When I went to clean out one of the bird nest boxes, I was surprised to find that a western bumble bee (Bombus occidentalis) had made its nest inside. Typically this kind of bumble bee makes its nest in the ground. But here was the clump of waxy cup-shaped cells described in the books nestled in the dog hair and sticks that the Bewick's wrens had been using to build their nest. (This could explain why the wrens decided to move out and rebuild their nest in the pot next to the back door.)
Alarmingly, all of the bumble bees, adults and juveniles, still developing in their wax cells, were dead. What killed them? Was is the cold weather snap we had in October/November? There was a garden spider in among the corpses. Had this small spider preyed on the bumble bee nest? I can't imagine that it could have gotten to the juveniles in their waxy cells. Was there some kind of mite or fungus that caused the death of all ages? I don't know.
During the winter the queen bumble bee hibernates in a protected location all alone. She will emerge in the spring to start a new colony and then usually die. I hope that there was at least one survivor from this western bumble bee nest. (More on bumble bees: xerces.org) Our native bees are vital pollinators and typically are not susceptible to the same diseases and environmental conditions that threaten European honey bees. Most are not aggressive and are not a threat to people.
We have noticed carpenter bees nesting in the yard before but this is the first time for bumble bees. I've wanted to create a native bee nesting site for some time. It seems now is the time to finally take on that project.