Leek Moth - A Pest of Allium Crops
Table of Contents
The leek moth (or onion leafminer), Acrolepiopsis assectella Zeller,
is an invasive alien species of European origin that damages Allium spp.
It was first identified in Eastern Ontario in 1993. The distribution of
the pest includes Asia, Africa, Europe and Canada. In 2001 and 2002, Canadian
Food Inspection Agency (CFIA) surveys found leek moth populations in a
localized area in Eastern Ontario and Western Quebec (National Capital
Region Ottawa-Carleton and l'Outaouais). In 2003, the CFIA conducted surveys
in Ontario in 16 areas outside of counties where leek moth had previously
been detected. This survey found a single adult in York County. In 2007,
OMAFRA staff installed pheromone traps in a number of locations in Central
and Southwestern Ontario. Leek moth was identified in four additional
locations: Centreville, Picton, Brighton and Bowmanville. Surveys in the
U.S. indicate the pest is not present in the continental U.S. but has
been positively identified in Hawaii.
The leek moth is considered a serious pest in some parts of Europe, with up to 40% infestation in areas where the insect has several generations per year. Where generations are limited to one-to-two per year, leek moth is sporadic and causes minor economic damage.
Figure 1. Garlic damage.
Figure 2. Onion damage.
Figure 3. Garlic damage and feeding larva.
Figure 4. "Window" damage on Onion an leaf.
In garlic, larvae also attack the scape (Figures 5 and 6).
Figure 5. Entry hole with subsurface tunnelling on a garlic scape.
Figure 6. Frass and damage to garlic plant.
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. Larval mines in the central leaves become longitudinal grooves in the mature plant. On leek, larvae prefer to feed on the youngest leaves but can consume leaves more than 2 months old. Leek moth larvae enter hollow leaves, such as those of onions and chives, to feed internally, creating translucent "windows" on the plant surface. Occasionally, larvae attack reproductive parts of the host plant but usually avoid the flowers, which contain saponins that inhibit insect growth. Affected plants may appear distorted and are more susceptible to plant pathogens. In general, damage is more prevalent near field perimeters.
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 (Figure 7).
Figure 7. Adult moth at
rest (side view).
Figure 8. Magnified leek
Figure 9. Leek moth
eggs on a garlic umbrel.
Larvae are yellowish-green with a pale brown head capsule and eight small grey spots on each abdominal segment (Figure 10).
Figure 10. Leek moth larva.
At maturity, larvae reach 13-14 mm in length. The reddish brown pupa is encased in a loosely netted cocoon (Figures 11 and 12).
Figure 11. Close up of
a pupa within its mesh cocoon.
Figure 12. Pupa on a garlic leaf.
Most cocoons are found on host plants but can be located on detritus (decaying plant matter) and on neighbouring vegetation.
There are three flight periods of leek moth per season in Ontario. In Europe, the insect can overwinter as an adult moth or as a pupa in various sheltered areas such as buildings, hedges and plant debris; field data collected in Ontario and Quebec have found the same. Adults become active and 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 are above 10°C-12°C. Females lay up to 100 eggs over a 3-4-week period. When eggs hatch, larvae enter leaves to mine tissues (leafminer stage). After several days, larvae move towards the centre of the plant where young leaves are formed. After several weeks of active feeding, larvae climb out onto foliage and spin their cocoons. Pupation lasts about 12 days, depending on weather conditions. Leek moth numbers and associated damage typically increase as the season progresses.
Figure 13. Leek moth life cycle.
Leek moth presence and activity can be monitored using commercially available pheromone trapping systems. Pheromone lures are placed in white delta or white wing traps installed around the field edge in mid-to-late April (Figure 14).
Figure 14. Leek moth pheromone trap.
In Ontario, the leek moth may be easily confused with the carrion-flower moth (Figures 15 and 16), a harmless native species present in Southern Ontario.
Figure 15. Carrion-flower
Figure 16. Leek moth adult.
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. Presently, the range of the leek moth does not overlap with that of the carrion-flower moth, but when it does, it will become important to distinguish the two species. Unfortunately, this requires microscopic examination of the mating organs by a trained insect taxonomist.
In Ontario, research has consistently found that pheromone traps alone can be used to properly time insecticide applications. Insecticide applications made 7-10 days following a peak flight of leek moth adults (determined through the use of the pheromone trap system) greatly reduce the leek moth population and amount of damage it causes.
Figure 17. Row covers installed over garlic rows.
Row covers can be easily removed during the day for weeding or scaping. As long as they are reassembled prior to dusk, there is little risk of leek moth entering the enclosures.
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