Xylosandrus germanus (Blandford, 1894) on Grapevines in Italy with a Compilation of World Scolytine Weevils Developing on Vitaceae

Simple Summary Xylosandrus germanus (Curculionidae: Scolytinae: Xyleborini) has been recorded attacking grapevines (cv Glera) in the Conegliano Valdobbiadene DOCG area (Vidor, Veneto, Northern Italy). Here, we documented the first case of this invasive species damaging Vitis vinifera in Italy, supplying the second record ever in Europe. The type of the attack is illustrated and the possible causes of the onset of the infestation are discussed. In addition, an updated checklist of world Scolytinae attacking Vitaceae is provided. A total of 34 scolytine species, in eight tribes and fifteen genera, are recorded. Abstract The invasive ambrosia beetle Xylosandrus germanus (Curculionidae: Scolytinae: Xyleborini) is recorded for the first time infesting wine grapes in Italy. The type of the attack is illustrated and the possible causes of the onset of the infestation are discussed. Furthermore, given the continuously increasing number of alien wood-borer beetles introduced worldwide, we provide and discuss the updated world checklist of Scolytinae attacking Vitaceae, and Vitis sp. in particular.

Xylosandrus germanus (Blandford, 1894), also known as the black stem borer, is a xyleborine beetle native to the Eastern Palearctic and Oriental region [16] that is now widely established in North America [13], Europe [17] and Oceania [18]. It is a wellfounded opinion that X. germanus was introduced and spread outside its territory of origin through timber trade and plants-for-planting [19]. This species may attack more than 200 species among plants and trees [20]. The attack consists of the excavation by a single foundress of an entrance tunnel, which is enlarged forming brood chambers with branch tunnels [21][22][23]. During the first days of the wood colonization, the maternal-boring activity is usually easily recognizable due to the emission from the entrance hole of a white frass cylinder [19,24]. The entire development of the offspring takes place in the chamber and each nest can host multiple individuals at different life stages; insect development takes approximately 30 days and the number of generations per year can vary between two and three depending on the environmental conditions [25][26][27].

Figure 1.
Schematic representation of the vineyard investigated (view from above). In green, the rows of grapevines; in gray, the concrete walls delimiting the field.

Literature Review
To compile and then review exhaustive literature on Vitaceae-related Scolytinae, we performed a careful search on Google Scholar and Scopus through the use of the keywords "Vitaceae", "Vitis", "Scolytinae" and "pest", integrated with the usage of the Bool- Figure 1. Schematic representation of the vineyard investigated (view from above). In green, the rows of grapevines; in gray, the concrete walls delimiting the field.

Specimen Collection and Spatial Data Analysis
The infested field was investigated in September 2019, a few days after the onset of the attack. Scolytine specimens were collected directly from grapevine trunks using soft forceps, and subsequently identified using the key provided by Faccoli [40]. Infested grapevines were marked on each row; four trunks were removed and opened in the laboratory to facilitate the images.
Photographs were taken using a Panasonic Lumix DMC-FZ200 camera equipped with a Lumix DMW-LC55 lens.
Data from grape plants in the field, along with the infestation intensity, were analyzed using the SADIE 'red-blue plots' methodology to detect spatial patterns in the symptomatic grapevines [41]. Infestation intensity was defined as the occurrence of attacked grapes on three contiguous grape plants per row for three rows (thus creating a squared plot of 3 × 3 plants).

Literature Review
To compile and then review exhaustive literature on Vitaceae-related Scolytinae, we performed a careful search on Google Scholar and Scopus through the use of the keywords "Vitaceae", "Vitis", "Scolytinae" and "pest", integrated with the usage of the Boolean operators AND, OR and NOT, and with the use of quotes for specific word combinations.
The tribes, genera and species provided in the results are listed alphabetically. The host plant taxonomy follows World Flora Online: A Project of the World Flora Online Consortium [42].

Species Distribution
The species distribution is based on the Catalog of Scolytidae (Coleoptera) IV published by Bright [43], integrated with records contained in publications subsequent to 2019. The distribution provided after each species name in Section 3.2 follows the system and acronyms adopted in Bright [43].

