See Sirex for genus-level diagnostic characteristics.
Sirex nitidus females can be distinguished from S. cyaneus and S. abietinus by the presence of a pit on the second annulus, and from S. californicus and S. noctilio by the relative length of the pulvillus on the second hind tarsomere. It is difficult to distinguish from European species S. torvus and S. atricornis without geographical range or host information as differences are very subtle. The most reliable way to distinguish S. torvus is by slightly denser and smaller pits on the gena (Schiff et al. 2012).
Individuals of S. nitidus fall into two color patterns. The common form, with reddish-brown femora, is found throughout the species’ range, while the black form, with mostly black femora, occurs only in Alaska, Yukon, northern British Columbia, and Alberta. There are also several intermediate forms in overlapping regions (Schiff et al. 2012).
Sirex species feed on trees of Pinaceae and Cupressaceae. Sirex nitidus is recorded on Thuja plicata (western red cedar), Abies balsamea (balsam fir), Abies lasiocarpa (subalpine fir), Larix laricina (tamarack), Picea engelmannii (Engelmann spruce), Picea glauca (white spruce), Picea mariana (black spruce), Picea rubens (red spruce), other Picea sp., Pinus contorta (lodgepole pine), Pinus ponderosa (ponderosa pine), Pseudotsuga menziesii (Douglas fir), and Tsuga heterophylla (western hemlock). The majority of specimens reared (88%) have been on Picea spp. (spruce) (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). The mycangia of S. nitidus individuals harbor either Amylostereum areolatum or A. chailletii fungus (Hajek et al. 2013).
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. nitidus is early July through early October, with most collections in September (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).
One rearing record includes emergence of a parasitoid, Ibalia ensiger (Johnson 1930).
World: North America. Interceptions have been made in New Zealand (Schiff et al. 2012).
North America: The range of S. nitidus extends through northwest regions of the United States and forested regions of Canada, from as far north as Newfoundland in the east, and Alaska in the west. The southernmost part of the range of this species includes the Rocky Mountains in the western United States (Schiff et al. 2012).
Map data from Washington State Department of Agriculture Entomology Collection.
Details about data used for maps can be found here.