A post script.
Any study of the causes of Bombus mortality might be useful. I suspect that
recent deaths are related to temperature fluctuations. Bees perhaps came out
of hibernation in January/February and have been searching for nectar in bad
weather conditions ever since.
In my neck of the woods I can attribute many deaths to road traffic
accidents: bees nest in local 'wild' sites but have to fly over a main road
to forage in domestic gardens. Since they fly low and slow they commonly get
smacked by speeding cars or buses.
Paul

> This is a good idea, since we have already had four dead bees in our house
> (three in the dining room one morning). I have no idea how they get in;
our
> windows are not open and the only holes we can see are vents where the
> fireplaces once were, leading to the bottoms of the disused chimneys.