In early July, 2005, we walked along the bank of the mouth of the Elwha River, in Washington State, on a sunny day and saw a bee which seemed to be carrying something and circling around as if searching for something. We stopped and watched it until we saw it land near a rock. Gently lifting the rock revealed the bee in a hole in the sand. This was a Leafcutter Bee, (one of three groups in the Family Megachilidae). Leafcutter Bees typically line their egg-laying holes with bits of leaf, separating egg chambers with circular pieces of leaf. The first video shows this Leafcutter Bee emerging from her nest hole (slide 6)

Borror, Delong and Triplehorn write, in An Introduction to The Study of Insects, 4ed, “It is not uncommon to find plants from which circular pieces have been cut by these bees.” Indeed, we did find a Bigleaf Maple nearby with leaf cutouts that exactly matched the size of the circular bits of leaf we saw under the rock (slides 4 and 5).

Minutes later, we noticed another, darker, bee flying around the same area. As we watched, it landed near the hole we were watching. In the second video you can see this bee pulling a leaf plug from the nest hole of the Leafcutter Bee, comically rolling over while holding on to the leaf wheel. Then she enters the hole, and turns around to deposit an egg in the hole (slide 7).

This was a Cuckoo leafcutter Bee, another megachilid, in the genus Coelioxys. They lay eggs in the nests of other bees, especially other megachilid bees. The larvae typically kill the host larvae (thus they are properly called parasitoids) and feed on the pollen stores. The name comes from the Cuckoo, a bird that lays eggs only in the nests of other birds.