Invasion of the box tree moth Cydalima perspectalis (Walker, 1859) in the Balearic Islands (Western Mediterranean, Spain): a matter of islands

The introduction of alien species in insular ecosystems is one of the main causes of population decrease and extinction of native biodiversity. This is the first time that the arrival of Cydalima perspectalis in the Balearic Islands has been reported, which makes it possible to complete the distribution of this Asian moth in Europe. The box tree moth was reported on three out of the four islands that compose the archipelago. However, it seems to be only established in Mallorca because of the wider distribution of Buxus balearica and B. sempervirens as the host plants.


Introduction
The box tree moth Cydalima perspectalis (Walker, 1859) is a Crambidae moth species from subtropical regions of Asia, including China, Japan and Korea (Inoue et al. 1982), as well as part of Eastern Russia (Kirpichnikova 2005). Since its first report in Europe in 2007 (Germany) (Krüger 2008), it has expanded rapidly throughout the continent occupying most of the European countries (EPPO 2018). Concerning to Iberian Peninsula of Spain, the first occurrence was in 2014 in Galicia province, particularly in the southern area of Pontevedra, although its occurrence had already been detected before in Cantabria province (Pérez-Otero 2014). Nowadays, it is distributed in the northwest, the center and the west of the Iberian Peninsula, including the provinces of Asturias, Barcelona, Cantabria, Gerona, Guipúzcoa, Madrid, Navarra, Pontevedra and Vizcaya (Gómez-Undiano et al. 2018).
It has been assumed that the main route of arrival and dispersion of C. perspectalis within the European continent is due to the commercial trade of ornamental plants and bushes such as Buxus sempervirens L. (Matošević, 2013). Furthermore, this moth species is considered a pest for plants of the genus Buxus L. since it has been documented as its main host plant   (Maruyma 1993;Brua 2013;Wang 2008). In addition, it should be noticed that some native European plants, such as Ilex aquifolium L. or Euonymus europaeus L. could potentially be affected by the box tree moth (Vieira 2020). This affection could be because of the activity of the larval stage that can induce severe defoliation and consumption of the bark, which in turn affects the plant's health, even causing death in some cases (Bella 2013).
Up to date, there are no published records of the occurrence of C. perspectalis in the Balearic Islands archipelago (Western Mediterranean, Spain), where the native species Buxus balearica Lam. inhabits in Mallorca and Cabrera archipelago. The present work provides updated information about the records of C. perspectalis in three out of the four largest islands of the archipelago during its first years of the colonization and the subsequent years, suggesting differences in the establishment success among islands.

Methods
A compilation of information about confirmed records of C. perspectalis was done to detect its first date of arrival and the trend in the colonization process. For each record, information about the date, the number of specimens if available and the source of observations was detailed (Supplementary material Table S1). The information obtained from the Bioatles database did not provide information about the number of observed specimens.
All records were plotted on a map to obtain the minimum convex polygon in each studied island (Mallorca, Menorca and Formentera) and the occupancy area calculated in km 2 .

Results and discussion
During the first three years of the presence of C. perspectalis in the Balearic Islands (2018-2020), up to 31 records were reported in three out of the four islands of the archipelago: Mallorca, Menorca, and Formentera ( Figure 1; Table S1). The species was determined according to the morphological and coloration traits described by Inoue (1982). The most common phenotype found was the one that presented hyaline white wings with a brown margin and white spot at the discoidal cell of the hindwings. The dark phenotype was also recorded, but with less frequency. There is only one species in the Balearic Islands that could be misidentified with C. perspectalis, i.e. the Crambidae species Palpita vitrealis (Rossi, 1894), but this species does not present the brown margin; instead it has a thin brown line in the front side of the hindwings.
In Mallorca, the occurrence area of the box tree moth was about 2978,85 km 2 , larger when compared with Menorca (706,45 km 2 ) and Formentera with   since these habitats are listed in Annex I of the Habitats Directive and must be protected all over Europe (Council Directive 92/43/EEC; habitat code 5110). Nowadays, the observations of C. perspectalis continue, detecting signs of impact in the landscape, such as most of the bushes being defoliated (Figure 3). These effects in Mallorca are becoming a worrying situation and even worse when synergistic effects, such as herbivory by feral goats, act on the bushes of B. balearica, an endemic species from the Mediterranean basin.
On the other hand, it is important to highlight that in Menorca Island, C. perspectalis was first detected in 2020 (Table S1). The detection of specimens of this species was done under a moth monitoring scheme. Up to three out of four moth monitoring stations showed evidence of the presence of C. perspectalis, recording a maximum of 47 specimens in a single night at one of the stations (Figure 2). In the following years (2021-2022), the moth monitoring continued at the same stations on this island, but no evidence of this species was recorded. Only an isolated record of larvae on B. sempervirens on April 3 rd , 2021, by one of the authors (MM) was reported despite the Citizen Science Campaign carried out by the Ministry of Environment and the University of the Balearic Islands to compile information about C. perspectalis distribution at the Balearic Islands. Finally, since 2018 only a single record of C. perspectalis has been reported in Formentera on B. sempervirens.
These differences between Mallorca and Menorca islands could be explained by the presence of the food plants for C. perspectalis. First, in Mallorca and the islet of Dragonera as well as the Cabrera archipelago, there is a large population of B. balearica. In addition, many ornamental specimens of B. sempervirens can be found around the island of Mallorca, specifically in public and private gardens and in public cemeteries. Finally, other host plants have been described as food plants for C. perspectalis including Aquifoliaceae and Celastraceae . Of them, only I. aquifolium occurs in Mallorca (Llorens et al. 2021), which would reinforce the stabilization of C. perspectalis at the island since it provides more host plants. On the contrary, there are no natural populations of B. balearica in Menorca or Formentera, although some specimens of B. sempervirens can be found in public and private gardens.
The pathway of arrival to the Balearic Islands has not been confirmed yet, but several possible hypotheses have arisen. Firstly, the natural arrival from mainland Spain, demonstrated in other further distance archipelagos from the continent, such as the Azores (Vieira 2020 and references therein); secondly, the arrival through nursery trade from the continent through cargo ships, or thirdly the passive transport on cruise ships. The potential areas of origin are still unknown. They could be eastern mainland Spain if the first introduction pathway hypothesis is confirmed or any European or Asian country of origin of the plant's trade under the second introduction pathway hypothesis.
Because of the occurrence of the native box tree species B. balearica at the Balearic Islands, the establishment of C. perspectalis, and its expansion in new territories around Europe, it is imperative to promote control actions against this invasive species, not only in the Balearic Islands but also in the European countries. These actions should be supported and coordinated with a precise record of observations so as to assess the colonization process. Also, these control actions must consider the possible impact on native invertebrates. In the case of trap-based control actions, traps must work specifically on the target species. There are specific pheromone commercial traps that have very low capture rates of non-target species. Finally, administrative actions such as the inclusion within the National and/or Regional Catalogue of Alien Invasive Species, as well as its declaration as a pest, would help to act against it and, consequently, to protect natural and native habitats.