Significant expansion of the distribution of the bumble bee Bombus moderatus (Hymenoptera: Apidae) in Alberta over 20 years1This paper is dedicated to the memory of Adolf Scholl.

Bombus moderatus Cresson, 1863 occurs in the northern and western regions of North America and reaches its southern limit in Alberta. In 1915, the southernmost record was Banff; by 1987, it had appeared in Kananaskis Country, 40 km southeast of Banff, and by 2010, it had spread 80 km farther east to become one of the more common bumble bee species in Calgary, where it had never been previously recorded. This represents a rate of spread over the last 20 years of about 4 km/year. The simplest hypothesis that can account for this change is that it is just a continuation of the natural expansion of its range since the end of the last ice age. An alternative hypothesis is that it is filling the niche vacated as a result of the decline in another species, Bombus occidentalis Greene, 1858.