A few other pests damage onions and leeks, and may be confused with leek moth:
External feeding on hollow leaves (Figure 1)
Leek moth will not feed on the outer layer of hollow leaves like this onion; this is not leek moth damage.
A good check is to split open hollow leaves and look for debris and frass. If inside is clean, leek moth was not there. Check inside folds of flat leaves such as garlic. Exception: they do damage the outer layer of garlic scapes.
Thrips (Figure 2 & 3)
Onion thrips are tiny and create shorter, more numerous whitened patches on leaves. They are hard to see with the naked eye, and are a very common pest of onions.
Salt marsh caterpillar (Figure 4)
These caterpillars are very hairy, even when tiny, and feed on the outside of the leaf. They are only occasional pests and tend to appear in spots in masses, causing localized damage. They are easy to control by simply crushing.
European corn borer (ECB) (Figure 5)
ECB prefers corn but in the spring of 2011 when corn was planted much later than usual due to weather conditions, we found ECB in some garlic, presumably since that was one of the few suitable crops up when the adult ECB’s arrived in late May.
Botrytis leaf blight (Figure 6)
Botrytis leaf blight is a fungus disease on onions favored by wet weather. The lesions resemble leek moth damage but when leaf is split open, the inside is clean with no debris.
Various diseases also cause white chlorotic spots on onion leaves that can be mistaken for leek moth injury. Split the leaf open and look for the characteristic debris and frass from the leek moth larvae. If the inside of leaf is clean, consider disease as a possibility and for more information visit the Vegetable MD Online: Onion Diseases Fact Sheet List.