See Sirex for genus-level diagnostic characteristics.
Sirex varipes females can be distinguished from S. abietinus, S. cyaneus and S. nitidus by the dark band on the dorsal surface of the hind tibia. Sometimes this characteristic dark band is also present on the fore and mid tibiae (Schiff et al. 2012).
Sirex species feed on trees of Pinaceae and Cupressaceae. Sirex varipes is recorded on species of Pinaceae: Abies amabilis (Pacific silver fir), Abies concolor (white fir), Abies grandis (grand fir), Abies magnifica (red fir), Abies lasiocarpa (subalpine fir), Picea engelmannii (Engelmann spruce), Picea sitchensis (Sitka spruce), Pinus ponderosa (ponderosa pine), Pseudotsuga menziesii (Douglas fir), and Tsuga heterophylla (western hemlock) (Schiff et al. 2012).
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. varipes is from early July through early October, with most collections in the months of August and September (Schiff et al. 2012).
World: North America. Interceptions have been made in England and New Zealand (Schiff et al. 2012).
North America: Sirex varipes is recorded west of the Rocky Mountains in British Columbia, Alberta, Montana, Idaho, Washington, Oregon, California, Nevada, and Arizona (Schiff et al. 2012).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Sirex varipes
Details about data used for maps can be found here.