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Approximately 7 to 12 mm in length, black and yellow abdomen with a yellow face. Identification to species is challenging, and specimens may easily be confused with S. ribesii, S. rectus (although their distributions do not overlap), or S. vitripennis. Cell cua1 is entirely covered with tiny hairs (i.e. trichose, see A on image), and cell bm is narrowly trichose along posterior margin (see B on image), in some but not all specimens. A fairly reliable characteristic is that the yellow bands of tergites 3 and 4 either do not quite reach the lateral margins, or only barely reach then anteriorly (see C on image, and compare to other images of Syrphus spp.). Males: The frons is either entirely yellow, or the upper half of it is dark gray and covered with a waxy, whitish powder (i.e. pruinose), with lower half of frons bright yellow. The hind femur is mostly black, with only the apical one-third to one-fifth black. Females: Similar to male, but with upper one-third of frons being a blackish colour, and the hind femur being totally yellow (Vockeroth 1992).

Poorly known, but probably multivoltine, due to its more southerly distribution in North America. Some researchers in Oregon have looked at the potential for using S. opinator as a biocontrol agent for cabbage aphids in broccoli fields (Western Region Sustainable Agriculture Research and Education 2003). Host choice of the aphidophagous larvae is probably mostly determined by the oviposition behaviour of the females, because the dispersal capabilities of most Syrphus larvae are relatively limited (Sadeghi and Gilbert 2000a,b). Females of a related species (S. corollae) were found to oviposit in response to the sole presence of aphid honeydew (Schneider 1969), although females may use both visual as well as olfactory signals for choosing an oviposition site. Females in this genus tend to choose large or growing colonies of aphids, to ensure a plentiful food source for their offspring (Sadeghi and Gilbert 2000a). Larvae are voracious predators, and can have significant impacts on aphid populations (Western Region Sustainable Agriculture Research and Education 2003). Parasitoids are unknown, although many parasitoids of other members of the genus Syrphus have been recorded, including (amongst many families) members of the Braconidae, Chalcididae, Proctotrupidae, Encyrtidae, and Ichneumonidae (particularly the subfamily Diplazontinae)(Schneider 1969).

Larvae are aphidophagous and polyphagous, and feed on a wide range of aphid species, including the pea aphid Acyrthosiphon pisum (Schneider 1969), the cabbage aphid Brevicoryne brassicae (Western Region Sustainable Agriculture Research and Education 2003), and the woolly aphid, Eriosoma lanigerum (Schneider 1969). Adults feed on the pollen and nectar of flowers.