The Asian bush mosquito, Aedes japonicus japonicus (Theobald, 1901), a potential vector of several pathogens, has recently established in North America and Central Europe. In 2013, it was found on the Slovenian-Croatian border, and during the following years, it emerged in more and more counties of northwestern Croatia. Surveillance of Ae . j . japonicus and other invasive mosquito species was subsequently extended both spatially and temporally in Croatia and neighbouring Bosnia and Herzegovina and Serbia. Mosquito collections were conducted in 2017 and 2018, based on adult trapping through dry ice-baited CDC traps and BG-Lure-baited BG-Sentinel traps, larval sampling through dippers and nets, and ovitrapping. Aedes j . japonicus specimens from collected samples were subjected to population genetic analysis by comparing microsatellite signatures and nad 4 DNA sequences between sampled locations and with data previously obtained from more western European distribution areas. Aedes j . japonicus immature stages were found at 19 sites in Croatia, two sites in Bosnia and Herzegovina and one site in Serbia. In Croatia, four new counties were found colonised, two in the east and two in the south of the previously known distribution area. A spread of 250 km could thus be documented within five years. The findings in Bosnia and Herzegovina and Serbia represent the first records of Ae . j . japonicus in these countries. Genetic analysis suggests at least two introduction events into the surveyed area. Among the locations analysed, Orahovica can be considered a genetic border. The individuals collected west of this point were found to be similar to samples previously collected in the border regions of Southeast Germany/Austria and Austria/Slovenia, while the specimens from more eastern Croatian localities, together with those from Bosnia and Herzegovina and Serbia, were genetically different and could not be assigned to a probable origin. Thus, introduction from Central Europe, possibly by vehicular traffic, into the study area is likely, but other origins, transportation routes and modes of entry appear to contribute. Further dispersal of Ae . j . japonicus to other parts of southeastern Europe is anticipated.

In research into mosquitoes in the mountain regions of Croatia, a new species of mosquito for Croatian mosquito fauna has been identified. The Culex pipiens complex is represented with four sibling species. Culex torrentium belongs to this complex and morphologically differs from the other species only in the structure of male genitals. The presence of this species is determined based on the male genital structure and confirmed by mitochondrial cytochrome c oxidase molecular analysis. Specimens of these species have been found in all three mountain areas covered by this research: Slavonian mountains, Gorski Kotar area and central Velebit Mt. The lowest altitude where we found this species was 159 metres above sea level (m a.s.l.) in Pleternica, and the highest 906 m a.s.l. in Baške Oštarije. At all sites, the species Cx. torrentium was sampled in community with other species, but most frequently together with sibling species of Cx. pipiens (most probably the subspecies Cx. p. pipiens).