Xylosandrus compactus (Eichhoff, 1875)

Garonna, Antonio P., Dole, Stephanie A., Saracino, Antonio, Mazzoleni, Stefano & Cristinzio, Gennaro, 2012, First record of the black twig borer Xylosandrus compactus (Eichhoff) (Coleoptera: Curculionidae, Scolytinae) from Europe, Zootaxa 3251, pp. 64-68 : 65-67

Xylosandrus compactus (Eichhoff, 1875)

To discriminate X. compactus from the other species of the genus Xylosandrus present in Europe, the following diagnostic dichotomous key to females, modified from Faccoli (2008), can be used:

1 - Declivital face of elytra steep and abruptly sloping from disc. Declivital striae with setae. Interstriae uniseriate punctate, with erect setae. 1.2–1.8 mm long......................................................................... X. morigerus (Blandford)

- Elytral disc gradually curving into declivity.................................................................................................................. 2

2 - Striae obsolete on declivity. Declivital surface dull, with dense, irregular, minute and uniformly distributed granules. Elytra with abundant hairs. Colour reddish brown. Large species, 2.1–2.9 mm long .................................................... ..................................................................................................................................... X . crassiusculus (Motschulsky)

- Declivital striae clearly visible. Declivital surface shiny, punctate, without granules. Colour dark brown or blackish. Smaller species, 1.5–2.3 mm ........................................................................................................................................ 3

3 - Striae on declivity with punctures deeply impressed in rows. Elytral declivity without strial setae. Body 2.3x longer than wide; larger species, 2.0– 2.5 mm long........................................................................... X. germanus (Blandford)

- Striae on declivity not impressed. Elytral declivity with strial setae. Smaller species, 1.4–1.7 mm long....................... ................................................................................................................................................ X. compactus (Eichhoff)

Distribution and host species. The native range of X. compactus is tropical and subtropical Asia, from where it has naturally spread or has been introduced to several countries of the Afrotropical Region, Oceania, southeastern USA and the Neotropical Region. X. compactus is a highly polyphagous species: over 200 plant species in at least 60 families have been recorded as hosts of the BTB, 137 species recorded alone from the Hawaiian islands ( Hara & Beardsley 1979). Brood chambers of the beetle were collected from 31 host species in Florida, ranging from orchids to shade and ornamental trees, among others Acer spp., Cornus sp., Magnolia spp., Persea spp., Platanus sp., Quercus spp. and Salix sp. ( Ngoan et al. 1976). The infested plant species we recorded for Italy add eight new hosts, listed according to the incidence of the detected attacks: bay laurel Laurus nobilis , holm oak Quercus ilex , European nettle tree Celtis australis , manna ash Fraxinus ornus , European chestnut Castanea sativa, Judas tree Cercis siliquastrum , white mulberry Morus alba , and laurestine Viburnum tinus . Moreover entrance holes only or incomplete and empty brood chambers were observed on several other species ( Emerus major , Phillyrea latifoli a, Tilia spp.).

General biology and preliminary data from Italy. Like all members of the tribe Xyleborini , X. compactus is xylomycetophagous; larvae live in wood and feed on the mycelium of symbiotic ambrosia fungi. Most ambrosia beetles attack primarily weak or unhealthy plants. However, X. compactus is known to attack healthy plants as well, which makes it a potentially serious pest to novel host species. Severe infestations can kill host plants, including large trees ( Tenbrink & Hara 1994). Apart from the worst scenario, the typical dieback of twigs can have considerable impact on the appearance of infested trees and shrubs. The species may develop two or more generations per year; adult females overwinter inside the damaged twigs ( Ngoan et al. 1976). Females dig a little cavity where they lay eggs in small clusters; after eclosion the larval stages develop together in the brood chamber. Egg and larval stages were described by Hara & Beardsley (1979). Each female remains with its offspring until development is completed. X. compactus is an arrhenotokous haplodiploid species. Males are rare, incapable of flight and do not leave the brood chamber, where mating with siblings occurs with high levels of inbreeding. As investigated by Ngoan et al. (1976) and Hara & Beardsley (1979) the life cycle is completed in about a month in warm climates. The sex ratio of males vs. females is about 1:10 and the mean number of offspring varies from 12.3 to 19.8. Moreover, these studies found that older females that had already reared one brood to the adult stage failed to excavate new galleries.

