Remote Scottish islands a stronghold for one of the UK's rarest insects

08/08/2006 00:00:00 Multiple nest sites of one of the UK's rarest bees have been discovered by RSPB Scotland on the Uists, revealing the islands to be the most important habitat in the UK for the species. More than ten colonies of the Northern colletes (Colletes floralis) were found on sandy dune sites including Berneray island off the north tip of North Uist, making it the most northerly nesting site for the threatened insect in the UK. The Northern colletes is a solitary variety of mining bee, and burrows underground into soft soil to build its nest where it stores nectar and pollen for its larvae. They differ from bumblebees and honey bees in having no workers. Although they do not co-operate with each other, they nest in what are termed ‘aggregations’ - the insect equivalent of rookeries.

Because of this it prefers gently sloping sandy banks and dunes, close to the herb-rich machair meadows familiar on the islands. The meadows host an enormously rich variety of wild flowers due to the fertile calcareous, shelly sand. It is because this soil produces a much higher diversity of flowering plant species that the bees appear to thrive in the habitat. Jamie Boyle, RSPB Scotland's Uist warden, said: ‘This is really great news and extremely encouraging for this struggling and very rare species. As well as in the Uists, there are only a few other isolated UK locations that the Northern colletes bee occurs, such as on the Ayrshire coast - where it was first discovered in the UK more than a century ago - on the Cumbrian coast, on Irvine Moor and on Machrihanish as well as off the northern coast of Ireland.’

Colonies discovered
Previously there were also just a handful of known nesting sites for the threatened bee in the Western Isles. But this year there has been a much more concerted effort to find the colonies, resulting in discovery of eight nests in one small area of north Uist, separate from another huge colony on north Uist and a further two large colonies on south Uist. RSPB Scotland staff have been joined by other wildlife enthusiasts searching for the bees, and have even found nests on some of the small islands such as Berneray.

Habits
Adults of the Northern colletes bee are active from mid-June to late August. The male bees emerge first a day or two before the females. The females are probably mated soon after emergence. The male then dies and the mated female constructs a nest burrow which can be up to 26 centimetres deep - a considerable excavation job for an insect just over one centimetre long. The females tend to lay in proximity to others, so the nest ‘aggregations’ are formed. They produce a secretion from glands in their mouths which they use to coat the inside of the burrow before laying their eggs in individual sealed cells. Each cell contains a food reserve comprised of regurgitated nectar and pollen that will feed the larva and then support the pupa through the winter while the bee develops. In June, the male bees emerge first and fly around the nests waiting for the females to emerge, and for the lifecycle to begin again

Worldwide distribution
Jamie added: ‘Outside the UK it is really quite an unusual insect in that it occurs in very low densities around the Baltic at sea level in Finland and in Sweden and in southern Norway in the Oslo area. But bizarrely, this little bee is also found in alpine habitats, although with very limited distribution in the Pyrenees, Carpathians and eastwards into the Altai.

‘The populations in Britain are of international significance because they are the only places that it occurs in the Atlantic bio-geographical zone, so they are of huge importance. What's more, we are talking about a bee that, as far as its European range is concerned, is likely to be threatened by climate change as a result of being driven off the tops of mountains in the south and finding its habitat no longer available as sea level in the Baltic area where it is present in very low numbers. As part of the big picture for the species, the population on the Hebrides is of enormous significance and worth making every effort to conserve.’

The Northern colletes is so rare that it is one of the species listed on the UK's Biodiversity Action Plan (BAP), with RSPB as a lead partner in efforts to try and conserve its UK population.