You would think that bumble bees (aka bumblebees) would be fairly well known. They are large, brightly coloured, industrious, more or less likable insects and well known for being good pollinators. Many dozens, perhaps hundreds, of scientific studies have been conducted on North American bumble bees and one would hope that the scientists were sure of their bee species. However, bumble bee colours are not to please us, but to warn potential predators that they taste terrible and have a large sting to boot. It helps to get the message across to birds and other insectivores when bees look more or less alike - hence the convergence in colour patterns that makes, for example the tricoloured bumble bees, so difficult to tell apart. This is officially known as Müllerian Mimicry (as opposed to Batesian Mimcry where tasty, harmless animals tend to look like nastier ones) and it occurs among the white-tailed Bombus (Bombus) species too.
The complex of white-tailed bumble bees has variously been called the terrestris-complex or the lucorum-complex after two well known Eurasian bumble bees. However, in 1990 two Swiss scientists and one from Calgary got together to show that the 'lucorum' in North America had different enzyme loci from European lucorum, but ones identical to the more mysterious Bombus moderatus Cresson, 1863, known from Alaska, the Yukon, the old Northwest Territories, northern British Columbia and down the Rockies to Jasper and Banff (the southernmost record) in Alberta. More recently it has shown up in Calgary and is featured at Robert Bercha's Insects of Alberta. Now this bee seems to have colonized Edmonton.
Thus we seem to have a name for our mystery Bombus - but unfortunately B. moderatus was identical in the enzyme loci to several other named Bombus species including one with an older name Bombus cryptarum (Fabricius, 1775). More recent studies by Andreas Bertsch and his colleagues have shown that Bombus moderatus from Alaska and Alberta and B. cryptarum from Europe have very similar, although not identical, 'barcodes' (a region of the mitochondrial gene cytochrome oxidase I). So, eventually what was once known as Bombus lucorum and now probably better called Bombus moderatus, may one day be called Bombus cryptarum.
Update: The BugWhisperer has found Bombus moderatus in his garden in Edmonton. Also, a friend collected several males to the west of Devon last Friday (19 August 2011). That is a probably a good sign, or at least will be if new queens of B. moderatus soon follow. Yesterday I noticed a number of males of what may be Bombus rufocinctus at my fireweed. I guess the bees think summer is coming to an end.
Update: Thanks to Cory Sheffield for pointing out this beautifully written article by Robin Owen at Mount Royal on Bombus moderatus.