The problem with bumble bee boxes is getting the bees to move in. Depending on the type of house and the location, average occupancy rates range from about 10% to 60%, but the number I see most often is 30%. So how can you in increase it?
One thing obvious about bumble bees is that they like birdhouses. Birdhouses frequently become bumble bee houses, but only after the birds have done all the work of collecting materials and building a nest. Bumble bees also like used rodent burrows for the same reason—they are completely furnished and move-in ready.
Commercial birdhouses often come with upholsterer’s cotton to attract bumble bees, but the bees prefer used nests every time. It could be they like grass and feathers better than cotton, or it could be that used nests have an odor the bees can detect which helps them find the nests in the first place.
Several people have recommended collecting rodent nests to line the bee boxes, but not being a real rodent fan, I’ve begun collecting abandoned bird nests for my bumble bee dwellings. The bird nests I have so far are from small birds that use small bits: moss, feathers, dog hair, threads, and grass. It seems to me that bees, being small themselves, would like smaller nest materials, so I decided to collect that type.
The literature on bumble bees is somewhat confusing. Many people have studied ways to attract bumble bees, but most papers show vastly different results. Most agree that buried boxes are better than ground level boxes which are better than just-above-ground boxes. Almost no one mentions birdhouses, but one of the most common queries I get is “How can I get rid of the bumble bees in my birdhouse?” The collection of bird-turned-bumble-bee houses at Oregon State University further confirms this common phenomenon.
Another thing people are doing is catching bumble bee queens in the spring, placing each one in bee box with a little honey and some nesting material, and locking them up for a couple days—sort of like saying, “This is your house now! Get used to it!” I’m not convinced this is a good idea—it seems intrusive—but it is similar to what we do with a package of honey bees. Maybe it’s worth a try? I’m still thinking on it.
Anyway, I mention this now because if you want to try using bird nests next spring, now is a good time to collect them—they are empty and haven’t yet been ruined by winter storms. I can’t guarantee results because I’m still in the “try it” stage, but it seems like an experiment worth pursuing.