Frit Fly

Key Points

• Dursban WG is the most effective treatment
• Dursban WG gives consistent protection
• Early application can prevent severe crop losses
• Applications to grassland in the autumn can eliminate the risk to spring-sown forage maize crops

Introduction

Frit fly — Oscinella frit — larvae can attack all cereal crops, including maize. Severe attacks in forage maize cause stunting and distortion of the plants. Many leaves fail to emerge properly and are badly twisted. Where leaves do emerge they are ragged and often appear to have been torn in to strips. Where attack is less severe, there may be little effect on growth, except for neat rows of small holes that become apparent as leaves expand.

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.

Biology

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. This spring generation is the most damaging to forage maize.

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.

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.

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 maize 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.

Product Choice

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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