The Asian tiger mosquito Aedes albopictus (Diptera: Culicidae) in Central Germany: Surveillance in its northernmost distribution area

Highlights

• Northernmost population of Aedes albopictus observed in Central Germany.

• Local winter temperatures allow long-term establishment of Aedes albopictus.

• Introduction possibly through plants from Italy.

Abstract

The invasive Asian tiger mosquito Aedes albopictus has recently been observed in southern Germany for the first time to reproduce and even overwinter north of the Alps. After the accidental capture of adult specimens in Jena, German federal state of Thuringia, in mid-2015, regular inspections brought forth developmental stages until autumn 2015, indicating local reproduction. Surveillance activities implemented in 2016 showed larvae already in early May, suggesting overwintering, and throughout the season until late October, although population densities remained low. Further sporadic specimens found in 2017 argue for establishment. Jena is located in Central Germany, north of all known distribution areas of Ae. albopictus, with the area of the municipality affected by the tiger mosquito characterised by a relatively mild climate. To check the suitability of the local climate for Ae. albopictus, winter temperatures, measured in a cemetery of Jena where larvae had regularly been found in 2015 and 2016, were analysed and compared with two sites of establishment in southern Germany. The conditions were similar at all three locations, suggesting that the Jena population might also be able to survive in the long term. While the municipality authorities have been informed and education of the Jena citizens to avoid producing potential breeding places has started, insecticidal control has not yet been implemented.

Introduction

The Asian tiger mosquito Aedes (Stegomyia) albopictus (Skuse, 1895) is an efficient potential vector of numerous human and animal pathogens, such as dengue virus, chikungunya virus, West Nile virus, Rift Valley fever virus and dirofilarial nematodes (Gratz, 2004; Paupy et al., 2009) and is therefore of paramount public and veterinary health concern. In Europe, it was made responsible for recent outbreaks of dengue and chikungunya (Schaffner et al., 2013; Calba et al., 2017; Succo et al., 2016; Venturi et al., 2017).

Originating from the Asian-Pacific region, Ae. albopictus was first reported in Europe from Albania in 1979 (Adhami and Reiter, 1998). Only some ten years later, it was discovered in Italy (Sabatini et al., 1990), from where it spread over large parts of the Mediterranean and beyond (Medlock et al., 2015). While both intercontinental and continental displacement is through transportation of eggs or larvae by the trade with used tyres and ornamental plants (e.g. Dracaena sp.), adults entering vehicles contribute to continental spread (Becker et al., 2017; Eritja et al., 2017; Hofhuis et al., 2009; Knudsen et al., 1996; Reiter, 1998; Walther et al., 2017). Since 2007, the species has been found on service stations along South German motorways (Becker et al., 2013; Kampen et al., 2013; Pluskota et al., 2008; Werner et al., 2012), and in 2014 and 2015, extended local reproduction was observed in the German cities of Freiburg, Heidelberg and Jena (Pluskota et al., 2016; Werner and Kampen, 2015; Walther et al., 2017). While in Freiburg and Heidelberg Ae. albopictus quickly developed alarming population densities which had prompted the implementation of control activities (Becker et al., 2017), detections of specimens in Jena remained sporadic.

Aedes albopictus is a thermophilic mosquito species which is adapted to both tropical and temperate climates, depending on its origin (Benedict et al., 2007; Hawley, 1988). Populations of temperate strains are known to have a higher cold hardiness than tropical ones which, in contrast, cannot overwinter as diapausing eggs (Hanson and Craig, 1994; Hawley et al., 1989). According to studies by Delatte et al. (2009), larval development does not take place below 10 °C and is optimal at temperatures between 25 and 30 °C. Larval survival rates are highest at 15 °C, and gonotrophic cycles are shortest at about 30 °C. In regions with relatively high summer temperatures, populations develop quickly, and densities reach their peaks early in the season, whereas in regions with low summer temperatures population growth is slow but steady, with a maximum later in the season (Alto und Juliano, 2001).

Future projection models suggest that climatically suitable areas for the establishment of Ae. albopictus will remain stable or increase until 2040 in western and Central Europe and, with some delay, probably also increase in eastern Europe (Caminade et al., 2012; Fischer et al., 2011, 2014). By contrast, living conditions are expected to deteriorate along the western Mediterranean coast of Spain (Fischer et al., 2014). Considerable discrepancies exist between different models, which predict France, Germany and western parts of the United Kingdom to become either persistently unsuitable or increasingly suitable (Fischer et al., 2014).

In this study, we describe the finding and population development of Ae. albopictus in the Central German city of Jena, which, to the best of the authors’ knowledge, represents the northernmost area colonised by this invasive mosquito species. Temperature profiles of the winter seasons 2015/2016 and 2016/2017 between locations with Ae. albopictus occurrence in Jena and in the Southwest German towns of Heidelberg and Freiburg, where the species is considered established, were compared.