 13–20 °C (mid-50s to 60s °F) emergence "progresses slowly", so the handbook advises incubating cocoons first, at 22–29 °C (72–85 °F), and best no lower than 20 °C.
- With days around 21–27 °C (the 70s °F) unemerged cocoons can go out about a week before first bloom without incubation; at room temperature males emerge in 24–48 hours.
Overall, nests with bees go out one to two weeks before expected bloom (Bosch & Kemp 2001). If they come out before there are flowers, "many will disperse to search for blooming plants and nest elsewhere." That's the mason bee season the planner dates for you.
Caring for mason bees and cocoons
These steps come from the USDA-ARS handbook (Bosch & Kemp 2001):
- Nest holes: about 7.5 mm (19/64 in) wide and at least 15 cm (6 in) long. Face nesting units south, east or southeast for more foraging hours, and don't move or swap them once bees start nesting — females get disoriented and leave.
- Mud: females need a mud source for their cell walls; if there's none nearby, dig a shallow trench and keep it moist during nesting.
- Sprays: some fungicides sprayed before nesting make females abandon the site en masse.
- Taking nests in: when only worn females (tattered wings, lost hair) are left and flights drop, remove nesting materials — parasites and predators are most common late in the season. Carry them with the entrances facing up so young larvae stay on their food.
- Summer: keep nests in a shady barn, garage or porch while the young develop.
Mason bee house care
Disease builds up in old nests. Chalkbrood spores "remain viable for years", so using new nesting cavities — paper straws or reeds — "is an essential measure for chalkbrood control". Wood blocks and grooved boards can be sanitised by soaking them for five minutes in household bleach (6% sodium hypochlorite) diluted 1:3 with water (Bosch & Kemp 2001). That's the core of mason bee house care: fresh tubes, clean blocks and nests stored away from late-season parasites.
How to store mason bee cocoons
Once the bees are adults in their cocoons (late summer to autumn), they need a cold winter to emerge on time. Bees cooled in September or October emerge by April or May; northern Utah bees "winter well at 39 °F (4 °C) for 180 to 200 days", and a refrigerator at 37–41 °F (3–5 °C) can hold them back until bloom (Bosch & Kemp 2001). Kept too warm for too long in autumn, adults burn their fat and may be too weak to chew out in spring. Keep humidity in check during development: trapped humidity makes pollen mould. Releasing loose cocoons cuts chalkbrood but makes more females wander off, so keep some in their nests (Bosch & Kemp 2001).
How this planner works
- Temperature band: below 12 °C "too cool to fly"; 12–20 °C "cool spring" (incubate first; put cocoons out 1–2 weeks before bloom); 21 °C and up "warm spring" (about a week before bloom).
- Dates: first bloom is the 1st of the month you choose, this year. Each female nests about 20 days; take nests in from about a month after bloom starts, once only worn females remain.
- Winter: cool cocoons in September or October (March or April in the Southern Hemisphere — our six-month shift) and hold them at 4 °C (39 °F) for 180–200 days.
- Same numbers everywhere: the 20-day female lifespan is the same figure the bee lifespan calculator uses.
The figures are for blue orchard bees from northern Utah; other populations and other Osmia species fly earlier or later, so adjust to your local bloom.
Frequently asked questions
How long do mason bees live?
A nesting female lives about 20 days on average (Bosch & Kemp 2001); counted from the egg, a mason bee lives about a year, most of it inside the nest.
What is the life cycle of a mason bee?
Spring: emerge, mate and nest. Summer: eggs, larvae and cocoons. Late summer–autumn: adults form in cocoons. Winter: adults rest until spring (Bosch & Kemp 2001).
When do mason bees emerge?
In spring with local bloom — from February at low latitudes to June in cold areas. Males first, females 1–3 days later (Bosch & Kemp 2001).
At what temperature do mason bees fly?
From about 12 °C (54 °F), even under cloud (Bosch & Kemp 2001).
How should I store mason bee cocoons?
Cool them in September or October and keep them at about 4 °C (39 °F) for 180–200 days; a fridge at 3–5 °C delays emergence (Bosch & Kemp 2001).
When should I put mason bee cocoons out?
One to two weeks before your trees bloom; about a week if days are around 21–27 °C (Bosch & Kemp 2001).
Do mason bees sting?
Rarely. Blue orchard bees are gentle (Bosch & Kemp 2001).
What size holes do mason bees need?
About 7.5 mm (19/64 in) wide and at least 15 cm (6 in) long (Bosch & Kemp 2001).
How many generations do mason bees have?
One a year (Bosch & Kemp 2001).
These calculators give estimates from research and extension figures, for education. Bees are important pollinators — never spray a swarm or a colony. For a swarm or bees in a wall, contact a local beekeeper, a bee-removal specialist or your Extension office (Mississippi State; UGA Extension). If a sting causes trouble breathing, swelling of the face or throat or faintness, call emergency services (UC IPM).