**1. Introduction**

Invasive alien species have always been a risk to ecosystems. They are a serious obstacle to the conservation of biodiversity, both globally and locally, as their stabilization and spread in new environments break the pre-existing balances. By coming into contact with a new environment, alien species can lead to a gradual degradation and alteration of the new habitat and the decline of indigenous species, until in some cases some of them become extinct [1–3].

The Asian yellow-legged hornet (*Vespa velutina nigrithorax* Du Buysson) is a social wasp, belonging to one of the 11 subspecies [4–6] of *V. velutina* originally present in Continental Asia [7], where it is native to subtropical and temperate areas of Indo-China [6,8].

The species established itself in non-native countries such as South Korea in 2003 [9] in the southern port town of Busan and Japan, on Tsushima Island in 2012 [10,11], in Kitakyushu City on Kyushu Island in 2015 [12] and on Iki Island in 2017 [13]. Arrived in France probably in 2004 along with garden pots imported from China [14,15], the species spread to neighboring countries. From France it reached the Navarra province and Basque country (Spain) in 2010 [16,17], Galicia [18] and Catalunya [19] in 2012, Majorca Island (Spain) in 2015 [20,21], the Minho province (Portugal) in 2011 [22], and Flobecq in the Hainaut province (Belgium) in 2011 [23]. In 2012, the Asian yellow-legged hornet was detected for the first time in Italy in the Liguria Region [24]; afterwards the hornet started to spread in this region mainly along the coastline [25–27]. In Piedmont Region (Italy) arrived in 2013 [25,26]. It was detected also in Veneto and Lombardy Regions (Italy) between 2016 and 2017, with no more reports in the following years, and in Tuscany Region (Italy) in 2017. By 2017, the species had colonized an area of at least 1,110 km<sup>2</sup> in Italy [27]. *V. v. nigrithorax* was firstly recorded in Germany in 2014 and a nest was found in Büchelberg (Rheinland-Pfalz) [28]. In 2016, few hornets were found in the United Kingdom, and in 2017 also in Netherland, Switzerland [29], and Scotland [30], Figure 1.

**Figure 1.** Presence of *V. v nigrithorax* in Europe estimated from several sources. Red areas indicate districts where hornets are established or have been reported in 2018 and 2019. Light-red areas show districts where hornets or nests have been exclusively spotted in the past until year 2017.

The spread of *V. v. nigrithorax* in Europe and in non-native Asian regions seems to respect the predicted climatic suitability maps modelled by Villemant et al. [31]. Global warming could worsen the current situation [32]. This hypothesis was confirmed by Rodríguez-Flores et al. [18]: High minimum temperatures, dew temperature, relative humidity and low maximum temperatures favor the occurrence and spread of *V. v. nigrithorax*. These conditions are common in coastal areas and can promote the rapid dispersal of this pest.

*V. v. nigrithorax* creates considerable damage to the environment and beekeeping activities. For this reason, the species has been included by European Union in the black-list of invasive alien species (Reg. EU 1141/2016) for which it is mandatory to develop surveillance plans and actions to limit its spread as well as control and containment strategies. The Japanese Ministry of the Environment added this hornet to the list of invasive alien species in 2015 [33].
