Adult L. donacis emerge in early spring. Lasioptera donacis has a symbiotic relationship with the saprophytic fungus Arthrinium arundinis. Female L. donacis collect spores of A. arundinis and deposit these along with L. donacis eggs inside the mesophyll tissue of giant reed leaf sheaths. Eggs are a pale translucent pink, elongate, tapered at one end and laid in clutches of 15–25.  As the fungus breaks down the tissue in giant reed leaves, L. donacis larvae feed on fungal mycelia through three instars. Third-instar larvae are orange, somewhat flattened, 3 mm long, and have very small head capsules. Pupation occurs within silken cocoons spun by third-instar larvae. Pupae are orange with darkened wing pads. When adults emerge, they leave pupal exuviae protruding from exit holes. Adults are very small, usually 2½ mm long. They are light brown with clear wings and long, delicate legs. Complete development from egg to adult can take just under four weeks, and in the native range, there are multiple generations per year.