Family common name: horntails
Genus: Tremex Jurine, 1807
The Siricidae are called “horntails” because of the 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 Tremex are large and slender with a cylindrical abdomen. The only North American species, Tremex columba, has at least four color morphs throughout its range, comprising combinations of brown, reddish-brown, and yellow. 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 (Cranshaw 2013).
A key to North American species of Tremex is included in Schiff et al. 2012.
Tremex can be confused with other Siricidae, especially Eriotremex, because of the similar coloration. They can be distinguished from other genera in the family by the single tibial spur, the lack of a ridge on the gena behind the eye, the number of antennal segments, and a short ovipositor. They can be distinguished from Eriotremex formosanus in North America by the lack of long golden-yellow hairs on the body (Schiff et al. 2012).
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.
Tremex fuscicornis is an introduced species to Australia and South America (Schiff et al. 2012). In Chile for example, established populations were discovered in 2000, and there has been significant damage recorded on walnut, poplar, maple, and willow trees (Parra et al. 2007). Though not yet established in North America, the generalist feeding behavior targeting several common trees and documented invasions makes this species a potential threat (USDA 1985).
In North America, Tremex species have been documented using a variety of host trees in several families (Schiff et al. 2012). Among them are Acer spp. (maple), Carpinus sp. (hornbeam), Robinia spp. (black locust), Castanea dentata (American chestnut), Fagus spp. (beech), Quercus sp. (oak), Carya spp. (hickory), Juglans cinerea (white walnut), Nyssa sylvatica (tupelo), Fraxinus sp. (ash), Platanus occidentalis (American sycamore), Malus sp. (apple), Pyrus sp. (pear), Populus spp. (cottonwood), Salix sp. (willow), Celtis spp. (hackberry), and Ulmus spp. (elm) (Schiff et al. 2012).
Female Tremex 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 T. columba, T. fuscicornis, and T. longicollis is the species Cerrena unicolor. Females reserve mucus in additional abdominal glands, but the function of these reservoirs is not well understood; the excreted mucus may encourage fungal growth, or like in the case of Sirex noctilio, may be phytotoxic and assist in decaying the host (Schiff et al. 2012).
Tremex have a unique oviposition behavior among the Siricidae. The female will drill new holes to place clusters of fertilized eggs, but lays unfertilized eggs in old emergence holes from a previous season (Schiff et al. 2012). Sawflies, like all Hymenoptera, are haplo-diploid, meaning that fertilized and unfertilized eggs result in female and male offspring respectively (Schiff et al. 2006).
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. There can be between 6–12 larval instars, depending on food availability, before reaching maturity, when the body measures about 2.5 cm in length (Schiff et al. 2012). The larvae bore galleries into the wood, up to 3 meters in length, running vertically on the tree just under the bark. 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 Tremex species varies from only a few months up to 3 years (Schiff et al. 2012).
World: Except for one species, Tremex is restricted to the Northern Hemisphere. Species range from Eastern Europe through Russia and India to East Asia, including Japan, Malaysia, and the Phillipines (Smith 1978). Tremex fuscicornis is introduced in Australia and Chile (Baldini 2002).
North America: Tremex columba ranges east of the Rocky Mountains as far north as Alberta and Nova Scotia, south to Florida. There are some collection records from California, where it may be adventive, and some interceptions on wood coming across the U.S. border with Mexico, indicating a possible population with a more southern range (Schiff et al. 2012).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Tremex
Details about data used for maps can be found here.