Urocerus

Taxonomy

Family: Siricidae
Family common name: horntails
Subfamily: Siricinae
Genus: Urocerus Geoffroy, 1762
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 Urocerus are generally large, about 2.5–4 cm in length, with cylindrical bodies (Camper and Cranshaw 2018). They are usually shining black with reddish-brown and sometimes light-colored or white areas. 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). The wings of Urocerus are commonly smoky, golden, or dark-colored, and occasionally sub-hyaline (Schiff et al. 2006).

Diversity

Worldwide, there are 33 described species. The diversity in North America is low, with only five endemic and one introduced species (Schiff et al. 2012).

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

Diagnostic characteristics

May be confused with

Urocerus can be confused with other Siricidae, especially Sirex 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 and lack of metallic reflections on black portions of the body (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.

Urocerus sah has become established in New Hampshire and Quebec. In its native range of West Asia and North Africa, it utilizes Abies spp. (fir), Picea spp. (spruce), and Pinus spp. (pine) as hosts. No specimens have yet been reared from North American hosts (Smith 1978, Schiff et al. 2012).

Host associations

Urocerus species have been documented from hosts in the Cupressaceae and Pinaeceae, including Chamaecyparis sp. (false cypress), Calocedrus decurrens (incense cedar), Thuja occidentalis (arborvitae), Thuja plicata (western red cedar), Taxodium distichum (bald cypress), Abies spp. (fir), Cedrus spp. (cedar), 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 Urocerus 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 two species of Urocerus in Asia (U. japonicus and U. antennatus) has been identified as Amylostereum laevigatum, and for five others in Europe, West Asia, and North America (U. californicus, U. flavicornis, U. gigas, U. augur and U. sah) as Amylostereum chailletti. Females reserve mucus in additional abdominal glands, but the function of these reservoirs is not well understood; the excreted mucus may encourage fungal growth. Urocerus species lay eggs a few at a time, spaced singly along a single long drill, with fungal hyphae between each one (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. At maturity, the body measures about 2.5 cm in length (Schiff et al. 2012). 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 Urocerus 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, Urocerus is restricted to the Northern Hemisphere. Species are found throughout North America, Europe, North Africa, and Asia. One species, U. gigas, has been introduced to South America and is recorded from Chile, Argentina, and Brazil (Smith 1978).

North America: Urocerus occurs throughout continental North America south to Mexico. Generally they are established where coniferous trees grow densely, most often along mountain ranges. Two species are widespread; one species is restricted to the west, and two are restricted to the east. One species, U. sah, native to West Asia, has become established in eastern North America (Smith 1978).

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

Details about data used for maps can be found here.

Urocerus flavicornis female lateral habitus; photo by J. Orr, WSDA

Urocerus flavicornis female lateral habitus; photo by J. Orr, WSDA

Urocerus flavicornis female dorsal habitus; photo by J. Orr, WSDA

Urocerus flavicornis female dorsal habitus; photo by J. Orr, WSDA

Urocerus californicus female dorsal habitus; photo by J. Orr, WSDA

Urocerus californicus female dorsal habitus; photo by J. Orr, WSDA

Urocerus albicornis female face; photo by Q. Baine, WSDA

Urocerus albicornis female face; photo by Q. Baine, WSDA

Urocerus californicus male lateral habitus; photo by J. Orr, WSDA

Urocerus californicus male lateral habitus; photo by J. Orr, WSDA

Urocerus californicus male dorsal habitus; photo by J. Orr, WSDA

Urocerus californicus male dorsal habitus; photo by J. Orr, WSDA

Urocerus californicus male face; photo by Q. Baine, WSDA

Urocerus californicus male face; photo by Q. Baine, WSDA

Urocerus albicornis fore wing; photo by Q. Baine, WSDA

Urocerus albicornis fore wing; photo by Q. Baine, WSDA

Urocerus albicornis female cornus; photo by J. Orr, WSDA

Urocerus albicornis female cornus; photo by J. Orr, WSDA