Infestations of these generally common flies occur on timothy grass. The larvae graze on the developing inflorescences within the shelter of the leaf sheaths. Symptoms of attack show clearly after the ear emerges, as sections of spikelets are either partly or totally grazed away; these damaged areas are often arranged spirally. Severe seed loss is reported mainly from the northern parts of continental Europe, especially France, Germany and Scandinavia (where the range of these pests extends into the Arctic Circle).
Adults are active in late April or early May. They occur in association with timothy grass, upon which pairs of mating adults may be found; they also rest on the leaf tips, characteristically head downwards. Eggs are deposited close to the stem on the upper surface of the uppermost leaves, parallel to the veins. They hatch 5-7 days later. The larvae then bore into the grass stem to feed on the developing, but still enclosed, inflorescence. After 2-3 weeks, when fully grown, they reappear and usually drop to the ground before pupating in the soil. There is just one generation annually.
Adult 5.0-5.5mm long, mainly dark grey; legs partly pale and reddish-brown. Egg 1 mm long, pale yellow, elongate, seed-like with four longitudinal ridges. Larva up to 8 mm long, lemon-yellow, maggot-like; anterior spiracles prominent and bifid, with several lobes; posterior spiracles slightly sclerotized and 3-pored. Puparium 5 mm long, dark brown.
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