For a long time there was thought to be only one species of Pollenia in North American but when there was a close examination of the collections six species were determined to occur here. The key, Cluster Flies (Calliphoridae: Polleniinae: Pollenia) of North America by Jewiss-Gaines et. al. (2012) has photographs of each and distributions. Even with this and a lot of peering at leg hairs I am not totally convinced which species that this was. I *think* that it is Pollenia rudis. This is the more common one and the one that species that was considered to be in North America prior to the splitting. All six species have been found in B.C.
It is assumed that cluster flies were introduced from their native Europe in soil in the ballasts of ships with the earthworm.