How Do Mason Bees Nest? Tubes, Mud and Eggs

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The person typing how do mason bees nest wants to know what happens inside a mason bee nest: where the female lays her eggs, what she puts in each chamber, and how she closes the nest. Mason bees are solitary. Each female builds her own nest in a small cavity such as a hole in wood or a hollow stem, and she seals every egg in its own chamber with mud. Oregon State University Extension describes the process as a repeated cycle of collecting nectar and pollen, laying an egg, and walling off the chamber with mud, which she repeats until there are several chambers in one hole.

Quick answer

• A female mason bee chooses a cavity, then collects nectar and pollen and packs them into a ball in the hole.
• She lays one egg on each ball, then seals that chamber with mud. She repeats this until there are 5 to 8 chambers in one hole.
• She caps the finished nest with mud. Each nest holds the chambers of one female, so there is no shared colony.
• Mud quality, pollen and nectar nearby, and clean nesting material decide whether the nest works. Pests can wipe out the pollen a larva needs.

What a mason bee nest is

The name “mason bee” refers to bees that build chambered nests in small holes or crevices using collected mud. The Oregon State University Extension guide (EM 9130) uses the same description. Its page on this topic says that “‘Mason bee’ is a general name for bees that build chambered nests in small crevices or holes using collected mud.” The Penn State Extension page on mason bee enemies adds that the bees “partition” cavities “into individual cells using mud” and that “These cells are provisioned with pollen and nectar for young bee larvae to eat.”

That last point explains the whole nest. A mason bee does not feed her young after they hatch. She stocks each cell before she closes it, so the larva has its food already in place. The nest is a set of prepared meals, each sealed in its own room.

Step 1: the female finds a site and starts foraging

The female’s life starts with mating. According to the Oregon State guide, males emerge first and wait to mate with the females, which emerge 2 to 5 days later. The guide says that “Once fertilized (typically after 3 to 5 days), a female mason bee locates a suitable nesting site and then immediately begins foraging.”

Foraging is the first part of the nest-building work. The guide says that “The female visits flowers to collect nectar and pollen (pollinating the flowers while doing this) and brings the pollen and nectar into the nesting hole.” Because she carries pollen from flower to flower, she also pollinates plants while she works. The guide lists fruit trees as a preferred source: “Mason bees prefer to feed on nectar and collect pollen from fruit trees like apple, cherry, and pear.”

The same guide makes a practical point about food supply. “If adequate nectar and pollen sources are not available, mason bees will not use the nesting site you have provided.” A tube placed in a yard with few flowers in spring is unlikely to be used, even if the tube itself is well made.

Step 2: building one chamber at a time

Inside the hole, the female forms a ball of nectar and pollen. The Oregon State guide describes it this way: “She then forms the nectar and pollen into a rough-shaped ball and lays one egg on it.” The ball is the larva’s food for its whole development, so its quality matters. The egg sits on top of the ball.

Next she seals the chamber. The guide says that “The female then collects mud and uses it to seal off the egg and nectar-pollen ball in a chamber.” Then she starts over. The repeating cycle is the core of the nest: “The female mason bee repeats the process of collecting nectar and pollen, laying eggs, and walling off with mud until there are 5 to 8 egg chambers per nesting hole.” When the nest is full, she closes the open end. The guide’s wording is short: “She caps completed nests with mud at the end.”

Each chamber is sealed separately. The mud walls are the reason the nest holds together, and they depend on the right soil, as discussed below.

Where the sexes go in the nest

The female decides the sex of each egg. The Oregon State guide says that “The female mason bee has control over the sex of each egg she lays.” It also gives a rule of thumb for position. “Eggs laid further than 3 inches from the entrance of the nesting hole are usually females, and eggs laid in the outer 3 inches of the hole are usually males.” The guide adds that “Typically, the ratio of male to female eggs is 2:1.”

The positions the guide describes are typical patterns. The sources do not give a fixed rule that every female follows, so a single nest may not match the rule exactly.

Mud: the material that makes or breaks the nest

Mud is not a decoration. The female needs it to make every wall between chambers. The Oregon State guide states: “The female bee needs access to moist clay to build the chamber walls in the nesting hole.” It also warns about soil that does not hold together: “Soil or mud that is too sandy is not suitable because it will crumble.”

If the yard soil is sandy, the guide suggests a mud source. “If your soil is not naturally high in clay, you can set out a container or small pile of mud that is high in clay. Wet one side of the pile to create mud that ranges in moisture from dry to very wet. The mason bees will choose the mud with the right moisture level.” It also says that “If clay mud is not available, then mason bees will not use the nesting site you have provided.” The guide recommends checking the mud during dry spells: “If it doesn’t rain for a few days, then add water to the clay mud to maintain the required moisture level.”

Why the nest can fail: parasites and pollen mites

A finished nest is also a food store for other animals. The Penn State Extension page on enemies of mason bees explains that “These pollen provisions, as well as the bee larvae themselves, attract parasites and other enemies that feed on the provisions or attack the larvae.” Among the pests it describes are hairy-footed mites. The page says: “In most cases, they feed on the pollen provision and cause the bee larva to starve, but in some cases may feed on the larva directly.” It also notes that “Multiple generations can occur per year, so a single female mite may give rise to thousands of descendants in a season.”

