Family common name: stem sawflies
Genus: Trachelus Jurine, 1807
The Cephidae are commonly known as stem sawflies because larvae feed and live within the stems of small herbaceous and woody plants. Many are considered pests, since this feeding behavior can damage or even kill the plant host (Shanower and Hoelmer 2004).
Stem sawflies of the genus Trachelus are generally slender, with slightly laterally compressed cylindrical bodies. They have a black head and black body often with yellow markings. There is a slight constriction at the second abdominal segment, giving it the gestalt of a wasp (Middlekauff 1969). Often the wings are darkened or violaceous. Average body length is about 10 mm (Ries 1937).
Worldwide, there are only eight described species, restricted to the North Hemisphere. Only one occurs in North America (Taeger et al. 2010).
All Cephidae can be distiguished from other sawfly families by the lack of cenchri. Trachelus can be confused with other Cephidae, especially Cephus and Calameuta, which were once treated as synonyms. They can be distinguished from Cephus and Calameuta by the presence of U-shaped bristles, pits on the 7th and 8th male sternum, and from other genera in the family presence of preapical spurs on the hind tibiae, flagellomere 3 subequal to 4, and the distinctly clavate antennae beginning past the midpoint (Ries 1937, Middlekauff 1969, Smith and Schiff 2005).
In North America, Trachelus species feed on cultivated grasses of the family Poaceae, most commonly Triticum spp. (wheat), but also Secale cereale (rye), and Hordeum vulgare (common barley). There are no records of this species feeding on non-cultivated or wild species of these genera or related plants (Shanower and Hoelmer 2004).
Female Trachelus oviposit into larger-diameter stems of grasses (Middlekauff 1969, Shanower and Hoelmer 2004). After hatching, the larva feeds on the vascular tissue of the plant, moving downwards towards the base (Shanower and Hoelmer 2004). Larvae are creamy white and grub-like in appearance. They lack abdominal prolegs, and thoracic prolegs are vestigial. Cephidae larvae possess a tubular dorsal horn on the posterior end of the body (Middlekauff 1969). As the larva feeds, it uses this horn to pack frass in the gallery behind it. At the base of the plant, the larve “girdles” the stem above it by chewing a V-shaped notch along the inner wall, then packs frass tightly between itself and this point. When the plant weakens and dies from the damage, it breaks off at this point and leaves a stub in the ground with a frass plug on top, keeping the larva safe inside (Shanower and Hoelmer 2004).
Larvae generally undergo a period of diapause, either over winter or through a dry season, then pupate inside the stub. After about 2 weeks, the adult chews or pushes its way out of the stub and emerges. In cultivated crops, adults live for 3–4 weeks and are univoltine (Shanower and Hoelmer 2004).
Trachelus tabidus, commonly known as the “black grain-stem sawfly” is considered a pest of wheat crops in North America, where it was introduced in the 1880s. Historically it had a large effect on crop yield, with infestation rates as high as 65% in the 1930s (Shanower and Hoelmer 2004). There is evidence that this species is being out-competed and displaced by another exotic stem sawfly, Cephus pygmaeus (Middlekauff 1969). A successful biocontrol program in the 1930s has generally reduced T. tabidus to a non-pest (Shanower 2004).
World: The genus generally occurs in the Mediterranean and Caucasus regions of Europe and West Asia, with 2 species ranging farther north into Central Europe, 1 species in Mongolia, and 1 species introduced to North America (Budak et al. 2017).
North America: Trachelus tabidus in North America is recorded from in the Mid-Atlantic region with records in Pennsylvania, Ohio, New Jersey, New York, Virginia, Maryland, and Delaware, and north into New England.
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Trachelus
Details about data used for maps can be found here.