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The secret lives of (mason) bees

A mason bee outside a bee house. Photo by Gail Hampshire/Flickr1 / 2
Outdoors Columnist Blane Klemek2 / 2

A few weeks ago I put out a bee house. I erected the small, wood structure on the south side of my home on a support column underneath the roof overhang.

I'll admit that I had my doubts about whether or not bees would actually find it, much less use it, but lo and behold, the "Bee Our Guest" tower bee house has attracted the attention of a small nondescript species of native bee known as mason bees.

The bamboo tubes that make up the interior of the bee house are what attracts mason bees. The hollow tubes, not much larger in circumference than a person's little finger and are about three to four inches long, are arranged on top of each other throughout the 12" tall and three or four-inch wide tower bee house.

Mason bees seek out natural cavities and holes in trees, rocks, and other objects to carry out their fascinating life cycles. These tiny solitary bees likely go relatively unnoticed, but if one pays attention to blooming flowers, you've likely seen them, but perhaps didn't pay much attention to them.

Belonging to the genus Osmia, mason bees get their name from their habit of using "masonry" material — mud — in constructing their nests. As mentioned, mason bees use naturally occurring gaps and cracks found everywhere in nature—be they cracks in rocks, inside woodpecker holes, or within hollow plant stems, including in the hollow tubes of artificial "bee houses." Other natural cavities used by mason bees include beetle burrows and other small holes in the ground.

There are over 300 species of mason bees found throughout the northern hemisphere. And according to the University of Minnesota, about 15 species of mason bees occur in Minnesota.

Among the earliest spring bees in the state, these small native bees are important pollinators for a wide variety of plants, including various trees and shrubs and a host of spring and summer herbaceous plants.

Among plants that depend on pollination from mason bees and other insects are maples, willows, plums, blueberries, and wild geranium, to name just some. Mason bees are also used commercially to pollinate cultivated apple, cherry, and plum trees.

Mason bees are early spring bees because of their unique lifecycle. Individual bees spend the entire winter inside of cocoons within the confines of the many different types of cavities they construct inside their nests.

And how all of this is done is very interesting.

It all begins in the spring (or at least we can begin our discussion here). An adult female, upon emerging from their cocoon, will quickly mate with a waiting male. Male mason bees are always the first to emerge in the springtime. After mating, the female embarks on finding a suitable nest-site for building her nest inside of. She collects pollen from blooming plants wherever she finds them, along with nectar, and carries this material to a cavity that she has found.

She then deposits her "pollen-nectar balls" in the rear-most area of her chosen tunnel, deposits a single egg on the ball of material, and then seals off the chamber with mud or chewed up plant material. Continuing in this manner, she makes another pollen ball, lays an egg on it, and then seals the chamber with mud once again. She repeats the process over and over until the entire tunnel is full of individual chambers of pollen balls, eggs, and mud. The ends of each of her many nest-tubes are plugged with mud.

After provisioning one tube or tunnel with her pollen-nectar balls/egg chambers, the female will locate another nest-site and repeat the process. In all cases, the female lays female eggs in the rear of her tubes and male eggs in the front of her nest tubes, hence the reason that males are the first to emerge each spring.

Upon hatching, mason bee larvae consume their individual pollen-nectar balls and soon after, they spin cocoons to begin the next stage of their life cycle — the pupal stage.

The pupa matures and hibernates inside the cocoon throughout the fall and winter—it isn't until the following spring that individual adult mason bees emerge to begin anew.

Indeed, well adapted to cold, northern climates, mason bees — these docile and attractive native hymenopterans — are all about pollinating plants and making little mud nests wherever suitable nest-sites exist, including your bee house, as we get out and enjoy the great outdoors.

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