Bumblebees only live one year at most, and queens often spend the winter in leaf litter or in the ground.
This is one more important reason why we should leave some leaf piles on the ground in the winter. They provide important habitat for bees and other insects. The article concludes:
The authors agree, and note that [in addition to warming] the paper shows habitat loss was also a driver of local extinction. Kerr stresses that “we’re not arguing against the role of habitat loss and pesticide misuse as [drivers] of decline… we think the case for those things is strong, but just different.”