The Leek Moth

An Introduced Pest of Allium Present in Ontario and Quebec

Table of Contents

1. Introduction
2. Hosts and Damage
3. Identification
4. Life History
5. Monitoring and Management
6. Related Links

1. Introduction

The leek moth (or onion leafminer), Acrolepiopsis assectella Zeller (Lepidoptera: Acrolepidae), a pest of Allium native to Europe, was first positively identified in eastern Ontario in 1993. The distribution of the pest includes Asia, Africa, Europe, and Canada. The leek moth is considered a serious pest in some parts of Europe, with levels of infestation up to 40% in areas where the insect has several generations per year. Where generations are limited to 1-2 per year, the pest is sporadic and causes little economic damage. Surveys conducted in 2001 by the Canadian Food Inspection Agency (CFIA) indicate that the insect is present and established in a localized area in eastern Ontario and western Quebec (National Capital Region Ottawa-Carleton and l'Outaouais). In 2002, a single moth was trapped Gwillimbury Twp, York, Co. No damage has been reported at that site.

For updated information on the 2002 survey, visit the CFIA annual survey report website. The Ontario and Quebec findings represent the first report of leek moth in North America. Surveys indicate the pest is not present in the US, though it is listed as an invasive insect species of concern (visit http://ceris.purdue.edu/napis/pests/lkmth/imap/usamap.html for U.S. survey information).

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2. Hosts and Damage

The leek moth is a known pest of Allium, the scientific name of the plant group that includes garlic and its relatives: chives, green onions, leek, onion, shallot, garlic, elephant garlic, wild garlic, etc. There are approximately 60 species of Allium (wild and cultivated) in North America, most of which are distributed in the West.

Leek is the preferred host of the pest, though other Allium crops can be attacked. The larvae will tunnel mines in the leaf tissue, sometimes causing distortion, and are reported to occasionally attack the bulb and stems. In garlic, the larvae will also attack the scape (Figure 1). Damage to the leaves of leek can make them unmarketable and damage to garlic cloves may predispose them to secondary bacterial or fungal diseases. Symptoms include mining and perforations (Figures 2, 3 and 4). On leek, larvae prefer to feed on the youngest leaves, but can consume leaves more than two months old. They bore through the folded leaves towards the centre of the plant, causing a series of pinholes on the inner leaves. Larval mines in the central leaves become longitudinal grooves in the mature plant. On onion, they feed inside the hollow leaves, forming "windows" on the plant surface. Occasionally, larvae attack reproductive parts of the host plant but usually avoid the flowers, which contain a saponin compound that inhibits the growth of the insect. Feeding at the base of the flower stalk may cause it to break off.

Damage is reported as being more prevalent near field perimeters. In eastern Ontario, there have been reports of damage from a limited number of organic producers; Allium producers following integrated pest management or conventional programs do not appear to be affected by the pest.

Figure 1. Frass (fecal matter) and damage to garlic scape

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3. Identification

The leek moth adult is a small (12-15mm wingspan; 5-7 mm long with wings folded at rest) reddish-brown moth with a white triangular mark on the middle of the folded wings (Figures 5 and 6) and with a scattered white dusting. As with many related species, the hindwings of the moth are heavily fringed and are pale grey to light black in colour. Eggs are white, 0.4 mm in diameter, and difficult to detect. Larvae are yellowish-green (Figures 7 and 8), with 8 small greyish spots on each segment, and possess a pale brown head capsule. They reach 13-14 mm at maturity. The reddish brown pupa is encased in a loosely netted cocoon (Figure 9). Most cocoons are found on the leaves but they can also be found on detritus (decaying plant matter) and on neighboring vegetation. The cocoon is similar in size and appearance to those found in other moth pests such as the diamondback moth; therefore, the presence of cocoons alone is not a positive indication of leek moth infestation.

