Evidently little is known about most of the bee families. There are not enough scientists to study them all. At the Ask a Biologist site I came across an article by John Alcock, professor at the ASU School of Life Sciences. Alcock first noticed the grey bees flying in low circles over open patches of sandy ground. While he watched, one of the bees landed, probed the soil with his antennae, and started to dig. The male bee uncovered a female digging her way up from her natal burrow and the two mated immediately.
This is the digger bee, Centris pallida. Many of our native bee species live underground. The female digger bee does all the hard work, burrowing into sandy and even gravely soil with her jaws and legs. She goes down about a foot then makes a right turn and excavates a brood cell. Sometimes multiple cells are constructed in a branching network.
The digger bee lines each cell with wax secreted from her body, sealing it from moisture and fungi. She then collects nectar and honey from the blossoms of ironwood and paloverde trees and fills each brood pot. She lays a single egg on top of the sweet, sticky food and seals the tiny den closed.
The egg hatches into a grub that eats all the food in the pot. The fat white prepupa lies curled in its cozy burrow for eleven months, waiting for spring. Then, in a double metamorphosis, the prepupa transforms first to a pupa and then an adult bee. The males dig out first and many may swarm above the nest site, awaiting the arrival of the females. Alcock states the bee’s sensitive antenna can smell the females as they approach the soil’s surface.
After mating, the bees feast on pollen and nectar from native blooming trees. The females will soon be digging and provisioning brood cells. I like to stand under the branches of an ironwood tree and listen to the rising and falling hum, a crescendo of digger bee buzz. You can try this if you live nearby, as these bees are non-aggressive. Or, see a short video of a female Centris pallida digging by clicking the link.