There is a series of papers by Hobbs and colleagues in the 1960s that
discussed habitat requirements for different Bombus species. Here's one
reference to get you started:
Hobbs, G.A. 1968. Ecology of species of Bombus in southern Alberta. VII.
Subgenus Bombus. Canadian Entomologist 100:156-164.
This is back when they were investigating suitability of Bombus for
management as pollinators.
David W. INOUYE
Dept. of Zoology, University of Maryland
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Phone:301-405-6946