Family common name: stem sawflies
Genus: Cephus Latreille, 1803
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 as this feeding behavior can damage or kill the plant host (Shanower and Hoelmer 2004).
Stem sawflies of the genus Cephus are generally slender, with slightly laterally compressed cylindrical bodies. They have a black head and black body with yellow markings. There is a slight constriction at the second abdominal segment, giving it the gestalt of a wasp (Middlekauff 1969). They are medium-sized, usually about 1 cm in length, and slow flyers, so it is easy to observe them where they occur (Ivie 2001).
Worldwide, there are 40 described species, restricted to the Northern Hemisphere. Diversity is low in North America, with only two known species (Taeger et al. 2010).
A key to Californian species of Cephus is included in Middlekauff 1969.
Cephus can be confused with other Cephidae, especially Trachelus and Calameuta which were once treated as synonyms. They can be distinguished from Trachelus by the lack of bristled pits on sternite 7 and 8 of the male, from Calameuta by the presence of long modified setae on sternite 8 of the male, 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).
Cephus pygmaeus is a pest of wheat crops in its native range of Eurasia, and in North America, where it was introduced in the 1880s (Shanower and Hoelmer 2004). Infestations in the Northeast United States have historically been as high as 45% in cereal and grain crops. This species may also be competing with and displacing another exotic stem sawfly, Trachelus tabidus in this same range (Middlekauff 1969). A successful biocontrol project initiated in the 1930s reduced the impact of C. pygmaeus, which is generally no longer a pest (Shanower and Hoelmer 2004).
The origin of the other North American species, Cephus cinctus, is a subject of some debate. The species was first collected in North America in the 1890s in Canada, and since then, the population has grown dramatically where it remains a significant pest of many grains. Research into biocontrol of this sawfly determined that there are no natural parasitoid predators in North America (Ivie 2001). This discovery, the lack of collections prior the late 19th century, and comparisons to Eurasian Cephus spp., suggested that Cephus cinctus is an introduced species that arrived in the Americas with early European immigrants. The species could be conspecific with C. hyalinatus (Ivie and Zinovjev 1996), and possibly C. camtschatcalis and C. zahaikevitschi (Hoelmer and Shanower 2004). However, recent research supports its status as a North American species, based on the larval biology requiring living or non-dry host material (Beres et al. 2010) and phylogenies identifying C. cinctus as a distinct monophyletic group from C. hyalinus and other morphologically similar species (Lesieur et al. 2016).
In North America, Cephus species feed on cultivated grasses of the family Poaceae, most commonly Triticum spp. (wheat), but also Secale cereale (rye), Hordeum vulgare (common barley), Phleum pratense (timothy), Less common host grasses of North America for this genus include wild varieties of Hordeum and Phleum, as well as wild or ornamental Agropyron spp. (wheatgrass), Beckmannia spp. (slough grass), Bromus secalinus (rye brome), Calamagrostis spp. (reed grass), Calamovilfa spp. (sandreed), Deschampsia spp. (hair grass), Elymus spp. (wild rye), Festuca spp. (fescue), Avena spp. (oat), and Stipa (feather grass) (Middlekauff 1969).
Female Cephus 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 have vestigial thoracic legs. All Cephidae larvae possess a tubular dorsal horn on the posterior end of the body (Middlekauff 1969). As they feed, they use this horn to pack frass in the gallery behind them. At the base of the plant, the larvae girdle the stem above it by chewing a V-shaped notch along the inner wall, then pack frass tightly between themselves 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 generally live for only 7–10 days and are univoltine (Shanower and Hoelmer 2004).
World: The genus is widespread throughout the Northern Hemisphere with species documented in North America, Europe, North Africa, Southwest Asia, and East Asia (Taeger et al. 2010).
North America: Cephus cinctus ranges west of the Mississippi River in continental United States and southern Canada. In North America Cephus pygmaeus occurs on the east coast of the United States and Canada (Middlekauff 1969).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Cephus
Details about data used for maps can be found here.