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Allium leaf mining fly was first detected in Britain in 2002, since when it has become a problem for allium growers in much of 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.
Allium leaf miner (Phytomyza gymnostoma) on leek (Allium porrum)
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.
Infestations are initiated by the adult fly:
Next seen is damage from the maggots (larvae) and the pupae:
The most obvious signs of a problem appear when rotting sets in:
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.
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.
Allium leaf miner has two generations a year:
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.
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