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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Welcome mason bees, Osmia spp.


After breaking through the earthen plug, a male hornfaced mason bee tidies up before searching for a mate.


Cardboard tubes and drilled firewood make suitable accommodations for mason bees.

Last week we visited the eastern tent caterpillars on the silk trail in search of tender cherry leaves. Ah, but tent caterpillars are not the only insects responding to Mother Nature’s belated wake-up call. One of the true delights of spring is the return of solitary bees. Several years ago I established a colony of mason bees by purchasing about 30 hollow cardboard tubes from a purveyor of bee paraphernalia. These tubes were rapidly occupied by grateful hordes of mason bees. Each year I have augmented my colony by drilling dozens of small holes in unused splits of maple and oak firewood. The bees have happily obliged by filling every gallery until my colony now numbers hundreds of bees.

In recent years with uncanny timing, the masons have reappeared within a day of the vernal equinox, March 21. This spring my first mason bee emerged on April 4, two weeks later than usual. A long winter and chilly spring appear to have delayed the mason’s spring debut. The saga of this year’s masons began last spring when mother mason bees filled brood chambers with food, laid their eggs, and then sealed the tubes with mud to keep out predators and parasites of their young. During spring, summer, and autumn, bee larvae developed within the galleries, dining on pollen cakes supplied by their mothers. Over the past week, as the morning sunshine warmed the tubes and logs, earthen plugs were dismantled as fresh new bees set themselves free from juvenile confinement.


Busy mason bees enter galleries head first, then remerge and do a 180 to add pollen from their abdomen to the pollen cake.

Like many of their kin, male mason bees complete development and emerge several days in advance of their future mates. This phenomenon, called protandry, is relatively common in the insect world and was noted by Charles Darwin in his famous work, "The Descent of Man." It seems that female mason bees are a highly sought after commodity and males that emerge early in a season have more opportunity to find and secure mates. Males that are slow to develop and emerge late may find all of the available ladies taken by earlier suitors. These latecomers may ultimately lose in the bee mating game. I was not surprised to see one of the first females of the season quickly captured, mated, and guarded by an eager male bee.


To prevent interlopers from mating with a female, males guard their mates long after mating is complete.

Mason bee larvae consume pollen cakes prepared by their mother.

In addition to being highly entertaining, mason bees provide valuable ecosystem services by pollinating a variety of native flowering plants and some that bear many of my favorite fruits and nuts such as apples, cherries, blueberries, and almonds. Female mason bees spend busy days gathering pollen and nectar from which they fashion pollen cakes. The future mothers then fill the cardboard tubes and wooden galleries with pollen cakes. Before each cake is sealed in a chamber, the female mason bee deposits an egg on it. Eggs hatch into tiny bee larvae that consume the cake as they develop and grow during summer and fall. They complete development during autumn, hunker down for winter, and are ready to emerge just in time for the return of spring.

Noxious bee-mites hitch a ride on the back of a hornfaced mason bee.  

This season as mason bees emerged from galleries, I noticed several festooned with hitchhiking phoretic mites. These nocent eight-legged parasites infest colonies of mason bees where they kill bee eggs and consume the pollen cakes fashioned by the mother bees as food for her young.  As bees exit their natal galleries, mites cling to the bee’s body and hitch a ride to a new nesting site where they can drop off and plunder provisions of these hard working bees. I recently read that a well-timed thermal treatment is likely to put a damper on these bee parasites and I plan to give it a try a bit later in the season. Like many other bees we have met in Bug of the Week, mason bees are gentle and not at all interested in stinging humans. I handled several adults and received a couple of cautionary bites, but never a sting.

Nesting materials for mason bees can be purchased commercially and I highly recommend creating habitats for these fascinating native pollinators.


References for this episode included “Bee Pollination in Agricultural Ecosystems” edited by Rosalind James and Theresa L. Pitts-Singer; “The significance of protandry in social Hymenoptera” by M. G. Bulmer; and “Control of the chaetodactylus mite, Chaetodactylus nipponicus Kurosa, an important mortality agent of hornfaced osmia bee, Osmia cornifrons Radoszkowski” by M. Yamada.