Frit fly — Oscinella frit — larvae can attack all cereal crops. Winter wheat crops following grass and late-sown spring oats are particularly susceptible. Feeding larvae kill the central shoot, causing the characteristic dead-heart symptoms, and are capable of moving from shoot to shoot. In spring oats it has been known for the developing grain to be damaged by feeding.
All areas of the UK are at risk from Frit fly attack and in the worse cases re-drilling will be necessary. Larvae present in ploughed grassland, or grass weeds in arable crops, attack winter cereals.
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.
The first action must always be to carry out a Risk Assessment. This can be done using the Dursban* WG Frit Fly Risk Assessment Chart (134KB PDF). Depending on the outcome of the Risk Assessment, Dursban WG should then be applied at the most appropriate rate in 200-1000 litres of water per hectare.
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