Dear Enemy Phenomenon in two invasive ant species, the Argentine ant and the giant needle ant

The Dear Enemy Phenomenon frequently occurs in ant community mosaics where neighboring colonies exhibit suppressed aggression in order to conserve resources. In landscaped areas of the North Carolinas piedmont, well established territories of the invasive Argentine ant (Linepithema humile) are becoming infringed upon by the giant needle ant (Pachycondyla chinensis), a comparatively new invasive ant in this habitat. Adjacent colonies of these two invasive species exhibit reduced aggression relative to spatially separate colonies. This situation is unique given that L. humile typically displace most ant species within their introduced range through competitive superiority. Replicated aggression assays between both neighboring colonies and distal colonies of these two invasive species were conducted in the both the field and laboratory. Aggression between adjacent colonies was significantly lower than between distal colonies. This was consistent between the lab and the field. Our results suggest the occurrence of the Dear Enemy phenomenon and provide the first evidence of the phenomenon involving two invasive ant species. At the very least, our results may provide insight into the mechanisms behind P. chinensis invasion as they continue to spread in this urban environment.