(Editor's Note: This article was originally published on May 18, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)

The Mason Bee [Osmia spp.] is not a honeybee. Although it is gregarious, preferring to nest near others of its kind, it is classified as a solitary bee, like the ground-dwelling and carpenter bees. There are no workers and queens among them; each female is a fertile egg-layer.

There are over a hundred different species of mason bees, many of them native to North America, unlike the imported honeybee. Perhaps best known is the Orchard Mason Bee [Osmia lignaria], but there are other species, such as Osmia cornifrons, the hornfaced bee. I have hornfaced bees, at least I think that's what they are, which originally came as rather a surprise, because I had ordered orchard bees, which are native to North America while the hornfaced bees are not. But their habits and benefits are much the same. Compared to the honeybee, the mason bees of both species are more effective pollinators of fruit trees and far more easy to keep. While the honeybee packs its pollen into cavities on its hind legs, the female mason bee uses a basket of hairs on her abdomen called a scopa. Not only does this hold more pollen, it comes more directly into contact with the stamens, ensuring more pollination per visit.

Most species of mason bees also emerge from hibernation in the spring, at just about the same time the pome and stone fruit trees are starting to flower. Their lifespan only lasts about six weeks, long enough to pollinate the season's fruit; by the time the fruit is set the next generation of bees is safely sealed into their nests, awaiting the next year's flowers. In every respect, the mason bee and the fruit tree appear to be made for each other's mutual benefit.

Mason bees are also unaggressive and almost never sting. Unlike honeybees, they have no communal hive, full of honey and a queen, to protect. Keeping them requires only minimal effort and expense. While gardeners might find them less useful, because they are usually dormant when the melons and squash need pollinating, for the grower of fruit trees they are a superior solution to the problem of the honeybee's decline.

I began keeping mason bees two years ago to pollinate my small home orchard of about a dozen trees. Because it is a mixed orchard, the trees are in bloom over several weeks, just at the same time [usually] as the bees are active. I ordered the bees online, and perhaps I might have done better to look for a supplier closer to home, to make sure of getting bees best suited to the climate in my zone, let alone the same species I ordered. There are now a number of companies that sell these bees and their nesting supplies. Some vendors also offer "inspected" bees, guaranteed to actually contain living bees.

Mason bees are shipped in the winter, when they are dormant in the pupa stage. Mine came in cardboard nesting tubes. I ordered six tubes, each containing about a half-dozen bees, and when they arrived in midwinter, I put them into the refrigerator, sealed in a plastic bag, to wait for spring. There are two reasons for doing this. First, because the bees might be susceptible to severe subzero temperatures - hornfaced bees are adapted to a warmer climate than northern Illinois. Second, to make sure the bees do not hatch too soon in case we have another unseasonably early hot spell, before my trees are in flower. The plastic bag keeps the bees from dehydration in the refrigerator. In late fall, after the bees are safely in their cocoons, I move them back to the refrigerator to wait for spring and the signs of blossoms about to break.


Along with the bees, I ordered a "bee kit" of a canister filled with 6 dozen additional paper-lined tubes and a shelter box to hold them. This is about the only thing you need to provide your mason bees - a place to nest. Unlike carpenter bees, they do not dig their own holes; in nature, they nest in holes that other insects or birds have made in trees. I placed my bee shelter on the east wall of my shed, about a dozen yards from the nearest fruit tree in my orchard, with the openings of the tubes facing south. The bees need early morning sun to raise their body temperature so they can fly off to work.

There are several different bee nesting systems now being sold, or you can make your own with a drill and a block of wood. The bees prefer holes about six inches deep with a diameter between ¼ and ½ inch. These can be drilled into wood, but not drilled all the way through; the back end of the hole should not be open. In England, people put up "bee posts" full of holes for bees to nest in.

