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IDENTIFY AND TREAT LEEK MOTH
Leek Moth mainly attacks leeks but can also attack onions, shallots, garlic and even chives. The moth is unremarkable and you are unlikely to notice it, at the moth stage of its life it does no damage.
The damage occurs when it is at the larva (caterpillar like) stage of its life cycle. The larvae are initially very small and at that stage they eat the surface of the leaves and then burrow into them. As their feeding enlarges them they begin to attack lower down the plant and start making holes in the main stalk and bulbs. Even then the damage may not be substantial but it weakens the plant and opens it up to wide variety of fungi which then cause rot to set in eventually killing the plant.
LIFECYCLE OF THE LEEK MOTH
The leek moth has two generations is any season in the UK and it all starts with moths overwintering in the surface of the soil and on surrounding plants especially where there is lots of falling leaves and other matter. When the weather warms up in April to May the adult moths mate and lay eggs (80 to 100 per adult female moth) on the leaves of leeks and other members of the onion family. The eggs are so small you are unlikely to see them at all, they are cream coloured and less than half a millimetre in size.
After a week or so the eggs hatch and the tiny larvae burrow into the surface of the leaves and then chomp their way down the insides of the leaves towards young developing leaves and the join between leaf and stem / bulb. When they have had their fill of leaf material they then pupate (change into fully grown moths) on the under surface of leaves. The second generation of moths emerge around August time to mate, lay eggs and then do more damage. In normal seasons it is the second generation which does most of the damage because by that that time there are far more of them.
In October time the the adult moths hibernate in plant debris ready to emerge the next spring.
IDENTIFYING LEEK MOTH DAMAGE
The first tell tale signs of leek moths (aside from identifying the larva stage as described below) are light are pin prick holes in the leaves and slim long light brown marks on the leaves where they have mined below the surface. Affected leaves should be removed and destroyed. Normally the worst damage occurs to younger leaves.
The larvae can attack the stem part of the leek and cause infections which can cause the surface to go slimy.
PREVENTING LEEK MOTH
Over the past five years leek moths have steadily moved northward over the UK. Previously limited only to southern coastal areas they now affect much of the south of England and has spread as far as North Wales. There are no pesticides available for the amateur gardener to prevent this pest so prevention is the key.
First, remove debris (leaves, twigs and the like) from areas where plants of the onion family will be planted. If you see any of the larvae on your plants remove them. They look like tiny caterpillars with front legs. They are yellowy-green with two lines of spots down their bodies. Their heads are slightly darker than their bodies.
Strong growing leeks can often outgrow an attack of leek moth so cultivate the plants well as described in other article on this website.
The final line of attack is to cover the crops with insect proof netting or horticultural fleece. This will stop the moths reaching the plants to lay eggs.