Sirex torvus

Taxonomy

Family: Siricidae
Subfamily: Siricinae
Genus: Sirex Linnaeus, 1760
Species: Sirex torvus Harris, 1779
Common names: none

Background

Sirex torvus is a Palearctic species with a typical metallic blue-black coloration and red legs (Schiff et al. 2012).

The European species previously identified as S. cyaneus has, through morphological and molecular analysis, been determined to be a separate species, and is now classified as S. torvus. Sirex cyaneus occurs in North America (Schiff et al. 2012).

Diagnostic characteristics

See Sirex for genus-level diagnostic characteristics.

Females:

Males:

May be confused with

In its range, S. torvus can be distinguished from S. juvencus and S. noctilio by the short ovipositor, dark antennae, and the completely light-colored tarsi (Benson 1943). Previously considered to be the same species, S. cyaneus is differentiated from S. torvus by the lack of pits on the basal part of the ovipositor and the shorter length of the ovipositor (Schiff et al. 2012).

Morphological and geographical variation

none recorded

Host associations

Sirex species feed on trees of Pinaceae and Cupressaceae. Sirex torvus is recorded from species of Pinaceae including Abies procera (noble fir), Abies alba (silver fir), Pinus sylvestris (Scots pine), Pinus spp., Larix europaea (European larch), Larix spp., and Picea spp. (spruce) (Smith 1978).

Life history

Female Sirex 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 (Johnson 1930, 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. The larvae bore galleries into wood, feeding 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 fungal symbiont is carried in specialized organs in female larvae that develop into the mycangia after metamorphosis (Schiff et al. 2012).

The documented flight period of S. cyaneus is early July through early September, with most collections in early August (Schiff et al. 2012). There is some evidence that trees with sustained damage, either from drought-related stress, weather, or other insect infestations, are preferred as hosts (Burnip et al. 2010).

The specific biology of S. torvus is not known.

Distribution

World: Sirex torvus occurs in most of Europe, as far north as Sweden, east to Germany, and west to the British Isles (Smith 1978).

North America: not recorded

Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Sirex cyaneus

Details about data used for maps can be found here.

Sirex cyaneus for many years was considered a Holarctic species. Recent research determined that European specimens of S. cyaneus are a different species, and these specimens have been named S. torvus. See Schiff et al. 2012 about this designation.

Sirex torvus female lateral habitus; photo by J. Orr, WSDA

Sirex torvus female lateral habitus; photo by J. Orr, WSDA

Sirex torvus female dorsal habitus; photo by J. Orr, WSDA

Sirex torvus female dorsal habitus; photo by J. Orr, WSDA

Sirex torvus male lateral habitus; photo by J. Orr, WSDA

Sirex torvus male lateral habitus; photo by J. Orr, WSDA

Sirex torvus male dorsal habitus; photo by J. Orr, WSDA

Sirex torvus male dorsal habitus; photo by J. Orr, WSDA