Leek Moth (Acrolepiopsis assectella) is an invasive pest of onions and other alliums that is established in parts of Canada, including Ontario. The small green caterpillars initially mine inside leaves, then feed deeper within folded leaves and sometimes enter the bulb. Their feeding produces translucent patches, ragged or shredded foliage and conspicuous frass. Damage to young plants can kill the growing point, while feeding on larger onions can provide openings for rot.

How to Fix It
Insect netting: Cover onions early with fine insect netting to prevent adult moths from laying eggs.
Hand-pick: Inspect damaged leaves and remove caterpillars when you find them.
Remove badly damaged leaves: Dispose of heavily infested foliage containing larvae or pupae.
Clean up after harvest: Remove allium crop debris and volunteer plants that can support the pest.
Rotate alliums: Move onions and related crops away from the previous year's planting where practical.
Bt: Bacillus thuringiensis var. kurstaki can control young caterpillars while they are exposed and actively feeding; it becomes less effective once they are protected deep inside plant tissue.