Emigration Raids by Slave-Making Ants: A Rapid-Transit System for Colony Relocation (Hymenoptera: Formicidae)

Polyergus lucidus Mayr is an obligate slave-making ant, found throughout north temperate regions of the world. Their slave raids are dramatic events in which columns of highly aroused workers penetrate nests of the related ant genus Formica, and carry the target colony's pupae back to their own nest (Marlin 1969; Talbot 1967). Although many of these pupae are consumed, varying numbers are reared through eclosion and become permanent members of the mixed-species nest. During the evolution of social parasitism, Polyergus workers lost the ability to participate in the ordinary chores of foraging, nest maintenance, and brood rearing, all of which are left to the Formica slaves. Raiding behavior in Polyergus has only been reported in the context of slave raids, or of intraspecific territorial raids (Topoff et al. in preparation). Field observations of colonies in late summer, however, have revealed an entirely new function of raiding behavior: the rapid transport of colony members during emigrations to new nests at the end of the slave-raiding season. During such colony movements, the low level of mixed-species ant traffic is periodically interrupted by the abrupt emergence of Polyergus workers, and their formation into a well-organized swarm. The Polyergus workers promptly "raid" the old nest, and transport adult Formica individuals to the new site. For such group processes, occurring in the context of nest relocation, we propose the term "emigration raid."