The leek moth (Acrolepiopsis assectella, syn. Acroplepia assectella), also called the onion leafminer, is a recently introduced pest of all things onion: onions, leeks, garlic, etc. In other words, alliums (Allium spp.). Originally from Europe and Asia, it was first found in Ontario in 1993. It is now abundant in many areas of eastern Canada and is spreading into the Northeastern United States. Even in its native Europe, its range seems to be expanding yearly, possibly because of climate change.
Know Thy Enemy
You’ll know your alliums are affected by this insect pest when the foliage begins to appear marbled with fairly large white patches or pierced with holes. Translucent “windows”, as seen in onions, are an even surer giveaway.
The tiny caterpillar begins its life on the outside of allium leaves. As the season progresses, it begins to migrate towards the heart of flat-leaved alliums (leeks, garlic, etc.), burrowing into their stem and leaf bases, rendering them useless. It also burrows into the flower stalk of garlic plants, causing it to die back (and to ruin your planned harvest of garlic flowers).
In the case of alliums with tubular leaves, like onions and scallions, it moves right inside the leaf, leaving the characteristic translucent windows that are so noticeable. It sometimes migrates down into onion and garlic bulbs, although more rarely.
Also, the openings pierced by the larva often allow other pathogens entry into the plant, notably rot, causing even more damage.
The small light green caterpillar with a dark head is most visible early in its cycle, in its crawler stage, that is while it is still on the outside of the leaves, although it can often be spotted worming its way through the tunnels it bore or inside the leaf, especially when the leaf is backlit.
You rarely see the adult, a tiny brown harmless-looking moth. Since it is nocturnal, you will probably never see it with its wings spread. Instead, it folds its wings tightly against its body, looking like a small piece of wood.
The iridescent white egg, laid under the leaves, is tiny and difficult to see. Doubly so in that the female usually lays a single egg before moving on. Finding one is much like looking for a needle in a haystack!
The next visible stage is a brown pupa surrounded by an airy mesh cocoon. It is quite easy to spot (when you know what to look for) as long as it is located on the plant. However, the caterpillar often migrates to the ground or to soil litter to pupate, making it difficult to spot.
The adults overwinter in the ground, among dead foliage, in rough tree bark, etc. In such cases, cold winter weather often kills them. However, they seem to be attracted to warmth in the fall and will often safely overwinter at the base of heated man-made structures. Since each female can lay over one hundred eggs, it only takes a handful of survivors to quickly cause major damage. Worse, there are two or three generations per summer, starting in late April or early May and ending in September, thus damage tends to increase as the summer progresses.
As long the population of leek moths remains relatively small, or the gardener does a good job of controlling them, it is still possible to get a reasonable crop of bulbs (scallions, onions, etc.). But for alliums grown for their edible foliage, like Welsh onions and leeks (the leek’s “stem” is in fact composed of overlapping leaves and is therefore foliage), even a moderate infestation is disastrous.
The control of leek moth begins the previous fall. After harvest, clean the area of all faded allium foliage and debris where adults can overwinter. Also replace any mulch with a fresh batch.
By far the best method of control for home gardens is the use of a floating row cover. For this to work, it is vital to apply crop rotation, planting alliums in a location where there were none the previous summer and therefore where there are no adults overwintering in the soil. Cover your crop with row cover as soon as it is planted (onions, leeks, etc.) or, if it was planted the previous fall (garlic), in the spring before it sprouts, and the adult moths will not be able to reach your plants to lay their eggs, solving the problem entirely.
Another possibility is to delay transplanting seedlings. The first generation of adult moths is active from late April to late June, depending on the local climate. If you transplant your seedlings to the garden after they are gone, at the end of June or in early July, the problem will be largely solved, because not only will the second generation will be smaller, it has to come from somewhere else. In fact, it often never even finds your plants. Obviously, this technique will not work in a community garden where other gardeners have been growing alliums since the beginning of the season.
Or skip a year. If you grow no alliums for a full summer, you’ll have completely eliminated all leek moths from your garden. Again, for this to work, your garden has to isolated from any other. The few moths that do find your alliums, if they do, will probably not be abundant enough to cause much damage. After a few years, when the population of moths begins to increase, skip a year again.
Plugging the Dyke
The above methods assume you’ll be taking preventive actions: you therefore have to have foreseen the infestation. But what if you find yourself with an infestation that is already underway?
Manual harvesting of caterpillars while they are still in the crawler stage (before they have moved out of reach inside the leaves and stems) will help a lot. And you can also remove and destroy pupae when you see them. However, most gardeners will want to apply some sort of insecticide treatment as well.
BTK is a bacterium that attacks the larvae and works very effectively if you apply it at the right time. BTK only affects butterfly and moth larvae (caterpillars) and will not harm beneficial insects. It has to be ingested by the caterpillar to be effective, though. Since adult leek moths don’t feed, there is no use spraying them with BTK.
Once the larva has penetrated inside the plant, however, BTK can no longer reach it. You therefore have to look for crawlers and repeat the BTK treatments while they are present. With up to 3 generations evolving at various speeds on the same plants, in some cases crawlers may be present most of the time and it may be necessary to treat weekly.
The choice of other insecticides safe enough for a use in a vegetable garden is limited. You can try pyrethrum, insecticidal soap, neem, or home-made sprays based on horsetail or rhubarb. All will work on crawlers and adult moths. None will be effective once the caterpillars are inside the plant.
You can also encourage the presence of earwigs in the garden, one of the rare predators that has proved effective in controlling leek moths.
In Europe, garden centers offer pheromone traps that specifically attract adult leek moths. They are not yet available in North America, but even then, they are mainly used to verify when adult moths are present so as to prepare upcoming treatments. They don’t appear to be very effective on their own in preventing infestations or reducing their impact.
What the Future Holds
Currently various state governments are considering the possibility of introducing leek moth predators, parasites and other pathogens from their native Europe into North America. Therefore perhaps one these introductions will reduce leek moth populations to a more acceptable level in the new future.
For now, in areas where it is present (it is still largely absent from central and western North America), the leek moth remains a major problem for home vegetable gardeners. I’ll leave it up to you to decide how to treat it.