Leek is fairly susceptible to pest attacks. The leek moth is clearly one of its main enemies! Here’s how to identify Acrolepiopsis assectella and fight it better. This bugger is often confused with both the leek leaf miner and the onion fly.
Acrolepiopsis assectella is a night butterfly with a wingspan of approximately 5/8 to 7/10 inch (16 to 18 mm). This Lepidoptera has an easily identifiable brown body. Its wings, very elongated, are also grayish brown and dotted with black. Two white spots stand out on the forewings while the hindwings are grayer.
The caterpillar, approximately 1/2 inch long (13 mm), is a yellowish-white or greenish color. Its body is sprinkled with black dots and light bands, the head has a dark tan hue. A few hairs cover its body. Leek moth attacks all alliums, namely leek, but also garlic, onion, shallot and chive, and even daylily in ornamental beds. That’s why it can be confused with the leek leaf miner (Phytomyza gymnostoma) , and with the onion fly (Delia antiqua), two diptera that cause more or less the same damage.
→ For further reading: Leek leaf miner, control and treatment and Onion fly, prevention and treatments
The leek moth usually spends winter well protected beneath heaps of dead leaves, among twigs or in bark crevices. Sometimes it even lays dormant as a pupa in unharvested leek.
The first flight of the leek moth occurs in April-May, as soon as nighttime temperatures approach 54 °F (12 °C). After mating, the female (who lives just over a month) lays hundreds of eggs which she deposits on leek leaves in the evening.
Ten days later, larvae hatch. It is at this stage that damage is most significant. They first attack the upper part of the leaf they hatched on, which quickly turns yellow. Then, the caterpillar burrows deeper into the leek stem which it gnaws at from the inside.
Satiated, it pupates into a spindle-shaped, see-through cocoon-like web, placed on the leaves. From these cocoons, a new generation of butterfly arises.
Ultimately, three generations can follow one another within the year. You can clearly track the three waves of adults fluttering around:
Then, in October, when temperatures drop, larvae and moths hibernate either in the adult state or in the pupal state. It is especially the last two generations that cause the most damage.
Leek moth is not a single attack but a succession of flights, and the later waves are the ones that ruin a crop. This table sets out each generation, when the adults are on the wing and what defence matches it.
| Generation | Adult flight | Damage | Main defence |
|---|---|---|---|
| First generation | mid-spring to late spring | Larvae hatch ten days after egg-laying and yellow the upper part of the leaf | Fine mesh net from late spring; transplant young plants only after early summer |
| Second generation | early summer to midsummer | One of the two waves responsible for the heaviest damage | Keep the net gap-free on arches; pheromone traps to prevent mating |
| Third generation | late summer to early autumn | Caterpillars reach the stem, which rots from the inside | Bacillus thuringiensis while larvae feed outside; otherwise cut the leek short |
Once temperatures drop in mid-autumn the moths hibernate, which is also when the netting can finally come off.
Often, the first sign of a leek moth attack is when holes and galleries appear on the underside of leaves. Foliage is covered with fuzzy white spots, and wilts.
Then, very quickly, caterpillars reach the leek stem which they start devouring. Since it gets wet because of sap and water, it starts rotting from the inside, sometimes causing the death of the plant.
In any case, crop development is considerably slowed down. Furthermore, the leek becomes inedible.
Split a suspect leek lengthwise: tunnelling into the white shaft often invites rot and may ruin the plant, while grazing confined to leaf tips is usually cosmetic and plants commonly recover.
Leek moth tunnelling wounds provide entry points for secondary soft-rot bacteria and fungi, so a plant that looks merely nibbled may later rot, though how quickly that happens varies with conditions.
As often, prevention and good practices can limit and even prevent infestations.
Grow carrot in between leek rows to keep moths away. In reverse, leek also keeps the carrot fly away. You can also grow plants that emit strong smells around your plots of leek. They confuse the moth, which can no longer find the leeks. Examples are fennel, celery, lavender, wild thyme…
If the infestation is real and caterpillars have settled settled, it’s not too late to fight the intruder off! But you must act before caterpillars penetrate to the heart of the leek:
Otherwise, if caterpillars have reached the stem, your only option left is to cut the leek short. Upper leaves should grow back as long as the caterpillar hasn’t burrowed too deep.
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