All the species of mason bees are so called because they use mud as plaster to make compartments inside their nests for their eggs. Each mud cell holds a single egg attached to a ball of pollen. The bee lays the female eggs first, one or two of them at the very back of the hole. After each egg is laid, she builds a wall of mud to separate it from the next. The males come last, near the front of the hole. When the nest is filled, she plasters a thicker plug of mud at the entrance to seal it off. For this reason, it is important that there be a nearby supply of mud for the bees. My shed sits near a pond where mud is always available, but I also have noticed that there were many bees excavating it from the drainage trenches between my strawberry beds - for once, an advantage of having poorly-drained soil. The mason bee mud plug is rough; if you find a smoothly-troweled mud seal, it was probably made by a wasp.

A six-inch hole holds between a half-dozen and a dozen eggs, depending on the species. The males, laid last, hatch first, followed by the females. As soon as they emerge, they are swarmed by the males, so that there are always mating pairs of bees at the nest during the first few days of activity. Afterwards, the mated females immediately fly off in search of pollen, while the males soon die, their sole function fulfilled.

A female mason bee can lay several dozen eggs, which means she can fill as many as six six-inch holes. The first year I had the bees, I was surprised to find that from my original six tubes of bees, not only had they filled all the tubes in the nesting canister, they had also filled some of the spaces between the tubes! [Visible in the close-up photo] Later, however, I began to suspect that as much as a third of these tubes had been filled by wasps. Still, there were certainly plenty of bees on the flowers of my trees the next year!Image

This spring, expecting even more bees to hatch, I bought two new canisters full of tubes. According to the experts, the bees would go first to the clean new holes to lay their eggs, and I would then be able to clean out the old canister and fill the tubes with clean new paper liners. This is important, because diseases, fungus, mites and other parasites, and different kinds of nastiness remaining in the old tubes can harm the developing new bees. You can see this for yourself if you clean out a used nest.

Unfortunately, my bees hadn't been reading the experts. Instead of emerging over several days, my bees seemed to be crawling all at once out of their tubes on the first day in the 70s and immediately falling orgiastically into each other's embrace. Before I could remove the old canister to clean it out, the bees were already crawling back inside with loads of pollen, despite the nice, clean new tubes close at hand. [In the top photo, the old cannister with emerging bees is above, the nice, clean new one below.]

I did worry last fall when I saw that about a third of the mud seals on the bee tubes had been broken open. I wasn't sure if it was wasps or some other predator breaking in, or newly-hatched wasps breaking out - which I now think was most likely the case. Still, I think I will cover the front of the bee shelter with netting after the tubes are all sealed, just in case of evildoers who might come around to prey on them. Most of the wasps who take over one of the nesting holes are not harmful to the bees, but they do have predators, notably a tiny 1/4-inch wasp called Monodontomerus, who uses its ovipositor to bore through into the bee cocoon where it lays its egg. The photo on the left shows a bee cocoon with the exit hole made by an emerging predatory wasp, and on the right are the tiny, long deceased wasps I found in a cocoon, who somehow died before they could emerge to attack a new generation of bees.


The weather can cause complications in beeland, too. Last spring we had a prolonged, deep late freeze that killed the blossoms on about half my trees. So many bees, so few flowers! The pear tree and apples were spared, though, and got all the attention of the bees. This year, in contrast, we had such a prolonged cold spring that not even the apricot, that perennial false starter, was blooming when the bees burst out of their tubes. I worried that the bees would die of starvation before the flowers opened, but again I need not have bothered - I don't know where they found it, but they were coming back loaded up with pollen the very first day they hatched. I do wish they'd get their fuzzy butts over to the fruit trees now that the flowers are starting to open. Next year, I'm planning to keep the bees in the refrigerator until I see some signs of blossom development on the trees. The neighbor's forsythia will have to be on its own.

I have to admit that I was surprised when I first discovered my bees were not the species I had thought they were. Osmia lignaria, the orchard bee, is a shiny blue-black color, rather resembling a fly. [photo right]Image

My bees have a faint yellow fuzz and striped abdomen, almost but not quite like honeybees. But from their behavior it is clear that they are mason bees. I figure it doesn't really much matter what they look like as long as they keep pollinating my fruit trees and coming back next season for more.Image

Reference: Griffin, Brian L., The Orchard Mason Bee. Knox Cellars Publishing, Bellingham Washington, 1999.