Frit fly generations vary from three in the south to two in the north and Scotland. Adult flies emerge from infested grass and cereals in May and lay eggs on leaf sheaths of seedlings. Larvae quickly hatch and bore into the centre of shoots to feed and then pupate.
The second generation emerges around mid-July in the south of England and eggs are laid on the panicles of late sown spring oats where the larvae feed within the grains and then pupate. The third generation emerges in late summer and lays eggs on cereals. Larvae feed through winter and pupate around March. This third generation is the most damaging to cereal crops.
Adult Frit fly are very small, only 2 – 3mm long, and shiny black in colour. Very small cream coloured, elongate eggs are laid in small groups at the base of shoots, often on the underside of the first leaf sheath. Larvae are yellowish-white in colour and are more or less evenly rounded at both ends. The front is normally slightly more pointed and the rear slightly more rounded. They possess two small tubes at the rear end (visible only when magnified) and are up to 5mm long when fully grown. Pupae are red-brown and often found within the damaged area of the plant.
Late-sown winter cereals and spring oats, following grass and arable crops with a high burden of grass weeds, are particularly susceptible to attack. Early emerged winter barley can be subject to egg laying by third generation adult flies.
Risk of attack by Frit fly can be reduced significantly by employing certain cultural techniques. Leaving a gap of 10 weeks between the previous grass crop, or grassy stubble, and drilling a cereal crop will reduce the risk significantly. Leaving a gap of at least 4 weeks between ploughing and drilling will help reduce the risk of attack. Spring oats should be drilled by mid-February in south-west England and by the end of March at latest elsewhere.
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