Very strange. We have always had a few southern (or vestal) cuckoo bumblebees - bombus vestalis - but for the last few weeks they have been so numerous they account for half of all bumblebee sightings and definately more than the buff-tail bumblebees that they predate on.
Cuckoo bumblebees will enter another bumblebee species nest and lay some eggs, using thier pheramones to influence the workers to look after them. Normally they do not take over a host colony completely like the bird cuckoo would, but co-exist with them. So it is very odd to have so many. It may be that last year a vestalis got really carried away and laid more eggs than norma. Logically, this is not sustainalbe as they will not all find host buff-tail nests and so next year I would expect the ratios to revert to normal.
You can tell these are cuckoos by two factors:
- no pollen baskets on thier back legs - because they get other workers to do all that for them
- very dark wings
They are large and striking bees but I feel a bit sorry for the poor buff-tails which are normally my most common bumblebee.