Behavioral mechanisms of social insect invasions

The spectacular ecological success of social insects stems in large part from their ability to form long-lived, protected colonies. This same attribute also helps explain how invasive social insects, which include some of the world's most problematic introduced species, are able to disrupt the ecosystems that they invade. Ecological success may be further enhanced when introduced populations undergo behavioral changes that increase colony size and longevity. Minor changes in worker-level behaviors can combine to greatly alter colony-level traits. Here we discuss two such examples. As aggressive predators, yellowjacket wasps (Vespula) have emerged as prominent invaders of both continental and island ecosystems. Western yellowjackets (V. pensylvanica), for example, now threaten endemic arthropod communities in Hawaii. Approximately twenty percent of Hawaiian V. pensylvanica colonies are perennial, an occurrence rarely observed in native populations. Perennial colonies can grow to be over ten times larger than annual colonies and have rates of prey retrieval commensurate with their larger size. Dramatic changes in colony size also occur in invasive ants, which are noteworthy because of their common (although not universal) tendency to form expansive supercolonies in their introduced ranges. This syndrome may help explain the ecological success of invasive ants because supercolonies avoid paying costs associated with intraspecific territoriality. Support for this hypothesis comes from naturally occurring territory boundaries of Argentine ants (Linepithema humile). Where opposing supercolonies abut, ants fight aggressively and remain genetically differentiated despite direct and prolonged contact. If shifts in colony-level attributes commonly contribute to the success of social insect invasions, then the task of predicting potential invaders becomes even more difficult.