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Status of the invasive species Aedes japonicus japonicus (Diptera: Culicidae) in southwest Germany in 2011

2012; Wiley; Volume: 37; Issue: 2 Linguagem: Inglês

10.1111/j.1948-7134.2012.00252.x

1948-7134

Katrin Huber, Björn Pluskota, Artur Jöst, Klaus H. Hoffmann, Norbert Becker,

Viral Infections and Vectors

Aedes (Finlaya) japonicus japonicus (Theobald, 1901) [Hulecoeteomyia japonica sensu] (Reinert et al. 2006) is one of four morphologically similar subspecies of the Asian bush or rock pool mosquito that is prevalent throughout Japan, Korea, Taiwan, eastern China, and Russia (Tanaka et al. 1979). First reports of Ae. j. japonicus outside its endemic area were from New Zealand where the mosquito was introduced via transoceanic containers carrying used tires from Japan (Laird et al. 1994). In 1998, the species was discovered in the northeastern United States (Peyton et al. 1999), most likely imported by the same mode of transportation. Since then, it has rapidly spread to 29 states in the U.S. (Fonseca et al. 2010). The first record of Ae. j. japonicus in Europe dates to 2000 when a few larvae were found in a storage yard of imported used tires in France (Schaffner et al. 2003). Since 2002, Ae. j. japonicus has been repeatedly observed at a second-hand tire company in southern Belgium (Versteirt et al. 2009). In Germany, the species occurred for the first time in 2008, presumably spreading from an infested area in northern Switzerland (Schaffner et al. 2009). In addition to its capacity to become an established pest, Ae. j. japonicus is a competent vector of West Nile virus (Turell et al. 2001, White et al. 2001, Kutz et al. 2003) and has been shown to be a laboratory vector of LaCrosse encephalitis (Sardelis et al. 2002a), eastern equine encephalitis (Sardelis et al. 2002b), and St. Louis encephalitis (Sardelis et al. 2003) viruses. After the detection of Ae. j. japonicus in 2008, a continuous larval monitoring program was initiated in southern Germany in 2009, starting with 155 municipalities close to the Swiss border (Becker et al. 2010). Our results concerning the establishment of Ae. japonicus japonicus in this area were confirned by the trapping of adult specimens in 2011 (Werner et al. 2012). In response to the detection of Ae. j. japonicus near the city of Stuttgart (Schneider 2011), the monitoring program was expanded to the entire federal state of Baden-Württemberg in 2011. The study was carried out from July 25 until September 7, 2011. This period of late summer was selected to ensure that the population density would reach its maximum. As cemeteries have been proven to be appropriate for the detection of invasive mosquito species (Vezzani 2007, Schaffner et al. 2009), the examinations were mainly of vases at cemeteries, as well as of used tires, rain water barrels and other water-filled containers at allotment gardens and farms. In order to obtain comparable results for the whole area under investigation (about 35,000 km2), a grid map of Baden-Württemberg was used, with each grid the size of ∼135 km2 (11 × 12.5 km). A minimum of 30 water-filled containers was analyzed per grid. A municipality was considered negative if no immature stages were found. If a cemetery was negative and located at the border of an infested area, the surrounding graveyards within the same grid as well as cemeteries of adjacent grids were also examined to get reliable results. To compare the current distribution of Ae. j. japonicus to the results of 2010, all of the 155 municipalities from the previous monitoring period were reinvestigated in 2011. All sampled immature stages were transported to the laboratory and reared to adult stages. For precise species determination, the keys of Yamaguti and LaCasse (1950), Tanaka et al. (1979), and Becker et al. (2010) were used. A total of 6,532 water-filled containers in 291 municipalities distributed over the whole area of Baden-Württemberg was sampled. Of these 291 municipalities, 54 (18.2%) were positive for Ae. j. japonicus. In the area in southern Baden-Württemberg, previously reported positive for Ae. j. japonicus, larvae of the Asian rock pool mosquito were only detected at 24 out of 50 sites (Figure 1). This result indicated that the colonized area decreased considerably from 2,200 km2 in 2010 (Becker et al. 2011) to 1,200 km2 in 2011. Furthermore, another large, apparently independent, infestation area was detected, ranging from the city of Stuttgart to the Swabian Mountains. In this ∼4,000 km2 area (Figure 2), the average larval density was ∼30 larvae/container and thus considerably higher than the densities of the southern region with only ∼15 larvae/container on average. Distribution of Ae. j. japonicus in southwestern Baden-Württemberg in 2010 and 2011. Distribution of Ae. j. japonicus in Baden-Württemberg in 