INVASIVE GARDEN ANTS · Evolution of hypersocial organisation in the invasive garden ant, Lasius neglectus

Final Activity Report Summary - INVASIVE GARDEN ANTS (Evolution of hypersocial organisation in the invasive garden ant, Lasius neglectus)

Human travel and commerce have lead to frequent introductions of exotic invasive species, which threaten the existence of many native ecosystems. Social insects and ants in particular, are exceptionally good invaders and are characterised by a fascinating transition of social organisation when they become invasive. Where ant colonies typically are family groups defending their nests against neighbours, invasive ants have universally lost territorial aggression, so that colonies form a single 'supercolony'. This 'peace treaty' with neighbouring nests is an effective alliance against native ants, which are quickly overwhelmed and eradicated by the joint invasive forces. The invasive garden ant, Lasius neglectus, is the most recently discovered invasive ant, described only 15 years ago. It is the first pest ant being able to invade temperate regions of Central Europe, where it has now spread to approximately 100 known sites. The aim of this project was to determine the invasion history of this ant and to reconstruct the evolution of its social transition towards invasive supercolonies. My collaborators and I did an extensive genetic analysis of 18 known populations. This included a phylogeographic tree to assess how the different introduction sites relate to each other, and an analysis of the genetic variation in individual populations to see how much variation was lost during the introduction events. Our data indicate that all populations likely originate from the same geographic area in Turkey, and that some invasive populations have recently split from other invasive populations. Such 'mother - daughter' populations of supercolonies can extend over ca. 1000 km. Knowing that invasive populations are often the starting point for new introductions (usually by human transport of potted plants) has important implications for biocontrol measures, which should be taken to prevent further spread of this pest ant. We also investigated whether supercoloniality evolved de novo in the invasive garden ant, or whether (pre)adaptations for this unusual form of organisation already existed in closely related non-invasive ants. The sister species, Lasius turcicus, occurs both at low and higher altitudes in Turkey. We found that the highland populations resemble typical ant species in having family based societies and founding new nests after a mating flight. However, the social structure of the lowland populations of L. turcicus was partly similar to that of the invasive sister, as mating occurs in the nest (without flight) and some colonies may form supercolonies. However, these L. turcicus supercolonies only comprise about 5 neighbouring nests, whereas in the invasive L. neglectus supercolonies can include thousands of nests and several localities. The lowland ants in Turkey thus already express some invasive features at a local scale, but have not yet made the step to become ecologically dominant over the entire native ant communities. We therefore expect that this lowland form would have a very high potential to become the next invasive ant in Europe, once it should be accidentally transferred out of its native range into a new environment. Why is it that invasive ants give up their territorial aggression and behave peacefully to members of neighbouring colonies of their species? We tested the hypothesis that invasive ants might have a lower potential to detect that an alien ant belongs to a different colony because the information encoded in body 'smell'- the most important cue for recognition in societies of nearly blind ants - has somehow been reduced. However, we found that the chemical profiles of invasive garden ant odours are equally informative than those in the lowland and highland forms of L. turcicus. We thus conclude that the ants apparently change their perception of 'familiar' versus 'alien' depending on the form of society to which they belong, with ants growing up in open societies being more tolerant to alien intruders.

Data: CORDIS, © European Union