Is Lasius neglectus invading The Netherlands?
About 25 years ago, inhabitants of a neighbourhood in Leiden complained about an invasion of small ants which entered their houses. By then, the nuisance already lasted 15 years (from about 1970 onwards). The species was formerly identified as Lasius alienus (Förster) by the Inspection of environmental hygiene (nowadays KAD: department of the control of pest insects). However, the ants in Leiden had a huge colony and lived in a different habitat than L. alienus. Several streets were occupied by the ants, indicating that the species is polygynous (many queens), while L. alienus nests only have one queen. However, at that time the identification as L. alienus was the only possible option, given the available literature. Only after 1990, when a new species was discovered in Budapest and formally described as Lasius neglectus Van Loon, Boomsma & Andrásfalvy, some workers of the colony in Leiden were identified as most probably belonging to the same invasive ant species. This species is polygynous and appeared to have outcompeted all other ant species at the type locality. During the 1990s L. neglectus was found in more countries and on more locations, often causing problems in houses and buildings. The species does hardly perform nuptial flights; young queens and males mate in the nests. Dispersion over long distances is assumed to occur by human transport only, probably with potted plants, soil or garden cuttings. Local dispersion occurs through nest budding. In 2009, the identification of the Leiden ants as L. neglectus was confirmed and some other old reports of 'L. alienus' in the archives of KAD also appeared to be L. neglectus. The species occurs in at least four other places in the Netherlands. The earliest introductions at the sites in Wassenaar and Leiden have probably taken place in the 1960s. In the summer of 2009, the distribution at the site in Leiden was studied. Nowadays the species covers an area of approximately five hectares. Expansion has been partly limited by its competitor L. niger (Linnaeus) and by a canal, although at some time the ants seem to have succeeded to cross a bridge. At this moment the species occurs in Europe nearly exclusively in towns and suburbs, which provide warm places in winter. However, the possibility that they will establish in a natural environment cannot be excluded. In that case the species probably will have a great impact on the local invertebrates. Nuisance by L. neglectus can be controlled by providing moist poisoned food (gels) at nest entrances. This food will be brought to the queens and their brood by the workers. At the same time, protecting competing ant species in the neighbourhood of the colony will be important. Measures should be taken to prevent the introduction of colonies to other areas.