Raised from decline: reassessment of the occurrence of Lasius neglectus (Hymenoptera: Formicidae), an invasive ant species, in Warsaw, Poland

Biological invasions are extensively studied, however some aspects of their long-lasting dynamics remain unknown. For example, populations of invasive species were often too hastily recognised as regressed or even collapsed. Here we pay attention to the need for long-term monitoring and identification of key factors that determine the possible success or collapse of the invasive populations to accurately develop management plans and reduce the ecological threats, using the example of the ant species Lasius neglectus in Warsaw, Poland. The invasive garden ant L. neglectus has been spreading rapidly in Europe since the 1990s. These ants established enormous supercolonies in many European cities, posing a threat to the local native ants. Over two decades the spread of this species has not slowed down. However, after an initial period of rapid development, at least some populations of this species entered a phase of stagnation or regression. This was demonstrated by the research conducted in the second decade of the 21st century in several cities (including Warsaw) of four European countries where over 93% of 29 supercolonies inspected at that time did not display invasive features. Our current inspection, carried out in Warsaw after 10 years from the previous one, showed that all five supercolonies known before 2013 – including two previously recognised as declined, one as stagnated and one as extincted – still exist and are doing very well. Moreover, we revealed several new localities of L. neglectus in Warsaw mostly represented by well developed supercolonies. The results are discussed in the light of the concept of boom-bust dynamics in biological invasions of ant and its practical implications.