Monitoring for Leek Moth

 We’ve been seeing increasing issues with leek moth, an invasive pest of alliums (garlic, leeks, onions, etc.) over the past several seasons. Leek moth cause damage through their larvae feeding on host plants, while the adult moths cause no direct damage. On alliums with flat leaves (garlics, leeks) the larvae feeds on the tops and inside of the leaves, as well as bores into the center of the plant leaving noticeable frass. In alliums with hollow leaves (onions, chives) the larvae will feed internally, producing translucent areas on the leaf known as "windowing". Leaf damage by leek moth larvae will reduce yields in affected crops, but they can also directly damage marketable portions of the plant. In garlic larvae will regularly bore into scapes and occasionally bulbs, as well as the stalk in leeks, rendering them unmarketable.

Larval feeding damage on garlic. Photo: Agriculture and Agri-Food Canada

Based on sampling performed in NS, we’ve observed up to 3 flights or 2 generations of leek moth. Depending on the region, the first flight occurs in late April to mid-May when overwintering leek moth adults emerge (last generation from previous year), typically corresponding to when temperatures reach 9.5^oC. The second flight is of 1st generation adults, beginning in late June and continues through mid-July. The third flight is of 2nd generation adults beginning in late-July to early-August. These dates give some general guidance as to when flights occur, but the best way to track leek moth on your farm is by trapping.

Pheromone traps are commercially available and allow you to determine if/when leek moths are active on your farm. Leek moth monitoring should begin in mid-April using commercially available pheromone traps, which attract adult males. Pheromone traps consist of a white delta trap, sticky pad and pheromone lure. The delta trap can be reused for multiple years, but the sticky pads should be checked and replaced weekly and the pheromone lure replaced monthly. The recommended number of traps is 1-2 traps per acre, and they should be placed at the edge of the upwind side of the field at crop canopy height.

The adult leek moth is a small (5-7mm in length) brown moth with a distinctive white triangle in middle of its wings when they are folded at rest and can be identified with a hand lens. Weekly counts should be recorded to determine when leek moth first appear and peak flight so management strategies can be timed correctly. Allium crops can also be scouted for feeding damage from leek moth larvae, but when damage has already appeared, it may be too late to effectively implement some controls.

Adult leek moth. Photo: David Fuller, University of Maine Extension.

One of the most effective ways to control leek moth is by using row covers. Research done in Ontario has found that row covers were just as effective as pesticides in reducing leek moth damage. These covers physically block leek moth adult females from laying eggs on the crop preventing establishment. Row covers should be applied before the first leek moth flight in the spring, so have row cover on by mid-April to achieve good control.   Row cover can then be removed after the leek moths stop being found from trapping, but covering may need to resume if leek moths appear again from later flights. Additionally, leek moths are only active at night, so row cover can be removed during the day to perform work in your alliums without risk of egg laying, as long as the row cover goes back on before dusk.

 There are also several conventional and biological pesticide products, some of which are organic, registered for use against leek moth on allium crops. Application of these insecticides is generally recommended 7-10 days following peak flight, determined through trapping, which targets larvae when they have newly hatched and are actively feeding. If leek moth controls are implemented early and effectively against the initial flight period, crop damage and populations can be significantly reduced as well as the severity of subsequent flights. 

For more information on where to get trap supplies and specific controls or pesticide products please contact Perennia’s Integrated Pest Management specialist, Matthew Peill, mpeill@perennia.ca.