Argentine ants (Linepithema humile) usually actively displace native ants through a combination of rapid recruitment, high numerical dominance and intense aggressive fights. However, in some cases, native ants can offer a strong resistance. In Corsica, a French Mediterranean island, local resistance by the dominant Tapinoma nigerrimum has been proposed as a factor limiting Argentine ant invasion. With the aim of evaluating the abilities of T. nigerrimum in interference and exploitative competition, this study tested in the laboratory the aggressive interactions between this native dominant ant and the invasive Argentine ant. We used four different assays between L. humile and T. nigerrimum: (1) worker dyadic interactions, (2) symmetrical group interactions, (3) intruder introductions into an established resident colony, and (4) a competition for space and food. This study confirms the ability of Argentine ants to compete with native species, by initiating more fights, using cooperation and simultaneously deploying physical and chemical defenses. However, despite Argentine ant fighting capabilities, T. nigerrimum was more efficient in both interference and exploitative competition. Its superiority was obvious in the space and food competition assays, where T. nigerrimum dominated food in 100% of the replicates after 1 h and invaded Argentine ant nests while the reverse was never observed. The death feigning behavior exhibited by Argentine ant workers also suggests the native ant's superiority. Our study thus demonstrates that T. nigerrimum can offer strong competition and so may be able to limit the spread of Argentine ants in Corsica. This confirms that interspecific competition from ecologically dominant native species can affect the invasion success of invaders, notably by decreasing the likelihood of incipient colony establishment and survival.

Social insects have been particularly evolutionarily successful: they dominate terrestrial ecosystems all over the globe. Their success stems from their social organization, where one or a few individuals reproduce, whereas others carry out different colony tasks. From an evolutionary standpoint, social species are particularly interesting because natural selection acts at both the individual and colony levels. Therefore, we might expect to see selection acting simultaneously on personality at the individual level and colony level. In this study, we tested whether captive colonies of the ant Aphaenogaster senilis exhibited different behavioral types and evaluated their consequences for intraspecific competition. Our results demonstrate that colonies of the same age exposed to standardized laboratory conditions did indeed have different personalities. In addition, we found that A. senilis demonstrated a behavioral syndrome that included proactive and reactive behaviors: colonies varied in their approaches to exploration, risk taking, food retrieval, and conspecific interactions. This syndrome appears to be associated with a trade-off between competition for food resources and temperaturerelated foraging risks. "Bold" colonies contained individuals who more readily explored novel environments, exhibited aggressive behaviors, and demonstrated higher food-retrieval efficiency during intraspecific competition trials. However, such colonies were also more risk prone: workers suffered higher mortality rates because they more frequently foraged over their critical thermal maximum. The trade-off we observed under laboratory conditions might be key in maintaining colony-level personality, thus driving local-level adaptations in collective behavior.

The Argentine ant (Linepithema humile, Mayr) is a highly invasive species that has successfully spread from its native range in South America across many zones of the globe. In Southern Europe, two continental supercolonies have been identified, the Catalonian supercolony and the main European supercolony spreading over 6,000 km. In Corsica, a French Mediterranean island, the Argentine ant has been present for 60 years. Here we compare patterns of intraspecific aggression and cuticular hydrocarbon profiles of Argentine ants in Corsica to three mainland European colonies. Chemical analyses reveal the existence of cuticular signature variations among the six study sites relative to a gradient of aggression. We find two distinct colony groups not belonging to the Catalonian supercolony, suggesting that the new population originates either (1) from an independent introduction event from the native range resulting in a third European supercolony, or (2), given the chemical proximity and the moderate level of aggression between the two groups, from an existing European population followed by a drift producing a division within the main European supercolony.