Stem borer (Diptera: Dolichopodidae) in Andropogon

This borer tunnels in the culm of the host, at least sometimes starting fairly high in the culm (approximately 1m above the ground in one example I examined). A photograph of a larva's tunnel in a dissected stem at a point several decimeters above ground level shows that the tunnel was approximately 1/8 of the culm diameter, with rough-textured walls and no significant solid frass accumulation.

The late-stage larva is a warm yellow color, elongate (roughly 12 times as long as wide) and cylindrical, the body tapering little except at the very front, the posterior end rounded. On its anterior end, the larva possesses a pair of ventrally-pointing toothed mandibles and, dorsad of these, an unpaired sclerotized plate whose exposed portion appears triangular in lateral view. (See Johannsen & Crosby (1913) for a drawing of similar features in a species of Thrypticus from another host.)

The larva finishes up its feeding at the base of the plant where it passes the winter and constructs a pupation chamber in the senescent culm. In one instance I observed, the chamber was several centimeters long and it occupied most of the diameter of the culm. Upon the emergence of the adult fly from this stem, the pupa was thrust partway out of the culm. Examination of the pupal exuviae revealed transverse rows of posteriorly-directed spines on the abdominal segments, which had perhaps helped the pupa gain purchase on the inner walls of the chamber.

Two adults reared in 2017 have not yet been identified beyond the genus level.