The University of Maine Cooperative Extension, which covers the closest documented growing region to New Brunswick, lists only one insect that actually chews on onion crops in this climate: the seedcorn maggot. It strikes early, right at seeding or transplanting, tunneling into the seed itself or the base of a young seedling before the plant ever gets a chance to establish. Everything else on the region’s pest list feeds on other plants entirely.
Which pests attack onion in New Brunswick
University of Maine Cooperative Extension documents ten insect pests across this region’s home landscapes and gardens: Browntail moth, Cabbage maggot, Colorado potato beetle, Emerald ash borer, Lily leaf beetle, Seedcorn maggot, Spotted wing drosophila, Squash vine borer, Striped cucumber beetle, and Winter moth. Read that list next to an onion bed and most of it drops away immediately, because each species is locked onto a completely different host plant.
Browntail moth and Winter moth are defoliators of oak, apple, and other hardwoods and shrubs; they have no interest in a low, strap-leaved allium. Emerald ash borer tunnels exclusively under the bark of ash trees. Lily leaf beetle is a specialist on true lilies and fritillaria, plants in the Liliaceae family, not the Allium genus where onions belong, so despite the name overlap it never touches a bulb crop like onion. Colorado potato beetle needs nightshades, potato foliage above all, and Cabbage maggot works the roots of brassicas like cabbage and broccoli, not onion roots. Spotted wing drosophila lays eggs in ripening soft fruit, Squash vine borer tunnels into cucurbit stems, and Striped cucumber beetle feeds on cucurbit leaves and flowers. None of these can develop on an onion plant.
That leaves Seedcorn maggot, the one species on this list with a genuine appetite for onion. It’s a soil-dwelling fly larva that isn’t picky about which crop’s seed it finds first, and onion seed sitting in cool, damp spring soil is exactly the kind of target it’s built for.
| Pest | Actual host | Feeds on onion? |
|---|---|---|
| Browntail moth | Oak, apple, other hardwoods | No |
| Cabbage maggot | Brassica roots | No |
| Colorado potato beetle | Potato, other nightshades | No |
| Emerald ash borer | Ash trees | No |
| Lily leaf beetle | True lilies, fritillaria | No |
| Seedcorn maggot | Seeds and seedlings of many crops | Yes |
| Spotted wing drosophila | Ripening soft fruit | No |
| Squash vine borer | Cucurbit stems | No |
| Striped cucumber beetle | Cucurbit leaves, flowers | No |
| Winter moth | Trees, shrubs | No |
When each one shows up, and what it does first
Seedcorn maggot activity in New Brunswick is tied to the ground itself, not to a calendar date. The adult flies are drawn to cool, moist, freshly worked soil, the kind onion beds sit in right after seeding or after transplants go in during early spring. Eggs get laid near the seed or the young stem, and the larvae start feeding almost immediately once they hatch, boring into the seed before it can germinate or into the base of a seedling before it can put on real growth.
What a gardener notices first is rarely the insect. It’s a bare patch where onion seed should have come up, or a scattering of seedlings that emerged fine but then wilted and toppled over within days, often before the first true leaf ever unfolds. Because the damage happens underground and inside the seed coat itself, by the time the row above ground looks wrong, the feeding is usually finished.
Soil that’s been recently amended with undecomposed organic matter, manure in particular, tends to draw egg-laying flies in higher numbers, which is why the same bed can behave differently from one season to the next depending on what went into it that spring. Seeding into cool, waterlogged soil right after fresh organic matter is worked in is the single biggest risk factor this pest responds to, more than any specific week on the calendar. Once soil warms and dries even a little, seedlings emerge faster and spend less time exposed at the vulnerable seed and root stage, which cuts the window this pest has to work in.
Telling the damage apart from everything else
Failed emergence and collapsing seedlings have more than one cause in an onion bed, and treating every gap in the row as an insect problem leads to the wrong fix. A few look-alikes are worth ruling out before assuming seedcorn maggot is behind it.
| Symptom | Possible cause | What distinguishes it |
|---|---|---|
| Seed never sprouts | Seedcorn maggot, or poor seed viability | Dig up the seed; a hollowed-out shell with a small legless larva inside points to the insect |
| Seedling wilts and falls over | Seedcorn maggot, or damping-off fungus | Insect damage leaves a tunneled or chewed stem base; damping-off leaves a pinched, water-soaked, but intact stem |
| Pale, weak seedlings that do emerge | Nitrogen deficiency or cold soil, not an insect | Uniform yellowing across the row rather than random gaps |
| Sudden wilting in one small dry patch | Water stress or shallow watering | Surrounding soil is visibly dry rather than wet and heavy |
| Broken or bent seedlings in a cluster | Mechanical injury from hoeing or animal traffic | Clean breaks or hoof/paw prints nearby, no feeding marks |
The one physical sign that actually confirms an insect is at work is the small, cream-colored, legless maggot itself inside the seed or at the base of the collapsed stem. Short of finding that larva, it’s worth describing the pattern, timing, and soil condition to a local extension office rather than guessing from the above-ground symptom alone.
What actually works here, and what is a waste of time
On a short growing season, timing and soil handling do more for onion seedlings than anything applied after the fact. Working fresh, undecomposed manure or green organic matter into a bed shortly before seeding is one of the clearest ways to draw egg-laying flies in, so finishing that kind of amendment well ahead of planting, and letting it break down first, removes a lot of the attraction. Waiting until soil has warmed and drained a bit before seeding, rather than rushing seed into cold, wet ground the moment it can be worked, shortens the exposed window during which seed and root are vulnerable.
A lightweight floating row cover laid down at seeding, and left on until seedlings have their first true leaves, keeps egg-laying adults off the bed entirely during the riskiest stretch. That’s a genuinely effective, low-cost move for a home garden. Rotating onion out of a bed that had a bad year the season before also helps, since the pest overwinters in the soil.
Spraying anything onto seedlings that are already wilting is largely wasted effort, because the feeding damage to the seed or stem base has already happened by the time symptoms show above ground. The decision that matters is made before or at seeding, not after wilting appears. Insecticide options, where they exist, and their legal use in a home garden setting, are questions for University of Maine Cooperative Extension or a local agricultural office, not something to guess at from a product label alone.
Where to check this for your own county
University of Maine Cooperative Extension is the service documenting this pest for the region closest to New Brunswick with published data, and their local offices are the right first call for anything specific to a garden here. Pest pressure isn’t uniform across a whole province any more than it is across a whole state: soil type, drainage, what was planted in a bed the year before, and even how close it sits to a woodlot or a field edge all shift how much risk a given garden actually carries.
A province-wide or state-wide list says a pest is documented somewhere in the region. It doesn’t say it’s in any one backyard bed. The next useful step is a call or a sample brought to the nearest county extension office, where staff can confirm what’s actually feeding on a specific planting rather than relying on a general list. That’s a more reliable answer than matching symptoms to a chart alone, especially for damage happening underground where the cause is easy to misread from the surface.