The Siricidae are called “horntails” because of the Siricidae sawflies; on tergite 10 in females, sternite 9 in males ">cornus, a short spine at the apex of the abdomen. Females in the family also possess a long, conspicuous ovipositor. This is often mistaken for a stinger, but it is harmless to animals and used for drilling into wood. This habit has inspired the other common name of this family: “woodwasps” (Klass 1974).
Horntails of the genus Xeris are generally large, but with high variation in size, ranging 9–35 mm in length. The body shape is slender and cylindrical. Females are generally larger than males, but because body size is highly variable within species, sexes are best distinguished by presence or absence of the long ovipositor (Goulet et al. 2015, Schiff et al. 2006).
Worldwide, there are 16 described species. The highest diversity is in western North America (Schiff et al. 2012).
A key to North American species of Xeris is included in Schiff et al. 2012.
This genus may be confused with other Siricidae, especially those of the genera Sirex and Urocerus. Xeris can usually be distinguished from these genera by the long ovipositor, a lack of a closed anal cell on the hind wing, and a lack of metallic reflections on the body (Schiff et al. 2006).
Siricidae are commonly intercepted in wood packing materials at ports of entry. The potential of larval stages to persist and develop for multiple years increases the potential of successful establishment from improperly treated or disposed of wood material. There are several records of intercepted specimens at ports, but no Xeris have become established outside their native range as of yet (Goulet et al. 2015).
Xeris species have been documented using host trees in the Cupressaceae and Pinaeceae, including Calocedrus decurrens (incense cedar), Thuja plicata (western red cedar), Juniperus occidentalis (western juniper), Crytomeria japonica (Japanese cedar), Cupressus macrocarpa (Monterey cypress), Abies spp. (fir), Cedrus deodara (Deodar cedar), Larix spp. (larch), Picea spp. (spruce), Pinus spp. (pine), Pseudotsuga menziesii (Douglas fir), and Tsuga heterophylla (western hemlock) (Goulet et al. 2015).
Most female Siricidae share a similar biology, in that they harbor symbiotic basidiomycete fungus in abdominal glands called mycangia. During oviposition, the site is inoculated with the fungus, which begins to decompose the surrounding wood. Larvae feed on the fungus, and in the process bore galleries through the wood.
Though Xeris larvae have the same feeding habits, the adult female’s mycangia harbors no fungus and is essentially non-functional. Instead, females oviposit into wood already inoculated by another Siricidae species. Xeris have been documented feeding on the fungus species Amylostereum areolatum and Amylostereum chailletti which are the species associated with the genera Sirex and Urocerus (Goulet et al. 2015). Xeris is also seemingly unique in its ability to feed on either of the two fungi without any observable preference. This generalism increases the number of suitable oviposition sites (Fukuda and Hijii 1997). Eggs are deposited in 1 small cluster per drill, 1–5 separate drills per insertion (Schiff et al. 2012).
Larvae are creamy white and grub-like in appearance with a dark head capsule. As with adults, larvae possess a short dorsal horn on the posterior end of the body. At maturity, the body measures about 2.5 cm in length (Schiff et al. 2012). The larvae bore galleries while developing through 6–12 larval instars, depending on food availability, until pupation and subsequent emergence. Throughout this process, the larvae use their horn to pack the tunnel behind them with sawdust. Emergence holes are perfectly circular. The life cycle of Xeris species varies from 1–3 years (Schiff et al. 2012).
World: All species of Xeris are restricted to the North Hemisphere and tend to have somewhat distinct ranges within Europe, North Africa, South Asia, and East Asia. There is no overlap of established species in the Western and Eastern Hemispheres (Goulet et al. 2015).
North America: Xeris is documented throughout boreal regions of Canada and the United States, with some species restricted to the Pacific Northwest region east to Colorado, and two restricted to the southwest U.S. into northern Mexico. One species, X. tropicalis, is known only from Chiapas, Mexico (Goulet et al. 2015).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Xeris
Details about data used for maps can be found here.