The Diprionidae are known as the conifer sawflies, though they are not the only family to use conifer trees as host (others include Siricidae, Pamphiliidae, Xyelidae, some genera of Tenthredinidae). Many are destructive pests as larvae, and so their biology has been more extensively studied than some other groups. Diprionids have stout bodies and distinctive antennal characteristics that make adults easy to recognize (Furniss and Carolin 1977).
Monoctenus is unique in the Diprionidae by using juniper trees as host. They are uncommonly observed, and, unlike closely related genera, seldom cause significant damage to their host (Rose et al. 2000).
Diprionidae are most easily recognized by the distinctive antennae of both sexes, but also are characterized by small, stout bodies. The subfamily Monocteninae can be separated from the Diprioninae (Diprion, Gilpinia, Neodiprion, Zadiprion) by the singular comb-like projection from each antennal segment, and by the fused veins 1A and 2A in the fore wing. It can be separated from Augomonoctenus by the truncate clypeus and the fore wing venation (Smith 1974b, Goulet 1992).
Larvae in North America feed on trees of Cupressaceae. Most species feed on species of Juniperus (juniper), and a few also feed on Thuja occidentalis (eastern white cedar) and possibly other Thuja sp. (Smith 1974b, Goulet 1992, Smith et al. 2010).
The biology of many species is unknown. Monoctenus fulvus females lay one egg at a time into the scale-like leaves of their host. After hatching, larvae feed on the foliage, usually at the new growing tips. Larvae are longitudinally striped and measure about 30 mm at maturity. When disturbed, larvae will rear up the head and thorax, similar to other Diprionidae. Mature larvae will drop to the ground and burrow into the soil to overwinter and pupate. Monoctenus are univoltine (Marlatt 1886, Smith et al. 2010). Phenology is variable (Smith et al. 2010).
World: Species are recorded from North America, in Europe, and in Japan (Smith 1974, Taeger et al. 2010).
North America: Monoctenus has two distinct ranges in North America. Three species occur in the east of Canada and the United States, from Quebec as far south as Virginia, west to Oklahoma; two species occur in central Mexico, one recorded in Durango, one recorded in San Luis Potosí (Smith 1974b, Smith et al. 2010).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Monoctenus
Details about data used for maps can be found here.