This pest has only been seen in the UK since 2002. The Allium Leaf Miner attacks leeks, onions (almost exclus3ively spring sown / planted ones), shallots and other members of the onion family. The first problem you will probably notice is that the leaves becomes distorted and don’t grow in the normal straight and upwards manner, some leaves will lie on the ground. If you look at the leaves carefully you may also see small, light green / grey marks on it in rows down the foliage – these are the points at which the fly stage of this pest fed on the sap.
Treatments for allium leaf miner
When the onions have been infested there is no treatment for them, chemical or otherwise. Remove all parts of the infected plants and burn them.
Preventing Allium Leaf Miner
Preventing the allium leaf miner from causing damage to your onions consists of preventing the adult flies from laying eggs on your plants. If you read the description of the lifecycle above it becomes clear that spring sown onions (as opposed to overwintered ones such as Japanese onions) are at risk from March to April and then again from mid-September to mid-October.
The most obvious method to prevent the flies laying eggs is to cover the crop with insect proof mesh / fleece during the two risk periods.
Another method is to plant onion sets after the first danger period has passed. This works with fast-maturing onion varieties which can be harvested before the second danger period occurs in September / October. Alternatively plant heat-treated onion sets (to avoid bolting) in small pots / modules in an unheated greenhouse or around the edge of the house (to avoid very cold temperatures) at the same time as you would normally outside. Plant them into their final positions in May and they will be ready for harvest before the second danger period occurs.
For those who grow onions from seed, sow the seeds in small pots / modules indoors and gradually acclimatise them to outside conditions in late April so that they can be planted in their final positions during early May.
For more information on Allium Leaf Miner follow this link.