allium leaf miner | Baldock Allotment Association

In the past, I had not been successful with onions or leeks because of the allium leaf miner. The adult flies appear from March to April and September to October and will feed on the leaves of your allium plants before laying her eggs. It attacks all members of the allium family. You can tell the female fly has fed on your leaves because you will see lines of white dots on the leaves on your alliums. The eggs will hatch and become larvae (small maggots) that will tunnel in the foliage, stems and bulbs of the allium plant exposing the plant to diseases and rotting.

So at the end of last year I thought I would protect my plants using insect mesh to stop the allium fly laying eggs on my allium plants. Back in October last year Monty Don on Gardeners’ World said to try planting onion sets in modules in the greenhouse as it will give the onions a head start and avoid cold and wet weather. I knew the allium fly would be unable to access the onions in the greenhouse over winter so I gave it a try.

In April, when the soil had warmed up, I decided to plant the onion plants out. I knew the allium fly could lay her eggs in April so I covered it with insect mesh (which is available at the BALGA Members’ shop).

Finally, it was a success! Have a look at the picture below:

Remember, there is no chemical available that will stop the allium leaf miner so it is best to use insect mesh in March-April and September-October. Perhaps you can try starting your onions in modules this year? It could give you a head start to the season next year and harvest onions early than usual!

Miles

Sadly, we have to report that allium leaf miner has reached deepest darkest Hertfordshire and has been spotted on both our sites here in Baldock. This pest first arrived in the UK back in 2002. It has come from mainland Europe and was confined for quite a time to southern and eastern counties but it has been gradually spreading since then.

And to make matters worse, allium leaf miner has two lifecycles each growing season and will attack all members of the allium family, particularly leeks and spring planted onions, shallots and garlic. There is no chemical protection available to allotment gardeners so the only thing that can be done is to cover crops with Enviromesh, and hope!

The adult flies appear in March or April, having overwintered in the soil. The females will feed on the leaves of your alliums before laying her eggs, usually near the base of the leaves of the plant. This can be spotted if you see lines of white dots on the leaves of your alliums.

The eggs hatch and the resulting larvae, which are tiny white, head and legless maggots, feed on the leaves before tunnelling (hence the name leaf miner) into the leaves. At this point, fully fed, the maggots pupate within the stem of the plant.

This second generation will hatch in September/October and again lay eggs at the base of the plant. It is this generation that will do the most damage to the now mature onion and leek plants. It will also overwinter in the soil ready to hatch the following spring.

It should be noted that most plants affected will tend to rot from a secondary infection from fungi or bacteria that develop in the damaged stems in the plant. The damage this fly can do is such that there can be no sign of your alliums being affected until the rotting is noticed.

So what can we do to protect our crops?
First off, employ a strict crop rotation policy. Do not grow alliums in the same area of your plot the following year.
Secondly, if your alliums do become infected, it is best to dig up the infected plants and burn them. As the pupae can survive over the winter, do not leave any infected plant debris on the soil.
Cover alliums with horticultural fleece, particularly when the flies are active in the spring and again in the autumn.
Plant out young plants after the danger of the first wave has passed and lift before the second generation is active. Whilst this may help with onions, leeks are in the ground for a long time so will not be ready for lifting in early autumn so covering them may be our only defence.

There is plenty more information online.
The RHS website
The National Society {Link updated Jan 2021}
Garden Focused

Tag: allium leaf miner

Successful Onions!

Allium Leaf Miner has Arrived!