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, and 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).
There are 13 described species worldwide. Just one introduced species occurs in North America (Taeger et al. 2010).
Diprionidae are easily recognized by the distinctive antennae of both sexes, and are characterized by small, stout bodies. Diprion can be separated from other genera in the family by the small cenchri, large mesoscutellum, and the completely bipectinate male antennae (Smith 1971b, Goulet 1992).
Diprion pini is a widespread pest of Pinus sylvestris (Scots pine) in the entire Palearctic region. It has a very similar biology to D. similis, in that it is a defoliator that feeds in large groups and has two generations per year. Additionally, D. pini can remain in diapause for several years, so it is difficult to determine or control the population size during any one year (Molet 2011). The estimated annual cost of the reduction in tree growth in Europe as of 1997 exceeds $6 million USD (Beaudoin et al. 1997). This species has more than one pine host, including many species like P. sylvestris that occur and are grown in North America for timber (Molet 2011).
Larvae in North America feed on several species of pine. The preferred host species is Pinus strobus (eastern white pine). Other hosts include Pinus banksiana (Jack pine), Pinus flexilis (limber pine), Pinus ponderosa (ponderosa pine), Pinus resinosa (red pine), Pinus rigida (pitch pine), Pinus taeda (loblolly pine), Pinus virginiana (Virginia pine), Pinus sylvestris (Scots pine) (Lyons 2014), Pinus monticola (western white pine), and Pinus contorta (lodgepole or shore pine) (Looney et al. 2016).
Diprion similis females oviposit into slits cut longitudinally along the needle, usually several in a line, and up to 19 on a single needle. After hatching, larvae feed gregariously from the tip of the needle to the base. Females have 6 larval instars while males have 5, and both sexes become more solitary around the third or fourth instar. During this phase, larvae move to other parts of the tree, or in cases of high abundance, to other nearby trees. At maturity, the larva builds a silken cocoon on a solid surface, usually in the tree. Depending on the generation, the prepupa may overwinter in the cocoon before pupating. It is bivoltine in most of its North American range, but has been recorded as univoltine in Asia (Lyons 2014).
Diprion similis is considered a pest of economic importance because outbreaks of this species in pine stands have historically caused significant damage. Coniferous trees do not regrow foliage every year, so heavy defoliation, especially in combination with other stress, can lead to mortality. A 1970 outbreak of D. similis in Ontario defoliated about 95,000 acres of white pines (Lyons 2014).
World: The species of Diprion are generally found in China, Korea, and Japan west to central Asia. Two species range farther west into Europe. One occurs in North America (Xiao et al. 1983, Taeger et al. 2010, Lyons 2014).
North America: Diprion similis has a mostly northeastern distribution. It was first recorded in Connecticut in 1914, and has since spread in the northern United States as far south as North Carolina and as far west as North Dakota. The first record in Canada was in Ontario in 1931. As of 2010, D. similis ranges throughout Ontario, Manitoba, Quebec, and Newfoundland (Lyons 2014). It was also recently detected in Washington State, where it appears to be widely distributed west of the Cascade Mountain range (Looney et al. 2016).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Diprion
Details about data used for maps can be found here.