Allium leaf miner

What it is

Allium leaf miner is a small fly, around 3 mm long, that was first found in Britain in the early 2000s and has since spread over much of England and Wales. It is now one of the most damaging pests of leeks on allotments, and it also attacks onions, garlic and chives.

Symptoms

Look first for lines of tiny white dots running along the leaves, usually near the tip. These are feeding punctures made by the female fly. Later, leaves become wavy, twisted or split, and the plant may rot. Peel back the outer layers of a leek and you will find tunnels running down towards the base and small, shiny, chestnut-brown pupae about 3 mm long tucked into the stem. Leek moth damage is higher up in the leaves and comes with larger caterpillars and net-like cocoons.

Cause

There are two generations a year. Adults emerge from late February to May and lay eggs in the leaves; the larvae mine down into the stems and bulbs, then pupate. Those pupae sit out the summer and a second batch of flies appears from September to November, attacking leeks and newly planted garlic and onions. The autumn generation overwinters as pupae in the plant or the soil.

Control

Lift and destroy badly affected plants, and do not compost them, as pupae survive in heaps. Lightly affected leeks can still be eaten once you have trimmed away damaged layers. Clear all allium debris from the bed before the next flight period. Chemical options for home gardeners are poor; prevention works far better.

Prevention

Cover alliums with fine insect mesh during both flight periods, roughly March to mid-May and September to November, making sure edges are buried. Plant leeks out after the spring flight and harvest some before autumn. Rotate alliums and avoid growing chives and onions next to leek beds that were infested.

Plants often affected

• Leeks
• Onions
• Garlic
• Chives
• Ornamental onion

Could it be something else?

These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.

• Carrot fly
• Leatherjackets
• Onion fly
• Parsnip canker

Months to keep an eye out: March, April, May, September, October and November.

Sources

Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.

• Wikipedia — Phytomyza gymnostoma
• Penn State Extension — Allium leafminer