Family common name: parasitic woodwasps
Genus: Orussus Latreille, 1797
The Orussidae are small, predominantly black sawflies with cylindrical bodies and globular heads (Eaton and Kaufman 2007). They are distinctive because they are the only sawfly family that is parasitic instead of phytophagous (Furniss and Carolin 1977). Current Hymenoptera phylogenies suggest that the Orussidae are the most closely related extant sawfly family to the suborder Apocrita, and specifically parasitic wasps (Vilhelmsen 2004).
Orussus are 5–14 mm in length (Furniss and Carolin 1977). They share several remarkable morphological characters with other Orussidae, including antennal insertions located extremely low on the face, reduced wing venation, and a relatively long and thin ovipositor (Vilhelmsen et al. 2014). Though it is the most speciose and widespread of the family, collections of this genus are uncommon (Vilhelmsen et al. 2014).
Worldwide, there are 28 extant described species (Choi et al. 2014). The lowest diversity is in the Southern Hemisphere, with only six species recorded from the Neotropical, African, and Australasian regions (Vilhelmsen et al. 2014). Five species occur in North America (Middlekauff 1983).
A key to North and Central American species of Orussus is included in Middlekauff 1983.
Orussidae are morphologically distinct among sawfly families because of the body shape and location of antennae on head. Orussus, however, is easily confused with other genera of the family. It can be distinguished by a lack of longitudinal carinae on the face between the compound eyes, lack of swelling on the hind femora, and the shield-shaped mesoscutellum (Middlekauff 1983).
In North America, Orussus are parasitoids that feed on wood-boring beetles of Buprestidae (Vilhelmsen et al. 2014). Species used as host include Buprestis aurulenta, Buprestis confluenta, Buprestis laeviventris, Buprestis rufipes, Chrysophana placida, Dicera divaricate and Polycesta californica (Middlekauff 1983).
The recorded “host” trees of North American Orussus, which are more likely to be the hosts of parasitized beetles, include Abies concolor (white fir), Abies magnifica (red fir), Acer rubrum (red maple), Acer saccharum (sugar maple), Acer macrophyllum (big leaf maple), Alnus sp. (alder), Betula occidentalis (water birch), Carya sp. (hickory), Fraxinus sp. (ash), Larix laricina (tamarack), Malus pumila (apple), Picea sitchensis (Sitka spruce), Pinus contorta (lodgepole pine), Pinus jeffreyi (Jeffrey pine), Pinus monticola (western white pine), Pinus ponderosa (ponderosa pine), Pseudotsuga menziesii (Douglas fir), Populus fremontii (Fremont cottonwood), Populus tremuloides (quaking aspen), Quercus californicus (California black oak), Tilia americanum (basswood) and Ulmus americanus (American elm) (Middlekauff 1983).
To find a suitable oviposition site, Orussus females run up and down fallen or standing dead or damaged tree trunks, using their antennae to tap repetitively on the log. The behavior of running up and down, with the wings folded and effectively hidden, superficially resembles the behavior of carpenter ants. In the field, these females have been observed spending up to 27 minutes on the same log patrolling for an oviposition site. Whether they are able to sense a host presence in the log by chemical signals or by vibrations caused by the tapping of antennae is unclear (Powell and Turner 1975).
Females oviposit into an established, frass-filled gallery, 5–15 cm away from a wood-boring insect larva. After hatching, the Orussus larva moves down the mine, sometimes feeding on frass, before beginning to feed as an external parasitoid on the wood-borer (Middlekauff 1983). Larvae are white, legless, and grub-like. The body is slightly flattened and tapers on either end (Vilhelmsen et al. 2014).
World: Representatives of Orussus are present in Europe, North America, Southwest, East and Southeast Asia, tropical Africa, the Philippines, and New Guinea (Middlekauff 1983)
North America: The species of Orussus in North America are mostly recorded from two distinct ranges, east and west - there are very few records for the plains states and provinces. The eastern range spans from Quebec and New England, south to Louisiana, and as far west as Ohio and Illinois. The western range includes British Columbia south to California, as far east as Ontario in the north, and some collections as far east as New Mexico in the south (Middlekauff 1983).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Orussus
Details about data used for maps can be found here.