Some of the bumblebees sleep in the open to defeat a parasite that will
eventually kill them.
By sleeping in the open they remain cooler slowing the parasites
development and allowing them to continue provisioning for their young
during the daytime.

http://www.colostate.edu/Depts/Entom...97/rempel.html
Bumblebees, Bombus spp., on the other hand appear to have successfully
mastered the use of
altered behavior for their own advantage. Mueller and Schmid-Hempel
(1993) reported that the
parasitized worker bumblebees stayed in the field overnight instead of
returning to the nest. These
workers spent significantly more time in cold areas than did
nonparasitized workers. The cold
temperatures experienced by the bumblebees retarded parasitoid
development and decreased the
parasitoid's survival chances. The parasitized worker's colony
benefited from the prolonged foraging in
the cold night air, and the worker had a prolonged life span as a
result of the reduced development rate
of the parasitoid. Poulin (1992) argues that these changes in
behaviors of parasitized bumblebee
workers are likely to be an adaptive response of the host resulting in
greater inclusive fitness. He
notes that this may be one of the few examples of Smith Trail's (1980)
kin selected-host suicide
hypothesis in practice in nature.




Ook wrote:
I have hundreds of bumblebees in my garden, and they are gentle like honey
bees and love my sunflowers and squash. Good polinators. I have noticed that
in the evenings, a lot of them find a nice cozy leaf or flower and curl up
and spend the night there. In the morning, I have a bunch of sleeping bumble
bees all over the garden. According to wiki they have colonies, but I'm
guessing they do not always spend the night there? Anyone else ever see
this?