Infestation Symptoms and Attack Incidence
In mid-September 2019, several grapevine plants showed serious symptoms of decline, such as loss of grapes, leaf staining and defoliation. Careful inspection of such plants revealed a massive attack by X. germanus, with abundant tunneling along the entire trunk of the infested plants and copious emission of white frass cylinders ("noodles") from the penetration holes. Infested plants showed a large number of tunnels concentrated near the graft union ( Figure 2a) and the head of the vine (Figure 2b); no attack was observed on cordons.
A peculiarity of the attack was the abundant secretion of gum by the plant, with the consequent formation of yellow-brown concretions (Figure 3a,b). After decortication, the stems of the infested vines showed a typical brown-black staining of the wood around each entrance hole (Figure 3c).
Once dissected longitudinally, infested trunks showed the Xylosandrus tunnels with several early settlement tunnels ( Figure 4a) and more complex structures with secondary tunnels and rearing chambers (Figure 4b,c). The dark color of the inner walls of the brood chambers suggested the successful establishment of the symbiont fungus A. grosmanniae.      Once dissected longitudinally, infested trunks showed the Xylosandrus tunnels with several early settlement tunnels ( Figure 4a) and more complex structures with secondary tunnels and rearing chambers (Figure 4b,c). The dark color of the inner walls of the brood chambers suggested the successful establishment of the symbiont fungus A. grosmanniae. In total, 70 out of 809 grapevines were attacked by X. germanus (8.7% of the total). The majority of the infested plants were concentrated on the west side of the field, close to a concrete wall delimiting the property. In this area, considering only the first five rows of vines, the infestation rate was 16%, with 13.4 attacked plants per row on average. The intensity of the attacks and their spatial distribution are illustrated in Figure 5. In total, 70 out of 809 grapevines were attacked by X. germanus (8.7% of the total). The majority of the infested plants were concentrated on the west side of the field, close to a concrete wall delimiting the property. In this area, considering only the first five rows of vines, the infestation rate was 16%, with 13.4 attacked plants per row on average. The intensity of the attacks and their spatial distribution are illustrated in Figure 5.

Curculionidae Scolytinae Attacking Vitaceae
A literature review allowed for the identification of 34 species of Scolytinae able to infest and develop on Vitaceae. Xyleborini is the most represented tribe, with 7 genera and 14 species (Table 1); Corthylini is the second most represented group with 3 genera, while Hypothenemus (Trypophloeini) includes the highest number of species attacking Vitaceae (11 Xyleborus Eichhoff, 1864 1 0

Xylosandrus germanus Infestation on V. vinifera
Given the peculiar pattern of attack that characterized our case study (i.e., attacks more concentrated on the grape plants close to the concrete wall and in the most depressed area of the vineyard), it is plausible that the major factor that determined the spatial position of the X. germanus infestations was stress due to flooding. The majority of the infested plants were, in fact, located in the most depressed part of the field (Figure 5), where it was more plausible that excess moisture had accumulated due to the heavy rains of the previous weeks. Furthermore, the concrete wall delimiting the field might have played a key role in stressing the rows of grapevines in its vicinity, working as physical barrier that probably limited the normal flow of rainwater and dispersion of residual moisture. Xylosandrus germanus is notoriously known to attack plants subjected to flood stress [89,90] and it is particularly attracted by ethanol, one of the most common volatiles released by stressed plants [91][92][93], including V. vinifera [94].
We did not observe any peculiar analogy with the infestation cases that occurred on Vitis in Germany [39] and China [30], possibly suggesting that the colonization of grapevines is regulated by a multiple-factors mechanism substantially depending on the environmental and agricultural conditions.

Scolytinae on Vitaceae
Vitaceae, and Vitis spp., in particular, host a relatively low number of Scolytinae worldwide; however, it is interesting to note how a great proportion of these species are widely distributed, naturally occurring or introduced on almost all continents. The majority of the taxa considered are polyphagous, possibly attacking Vitaceae sporadically or only under certain conditions. Furthermore, the association between some species of Scolytinae and the Vitaceae still remains to be clarified, especially in regard to poorly known species and species whose records were sporadic and never reconfirmed.
However, Xyleborini members must be considered species of primary phytosanitary and economic interest as they are already present (also as invasive) in the areas suited to viticulture and already documented as pests of Vitis spp. (see above).

Conclusions
Sustainable production remains one of the major challenges in modern viticulture, especially in regard to pest management [95]. In an ever-changing world increasingly subject to climate change and human-assisted movements of exotic fauna, understanding the association between the incidence of attacks on grapevines and the emergence of wood-boring pests becomes of primary importance. Extreme weather events such as those that have characterized recent years (heavy rains, sudden frosts and prolonged drought) may predispose V. vinifera, an already susceptible plant, to further stressful conditions and consequently make it more attractive to pests, including woodborers. What we have reported here may not just be a solitary case, but could be a first warning to be considered in the phytosanitary management of vineyards. Xylosandrus germanus is widespread in Italy and the rest of Europe, and it is plausible that, in a territory densely subject to viticulture and close to forested areas, such as our study area, it may become a constant presence in our vineyards in the near future. Furthermore, other invasive species such as Xylosandrus crassiusculus deserve just as much attention.