Damage, Monitoring and Detection

The leek moth larva is a small, leaf-mining caterpillar. The first generation (May-June) feeds on the leaves. The worst damage is done by the second generation (July-August) as it continues to damage emerging leaves (Figure 1) and moves towards the bulb. Feeding damage stunts plant growth, introduces rot and can compromise the storage life of onions and garlic. In New York leek moth populations have been found in onions, garlic and leeks.

Figure 1. Damage to leeks. Photo: Lorraine Chilson.

Where to look on various crops

On crops with hollow leaves (onions, shallots and chives), the larvae feed on the inside tissue, leaving characteristic 'windowpane' damage to the leaves (Figure 2 & 3). Split open damaged leaves and look for frass (excrement) and debris (Figure 4). Even after the larvae have left to pupate, the telltale debris remains visible (Figure 5).

Figure 4: Onion leaf split open, larva and debris visible. Photo: Amy Ivy, Cornell University.

Figure 5: Telltale debris. Photo: Amy Ivy, Cornell University.

On garlic and leeks, larvae feed on the leaf surfaces and sometimes tunnel through the leaves (Figure 6). They are often found in the protection of the folded leaves on leeks and garlic (Figure 7). In June in hardneck garlic, damage will be the most noticeable on the garlic scapes (Figure 8). On both crops check the newest leaves as well (Figure 9).

Figure 6: Damage to leeks.

Figure 7: Damage to garlic inside fold of leaf. Photo: Amy Ivy, Cornell University.

Figure 8: Damage to garlic leaf and scape. Photo: Amy Ivy, Cornell University.

Figure 8a: Damage to garlic scape. Photo: Scott Lewins, UVM.

Figure 9: Leek — Check newest leaves for damage.