Identifying and Managing Leek Moth, a Pest of Allium Crops

The leek moth is an invasive alien species from Europe which has become established in eastern Ontario and parts of Quebec. It continues to expand its range, moving west toward the Great Lakes region and east toward the Maritimes. Recent finds include Prince Edward Island (2008) and New York State (2009).

Although leek moth does not appear to have reached large allium production areas in Ontario, we anticipate this pest will eventually find its way. Early recognition of leek moth infestations is important in keeping populations at manageable levels.

How does leek moth damage plants?

The first sign of leek moth usually comes in the form of damaged crops. On alliums with flat leaves, including leeks and garlic, larvae feed on top of and inside the leaf material. They bore through folded leaves towards the centre of the plant, causing a series of pinholes on the inner leaves. In the case of garlic, larvae feed on the scapes and the stalks. Larval mines in the central leaves become longitudinal grooves in the mature plant. Leek moth larvae enter hollow leaves, such as those of onions and chives, to feed internally, creating translucent "windows" on the plant surface. Bulbs are occasionally affected.

What does leek moth look like?

Examination of affected plants may reveal a yellowish-green larva with a pale brown head capsule and 8 small grey spots on each abdominal segment, or a reddish-brown pupa encased in a loosely netted cocoon. Most cocoons are found on host plants but can be located on decaying plant matter and neighbouring vegetation. The adult leek moth is a small, reddish-brown moth with a white triangular mark on the middle of the folded wings. It has a 12-15 mm wingspan and is 5-7 mm long with wings folded at rest. The hindwings of the moth are heavily fringed and are pale grey to light black in colour.

What crops are affected?

The leek moth is a known pest of alliums, including onion, garlic, leeks, chives, green onion, shallot, elephant garlic, wild garlic, etc. There are more than 500 species of allium worldwide with approximately 60 species (wild and cultivated) in North America.

How can I manage leek moth?

There are three flight periods of leek moth per season in Ontario. The insect overwinters as an adult moth in various sheltered areas such as buildings, hedges and plant debris. Adults become active and emerge in the spring when temperatures reach 9.5°C. Leek moth presence and activity can be monitored using commercially available pheromone trapping systems. These should be installed around the field edge in mid-to-late April. Insecticide applications made 7-10 days following a peak flight of leek moth adults can reduce the population and amount of damage it causes. At present, only Warrior (lambda cyhalothrin) is registered (garlic, including elephant; leek; dry bulb onion; green
onion; Welch onion; and shallot). Other candidate pest control products remain in the minor use system.

Cultural controls may be effective in reducing populations below damaging levels. These include:

• crop rotation / positioning susceptible crops away from infested areas
• delayed planting
• removal of old and infested leaves
  
• early harvesting (avoid damage by last generation larvae and population build-up)
• destruction of plant debris following harvest

Research in Europe and Ontario has consistently shown that lightweight floating row covers can protect developing plants from leek moth damage. The use pattern of row covers must be adapted to the crop for which it is being used. Row covers should be placed out early in the season with enough slack to allow the crop to push up the cover. The cover can be kept in place with a variety of methods including trench and dirt, fire hose filled with water, sandbags, washed stone bags, etc. This allows the cover to be removed during the day for weeding and harvesting (example: garlic scapes) and then reinstalled before the leek moth flight during the evening and early morning.

While leek moth is not considered a quarantine pest in Canada, it is considered one by the United States (US), resulting in impacts to Canadian exporters. All Allium spp. with green parts such as leeks and green onions (scallions) destined for the US must be certified free of any life stage of the pest. The impact of the positive finds in New York State in 2009 and 2010 to the quarantine status are unknown at this time.

Figure 1. Leek moth stem damage.

Figure 2. Leek moth garlic scape boring.

Figure 3. Leek moth larvae in garlic stem.

Figure 4. Leek moth pupae.