Q What is allium leaf miner?
A The adult allium leaf miner is a small fly, Phytomysa (formerly Napomyza) gymnostoma, but you are unlikely to spot it as it only 3mm long, and inconspicuous. The damage is done by the maggot-like larvae, which tunnel into the leaves.
Caption: Allium-leaf miner damage causes the plant to rot
Q Where does it occur?
A In the UK it was only found around Wolverhampton until recently, but it now appears to be spreading, and has been recorded in Cheshire, London, Shropshire, Staffordshire, Warwickshire and Worcestershire. Interestingly, it has only been found in gardens and allotments, not in commercial crops.
Q Where has it come from?
A Allium leaf miner came originally from Central and Eastern Europe, where it's a serious pest on leeks, onions and chives. It has been spreading outwards in the last 20 years and is now found throughout Europe. It was first identified in Britain in 2003 on leeks in a garden in Wolverhampton.
Q What plants does it attack?
A So far, this pest has only been found here on winter leeks and spring-sown onions. But it can affect chives, garlic, salad onions, shallots and ornamental alliums.
Q What damage does it do?
A The first sign of attack is often rotting towards the base of the leaves caused by fungi or bacteria invading the wounds caused by the miners. If visible, the mines themselves are narrow and linear. With onions, the leaves may become deformed, curled or wavy. If left unchecked, secondary rotting can soon destroy the whole plant.
Q What are the early warning signs?
A Female adult flies feed by puncturing the leaves, and this produces a row of white spots towards the tips of the leaves in spring and autumn. This is easier to see in onions and chives than in leeks, but is still not very distinctive.
Q How do I recognise the pest?
A The larvae are off-white maggots around 5mm long. They often go undetected, and, by the time the damage is spotted, they have turned into orange-brown pupae, 3-4mm long.
These pupae can be found towards the base of the leek or onion either embedded in the mines or wedged between the leaves - you need to peel back the outer layers to see them.
Q When do attacks occur?
A Leaf mining takes place mostly from April to May and from October to November. However, the damage is most obvious later, once rotting has set in, so is usually seen in overwintering leeks between December and February, and in onions in June.
Q How serious is it?
A Commercially, allium leaf miner would probably be controlled by pesticides already routinely used to control other pests, but for gardeners and organic producers it can be very damaging.
Q Do I need to tell anyone about it?
A No. When it first appeared, allium leaf miner was a notifiable pest, but now the Food and Environment Research Agency (FERA) has concluded it's not practical to try to control it as it would just be re-introduced.
Q Could I mistake it for anything else?
A The rotting and disintegration that follow leaf-miner damage can also appear following damage caused by leek moth or onion fly, both of which can also attack the whole allium family.
Leek moth caterpillars are yellow-green, and have legs and a distinct dark head. The pupae, found on the leaves, are 10mm long, cream with a silk covering.
Onion fly maggots reach about 8mm long and cluster in the base of the bulb. Damage occurs between June and September and is usually first noticed when the leaves turn yellow. They pupate in the soil. for more information.
Q What should I do with affected plants?
A Once rotting sets in, the plants won’t recover. Undamaged parts can still be eaten, but affected onions can't be stored. If they're too badly damaged to be used, affected plants are best consigned to your local council's composting scheme, which will destroy the pest, or buried deeply so the adults can't emerge from the pupae.
Q What is its life cycle?
A The adult flies emerge from pupae in spring to feed, then lay eggs at base of the leaves. The larvae emerge and make tunnels into the leaves, feeding for a month or so, then pupating. A second generation of flies emerges in autumn, and it's their offspring that will pupate over winter.
Q How do I control it and prevent future attacks?
A Once the miners have burrowed into the crop, there's little you can do. There are no insecticides approved for treating the pest, even if you knew it was there. Timing your crops can help avoid attacks. The flies lay their eggs between the end of February and the end of April, and again from the end of September to the end of November. This means crops raised between May and September should escape damage.
At other times, the best approach is to use a physical barrier to keep the flies at bay. In winter and early spring you can use horticultural fleece. In summer, fleece can make the plants too hot and tends to keep the rain off, so insect-proof mesh is a better choice.