queens appear to maintain dominance purely by aggressive behaviour, though it is believed that a dominant queen secretes a pheromone that suppresses the glands in workers that would otherwise lead to their ovaries developing.
In many species the queen is bigger than the workers, she uses her size to dominate workers by opening her mandibles and
head-butting the most dominant worker from time to time. This is usually sufficient until unfertilised eggs are laid, or a
worker's ovaries develop.
Although bumblebees produce honey, the quantity
produced is not enough to make it worth while domesticating them as has been
done with honey bees. Even in the largest nests they is usually just a few teaspoons of honey.
So how do you know if you are
upsetting a bumblebee? Well it's quite simple really. If the bee is on a flower
or other surface and is feeling threatened it will raise one of its middle
legs. This is a sign that you are too close and should back off a bit.
The Bombus impatiens in the photograph on the right is showing this defensive behaviour. B. impatiens is native to the eastern U. S., but is sold commercially in the western states as a pollinator for glasshouses etc. A visitor to bumblebee.org kindly allowed me to use this image.
In cold weather a bumblebee feeling threatened may fall to the ground to avoid you, as it hasn't built up
enough heat to fly off. It is said that bumblebees don't like human breath, so
if you want to observe one closely then don't breathe on it.
While marking and measuring bumblebees I noticed that one would
sometimes give out a high pitched buzz similar to that when they sonicate to
dislodge pollen from tomatoes and other similarly constructed flowers. The bee
was trapped at this time with no chance of flying away, so I have always
believed that it was angry, perhaps I was holding it down too firmly, or
perhaps it was just fed up being handled.