A multilane highway of ants marching across your kitchen floor is the last thing you want in your home, but this is the situation faced by many Californians and southern Europeans since the Argentine ant invaded. According to Jules Silverman, from North Carolina State University, these ants are far less problematic in their native environment. But when they arrived in the northern hemisphere, something changed; the ants' genetic diversity declined,they became less aggressive towards neighbours and formed massive supercolonies, wiping out native ant species. Curious to know how the Argentine ant has been so successful, Silverman wondered why the invaders'colonies were so much larger than in their native environment. Could individual Argentine ant colonies have fused to form supercolonies by adopting queen and worker ants from elsewhere(p. 1249)?
But, according to Silverman, ant colonies don't usually accept outsiders. Each individual is coated with a blend of hydrocarbon compounds, unique to their home colony, which they use to distinguish between colonymates and strangers. But what if individual colonies were closely related and their hydrocarbon cocktails were similar, could the colonies accept each other and fuse to form a supercolony? Teaming up with Gissella Vásquez and Coby Schal, Silverman decided to test out whether invasive Argentine ant colonies accept visitors from other colonies if their hydrocarbon signatures are similar.
Fortunately for Silverman, the Argentinian invaders that have made it as far as North Carolina have retained their genetic diversity with varying degrees of aggression towards other colonies. So Vásquez collected workers and queens from 4 colonies at different locations in the state before teaming up with entomologist Schal to analyse their hydrocarbon cocktail. Having already established the relative aggression levels of each colony,Vásquez found that the hydrocarbon cocktails of queens that were accepted into a colony were very similar to the adopting colony's own cocktail; while hydrocarbon mixtures of the queens which were attacked differed most from the new colony's mixture. So queens can be adopted by a new colony, but only if their hydrocarbon signatures are similar to their new colony's signature.
Having identified the colonies that would accept queens from elsewhere,Vásquez tested whether an adopted queen's hydrocarbon cocktail changed after she had been accepted into another colony, and found that after two weeks it had changed slightly to resemble the cocktail of her new home. The team suspect that the insects could have modified how they synthesise the hydrocarbons, but add that the queens could have picked up hydrocarbons from their new nestmates.
Finally, Vásquez confirmed that the colony uses the hydrocarbon cocktail to recognise impostors by replacing a queen's hydrocarbon coating with that of a colony that her nestmates respond to aggressively. Having returned the queen to her nestmates, Vásquez saw the workers attack her aggressively; they no longer recognised her thanks to her new coating. The queen's hydrocarbon cocktail was essential for recognition.
Having found that it is possible for Argentine ant colonies to accept outsiders if their hydrocarbon signatures are similar enough, Silverman suspects that colonies could fuse and suggests that this could explain in part how Argentine ant gigantic supercolonies have developed.