Leek Moth

Jean Price - Plot 4

Did you have problems with your leeks this year?  We did, and so did a lot of people that I have spoken to.  Quite a few allotment sites in Trafford think that they have been affected by Leek Moth, for the very first time.  Leek moth is mainly a problem in southern England but it is spreading north.  Here is what the RHS have to say about it:
Quick facts
Common name Leek moth
Plants affected Leek, onion, shallot, garlic
Main symptoms White patches on leaves, with young plants rotting and dying. Small caterpillars may be seen in the plant tissues
Caused by Caterpillars of a small moth
Timing April-October
 
What is leek moth?
The adult leek moth is an inconspicuous very small (5-6mm) brown moth. Its larvae (caterpillars) feed on leeks and similar crops.
 
Symptoms
Damage from leek moth caterpillars appears as;

• White patches developing on the foliage where the caterpillars have eaten the internal tissues
• Tunnels in the stems of leeks and bulbs of onions, shallots and garlic
• Affected plants often develop secondary rots and young leek plants may be killed

Note that these plants are also attacked in a similar manner by larvae of a fly, known as the allium leaf-mining fly (allium leaf-miner);

• Leek moth caterpillars are creamy-white with brown heads and small legs. They do not usually pupate within plant tissues
• Larvae of the leaf-mining fly are white, headless maggots with no legs. Cylindrical brown 3mm long pupae are likely to be found embedded in the stems and bulbs

 
Control
Non-chemical control
The female moths can be prevented from laying eggs by covering susceptible plants with horticultural fleece, or an insect-proof mesh such as Ultra-Fine Enviromesh. Crop rotation should also be practiced to prevent potential build-up of moth populations under the fleece. Look for the white, net-like silk cocoons on the foliage and squash them.
Chemical control
None of the pesticides currently available to home gardeners for use on leeks and onions will give effective control of leek moth.
 
Biology
Leek moth has two generations during the summer with larvae damaging the plants;

• From the first generation in May-June
• And the second generation in August-October
• The second generation is the more numerous and damaging

Initially the caterpillars mine the foliage but the older larvae bore into the stems and bulbs.
When fully fed, the caterpillars are 11 mm long. They come out of the plant and pupate within net-like silk cocoons that are spun on the foliage.
Adult moths emerge in autumn and overwinter in sheltered places.

There is a much more detailed study here.  leek-moth-project-kdunn.pdf