Factors contributing to the dear enemy phenomenon between two invasive ant species, the Argentine ant and the Asian needle ant

The Dear-Enemy phenomenon can occur in ant community mosaics where neighboring colonies exhibit suppressed aggression in order to conserve resources. In landscaped areas of the North Carolina Piedmont, the Asian needle ant (Pachycondyla chinensis), a comparatively new invasive ant in this habitat, persists within well-established territories of the invasive Argentine ant (Linepithema humile). Adjacent colonies of these two invasive species exhibit reduced aggression relative to spatially separate colonies even though L. humile is noted for displacing resident ants within its introduced range. This example of Dear Enemy Phenomenon is unique given that L. humile typically displace most ant species within their introduced range through competitive superiority. We recently discovered that by separating nests of the two cohabiting species we found that, over time, these ants displayed high levels of interspecific aggression upon re-introduction while nests with continued contact displayed low or no aggression. We suggest that “interspecific neighbor” recognition cues are altered when contact is restricted. Elucidating mechanisms behind the Dear Enemy Phenomenon can provide insight into the ability of a new invasive to expand its range from forested to urban environments.