In early July, 2005, we walked along the bank of the mouth of the Elwha River, in Washintton State, on a sunny day and saw a bee which seemed to be carrying something. We stopped and watched it until we saw it land near a rock. Gently lifting the rock revealed a bee in a hole in the sand (1). Note the two circular bits of leaf and the bee’s hairy hind end sticking out of the hole. We took a short video of this bee as it continued to work the hole, finally emerging and flying away (2). At the very end of the movie, note that the bee’s wingbeats create enough downdraft to flip a leaf circle. Image 3 is a zoomed close up of the hole the bee was in. You can see bits of leaf on the walls and blobs of yellow pollen. We never did get a still of this individual. This is a Leafcutting Bee, (Family Megachilidae), which typically line their egg-laying holes with bits of leaf, and separate egg chambers with circular pieces of leaf.
According to Boror, Delong and Triplehorn, 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 with leaf cutouts (4) that exactly matched the size of the circular bits of leaf we saw under the rock.
Minutes later, we noticed another bee flying around the same area (5). As we watched, it landed near the hole we were watching. In the movie (6) you can see this bee pulling a leaf plug from the nest hole of the Leafcutting 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.
This is a Cuckoo Leafcutting Bee, another megachilid, in the genus Coelioxys. Cuckoo Leafcutting Bees lay eggs in the nests of other bees, especially other megachilid bees. The larvae do not parasitize the host larvae, they just compete for food. The name comes from the Cuckoo, a bird that lays eggs only in the nests of other birds.