Range expansion in an introduced social parasite-host species pair.
Dispersal in social parasites is constrained by the presence of suitable host populations, limiting opportunities for rapid range expansion. For this reason, although hundreds of ant species have expanded their ranges through human transport, few obligate social parasites have done so. We test the hypothesis that social parasites expand their ranges more slowly than their hosts by examining the spread of an introduced social parasite-host species pair in North America - the workerless ant Tetramorium atratulum and the pavement ant T. immigrans. In doing so we report a new range extension of T. atratulum in the interior US. Consistent with host limitation on dispersal, we found a time lag ranging from several years to over a century between the arrivals of the host and parasite to a new region. The estimated maximum rate of range expansion in the parasite was only a third as fast as that of the host. We suggest that relative to free-living social insects, social parasites may be less able to rapidly shift their ranges in response to changes in habitat or climate.