Leek Moth on Garlic: Identification, Prevention, and Control

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Robert Pavlis

Garlic crops are now at risk from the destructive leek moth, a pest known for causing significant damage and reducing yields. If you’re noticing holes in your garlic leaves or frass (insect excrement), you might be dealing with this moth. It’s crucial to identify and prevent these pests early on to ensure a healthy and bountiful garlic harvest.

The leek moth also attacks onions and leeks and the information in this post also applies to these crops.

top of a garlic plant showing chew marks on the scape
Leek moth damage on garlic scape and upper leaves, source: U. of Vermont

Understand the Pest

Gardeners are far too quick to reach for a possible solution when they think they have a pest problem. It is much better to take a breath and follow these steps:

  1. Identify the pest. If you don’t know what it is, you won’t be successful in controlling it.
  2. Understand it’s life cycle, which helps you know when and how to tackle the problem.
  3. Before doing anything decide if action is needed. In many cases it isn’t and you can simply learn to live with the pest.

Identification of Leek Moth

Leek moths can wreak havoc on garlic crops, causing visible damage that can impact your harvest. Understanding how to identify these pests is crucial for timely intervention and effective control. Let’s get into the physical characteristics of leek moth adults, eggs, larvae, as well as the telltale signs of infestation you should watch out for.

Physical Characteristics

Leek moth adults are small, only 1/4″ (6 mm) long. They have brown wings with a recognizable white diamond on their back but you will rarely see one because they’re nocturnal. The eggs are even smaller, oval-shaped, and are usually laid on the underside of garlic leaves.

Growing Great Tomaotes, by Robert Pavlis
dark gray moth with a white shape on its back
Adult moth with a white diamond mark on its back, source: Patrick Clement

The eggs hatch into pale green larvae with darker heads. When they are fully grown they pupate by forming a small net covered cocoon.

two larvae on a shewed up garlic leaf
Larvae chewing on leaves, surrounded by frass, source: Biobee

Signs of Infestation

These common signs are fairly easy to spot.

  • In alliums with flat leaves such as garlic and leeks, the larvae are usually located in the leaf axles at the top of the plant where they find tender tissue to eat and the leaf provides it protection from predation. On alliums with hollow stems like onions, the larvae can be found inside the stem.
  • Pupae will be located lower down on the plant and are usually located on the outside of the leaf.
  • Irregular holes in the leaves are a good sign that larvae are present, as is the frass (insect poop) which tends to be deposited in leaf axiles.
  • In hard-neck garlic a deformed scape is sure sign of leek moths.
oval shaped cocoon covered in a lacy shell
Leek moth cocoon showing the outer webbing, source: Patrick Clement
YouTube video

Life Cycle of the Leek Moth

The number of generations and the time spent in each stage of a cycle is very much temperature dependent. In zones 5-7, the moth generally has two larvae stages and three adult stages, with the third adult overwintering. Each life cycle can take from 3 to 6 weeks to complete. The first cycle of larvae are present from mid-May to mid-June and the second cycle in mid-July to mid-August. Warmer climates can have 4 adult stages.

The leek moth usually overwinters as an adult hidden in various places where it gets some protection from the cold, but it can also overwinter as a cocoon. In both cases the adult moth becomes active in early spring, mates, and starts laying eggs. Egg laying takes place over several weeks, after which the moth dies.

Eggs hatch out into larvae (caterpillars) that start to feed. They are quite small, but do quite a bit of damage to young leaves. Once fully grown, they pupate and continue their development. Then they hatch into adults and the cycle repeats itself.

lines showing the progression from one life stage to another
Life Cycle of leek moth, source: Ontario Government

Eggs for both larval stages are laid on the top growth meristem (new growth at the top). The first larval stage stays at the top of the plant. The second larval stage also starts at the top of the plant but it can find its way into the bulb (see below). “If leek moth caterpillars are feeding on allium leaves around the time of harvest, they will move into the bulbs as leaves dry down during curing. Feeding damage and exit holes on bulbs while in storage can significantly reduce their marketability and open the bulbs to secondary infection”.

garlic bulb that is mushy, with a white grub in the center
Larvae has made its way to the bulb, source: Søren Holt

Will the Larvae Damage Garlic Bulbs?

The larvae may or may not damage the bulb, depending on climate and seasonal variations. If the larvae does not reach the bulb, it won’t damage it directly, so the real question we need to ask is, when and how does the larvae reach the bulb?

Microbe Science for Gardeners Book, by Robert Pavlis

It is important to understand that even if the larvae does not reach the bulb it still harms the garlic bulb by reducing the top growth. This reduces photosynthesis and in turn results in smaller bulbs.

The first larvae generation does not try to travel down to the bulb, preferring instead to munch on the tender new growth at the tip of the plant. The second generation also stays at the top until the plant starts to die back. It is then forced to feed on the tender tissue inside the stem, eventually eating its way to the bulb.

