Alien arthropod species play a detrimental role in natural communities and ecosystems where they are introduced, as they can have high economic and environmental impact (e.g. Pyšek and Richardson 2010, Vaes-Petignat and Nentwig 2014, Early et al. 2016).

The genus Juglans is distributed in Asia, Europe and America and comprises approximately 22 species (Zhang et al. 2019). In Italy, the 4930 hectares of English walnut, Juglans regia L., 1753 (Fagales, Juglandaceae) produce about 15,490 tonnes of fruit (FAOSTAT 2021). Arthropod pests attack J. regia on both vegetative and reproductive plant parts and the main species belong to insects and mites, such as leaf gall mites, aphids, scales, flies, bark beetles and moths (Sharma et al. 2012). A novel insect species recently raised attention on the panorama of walnut moth pests, named the Asian walnut moth, Garella musculana (Erschov, 1874) (Lepidoptera, Nolidae). Garella musculana is an alien species that originated from Central Asia and is now spreading in Europe, developing on Juglans species like the English walnut, J. regia and the black walnut, Juglans nigra L., 1753 (Fagales, Juglandaceae) where larvae feed on shoots, buds and fruits (EPPO 2005, Bostancı et al. 2019, Bozkurt et al. 2020, Bostancı et al. 2021, EPPO 2021a). Previous records state Prunus dulcis (Miller) (Rosales, Rosaceae) and Populus spp. (Salicales, Salicaceae) as host plants for this pest (Esonbaev et al. 2020, Robinson et al. 2023), but these plant taxa still require confirmation, especially in new invasion areas.

Garella musculana old records cover territories of Afghanistan, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan and the north of Iran, which likely represent the native area of the moth and it probably also occurs in China and Pakistan (Fibinger et al. 2009, EPPO 2021b, Lepiforum.org 2023, Fig. 1). Notably, records of G. musculana in the Oriental Region and Eastern Palaearctic require verification due to the presence of its eastern closely-related taxon, Garella ruficirra (Hampson, 1905) (Lepidoptera, Nolidae), with which it may be misidentified (Scaccini et al. 2023). Recent records of the Asian walnut moth identify this pest in the Kashmir Valley, India at least from 2011 (Khan et al. 2011), while for Eastern Europe and the Middle East in Crimea and United Kingdom since 2008 (Sviridov 2008, CABI 2022), in Turkey since 2015 (Yıldız et al. 2018), in Bulgaria since 2016 (Beaumont 2018), in Romania since 2018 (Bostancı et al. 2021) and in southern Russia and Italy mainland since 2021 (Fibinger et al. 2009, EPPO 2021b, Romanchuk and Kolesnikov 2021, Scaccini et al. 2023, Fig. 1). The report for the UK in 2008 (CABI 2022) needs to be checked further, since it was not reported in the other published material we checked.

Figure 1.  

Global distribution of Garella musculana in the native range (Central Asia, dark green) and in the invaded range retrieved from bibliographic data, by year of first detection. Base map from https://www.mapchart.net/italy.html.

Although for some countries, G. musculana findings were occasional, for others, the Asian walnut moth is more frequently detected (e.g. Yoğurtcu et al. 2018, Yoğurtçu and Kaçar 2018, Bostancı et al. 2019, Yıldız and Ayberk 2019, GBIF 2023) and, in some areas, it is a known pest that affects walnut production (Orozumbekov and Moore 2007, Sangov 2015, Yoğurtcu et al. 2018, Bragard et al. 2021, Yoğurcu and Kaçar 2022).

This study aimed to assess the current distribution of G. musculana in Italy after its discovery in 2021, mainly by ad hoc field surveys in the Italian territory, focusing on the area where the species was found for the first time. It is shown that, despite the recent first records, this species is more common and widely distributed in this country than previously expected, possibly representing a new threat to walnut cultivation.

Data mining and field surveys

Species identification

Distribution map

Asian walnut moth distribution data, used to describe its global distribution, were included in the map in Fig. 1. In Italy, data mining ended in only one case of moth occurrence, an adult collected in a private garden in Vigonza, Padua Province, on 30 March 2022 (Forum Natura Mediterraneo 2022). Garella musculana presence covered 79 points of the 151 surveyed, mostly in the Veneto Region (70, 58.33% of sampled points for this region), with some records in Emilia Romagna (9, 45.00% of regional points), where symptoms potentially ascribed to G. musculana were observed. This species was not recorded in Lombardy or Friuli-Venezia Giulia Regions. In total, 52.32% of the points resulted in infestation by the Asian walnut moth (Fig. 2). The altitudes at which G. musculana was recorded ranged from sea level up to 99 m a.s.l., with an average of 11.49 ± 2.26 m a.s.l. The dataset with the detailed distribution data is available in Suppl. material 1.