2011; circles represent the municipalities investigated, light gray circles: negative for immature stages, dark gray circles: positive for immature stages. In contrast to other countries, where Ae. j. japonicus is steadily spreading its range of distribution (Morris et al. 2007, Schaffner, personal communication), a decrease in the distribution of this species from 2010 to 2011 was detected for Germany. As precipitation has been shown to have an impact on the population dynamics of other invasive species, such as Ae. albopictus (Alto and Juliano 2001), the notable reduction of the infested area in southern Baden-Württemberg (Becker et al. 2011) might be due to the dry spring in 2011, which was the second driest recorded in Germany. Despite this, water-filled containers in cemeteries are possible breeding sites during the entire season. The disappearance of Ae. j. japonicus from the southern part of the Upper Rhine Valley, which is characterized by its exceptionally mild climate (Liedtke and Marcinek 2002), makes it unlikely that temperature accounts for the decrease. Nonetheless, it is the most favorable region for invasive species like Ae. albopictus in Germany (Pluskota 2008). This is shown by the fact that a municipality on the high grounds of the Black Forest, situated at ∼1,200 m above the sea level, is still positive for larvae of the Asian bush mosquito. Most probably, the lower larval densities at the periphery of the infested area lead to a fluctuation of colonization by Ae. j. japonicus, which results in a certain vagueness concerning the boundaries of this area. In addition to the infested area close to the Swiss border, another hotspot was found near the city of Stuttgart. Ae. j. japonicus had already been reported in this region in 2010, but only five sites southeast of Stuttgart had been analyzed, so that year's dimension of this infested area is unclear (Schneider 2011). The close-meshed monitoring used in 2011 revealed a distribution area of ∼4,000 km2 (Figure 2), which is larger than the colonized areas of Switzerland (Schaffner et al. 2009) and southern Baden-Württemberg combined. It is striking that the larval densities in the newly-discovered distribution area were much higher than those of southern Baden-Württemberg. This could be explained by the more favorable climatic conditions in comparison to the colonized areas in southern Germany (Müller-Westermeier et al. 1999). The mode of dispersal remains unclear, as there is no obvious connection to the southern populations. One possibility could be a second independent source of introduction via the international airport in Stuttgart (Figure 2), which is located in the center of the colonized area. Additionally, the trade of infested used tires within Europe or Germany could be a possibility for distribution, similar to the mode of dispersal in North America (Lounibos 2002). However, any saltatory dispersion would be human-caused, as the distance cannot be covered by an active spread of the Asian bush mosquito (Fonseca et al. 2001). In order to find indications for the favored source of introduction, German Ae. j. japonicus individuals will be genotyped for comparison to other populations in Europe and worldwide. Regardless of the mode of introduction, the occurrence of Ae. j. japonicus in an agglomeration, like the region around the Stuttgart area with its 2.8 million inhabitants, bears increased risks due to the vector competence of this species. The role of Stuttgart as an important commercial and industrial region provides numerous opportunities for the spread of the Asian bush mosquito to other parts of Germany and Europe. Therefore, measurements to reduce population densities and to prevent any further spread need to be implemented as soon as possible. Public information campaigns in the affected municipalities will be initialized in order to raise awareness, similar to campaigns in Spain concerning the control of Ae. albopictus (Eritja and Marquès 2009). A reduction of the breeding sites of Ae. j. japonicus on private and public properties will be attempted by the application of Bacillus thuringiensis israelensis. Apart from the necessary control measures, continued surveillance for Ae. j. japonicus is crucial to estimate the future development of its distribution and abundance. Further studies will provide more information about the bionomics of this species, in order to assess its vector potential for several pathogens. We thank our KABS co-workers, Wolf Peter Pfitzner, Kalyani Rösch-Manneh, Wolfgang Hauck, Ursula Ernst, Udo Fromme, Cristina Czajka, Max Huber, Stephan Mende, and Dominik Soyk for their support in evaluating the breeding sites and collecting mosquitoes. Thanks also to Minoo B. Madon (Los Angeles, CA, U.S.A.) and Dr. Stefanie Müller (Bernhard Nocht Institute for Tropical Medicine, Hamburg) for their comments on the manuscript.