Figure 9. Close-up of pupa in mesh-like cocoon (5-6 mm long)

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4. Life History

There is no published scientific information regarding the biology of the leek moth in Ontario. Researchers at Agriculture and Agri-Food Canada (AAFC) have reared the leek moth successfully through two generations on chives grown indoors, with a third generation underway in September when the plants died from overeating by the larvae. There are possibly two to three generations per summer, although this remains to be verified under field conditions. Information from Europe indicates the insect overwinters as an adult moth in various sheltered areas such as buildings, hedges, and plant debris. In northern Europe, the leek moth may survive the winter as a pupa. They emerge in the spring when temperatures reach 9.5°C, and mate shortly thereafter. Eggs are laid singly on lower leaf surfaces whenever night temperatures do not fall below 10 -12°C. Females will lay up to 100 eggs during their 3-4 week life span. Development from egg to adult is 25-31 days at 8-9°C, with an optimal temperature of 20-25°C. In France, there are 4-6 generations depending on the area. In southern Ontario, it is estimated that 2-3 generations may survive. A CFIA pest risk assessment (PRA) revised in 2002 has rated the overall potential impact of the leek moth as low.

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5. Monitoring and Management

The species range, which potentially includes the southern half of Canada and much of the United States, appears to be expanding. A pheromone lure for monitoring adult activity is available commercially. In Ontario, the leek moth may be easily confused with the carrion-flower moth (see Figures 10 and 11), a harmless native closely related species present in southern Ontario. Surveys to detect the leek moth use pheromone traps to catch adult males; it is not known whether the carrion-flower moth could be attracted to the pheromone lures set to detect the leek moth and thus confuse monitoring results. At present, the range of the leek moth does not overlap that of the carrion-flower moth, but when it does, it will become important to distinguish them. Unfortunately, this requires microscopic examination of the mating organs by a trained insect taxonomist.

The established population in the capital region of Ontario appears to be affecting a limited number of organic growers, possibly due to a lack of registered pest control products and effective cultural intervention. Estimates of damage to affected growers in 2002 are not available, but the limited number of reports indicate the pest did not cause significant economic losses under local climatic conditions. The potential impact of leek moth establishment to growers in other Allium production areas is undetermined, but measures to prevent the spread of this insect should be undertaken to avoid losses in other regions of the province. The PRA conducted by the CFIA has identified the aerial portions of green bunching onions and leeks as potential pathways for movement; however, damage from larval feeding in the form of holes and leaf mining is easily identifiable upon inspection.

At present, there are no registered pest control products in Canada for management of leek moth. Allium crops grown in Ontario are already subject to insecticide applications to control other crop pests (onion maggot, thrips, other). Information from Europe indicate that pyrethroids and Bt products are effective tools for the management of infestations. Insecticides are rarely required in the United Kingdom. Cultural controls including crop rotation, delayed planting, removal of old and infested leaves, destroying any obvious pupae or larvae, early harvesting (to avoid damage by last generation larvae and population build-up), positioning susceptible crops away from infested areas and destruction of plant debris following harvest may be effective in reducing populations below damaging levels. German literature suggests covering leeks with netting prior to female activity and cutting off all outer leaves before the winter leaves appear in late season may reduce damage to leek. In Europe, a number of predators, parasites and pathogens are known to attack the larvae and pupae of the leek moth. However, the impact of these biological control agents on leek moth populations has not been evaluated in Ontario to date.

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Photo Credits:
Jean-Francois Landry, Agriculture & Agri-Food Canada (AAFC) # 2, 3, 5, 6, 7, 10, 11, 12
OMAFRA 1, 4, 8, 9

Related Links

• CFIA annual survey report website
• U.S. survey information
• Leek moth, Onion moth
• Leek Moth Data, North American Plant Protection Organization
• Leek Moth, U.S. National Pest Information System
• Acrolepiopsis assectella Zeller
• Canadian Food Inspection Agency Science Branch: The Leek Moth (Acrolepiopsis assectella)

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