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.

Invasive species can encounter environments different from their source populations, which may trigger rapid adaptive changes after introduction (niche shift hypothesis). To test this hypothesis, we investigated whether postintroduction evolution is correlated with contrasting environmental conditions between the European invasive and source ranges in the Asian tiger mosquito Aedes albopictus. The comparison of environmental niches occupied in European and source population ranges revealed more than 96% overlap between invasive and source niches, supporting niche conservatism. However, we found evidence for postintroduction genetic evolution by reanalyzing a published ddRADseq genomic dataset from 90 European invasive populations using genotype–environment association (GEA) methods and generalized dissimilarity modeling (GDM). Three loci, among which a putative heat‐shock protein, exhibited significant allelic turnover along the gradient of winter precipitation that could be associated with ongoing range expansion. Wing morphometric traits weakly correlated with environmental gradients within Europe, but wing size differed between invasive and source populations located in different climatic areas. Niche similarities between source and invasive ranges might have facilitated the establishment of populations. Nonetheless, we found evidence for environmental‐induced adaptive changes after introduction. The ability to rapidly evolve observed in invasive populations (genetic shift) together with a large proportion of unfilled potential suitable areas (80%) pave the way to further spread of Ae. albopictus in Europe.

Summary Lowland riverine floodplains are among the most dynamic and heterogeneous of ecosystems, but many have been extensively modified. Their provision of important habitats provides a key motivation for restoring natural flow regimes (in terms of magnitude, frequency, duration, timing and rate of change of hydrological conditions). Despite general recognition that aquatic insects respond to changes in water availability, there is little agreement about the influence of flow regime components on community structure in floodplains. We analysed a 6‐year (2005 and 2007–2011) data set of aquatic insects (Heteroptera and Coleoptera) in a large Danubian floodplain characterised by pronounced hydrological variability. We hypothesised that abundance and assemblage structure would depend on flow regime components. In particular, we predicted that high‐flow events in the mainstream and concurrent high‐magnitude floods of prolonged duration would have a positive effect on diversity and abundance and that insect assemblages would differ between phases of hydrological connectivity. In addition, we assessed whether the survival of two threatened aquatic insects, Graphoderus bilineatus and Berosus geminus, depends on hydrological connectivity. Abundance and species richness of Heteroptera and Coleoptera increased during years with frequent extremely high flood pulses and prolonged flooding. Multivariate ordination showed subtle relationships between hydrological conditions, hydrological connectivity and insect assemblages with number of days in flood providing the best fit. Spring floods had a more pronounced effect on the assemblages than autumn floods. The time lag between flood occurrence and its apparent effect on aquatic insects was 17–31 days, reflecting insect life cycles. The abundance of G. bilineatus (Coleoptera: Dytiscidae) was affected by mean water level during the month preceding sampling, whereas the abundance of B. geminus (Coleoptera: Hydrophilidae) was related to the occurrence of prolonged high‐magnitude floods. The current water regime of the Danube River favours generalist species with high dispersal capacities and broad niches and food resources. Changes in flow management practices could remove the positive influence of floods on aquatic insect communities and lead to losses of threatened species that depend on these river–floodplain habitats. Our results contribute to the understanding necessary for appropriate evaluation and prioritisation of international floodplain conservation efforts.

Improvement of morphological and molecular identification methods allows the detection of new species of mosquitoes. The mosquito fauna of Croatia currently includes 52 species, belonging to eight genera, including Anopheles (12 species), Aedes (24 species), Coquillettidia (one species), Culex (seven species), Culiseta (six species), Orthopodomyia (one species), and Uranotaenia (one species). This is an updated checklist, which includes five new species found in Croatian mosquito fauna. Two of these are invasive mosquito species, Aedes albopictus (Skuse, 1895) and Aedes japonicus (Theobald 1901), which are spreading across Europe and Croatia. The other three species, Culex laticinctus (Edwards 1913), Culex torrentium (Martini 1925), and Anopheles daciae (Linton, Nicolescu & Harbach 2004) are autochthonous species which haven't been recorded so far. Since there are several more invasive species spreading across Europe, we assume that this is not the final list.

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