Allium leaf mining fly was first detected in Britain in 2002, it has become widespread in England and parts of Wales. The larvae bore into the stems and bulbs of leeks, onions, chives and garlic. Affected plants often develop secondary infections and rot.

Quick facts
Common name Allium leaf miner
Scientific name Phytomyza gymnostoma
Plants affected Leeks, onion, chives, shallot and garlic
Main symptoms Lines of white mines on leaves, maggots or brown pupae in the stems and bulbs
Caused by Maggots of a leaf mining fly
Timing March-June and September-November
​
What is allium leaf miner?Allium leaf miner can feed on leeks, onion, chives, shallots and garlic. The larvae feed within the leaves, the initial feeding damage can be followed by secondary rots making the crop inedible.


SymptomsThe adult fly can fly in from some distance:

• The greyish brown flies are 3 mm long and appear similar to many other species of true fly
• Before laying eggs, the female flies feed by making punctures in the leaves and sucking up the exuding sap
• This causes distinctive lines of white dots on the foliage

Next seen is damage from the maggots (larvae) and the pupae:

• The larvae are white, headless maggots without legs
• Note: Similar damage is caused by caterpillars of the leek mothbut that insect has creamy white larvae with brown heads and small legs
• The cylindrical brown pupae are about 3 mm long and found embedded in the stems and bulbs
• In spring presence of the fly larvae can cause twisting and distortion of the leaves this can be similar to distortion caused by changing weather conditions in the spring
• The most obvious signs of this insect appear when rotting sets in, this is particularly prevalent in autumn:

• Plants affected by allium leaf miner or leek moth tend to rot due to secondary infections from fungi and bacteria that develop in the damaged tissues

ManagementPlants can be effectively protected by covering them with an insect proof mesh at times when the adult flies are active and laying eggs. Peak adult activity is March to April and September to November. Crop rotation must be used, as adult flies might emerge from pupae underneath the covering if susceptible plants are grown in the same piece of ground in successive years.


BiologyAllium leaf miner has two generations a year:

• First generation female flies lay eggs on the stems or base of leaves during March and April
• The second generation repeats the process in late summer and autumn, this generation is usually the most damaging

The maggots bore into the foliage, stems or bulbs of their host plants and, after a couple of weeks, are fully grown and ready to turn into brown pupae. Pupation takes place mainly within the stems and bulbs during summer and winter but some pupae may end up in the soil, especially where plants have rotted off.