While finding a pile of dead bumblebees under your Buddleia is a disturbing sight, at this time of year it is actually quite natural. So what exactly is going on?
Towards the end of the summer the queen bumblebee in each nest will start to
produce male offspring and new queen bumblebees. The males will leave the nest soon after they have grown and never return. Their primary role is to mate so they will spend their time in search of new queens from other nests.
The new queens will leave the nest during the day to feed and mate but will return each night to the safety of the nest. Once the new queen has mated (an acrobatic process, which most queen bumblebees only do once – see photo) she will spend her time feeding in preparation for hibernation. Newly mated queens are the only members of the colony to hibernate over winter – more on this next month.
Once the new queens have left the nest to hibernate, the rest of the colony will start to die. This includes the old queen, the female workers and the males. It is common to find the dead and dying bees near to flowers – when they are close to the end of their short lives, they become lethargic therefore their natural instinct is to feed on nectar.
There is more information about this on our website, here: http://bumblebeeconservation.org/about-bees/faqs/finding-dead-bees/
Lifted from the Bumblebee Conservation Trust August 2013 newsletter