Figure 2.  

Garella musculana in Italy. (A) Present distribution of G. musculana in Italy including the first two records for the country, also considering sampling points with no records. (B) Pie chart representing the amount of the three categories, based on original data appearing in the map.

Concerning host plants, the Asian walnut moth was collected from J. regia (54.23% of cases attributed to J. regia) and J. nigra (12.50% of J. nigra), while no records were shown for poplar or almond trees. Shoots of infested trees were yellowing, wilting, desiccating and excavated by the larva, with the presence of dark frass and seta at the entry hole, which is often on the leaf axil (Fig. 3A). Usually, a single larva inhabits the gallery (Fig. 3C, D). In some cases, similar symptoms were found even on the leaflet or leaf petioles, where only small larvae dig in plant tissues given their small size (Fig. 3B). Besides, in two cases, only G. musculana attack was reported on J. regia fruits.

Figure 3.  

Damage by Garella musculana of: (A) a 1-year-old Juglans regia shoot and (B) on a leaf. The arrows indicate the presence of larval frass and seta at the gallery entrance. (C) A mature larva in the gallery of a J. regia shoot and (D) a small larva in the entrance of its gallery in the leaf axils, opened to show the insect inside (Veneto Region, 2022). Photo credit: D. Scaccini.

Garella musculana findings reported here refer to walnut orchards (87.34% of the finding points), isolated walnut trees and hedgerows (10.13%) and private gardens (2.53%). Despite in the aforementioned case of an adult found in a private garden, all the other observations refer to immature forms, with larvae found from the end of June to the first ten days of September. In some cases, pupae were also found on trunks in loose bark crevices.

Despite the first assessment by EPPO (2022) referring to the Asian walnut moth as transient for the Italian territory, this information should be updated while considering the results reported in this study. The current distribution of the Asian walnut moth suggests tracing back its establishment in the Italian territory as early as a few years before the first findings in 2021. Further investigations are, however, required.

In this study, points on a map where G. musculana presence was confirmed are separated from those where only symptoms were observed. Indeed, sometimes G. musculana symptoms may be misidentified with those of Zeuzera pyrina (Linnaeus, 1761) (Lepidoptera, Cossidae), an indigenous carpenter moth that develops on twigs and small branches of many host plants (Yakovlev 2012). This and other carpenter moth species are, in fact, common, serious pests of agriculture and forestry that affect both native and introduced plant species worldwide (e.g. Yakovlev 2012, Tavares et al. 2020, Yadav et al. 2020, Scaccini et al. 2021). However, the hypothesis of moth misidentification is unlikely since shoots damaged by the larvae of Z. pyrina differ from those hosting G. musculana by the presence of sap emission for Z. pyrina and not for G. musculana and more seta and frass/faecal pellets on the entrance of galleries in the case of the latter. In the surveyed sites, plant infestation by the Asian walnut pest was more relevant than those observed for carpenter moths, which, in turn, concerned only a few sites. Furthermore, in the case of moth larvae findings in walnut shoots, Z. pyrina larvae cannot be confused with those of G. musculana since they are yellow with dark spots, differing from those of the alien pest.

The Asian walnut moth damage on plants was consistent with those reported by previous studies, with typical damage on shoots (e.g. EPPO 2005, Bostancı et al. 2019, Bostancı et al. 2021, Scaccini et al. 2023) and attacks on fruit that, however, were only sporadic in the surveyed territory. Damage was observed on J. regia and J. nigra only, but it should be considered that more time has been devoted to these species in the sampling effort. In the case of pupae, they were detected under loose bark and in other refuges on the trunk, especially for old walnut trees that have more of these refuges. The first adult of the season was found in late March, confirming what was previously reported, with first adult appearance known from the beginning of April (EPPO 2005, GBIF 2023). However, G. musculana biological features and phenology in Italy are not yet well understood.

Field surveys showed that G. musculana is widely distributed in north-eastern Italy and particularly in Veneto Region. Future studies should be dedicated to the investigation of biological features of this pest and to better understand its distribution. Finally, the impact on cultivated plants has to be researched in Italian walnut orchards in order to plan suitable management tactics against this alien pest.

We are grateful to people who helped us during field surveys and collected more data on G. musculana distribution in Italy, especially Cesare Bendandi, Eugenio Cozzolino, Luca Fagioli, Antonio Fiorin and Alessio Signori, the farmers and other technicians who hosted our surveys in the orchards. We thank Francesco Sanna (TESAF – University of Padua) and Filippo Simoni (DAFNAE – University of Padua) for helping during walnut inspections in the field. We would also like to thank Matteo Marchioro, Alberto Mele and Enrico Mirandola (DAFNAE – University of Padua) for their help in building the distribution maps. The present work was supported by Regione Veneto U.O. Fitosanitario.