Although they are not generally known as mason bees, the members of the genus Dianthidium certainly outshine Osmia bees when it comes to actually building something. Dianthidium bees collect pebbles, stones, and gravel, then cement them together to form a structure that can be pounded by the elements. The resulting nests are both impressive and beautiful.
This past summer I was fortunate to find several Dianthidium bees at the home of Larry and Naomi Price in Prineville, Oregon. They have a sweet little patch of Grindelia (curly-cup gumweed) alongside a gravel road. The patch attracts the most amazing collection of native bees I have ever seen. Among the blooms, I found some colorful Dianthidium bees back in July.
Another member of the Megachilidae
As you might guess, Dianthidium are related to Anthidium in the family Megachilidae. The name means “another Anthidium,” signifying their close relationship and many similarities. But while most Anthidium species nest in pre-existing cavities, some (but not all) of the Dianthidium build from scratch.
The ones that build choose a rock, a twig, or even an exposed root in an underground cavity, and begin by gluing a foundation layer onto the chosen surface. They collect pebbles, tiny stones, and “rocklets” to build with, and cement them together with plant resins and chewed leaves. Then they collect tiny particles—sand or soil—to use like grout wherever reinforcement is needed.
A completed stone hut
The pebble nest shown at the bottom of the page was photographed in Arizona by Patti Zigler. Bear in mind that the bees in my photo are not necessarily the same species of Dianthidium as the ones that built the nest. Nevertheless, it will give you a very good idea of what these bees look like and what they can do.
As you can see, the exposed nest is built on a tree branch. The entrance hole is at the bottom so precipitation does not drip onto the brood. Although these bees are found across the United States and into Canada and Mexico, the ones that build nests like this are most likely to be found in drier areas where there is limited rainfall to erode the structure. When I look closely at the rocklets, it seems improbable that a little bee about 5–12 mm long could collect them. Bees are truly amazing.
One waspy-looking bee
Like all bees in the Megachilidae family, Dianthidium carry pollen in an abdominal scopa which you can see in the first photo. The integument of these bees is usually black with contrasty yellow markings, making them look especially wasplike. They are solitary bees that forage on many plants, but they have a preference for those in the Asteraceae family, like the Grindelia shown below.
Grindelia offers a rich source of nectar and pollen, but it also offers plant resins which can be used as glue and waterproofing. In fact, after touching these plants, my hands smell strongly of turpentine—or something similar—for hours afterwards.
Keys to identity
For those of you who like to identify bees, Dianthidium have a pronotal lobe that is translucent rather than opaque, they have a mid-tibial spine on the hind leg, and they have an arolium between the tarsal claws that Anthidium are lacking.
I extend a special thanks to Patti Zigler for finding, photographing, and posting the pebble nest.
Honey Bee Suite