What is allium leaf miner?

Allium leaf miner can be a problem on many common crops: leeks, onion, chives, shallot and garlic. The initial damage is done by the maggots, but secondary fungal and bacterial infections often cause the most noticeable rotting.

Symptoms

Infestations are initiated by the adult fly:

• The greyish brown flies are 3mm long
• 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
• These make tunnels in the foliage, stems and bulbs of their host plants
• Note: Similar damage is caused by caterpillars of the leek moth but that insect has creamy white larvae with brown heads and small legs
• The cylindrical brown pupae are about 3mm long and found embedded in the stems and bulbs

The most obvious signs of a problem appear when rotting sets in:

• 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

Control

Non-pesticide control

Plants can be 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 October 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.

Pesticide control

None of the pesticides available to home gardeners for use on leeks, onions and allied plants is likely to give control of allium leaf miner.

Biology

Allium 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 October to November, 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 fed 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.