Last year I found a few specimens of Bombus bohemicus, which is parasitic on Bombus lucorum. Today I noticed the head of a bumblebee poking out from some grass:
After a few minutes' observation it was clear that I had another Bombus bohemicus in front of me, so they clearly managed to find a lucorum nest last year.
The main identification feature is the yellow tail band which almost mirrors the curved golden band on the thorax. Another give-away is the total lack of pollen baskets on the legs.
These cuckoo bumblebees find a nest where the host queen has already laid eggs and then a fight to the death ensues. The cuckoo females then lay their eggs in the host nest. The host workers hatch out and then proceed to feed the young of the cuckoo bees for the duration of the season.
I find this total destruction of the host to be a rather dramatic form of parasitisation, since the parasitised nest will not produce any hosts for the following year. Strictly, these cuckoo bees are kleptoparasites, since they don't live off the body of the host: they 'merely' steal the pollen that would have been used to feed the host larvae.
Status of Bombus bohemicus in Ireland: threatened.