Mason bee house basics: setup, nesting tubes and yearly care

Mason bees are among the easiest native bees to support. They are active early in the season, they pollinate fruit trees and spring wildflowers, they rarely sting and they will nest in a simple house. But a poorly built house, or one that is never cleaned, can do more harm than good. This guide covers the basics.

Meet the mason bee

Mason bees belong to the genus Osmia. The blue orchard bee is a well-known native species, and other native Osmia are found across the country. They are solitary: each female builds her own nest instead of working in a colony. There is no hive, no honey and no queen, so there is little to defend. Males do not sting at all, and females sting only if pinched or trapped.

Mason bees fly for roughly a month or two in spring, when fruit trees, redbuds, willows and early wildflowers bloom. They carry pollen on the underside of their abdomens rather than in leg baskets, which is why they are very effective pollinators.

How a mason bee nests

1. A female finds a hollow tube or hole.
2. She gathers pollen and nectar, forms a ball and lays an egg on it.
3. She seals the cell with a wall of mud and repeats, filling the tunnel with a row of cells.
4. She plugs the entrance with mud and leaves.
5. The egg hatches, the larva eats its pollen ball and spins a cocoon, and by late summer it becomes an adult inside the cocoon.
6. The adult stays inside through winter and chews out the following spring.

This explains why mud matters and why the tubes need maintenance: the cocoons sit in the tunnels for almost a year.

Choosing a house

University extension services, such as Utah State University, commonly recommend the same design features for cavity-nesting bees:

• Tunnel diameter. About 5/16 inch is a popular size for mason bees, and a mix of sizes from roughly 1/8 to 3/8 inch supports other small bees.
• Depth. Roughly 3 to 6 inches. Tunnels should be blocked at the back so only one end is open.
• Smooth interiors. Splinters can damage wings.
• Replaceable or removable liners. Paper tubes or grooved wood boards that can be opened make yearly cleaning easy.
• A roof. An overhang keeps rain off and the nest cooler.

Avoid houses made only of bundled bamboo that cannot be opened, pinecones stuffed in a frame or tiny drilled holes in thin wood. These look pretty and may attract insects, but they are hard to clean, can hold moisture and may build up parasites.

Where to put it

• Face the morning sun. A southeast-facing spot warms bees early. In hot climates provide shade in the afternoon.
• 3 to 6 feet off the ground, on a post or wall that does not sway.
• Within a short flight of flowers. Mason bees do not travel far from the nest.
• Near mud. A patch of moist, clay-rich soil or a shallow tray of mud is essential. In dry areas, keep it damp.
• Out of the rain and sprinklers.

When to set it out

Put the house up in early spring, before the bees emerge, or when the first fruit trees begin to bloom. If you buy cocoons, follow the supplier's instructions for timing, and place them in a protected box near the house. Keep the house out of reach of birds that might pull tubes apart.

Care: every year

This is the step most people skip.

1. Late fall or winter: bring tubes with sealed cocoons into an unheated, protected spot such as a garage or shed, or leave the house in place if you will not harvest.
2. Clean or replace tubes annually. Either replace paper liners, or open the trays, remove the cocoons and clean the tunnels.
3. Watch for pests. Mites, parasitic wasps and mold can build up. If tubes look moldy or full of debris, discard them.
4. Spring: put clean tubes and the cocoons back out before the bees emerge.

Cocoon harvesting is optional and takes more effort. If you do not wish to do it, replace the tubes each year and let nature handle the rest.

Other bees that use bee houses

Leafcutter bees fly in summer and seal their nests with pieces of leaves. Small carpenter bees and some resin bees may use larger tunnels. Wasps, including beneficial solitary wasps, may also move in. Most are harmless to people.

What mason bees need beyond the house

Flowers that bloom in spring, such as fruit trees, redbud, wild plum, willow, dogwood and spring wildflowers, are essential, and later blooms help other bees. Do not use pesticides near the house, including systemic products that can persist in plants. Leave some bare soil, stems and leaf litter for bees that do not use houses. Read about them in the native bees guide.