> And I can say from personal experience that failure of bumblebee colonies is very common, for reasons unknown

I concur with that. About ten years ago, I was helping a Cornell grad student establish and maintain bumble bee colonies in an outbuilding at the University. My contribution consisted mainly of renovating the building, installing some tables, and supplying her with honey bee collected pollen. 

She would catch bumble bee queens in the spring and install them into small wooden nest boxes. She had up to about 30 of these, set about on the large tables. These were not free flying colonies, so they had to be fed constantly: sugar syrup and bee pollen. The bees seemed content to live and grow in this odd arrangement.

There were a few parasitic wasps as I recall. I don't know how they found the nests. Anyway, late in the summer after the experiments were done, she would set them out of doors. They were mostly dead in a week, due to being preyed upon by all sorts of predatory insects. It's a bee eat bee word out there.

Pete

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