The specimens collected from old dried flagged twig terminals in late March 2011 at Portici were overwintering females, while in late May, in June and first half of July, in both localities, the investigated populations were represented by all developmental stages of both sexes in new wilted twigs ( Figs. 2 , a–c). Progeny and sex ratio both seem affected by the different host plant. The number of callow specimens collected in brood chambers ranged from 3–11 (6.08±1.82; n=25) to 20–36 (24.48±5.7; n=20) with the lowest interval of values from holm oak and the highest from bay laurel. The overall sex ratio rate recorded for bay laurel in July was 1m: 15f. Adults of BTB were collected but in low numbers from the other recorded host plants ( C.australis , n=91; F. ornus , n=26; C. sativa , n=21; C. siliquastrum , n=14; M. alba , n=18; V. tinus , n=7).

Entrance holes were about 0.8 mm in diameter, usually found on the lower surface of the twigs ( Fig. 2 , d). Females built a single brood chamber where the eggs were laid, digging the pith on both side of the short entrance tunnel. Infested twigs had an average diameter ranging from 3 to 9 mm. Leaves on infested twigs turned dull green, curled inwards and wilted (flagging of twigs) about 3–4 days after initial attack of a female. Most infested twigs turned brown in 2–3 weeks. Inside the brood chamber, the ambrosia fungus formed a 0.1–0.5 mm thick lining continually grazed by larvae and adults. The host is damaged both by tunnelling and by the introduction of ambrosia fungi. Infestation by one female is sufficient to kill the twig well below the brood chamber. Blackish stain developed in xylem adjacent to this fungal growth area ( Fig. 2 , e). Attacks on woody host stems, larger than 15 mm in diameter and up to 30 mm, were also recorded, but without associated reproductive galleries.

X. compactus has established a potentially harmful breeding population in Italy, one which could have dramatic effects on both survival of evergreen shrubby undergrowth and aesthetic appearance of mature trees in some urban woods, where holm oak and bay laurel could be seriously damaged. The richness in biodiversity of Mediterranean ecosystem and typically milder winter than elsewhere in continental Europe are two factors which aid in the establishment of invasive species in Italy ( Underwood et al. 2009). Therefore, the Mediterranean Basin is disproportionately rich in exotic scolytines and Italy is the European country with the highest number of exotic ambrosia beetles ( Marini et al. 2011). This record of X. compactus from Italy raises the number of exotic Xyleborini established in Europe to 10, a bit more than 40% of the total fauna of European ambrosia beetles, which at the present includes 24 species ( Faccoli et al. 2009; Kirkendall & Faccoli 2010).

Climatic and vegetational conditions of the region where the species has been recorded are certainly suitable for X. compactus , and it might have been already established in the region for almost two years. The easy acceptance of new hosts and its plasticity (environmental adaptability) might allow X. compactus to spread quickly into new territories. The beetle is considered an exceedingly destructive species boring into the most vigorous growing, succulent twigs ( Wood 2007). Based on the essential traits of the biology of X. compactus , there are concrete risks of economic damage deriving from its further spread. Theoretically, a xyleborine population could be established via a single, unfertilized female. In most haplodiploid species, newly matured females mate with siblings before emerging from the host and are thus able to establish new populations from very few (or even a single) individuals ( Kirkendall 1983; Jordal et al. 2001). Further specific investigations will be carried out to establish the effective distribution and the impact of the black twig borer to forest, fruit and ornamental trees in Italy.

Figures

FIGURE 2. Xylosandrus compactus (Eichhoff): a, bay laurel wilted twigs; b, ambrosia fungus in brood chamber; c, brood chamber with female pupal stages and male larvae in bay laurel twig; d, entry hole on holm oak twig; e, black stain infecting holm oak xylem.