Quebec gardens see onion damage from three insects with real track records nearby: onion maggot, leek moth, and seedcorn maggot. All three feed on the onion family specifically, not on random garden greenery, and all three show up while the plant is still building leaves, long before a bulb has formed. What decides how bad the damage gets isn’t luck. It’s whether the planting date and the first pest flight line up.
Which pests attack onion in Quebec
The species list documented for this region comes from Cornell University CALS Insect Diagnostic Laboratory, UNH Cooperative Extension, and University of Maine Cooperative Extension. Quebec doesn’t run its own province-wide insect tracking program the way these three do, so the closest verified picture of what’s actually flying and feeding in a climate this similar comes from the bordering states: New York, New Hampshire, and Maine. Their growing seasons, frost patterns, and insect calendars sit close enough to southern Quebec that their published records are the most honest stand-in available.
Out of the much longer roster these three services track across all garden crops, just three feed on onion specifically: onion maggot, leek moth, and seedcorn maggot. Onion maggot is the classic root feeder, laying eggs at the base of young plants so larvae can tunnel straight into the bulb. Leek moth is newer to the region and chews foliage and the neck of the plant, sometimes boring down into the bulb itself. Seedcorn maggot isn’t picky about crops, but it will attack onion seed and seedlings underground before they even break the surface, especially in cool, wet soil right after sowing.
Everything else on the broader pest list documented for this region has no business on an onion bed. Colorado potato beetle wants potato and tomato foliage, not the thin, waxy leaves of an onion. Squash vine borer targets the hollow stems of squash and pumpkin, plants an onion patch simply doesn’t have. Japanese beetle skeletonizes roses, grapes, and turf roots, not alliums. Cabbage maggot and swede midge are brassica specialists, drawn to the mustard-oil compounds in cabbage, broccoli, and their relatives, a chemical signature onion doesn’t produce. If a gardener spots one of these insects on an onion bed, it’s either passing through or feeding on a neighboring crop, not on the onion itself.
When each one shows up, and what it does first
Timing here runs on day length as much as on temperature, because bulbing itself is triggered by how long the sun stays up, not by how warm the soil gets. At the latitude of Montreal, the longest day of the year delivers 15 hours and 29 minutes of daylight, which matters because it determines which onion varieties even attempt to bulb in this region, and by extension how long the plant spends in its vulnerable, leaf-only stage where these three pests do the most damage.
Seedcorn maggot is usually the first sign of trouble, and it shows up before a gardener even sees a seedling. The symptom is a gap in the row: seeds that never emerge, or a seedling that comes up wilted and yellow with a hollowed-out base just below the soil line. Cool, wet spring soil right after sowing is when this insect does its work, tunneling into the seed or the young root before the plant has a chance to establish.
Onion maggot follows a few weeks later, once transplants or sets are settled in and putting out new leaf growth. The first sign is a plant that wilts in the heat of the day even though the soil isn’t dry, followed by yellowing at the leaf tips that spreads downward. Pulled up, the bulb or root shows soft, discolored tunnels, sometimes with a small white larva still inside. This is the stage where an entire section of a row can go down together, because the adult fly tends to lay eggs on several plants in the same area.
Leek moth shows up later in the season, once the plant has a good stand of upright leaves. Its first visible sign is small, elongated notches or windowpane-thin patches chewed into the leaf blade, quite different from the wilting caused by the two maggot species. Left alone, the larvae move down toward the base of the plant and can tunnel into the neck or the top of the bulb itself.
Telling the damage apart from everything else
Onion beds fail for plenty of reasons that have nothing to do with insects, and the first job is ruling those out before blaming a pest. Checking the root, not just the leaf, is the single habit that separates a correct diagnosis from a guess.
| Symptom | Likely cause | What settles it |
|---|---|---|
| Yellow leaf tips, whole plant wilts in daytime heat | Onion maggot or simple drought stress | Pull one plant: tunnels and a larva mean maggot; dry, intact roots mean water |
| Seedling never emerges or collapses right at ground level | Seedcorn maggot or seed rot from cold, wet soil | Dig at the seed spot; a hollowed seed or larva confirms the insect |
| Notched or papery patches on leaves | Leek moth feeding | Look for small green larvae inside the leaf fold or at the base |
| Pale, thin leaves overall, slow growth | Nitrogen deficiency or poor drainage | No tunnels, no larvae, and damage is uniform across the whole bed, not patchy |
| Bulb rot with a bad smell but no tunnel | Fungal or bacterial disease, not an insect | Soft, slimy tissue without any visible larva or exit hole |
A gardener who isn’t sure which of these applies shouldn’t guess from a photo online. A local extension office can look at an actual sample and confirm it in a way a written description never fully can.
What actually works here, and what is a waste of time
On a growing season this short, timing beats almost everything else. Installing floating row cover before transplanting, not after, is the single move that keeps the adult flies of onion maggot and seedcorn maggot from ever reaching the row to lay eggs in the first place. Once a fly has already laid eggs at the base of a plant, no cover placed afterward undoes that.
Crop rotation helps for the same reason: these insects overwinter in or near the soil where onions grew the previous year, so moving the bed even a modest distance cuts down how many adults emerge close enough to find the new planting. Clearing crop debris in fall, rather than leaving cut leaves and rotted bulbs in place over winter, removes another overwintering site for maggot pupae.
What tends to waste a gardener’s time is spraying anything without confirming which insect is actually present, since the three species behave differently and respond to different timing windows. A treatment aimed at leaf-feeding leek moth does nothing for a root-feeding maggot, and vice versa. Any decision about a specific product, including whether one is even registered for use on onion in Quebec, belongs to the regional extension service, not to a general gardening article. They know what’s currently approved and what isn’t, and that list changes more often than most gardeners expect.
Where to check this for your own county
Presence and pressure from these three insects shift from one county to the next, even within a single state or province, because local soil moisture, previous crop history, and even how close a garden sits to a wooded edge all change how many adults are around in a given year. What’s documented at the regional level by Cornell University CALS Insect Diagnostic Laboratory, UNH Cooperative Extension, or University of Maine Cooperative Extension tells a gardener what’s plausible nearby, not what’s definitely in a specific backyard.
The most useful next step is contacting the county extension office closest to the garden in question, or the closest equivalent regional agricultural office for a Quebec address, and asking whether onion maggot, leek moth, or seedcorn maggot has been reported locally this season. Many of these offices also accept physical samples or clear photos for identification, which settles the question faster than trying to match symptoms to a chart from a distance.
A garden ten miles away can have a completely different pest history than one on the other side of a county line, especially if one sits near an old onion field and the other doesn’t. That local knowledge, not a general regional list, is what actually decides what to plant for and when to cover the row.