Exploring the Mason Bee Life Cycle

Should You Buy Mason Bees?

Reading Time: 5 minutes

In the shivery days of early spring, long before honey bees peek from their hive entrance, early mason bees remind us that sunny days lie ahead. Often mistaken for flies, mason bees are some of the earliest spring fliers. But the timing of the mason bee life cycle varies with each individual species — and we in North America have an enormous variety.

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This male Osmia is resting on a leaf. If you see a mason bee sunning itself on the ground or perched on leaf litter, it’s probably a male.

The term “mason bee” is confusing because it can mean several different things. In the broadest sense, a mason bee is any bee that collects materials from the environment to use for building a nest.

The choice of materials depends on the species, but it can include pebbles, mud, fibers, resins, petals, leaves, and even manmade materials such as builder’s caulk. What these bees have in common is a way to collect and carry their treasures and the skills to use them.

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In more common usage, the one I will use here, “mason bee” refers to bees in the genus Osmia, usually Osmia lignaria, but sometimes others. This can be confusing. While there is only one species of Apis in the entire western hemisphere — Apis mellifera — North America alone is home to about 150 different Osmia species. When you use the term honey bee, everyone knows exactly what you mean, but the term mason bee is vague and variable, like the word “dog” or “chicken.”

The type of mason bees in your garden will depend on where you live. Even the most common type, Osmia lignaria, comes in two forms — the east coast version and the west coast version.

Still, the very thing that makes them confusing also makes them fascinating. During the course of year, you may have several different types of mason bees in your garden, ranging from midnight black to metallic green and blue.

Details of the Mason Bee Life Cycle

However variable mason bees are, they have a fairly consistent life cycle. Nearly all mason bee species are cavity dwellers, which means they nest in above-ground spaces. Usually, they seek pre-drilled holes in trees or stumps, hollowed out stems, or old beetle burrows. However, they tend to be eclectic in their choices and will occasionally use keyholes, light sockets, electrical outlets, and wheel wells. The ones at my house are crazy about the drain holes beneath the vinyl windows, and I’ve even seen them nest inside a honey bee hive.

When a mason bee lays eggs in a tunnel, she puts the female eggs in first. The last two or three eggs she lays, those nearest the opening, are males. This arrangement means the males emerge first in spring. After emergence, the males sip nectar from flowers but spend most of their time cruising near the nests, waiting for females to emerge. When he spots a female, the male mates immediately and then waits for another. Unlike honey bee drones, male mason bees can mate as many times as they wish.

Once mated, the female begins nest building by searching for a suitable cavity. She tends to search very close to her birthplace, often using the same cavity she emerged from. This allows us to easily increase a population of local bees because they tend to stay put. On the other hand, it means parasites can accumulate in overused nesting cavities, something we sometimes need to control.

After she selects a nest, the female begins to collect pollen for her young. She goes from flower to flower, filling her abdominal scopa. When the scopa is full, she returns home and erects a mound of pollen in the back of the cavity. She flies back and forth between flower and nest until she has enough pollen to feed a larva, then she backs into the cavity and lays an egg on top of the mound.

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Mating occurs as soon as the females emerge. The males are slightly smaller and hairier than the females. They also have a mustache and very long antennae.

Putting the Mason in Mason Bee

At this point the masonry begins. Depending on the species, the female flies off to collect her materials of choice. For Osmia bees, this is usually mud, mud mixed with fine gravel, or mud mixed with chewed leaf bits. She uses this concoction to build a partition that encloses the pollen and egg in its own chamber. Once the chamber is complete, she repeats the entire process.

This exhausting work continues for the life of the mason bee, which totals roughly four-to-six weeks. Once she dies, the eggs hatch, the larvae eat the pollen mound, and the immature bee overwinters as either a larva or a pupa, depending on species. As a rule of thumb, we can watch any particular species of Osmia for about two months before it disappears for the rest of the year.

Nearly all adult bees are active for a mere four-to-six weeks, including honey bee workers and drones. We may think that honey bees live longer, but it is only the colony that persists, not individual bees. Only a honey bee queen has the potential to live much longer.

What Do Mason Bees Pollinate?

The most popular mason bees are those that emerge early and pollinate the first spring crops, including fruit trees and berries. They are important pollinators because honey bees are heat lovers that often ignore early crops unless the weather is warm and dry. But since the plants won’t wait for warm and dry, mason bees make an excellent addition to many farms and gardens.

However, other Osmia species will emerge just as the first ones disappear. Sometimes called “summer masons” these bees are often smaller and more discreet. But if you are open to multiple species, and provide tunnels of various diameters, you can often attract these to your mason bee housing as well.

Where Can I Buy Mason Bees?

Buying mason bees is never a good idea because the cocoons may contain parasitic wasps. If those unwelcome guests become established, they can decimate a population of mason bees in one season. And even after the masons are gone, some of the parasites may infest other species. It is not worth the risk. Remember, by moving honey bee colonies and packages all over the country, we have spread their diseases and parasites into every backwater and city in America. We should learn from this unfortunate mistake and not repeat the process with our native bees.

Unlike honey bees, which have a foraging distance of miles, mason bees have very short foraging ranges. Because each environment is unique, even local shipping is not good for their health. A number of conservation groups are adamantly opposed to moving native bees, no matter how short the distance. Since I have seen commercial installations ravaged by mites and parasites, I have to agree. It’s best to be patient and allow the mason bees to come to you.

What is your experience with mason bee cocoons? Have you ever had unwanted creatures emerge?

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