Sirex obesus

Taxonomy

Family: Siricidae
Subfamily: Siricinae
Genus: Sirex Linnaeus, 1760
Species: Sirex obesus Bradley, 1913
Common names: none

Background

Sirex obesus is a somewhat rare species from the southwestern United States with a stout, dark body (Schiff et al. 2012).

Diagnostic characteristics

See Sirex for genus-level diagnostic characteristics.

Females:

Males:

May be confused with

Sirex obesus females can be distinguished from S. areolatus and the dark form of S. californicus by the length of the second hind tarsomere and by the dense gena pits. The males can be recognized by the black antennae and the yellow-tinted fore wing (Schiff et al. 2012).

Morphological and geographical variation

none recorded

Host associations

One specimen of S. obesus was collected from Pinus ponderosa (ponderosa pine) (Schiff et al. 2012).

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 flight period of S. obesus is from late July through late September (Schiff et al. 2012).

Distribution

World: North America

North America: Sirex obesus is only known from Arizona and New Mexico in the southwestern United States (Schiff et al. 2012).

No specific locality data was available for mapping the range of this species at the time of publication.

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

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

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

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

Sirex obesus male lateral habitus; photo by H. Goulet, CNC

Sirex obesus male lateral habitus; photo by H. Goulet, CNC

Sirex obesus fore wing; photo by H. Goulet, CNC

Sirex obesus fore wing; photo by H. Goulet, CNC

Sirex obesus male antenna; photo by H. Goulet, CNC

Sirex obesus male antenna; photo by H. Goulet, CNC

Sirex obesus head; photo by H. Goulet, CNC

Sirex obesus head; photo by H. Goulet, CNC

Sirex obesus ovipositor pits; photo by H. Goulet, CNC

Sirex obesus ovipositor pits; photo by H. Goulet, CNC