What’s Eating My Leeks?

Something is eating my leeks, sadly it’s not me.

I noticed my leeks started to look poorly recently (they have a touch of leek rust but that’s pretty standard for my allotment site), they’re shrivelling before my eyes with damaged stems and dying leaves. I had a good look at them and poking around revealed some rather yukky looking culprits.

*Sorry for poor picture quality on some of the images, I enlarged them for clear viewing of the tiny pests*

I’d already guessed it could be the work of allium leaf miner maggots or leek moth caterpillars, but I found both. As you can see from the enlarged photos they’re just as wormy as each other, but if you look carefully they are very different. Larvae of the allium leaf miner fly is a maggot, headless with no feet and has a creamy white body, whereas the leek moth caterpillar has a light brown head, tiny legs and has a creamy white body with dots down the length of it. Both are a problem to leeks and other members of the allium family such as onion and garlic.

The adult allium leaf miner is a greyish black fly, before laying eggs the female fly feeds from the leaves causing distinctive vertical lines of white dots in which sap emerges, she then lays eggs, inserting them into the leaves. The eggs hatch into maggots which tunnel into leaves and stems causing the plants to collapse or wilt, after a couple of weeks the maggots are fully fed and ready to turn into brown pupae which are cylindrical and approximately 3mm long. Pupation takes place mainly within the stems but some pupae may end up in the soil, especially where plants have rotted off. From what I understand there are two generations of allium leaf miner fly; in spring from March to May adults from the second generation of the previous year emerge from pupae inside overwintered leeks or from the soil. The second generation of flies do not usually hatch out until end of August to September, this is to protect them from hot summers, timing of the emergence seems to be temperature dependent. 

Mainly a pest of leeks, leek moth is a small mottled brown moth which is rarely seen in the act of laying eggs, it overwinters in soil and leaf debris and then surfaces to lay eggs on the leaves of leeks as the weather warms. The eggs are so small you are unlikely to see them, the damage is done by caterpillars after hatching which feed on the leaves first and then tunnel inside the stems. Leek moth has two generations during the summer, May to June and August to October, the second generation being the most damaging. Affected plants will show signs of white patches on leaves which is damaged internal tissues, young plants often rot and die. The caterpillars eventually pupate on leaves of alliums or nearby plants inside a loosely woven cocoon.

Many allotment holders on my site are having the same problem with their leeks, and it’s particularly bad this year. The only way to successfully grow leeks on my allotment in future is to be very strict with crop rotation and to cover with good quality insect netting, something like Enviromesh or fleece from the moment they’re sown or planted out. The covering will have to be kept in place for the entire growing season.

As for my poor leeks right now, I’m not holding out much hope for them. They’re of a good size so they may recover if I keep on top of picking the pests off but probably not all, it just depends how far down the shank/stem the little grubs have damaged. One method to try is cutting all the top growth off right down to the base and hope the maggots or larvae are not already in the stem, the leeks will grow back perfectly fine but I think for mine they are already past this stage. If you decide to follow this advice be sure not to compost the leaves, take them home and burn.