Allium leaf miner

The allium leaf miner (Phytomyza gymnostoma) was first detected in Britain in 2002, since then it has spread in the Midlands and has also been found in Surrey. The larvae bore into the stems and bulbs of leeks, onions, chives and garlic with devastating consequences. Affected plants often develop secondary infections and rot.

Symptoms
The first sign of an attack from allium leaf miner is the presence of the adult fly, which are greyish-brown and 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. The larvae are white, headless maggots without legs that make tunnels in the stems and bulbs of their host plants.

Note: Similar damage is caused by caterpillars of the leek moth but that pest has creamy white larvae with brown heads and small legs.

But perhaps the most obvious signs of a problem appear when rotting sets in; plants affected by both allium leaf miner and leek moth tend to rot due to secondary infections from fungi and bacteria that develop in the damaged tissues. On closer inspection, cylindrical brown pupae may also be found embedded in the stems and bulbs.

Non-chemical
Plants can be protected by covering them with horticultural fleece, or an insect-proof mesh such as Ultra-Fine Enviromesh, at times when the adult flies are active and laying eggs (March to April and October to November). Crop rotation must be used, as adult flies might emerge from pupae underneath the fleece covering if susceptible plants are grown in the same piece of ground in successive years.

Chemical
None of the pesticides currently available to amateur gardeners for use on leeks and onions is likely to give good control of allium leaf miner.