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A medium-size moth (3.5-4.0 cm wingspan) with pale yellow brown to deep rust brown forewings with a darker shade along the cubital vein and a prominent row of dark dots marking the postmedian line where it crosses the veins, and a dark shade in the middle of the outer margin. The hind wings are tan to almost white, much paler than the forewings. Similar to Archanara oblonga, but subflava is generally much lighter in color, has much paler hindwings and lacks the row of terminal dots on the forewings. The illustrated specimen on the right is from the Moths of Canada website; the dark specimen is from Grant Co., WA.

Adults are nocturnal and come to light. There is a single brood each year. The larvae are borers in cattails, rushes and bulrushes (Typha, Scirpus). The life history is very similar to that described for A. oblonga.

The Subflava Sedge Borer may be found far from its wetland habitat and hostplants. The reedy ponds and marshes where this moth breeds are scattered across the landscape like islands in a sea. Many of these are intermittent, appearing and disappearing with changes in moisture regimes, and it is not surprising that subflava disperses to colonize new sites, as do other aquatic insects such as Dytiscus water beetles.