grass sheathminer fly

Grass sheathminer fly is a common, small fly known for its distinctive larval feeding habits. It occurs in the United States, southern Canada, Mexico, Central America, Brazil, and Puerto Rico.

The larvae have been recorded feeding on many wild and cultivated species in the family Poaceae, including barley, bluegrass, brome, corn, reed canarygrass, goose grass, oat, orchard grass, panicgrass, rye, timothy, wheat, and wheatgrass. The young larvae feed on the parenchyma (inner tissue) within the blade of grass, leaving a narrow visible trail (mine). They feed downwards, eventually entering the leaf sheath and sometimes even the stem. This is the behavior that gives the fly its common name. The larvae pupate within the mine.

For small flies like Agromyzidae, wing length is often given in taxonomic descriptions as a measure of size, rather than body length. For Cerodontha dorsalis, the wing length of adults is variable, ranging from 1⁄16″ to ⅛″ (2.4 to 3.2 mm).

The frons (upper part of the front of the head, corresponding to the forehead) is orangish yellow, The genae (cheeks) and face are also orangish yellow. The genae are broad and are about one-third the height of the eye. On the antennae, the third segment is long and black, with a short spine at the upper corner.

There are two geographically isolated color forms. The eastern form is more pale and the western form is more black. Though it was investigated in some detail, the reason for the difference has not been satisfactorily explained. Only the typical (eastern) form occurs in Minnesota. The following description is of the eastern form.

The mesonotum (upper part of the thorax) is matte grayish black on the front half, orangish yellow on the rear half. There is a transverse suture (horizontal groove) across the mesonotum. On each side of the mesonotum, there is a longitudinal row of dorsocentral bristles (dc), consisting of three longer bristles located behind the transverse suture and one longer bristle located in front of it. There are no acrostichals (minute hairs) between the dorsocentral bristles. On each side of the thorax, the notopleuron (ttriangular area immediately below or to the side of the transverse suture) is yellow.

The scutellum (the plate between the abdomen and the thorax) is yellow in the center and black on the margins. It has only one pair of bristles.

On each leg, the femur (third segment) is bright yellow.

The wings are clear. The subcosta vein runs parallel to the R₁ vein and reaches the costa (vein on the leading edge of the wing), rather than terminating in the R₁ vein. Vein M₁₊₂ ends nearest to the wing tip.

Wing length: 1⁄16″ to ⅛″ (2.4 to 3.2 mm)

Species in the family Poaceae, including barley, bluegrass, brome, corn, reed canarygrass, goose grass, oat, orchard grass, panicgrass, rye, timothy, wheat, and wheatgrass

Multiple overlapping generations per year: May to October

The larvae pupate within the mine.

Sources

24, 29, 30, 82, 83.

Dan W. Andree
6/15/2025

Location: Twin Valley Prairie SNA

This tiny fly like insect landed on the prairie violet out at the Twin Valley Prairie SNA. Just thought I'd share it. Funny little creature. Not a detailed enough insect so not expecting you to identify what it is just thought you may find it interesting.