Identification and life cycle of the brassica leaf miner on vegetables and oilseed rape

Brassica leaf miner causes severe damage to vegetable brassicas, including salad crops such as rocket and tatsoi. This species will also cause damage to oilseed rape but rarely justifies insecticide treatment.

• Adults are generally most abundant in September

Scientific name: Scaptomyza flava

Adults are pale brown with faint stripes on the thorax and red eyes. The wings are about 50% longer than the head and thorax combined. It is about 3 mm long with a wingspan of 6 mm.

Eggs (0.3–0.4 mm long, 0.2 mm wide) are laid singly, although close together.

Larvae are cylindrical maggots, which becomes greenish and 0.4–5 mm long.

Pupae are brown and about 3 mm long.

Nov–Mar: Brassica leaf miner overwinters as pupae in the soil.

Jun–Aug: Adults are generally most abundant in September, although local peaks of activity can occur in July or August. Females can lay more than 300 eggs in punctures made within the lower surface of the leaf. The number of generations per year in the UK is unknown.

Jul–Oct: Hatching larvae feed between the upper and lower surfaces of the leaf. They move towards the midrib and create a long narrow mine that expands into a large, irregular white/yellow blotch. Several larvae may occupy the same mine. Typically, mines are visible between July and October. In smaller leaves, the mine lies in the centre of the leaf and often touches the petiole. In larger leaves, the mine is to one side of the midrib. Frass (droppings) is usually deposited in green clumps near the margin of the mine.

Jul–Oct: Larvae usually drop to the ground to pupate, but sometimes a separate pupation leaf mine is used.

Non-chemical control

Cover vegetable brassicas with insect-proof netting when the adult flies are active and laying eggs.

Monitoring

Yellow water traps or white sticky traps can be used to monitor adults. No forecasting systems have been developed.

Thresholds

None established.

Insecticide resistance

Resistance to pyrethroids has been confirmed in the UK.