It is all quite demoralising, watching this happen, when leeks used to be a relatively easy crop to grow through the winter. The pest is allium leaf miner, and was first sited in the Midlands in 2002. The maggots are cream coloured, not to be confused with the leek moth caterpillar which has a brown head. Garden Organic last did a survey of this pest in 2011, in conjunction with the Organic Growers Alliance, so we thought that it would be useful to conduct another survey to see just how far it has spread now.

The results tell an interesting story. A few people recollect seeing it as long ago as 2009, but the incidence seemed to increase most rapidly after 2013. It has been slowly spreading out from two centres around the Midlands and London but nobody north of Manchester in our survey reported a problem. Either it doesn’t thrive under cooler conditions, or it hasn’t reached those areas yet.

Leeks were by far the worst crop affected, with 78% of sites reporting a problem, although garlic and onions suffered some damage too. Most damage was caused between September and December. This is the main time when the fly lays its eggs, so it is best to cover your crops with fine mesh if you know there is a problem in your area.

Even though there were problems in the majority of leek crops, only 27% of people covered their crops to protect them. However, many of the people who currently take no action said that they would cover their crops in the future, now that they were aware of what the pest was.

Not everybody knows about this pest, so it is important that people are given reliable information as to how to identify and prevent it. For full details of the results and more information on allium leaf miner, download the report here.