Pest
Leek moth
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Watch for | Watch for in April | Watch for in May | Watch for in June | Watch for in July | Watch for in August | Watch for in September |
What it is
Leek moth is a narrow-winged brown moth about 1 cm long with a pale triangular mark on its folded wings. It has long been common in southern and eastern England and is now found further north. The caterpillars damage leeks most, but onions and garlic are also hosts.
Symptoms
Young caterpillars mine inside the leaves, producing pale, papery or silvery patches. Older ones feed on the surface and burrow towards the heart of the plant, leaving ragged holes, brown scorched-looking tissue and droppings in the leaf folds. The caterpillars are yellowish green with a brown head and grow to about 12 mm. Look too for loose, net-like white cocoons stuck to the leaves. Damaged leeks often rot in wet weather. Allium leaf miner, by contrast, leaves rows of dots, legless maggots and small brown pupae lower in the stem.
Cause
Adults overwinter in sheltered places and lay eggs on allium leaves from spring. Caterpillars feed for a few weeks, spin cocoons on the plant and emerge as a second generation in midsummer, with a third sometimes following in a warm year. Damage is usually worst from July to September.
Control
On a small scale, check leeks weekly and squash caterpillars and cocoons. Cut off and dispose of badly damaged leaves. Plants that have been bored to the heart are best pulled and destroyed. Garden insecticides are rarely justified for this pest.
Prevention
Cover leeks and onions with fine insect mesh from April to the end of September, supported clear of the leaves so moths cannot lay through it. The same cover also stops allium leaf miner. Rotate crops, and clear allium debris at the end of the season so overwintering moths have fewer hiding places.
Plants often affected
Could it be something else?
These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.
Months to keep an eye out: April, May, June, July, August and September.
Sources
Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.