At about the same time as the
queen starts laying unfertilised eggs that will produce males, the ovaries of
some workers, usually those performing household duties, may develop. It is
possible for workers (without mating) to lay unfertilised eggs that will
develop into males, however workers cannot produce queens or other workers.
Some workers try to lay eggs of their own, and may even attempt to eat eggs
laid by the queen (eating their own sisters). This leads to aggression between worker and worker, and
worker and queen. In many cases the more persistent workers will succeed in
laying some eggs that will reach maturity. The queen will also attempt to eat worker -laid eggs (her own grandsons) and head butts and bites her daughters to try to maintain her dominance.
A recent study on Bombus terrestris showed that 10% of adult male bumblebees came from worker-laid eggs.
The photograph on the right shows a Bombus hortorum male who has spent the night inside a flower, and the one above right shows a male resting under a flower head.
Males do not return to the nest once they have left it, so spend their nights either inside or hanging under flower heads. In the morning they are often very lethargic and may appear to be ill, but this is normal. They just need to get up heat by drinking nectar or being warmed by the sun or both.
Have a look at his rear leg - it has no pollen basket. Only males and cuckoo bumblebees have no pollen basket.