Family common name: stem sawflies
Genus: Janus Stephens, 1829
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 Janus are generally black, slender, with slightly laterally compressed cylindrical bodies. Females are about 6–12 mm in length, while measurements for males are slightly smaller. The ovipositor of Janus females is generally short (Ries 1937).
A key to North American species of Janus is included in Smith and Solomon 1989.
Species of Janus are often confused with other Cephidae, especially similar genus, Phylloecus. It is distinguished from Caenocephus by the presence of the preapical hind-tibia spurs and presence of vein 3r-m in the hind wing. It can be distinguished from Phylloecus by the presence of a basal lobe on the tarsal claws and the pair of preapical hind-tibia spurs (Smith 1986).
In North America, the hosts of Janus are from several unrelated groups, including Populus spp. (poplar), Salix spp. (willow), Ribes spp. (currant), Viburnum spp., and Quercus spp. (oak) (Hansen 1986).
Some Janus species in North America are pestiferous. Janus integer, also known as the “currant stem girdler” can cause significant damage to crops of currant and gooseberry (Petroski et al. 2003). Janus abbreviatus, also known as the “willow shoot sawfly” is associated with shoot mortality of nursery willow, poplar, and cottonwood trees (Solomon and Randall 1978).
Female Janus oviposit eggs singly into stems of shrubs and young trees (Middlekauff 1969). After oviposition, the female girdles the stem above the egg using the ovipositor “saw” to make punctures around the stem. This effectively kills the shoot above the feeding site (Solomon and Randall 1978). Larvae are creamy white and grub-like in appearance. They lack any abdominal prolegs, and thoracic legs are vestigial. Cephidae larvae possess a tubular dorsal horn on the posterior end of the body (Middlekauff 1969). As the larva feeds, it tunnels downwards through the cambium and pith of the stem, while using this horn to pack frass in the gallery behind it. As it tunnels it kills the buds that it passes (Hansen 1986).
Larvae generally undergo a period of diapause, over winter, in a white, papery cocoon, then pupate inside the hollow stem (Hansen 1986). In the spring, the adult chews or pushes its way out of the stub and emerges. All species that have been studied are univoltine (Solomon and Randall 1978).
World: This genus occurs throughout Europe, North Africa, and Asia (Smith 1993).
North America: Most species of Janus occur on the East Coast, throughout Southeast Canada and Northeast United States, west as far as Illinois and south to Virginia. One species, J. rufiventris, occurs in the Pacific Northwest of the United States with collections in California, Oregon, and Washington (Taeger et al. 2010, Smith and Solomon 1989, Middlekauff 1969).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Janus
Details about data used for maps can be found here.