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Great Yellow Bumble Bee, Balranald Nature Reserve, North Uist. Via david ian

 

A number of our rarer bee species can be found in the Western Isles, most notably B. distinguendus, or the Great Yellow Bumble Bee, an exceedingly rare species now found only in the Hebrides, Orkney, Shetland and tiny populations on the mainland coast of north Scotland.

Other species that can be seen include B. magnus, B. pascuorum, B. ruderarius, B. lapidarius, B. lucorum, B. soroeensis, B. hortorum and B. jonellus. B. muscorum, now very rare in England, is one of the commonest species, occurring on most islands. In the Outer Hebrides can be found the subspecies B. muscorum smithianus, a most impressive beast with a chestnut coloured thorax and black underside.

The Great Yellow Bumblebee is one of a number of bumblebee species to have undergone a drastic reduction in range and abundance as a result of loss of habitat in the modern agricultural landscape. Agriculture, usually in the form of crofting, in the Western Isles remains much less intensive than in most of the UK, and that is why so many rare species survive. 

In the Western Isles the Great Yellow is typically associated with the clover-rich machair. The western coasts of the Hebridean islands supports this very rare habitat, which is fantastic for wildflowers and bumblebees. Essentially consisting of vegetated sand dunes, managed by low-intensity grazing and occasional planting of arable crops, the low soil fertility ensures that Fabaceae such as red clover and kidney vetch flourish, along with drifts of other flowers. Here the bumble bee feeds on bird’s foot trefoil early in the season before moving on to clover and then at the end of the summer to knapweed.

So far all nests found in the Western Isles have been underground, and those that have been successfully excavated have all been in adapted mouse nests. The numbers of workers of the great yellow bumblebee are lower than most species and the bee itself is larger than most. 

Bumblebee colonies are annual. Fertilised queens establish nests in spring. Several generations of workers (non-breeding females) provide the nest with nectar and pollen, until later in the season males and fresh queens (females destined to breed) are produced. Mating occurs and all die except the fertilised queens, which hibernate underground. The breeding cycle of the great yellow bumblebee is short allowing successful colony formation and production of males and queens within the relatively short northern summer. To some extent this is compounded by their predilection for machair flowers, which are not abundant until June or July.

 

http://www.bumblebeeconservationtrust.co.uk/