Sirex

Taxonomy

Family: Siricidae
Family common name: horntails
Subfamily: Siricinae
Genus: Sirex Linnaeus, 1760
Subgenera: none

Background

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 Sirex are generally large, about 2.5–4 cm in length, with cylindrical bodies (Camper and Cranshaw 2018, Schiff et al. 2012). They are usually black with metallic reflections, and some have pale reddish and reddish-brown areas (Schiff et al. 2012). 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 (Camper and Cranshaw 2018).

Diversity

Worldwide, there are 28 described species. The highest diversity is in North America, with 13 endemic and one introduced species (Schiff et al. 2012).

The revision by Schiff et al. 2012 covers all Sirex species of the Western Hemisphere. Characters used in this revision were taken from previous work in Ross 1937 and Benson 1943. Throughout the Sirex species pages, and in the key to Sirex species, we primarily cite Schiff et al. 2012 because of its use of more modern terminology and current taxonomy.

A key to North American species of Sirex is included in Schiff et al. 2012.

Diagnostic characteristics

May be confused with

Sirex can be confused with other Siricidae, especially Urocerus, because of the similar coloration. They can be distinguished from other genera in the family by the lack of ridge on the gena behind the eye, a lack of light-colored spot on the dorsal portion of the gena, and lack of constriction at the base of the Siricidae sawflies; on tergite 10 in females, sternite 9 in males ">cornus (Schiff et al. 2012).

Exotic pest species of concern

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. One species is established and invasive in North America, South America, South Africa, New Zealand, and Australia (Schiff et al. 2012). Read more details on the species factsheet: Sirex noctilio.

Host associations

In North America, Sirex species have been documented using host trees in the Cupressaceae and Pinaceae, including Cupressus spp. (cypress), Taxodium distichum (bald cypress), Juniperus spp. (juniper), Calocedrus decurrens (incense-cedar), Thuja plicata (redcedar), Sequoia sempervirens (coast redwood), Abies spp. (fir), Larix spp. (larch), Picea spp. (spruce), Pinus spp. (pine), Pseudotsuga menziesii (Douglas fir) and Tsuga heterophylla (western hemlock) (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. The fungal symbiont for three species of Sirex (S. noctilio, S. nitobei, and S. juvencus) has been identified as Amylostereum areolatum, and for six others (S. cyaneus, S. imperialis, S. aereolatus, S. californicus, S. nigricornis, and S. varipes) as Amylostereum chailletti. Females reserve mucus in additional abdominal glands, but the function of these reservoirs in species other than Sirex noctilio is not well understood. Sirex noctilio uses the mucus as a phytotoxin to weaken the tree, while the mucus in other species may just encourage fungal growth. Sirex species drill holes in trees for oviposition, but often drill holes without depositing an egg and/or fungus (Schiff et al. 2012). Eggs are laid singly (Klass 1974).

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 (Camper and Cranshaw 2018). The larvae bore galleries 5–20 cm long 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 fungal symbiont is carried in specialized organs in female larvae that develop into the mycangia after metamorphosis. The life cycle of Sirex species varies from 1–3 years (Schiff et al. 2012).

Adult males have been observed swarming and waiting for a female to enter the swarm for mating. 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). In some cases, females oviposit in fire-damaged trees. Adult Siricidae are not well-studied (Schiff et al. 2012).

Distribution

World: Except for one species, Sirex is restricted to the Northern Hemisphere. Species are found throughout Europe, Western Asia, India, China, Korea and Japan. Sirex noctilio is documented in all the regions above plus Mongolia, as well as being introduced and established in South Africa, New Zealand, Australia, and North and South America (Smith 1978).

North America: Sirex occurs throughout Canada and the United States. Several species are restricted to Mexico, Honduras, Guatemala and the Caribbean islands. Generally Sirex are established where coniferous trees grow densely, most often along mountain ranges in the boreal north, far west, far east, and southwest desert of continental North America, with one species occurring above 1000 meters in the Greater Antilles of the Caribbean (Smith 1978).

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

Details about data used for maps can be found here.

Sirex californicus female dark form lateral habitus; photo by H. Goulet, CNC

Sirex californicus female dark form lateral habitus; photo by H. Goulet, CNC

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

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

Sirex nitidus female face; photo by H. Goulet, CNC

Sirex nitidus female face; photo by H. Goulet, CNC

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

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

Sirex imperialis male dorsal habitus; photo by H. Goulet, CNC

Sirex imperialis male dorsal habitus; photo by H. Goulet, CNC

Sirex imperialis wing; photo by J. Orr, WSDA

Sirex imperialis wing; photo by J. Orr, WSDA

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

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

live Sirex californicus female, photo by H. Goulet, CNC

live Sirex californicus female, photo by H. Goulet, CNC