Natural history of an invasive ant and its associated mites

The success of invasive species is generally linked to interactions with co-occurring species in their native and introduced ranges. Within their native range, species have co-evolved with predators, parasites, and pathogens that limit their population growth; if these so-called “enemies” are absent in a species’ introduced range, their populations may expand unchecked (the “enemy release hypothesis”). Conversely, invasive species may bring with them new parasites and pathogens that spillover to infect potential competitors in their introduced range, which could then pave the way for their invasive success (the “parasite spillover hypothesis”). Here we evaluated each of these hypotheses using the Asian needle ant, Brachyponera chinensis, and their associated mites as a model system. Ants are often hosts to a range of phoretic and parasitic mite species that sometimes display tightly co-evolved relationships. We sampled needle ant populations in Japan, where they are native, and North Carolina, where they are invasive, to determine the abundance and diversity of mites associated with individual workers. In total, we identified nine species of associated mites, all of which were found in their introduced range, suggesting that needle ants are not undergoing enemy release. We then investigated whether mites were found in co-occurring insects that nest in logs with needle ants, but we found no evidence of spillover. Finally, we compared the seasonal abundance of ant-associated mites and their affinity for dispersing sexuals, and we found evidence of tightly evolved as well as loose relationships between parasites and their hosts.