The Oregon State guide describes the same problem from the cocoon side. “Pollen mites feed on the pollen that the mason bee needs for development.” Cleaning matters for this reason. The guide says that “Cleaning the cocoons helps reduce bee mortality the next year by removing pollen mites, wasps, dermestid beetles, and fungal pathogens that feed on the pollen, nectar, and developing adults inside.” Chalkbrood, a fungal disease, is another risk. The Penn State page says that “The adult bees spread the chalkbrood spores to pollen stores while provisioning the nest, which the bee larvae later consume.” It adds that “After the spores enter the larval gut, they begin to compete with the larva for pollen, which causes it to starve.”

The practical lesson in both sources is simple: a nest that is left full of old material for years is more likely to host pests. A nest that is cleaned and stored properly gives the next generation a better start.

Yearly cycle: from the nest to storage

Mason bees have one generation per year, according to the Oregon State guide. That means the nest you see in spring holds the eggs laid that season. The guide notes that the cocoons carry a signal for the next year: “The old cocoons produce a scent that encourages the newly emerged females to choose that nesting house to lay their eggs.” This is one reason the same nesting site can be reused, provided it is cleaned.

For storage, the guide says that “Mud-capped tubes and laminated blocks can stay in place outside, but you can have better success if you move the tubes to protected storage, such as inside sealed paper bags, cardboard boxes protected from mice, or plastic boxes with small ventilation holes.” It also says that “Removing unused nesting materials prevents summer cavity-nesting bees or other insects from using any empty nests.”

How we chose these sources

We limited this post to two university extension pages that describe mason bee nesting in their own words: the Oregon State University Extension publication EM 9130 and the Penn State Extension page on mason bee enemies. Both were opened on 2026-10-30 with a plain HTTP request. Every quotation above was checked against the fetched text after the HTML was removed. We did not use any product listing, and we did not use blog posts as evidence.

What we could not verify

• The sources do not give a single tube diameter for every mason bee species. The guide describes the process, not a size chart.
• The sources do not say how far a female travels from her nest to forage, so this post does not give a distance.
• The sources describe the egg position rule as a typical pattern. They do not state how reliable it is for every female.
• The sources do not state how many days a finished nest takes to complete. The guide says only that the female repeats the cycle until the hole is full.

Mistakes to avoid

• Using sandy soil alone. The guide says sandy mud crumbles, so the female may not build walls.
• Placing a nest where there are no flowers. The guide says the female will not use a site without enough nectar and pollen nearby.
• Leaving old nests in place for years. The Penn State page describes how pests and parasites feed on stored pollen and larvae.
• Forgetting to store the tubes. The guide recommends protected storage over the winter, not an open yard.

Checklist before you give mason bees a nest

• Check that clay mud is available nearby, or set out a pile of clay that you can wet.
• Confirm that flowering fruit trees or other pollen and nectar plants bloom in spring nearby.
• Plan to clean the cocoons in fall and to store them in a protected place.
• Look for pollen mites and fungus when you open a nest, since both feed on the same food the larva needs.

Related reading on this site

If you want to bring more pollinators into the yard, our guide to planting a pollinator garden for butterflies covers flowers that feed adult insects through the season. For birds that also visit flowers, see how to attract hummingbirds to your yard.

FAQ

Do mason bees sting?

The two sources in this post focus on nesting, not stinging behavior, so this post does not make a claim about stings. What the sources do describe is how the female spends her time: foraging for nectar and pollen, building mud walls, and sealing each chamber. Check a regional extension page if you need a statement on stings for your area.

How many eggs does one female lay?

The Oregon State guide describes a nest with 5 to 8 egg chambers per hole. It does not give a fixed number of eggs for every species, so treat that range as the pattern the guide describes.

Why do mason bees need mud?

The female uses moist clay to build the walls between chambers and to cap the nest. The guide says that sandy soil crumbles, and that without clay mud the bees will not use the site.

When should I clean the nest?

The Oregon State guide describes cleaning the cocoons in fall. Its stated reason is to reduce losses from pollen mites, wasps, beetles and fungus the next year.

Can the nest stay outside all year?

The guide says the mud-capped tubes and laminated blocks can stay outside, but storing them in a protected place gives better results. The guide lists paper bags, mouse-proof cardboard boxes and ventilated plastic boxes as examples.

Bottom line

A mason bee nest is a row of sealed chambers, each with one egg on a ball of pollen and nectar and each walled off with mud. If the yard has flowers and clay mud, and you clean and store the nest each year, the female has what she needs. The rule to remember is simple: no clay mud and no pollen means no nest.

Sources

• Oregon State University Extension Service, “Mason Bees” (EM 9130), https://extension.oregonstate.edu/pub/em-9130, opened 2026-10-30.
• Penn State Extension, “Enemies of Mason Bees”, https://extension.psu.edu/enemies-of-mason-bees, opened 2026-10-30.