Allium leaf miner

The larvae (maggots) of the tiny allium leaf miner (leaf mining fly Phytomyza gymnostoma) can devastate allium crops, particularly leeks. It can also attack onions, shallots, chives, garlic and ornamental alliums. This pest was first noticed in the UK in the West Midlands in 2003. It is gradually spreading to other areas.

Be aware: Similar damage can also be caused by the leek moth (Acrolepiopsis assectella) For more information on this pest, see our Leek moth factsheet.

Typical symptoms

• Lines of white spots on leaves where maggots have been feeding
• Creamy white, legless maggots, 5-8mm long in leaves and bulbs
• Leaves split and plants distorted
• Browny orange pupae, 3-4mm long, in leaves and bulbs
• Plants rotting.

Check alliums regularly for mining damage, and split leaves. Peel back leaf layers to check for pupae; slicing leeks, onions or garlic longitudinally in November to March may also reveal the pest.
When plants are in the ground, the damage is more obvious once secondary rotting takes place, on leeks between December and February, and on onions in June.

Description of pest

The adult fly is only 3mm long, so you are unlikely to notice it. The larvae (maggots) are slim, 5-8mm long and legless. The pupae are a shiny brown, cylindrical and 3-4mm long.

Life cycle

The fly has two generations a year. Adults lay eggs in March/April; the next generation lay in September to November. The exact timing will differ in different areas.
This pest overwinters as pupae, attached to Allium (onion) family plants, plant debris or in the soil. In the spring, the adults emerging from the pupae lay eggs at the base of host plant stems. The first generation of larvae feed in April and May. The second generation is likely to peak in mid-september.

Prevention and control

• Encourage natural predators: Garden organically, avoid harmful sprays and provide food and shelter for wildlife. Birds, bats, hedgehogs, frogs and beetles will eat the various stages of the fly.
• Good hygiene: don’t accept leek, onion or other allium family plants from other people unless you can be sure that they are not infected. And don’t pass on plants that might be infected – from transplants to mature crops.
• Cultural control: Clear away all plant debris at harvest. Dig over the soil to disturb over-wintering adults and pupae.
• Use a crop rotation: Although this pest usually pupates on/in the plant, some pupae will fall to the ground. If you then plant susceptible plants in the same place next year, and use a crop cover, pupae in the soil may hatch out under the cover to infect the new crop.
• Crop covers: Cover susceptible plants with an ultra fine mesh cover (the fly is tiny), or horticultural fleece to prevent adult flies laying eggs on the crop. As plants need to be protected for a considerable time, a mesh cover is preferable to horticultural fleece as it allows greater air circulation. Remember to cover seedlings in the ground and in trays as well. Ensure the cover is firmly fixed so it will not blow off in the wind.
  You could in theory remove the cover in June until August when the fly should not be active, but that is a period when leek moth is also active, if that is also present in your area!

• Disposal of infected plants: Soak plants in a bucket of water for a couple of weeks, then add the mush to a compost heap, or bury them in a deep hole. Do not put them straight into your compost heap. Alternatively, put them in your green waste collection bin.

Developing new methods of control.

Several organic farmers in Slovenia are spraying whey (non diluted), with some success. Spraying needs to take place when adults are active and females egg-laying (March/April and October/November) and when larvae are found feeding in the upper parts of the leaves.

We are always interested to hear from anyone who has devised other methods of dealing with this relatively new pest. If you have any suggestions to pass on, please do email editor@gardenorganic.org.uk

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