Dear Sibe

For the last two years I have had greyish grubs in my leeks. What are they and how can I prevent them in future crops?

Regards,
Brian Silsbury

Our Gardening Expert replies:

Hello Brian,

Two candidates spring to mind for the grubs in your leeks. The first is Onion Fly, which can also attack leeks as well as onions and shallots. The symptoms are that the plants wilt and die and older plants may also rot. Damage usually occurs in mid-summer and is caused by the pupae of the Onion Fly. These emerge in May, when they lay eggs on the leaves of onions and leeks. They then burrow into the plants and feed for up to a month before returning to the soil to pupate. Further generations emerge throughout the summer and autumn but with increasing numbers of these staying dormant until after the following winter.

The other possibility is Leek Moth. These tunnel through the leaves and eat downwards into the stem causing rotting. Eggs are laid when temperatures warm up in the spring but after about a month the grubs pupate on the leaves in cocoons rather than in the soil. Further generations follow throughout the year.

The treatment for both of these pests is similar. If you cultivate the soil during the winter you will disturb any over-wintering pupae and hopefully give a few robins a winter snack. The Leek Moth adults over-winter in plant debris, so for these you need to have a thorough clean up at the end of each year before cultivating the soil.
Otherwise, it is a matter of keeping a careful watch on your plants as they develop and removing and destroying any affected plants as soon as you see any sign of attack. This can be particularly effective as it stops further generations following after the first wave.

Regards,
Sibe