What determines how far down the stem the larvae gets? Time of year and weather.

Garlic is normally harvested mid July to early August in zone 5. If eggs are laid near harvest time the larvae don’t have enough time to reach the bulb before harvest.

If temperatures or moisture levels cause the garlic to senesce (top dies back) early, it forces the larvae to head down the stem towards the bulb sooner. Once there, they can continue to feed as the garlic matures and even after it is harvested.

Travis Cranmer, Vegetable Crop Specialist at the Ontario Ministry of Agriculture (OMAFRA) has found that in Ontario (zone 5), the larvae rarely reaches the bulb. Any larvae found in storage in August or September is likely to be the Indian meal moth and not the leek moth.

If you find the leek moth early in the season it is a good idea to check local agriculture offices to understand the likelihood of larvae reaching the bulb in your area.

Geographic Areas that are Affected

Scientists have estimated the regions which will be affected by the leek moth, as shown in these maps (dark brown = major pest). It is predicted to be a serious allium pest.

maps of Europe and North America showing most of the regions in red which reflects areas where it is likely to find the leek moth
Potential distribution of leek moth, by Peter Mason et al

Prevention and Control

Leek moth infestations can be controlled and almost eliminated. The key to success is to understand the life cycle and to apply controls at the appropriate time. For example, there is no point in spraying when the larvae is not actively feeding. If you are using row covers you can leave them off during the day because the moth is nocturnal.

Crop Rotation

Crop rotation works well on farms where you can move a crop a long distance, but it does not work for most backyards. The leek moth travels about 220 yards (200 meters) in their life time, which is not a great distance, but it’s farther than most backyards.

Sanitation Practices

Remove any crop residues or debris after harvesting. This eliminates potential hiding spots for pupae and adults.

Floating Row Covers

Floating row covers serve as a valuable shield for your garlic plants, acting as a barrier that physically blocks leek moths from reaching and laying eggs on the foliage. These lightweight, breathable fabrics allow sunlight, water, and air to penetrate while keeping pests at bay. Draping row covers over garlic creates a protective enclosure that maintains a favorable environment for plant growth while deterring unwanted insect visitors.

To maximize the efficacy of row covers proper timing and application are crucial. It’s recommended to install row covers early in the growing season, ideally before leek moth activity starts. Secure the covers firmly to the ground to prevent pests from finding entry points and ensure a snug fit that minimizes gaps or openings.

Row covers can be removed during the day while you are working on the crop, but should be replaced over night. They can also be removed between growth cycles once you are sure the moth is not actively laying eggs. Monitoring traps are used in agriculture to get the timing right, but in the backyard you will have to use the calendar and your own experience.

Systemic Insecticides

Agriculture has access to systemic insecticides that are very effective (see table below), but these may not be available to homeowners.

Biological Control Using BTk

Bacillus thuringiensis var. kurstaki (BTk) offers a natural and environmentally friendly solution for managing leek moth populations. BTk is a microbe produced protein that targets lepidopteran (moths and butterflies) pests while remaining safe for other insects, animals, and humans. It disrupts the pests’ digestive system, leading to their demise without harming the plant or ecosystem.

To be effective the larvae must consume the BTk. Therefore its important to spray it during the larval stage and to spray the part of the leaves where the larvae feeds. There is no benefit spraying the lower part of a garlic plant because the larvae is at the top. BTk will also not harm the cocoons because they are not feeding.

This solution is also not effective for alliums such as onions that have hollow stems. In such cases the larvae lives inside the stem and a non-systemic spray won’t penetrate the stem.

A common belief is that BTk contains the actual bacteria but that is a myth.

Is BTk Effective Against Leek Moth?

Several synthetic chemicals and biological chemicals have been tested on leeks to see how effective they are. The synthetics are very effective showing mortality rates in the 80% to 90% range.

Of the biological agents, spinosad worked fairly well with mortalities around 70%. Neither Neem or BTk was very effective, with both having mortality rates of about 12%.

Although many online sources recommend BTk, it is not really the best solution.

Keep in mind that leeks and garlic have flat leaves and the larvae lives mostly on the outside of the leaf giving good exposure to products like BTk. On onions the pest lives inside the hollow stem and therefore agents like BTk are even less effective.

What Should a Gardener Do?

Familiarize yourself with the above information and keep a close eye on your garlic. I’ve grown garlic for 40+ years and never had a problem but last year I did have some plants infested. This is a new pest that was first found in Canada 20 years ago and is now being reported by many gardeners.

Prevent the problem before it starts by using row covers and BTk or spinosad spray. Timing for both is critical. Spraying too early or too late is a waste of time and money.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

1 thought on “Leek Moth on Garlic: Identification, Prevention, and Control”

  1. Great gardening advice as usual Robert. I will share this with my garden club. I visited a garden just last week with deformed scapes and we were puzzling over what the problem could be.

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