Four groups go after onions in the Northwest Territories: cutworms and wireworms in the soil, onion maggots at the base of the plant, thrips on the leaves, and rot and rodents once the bulbs are stored. Which one you meet depends less on the calendar than on how big the plant is. Daylight sets that clock in the far north, so reading your onion’s growth stage tells you more than any pest chart.
The short answer for Northwest Territories
Yellowknife sits at 62.46 degrees north, and on the longest day of the year it gets 19 h 30 of daylight. That figure matters more than any temperature reading. Onions do not bulb because the soil warms up. They bulb when the day reaches a set length: about 12 hours for a short-day variety, 14 to 16 hours for a long-day one (a threshold published by Clemson Cooperative Extension). At Yellowknife’s latitude every onion type gets past its trigger. What differs is how much plant exists when the trigger arrives, and a plant’s size is what decides which pests can hurt it.
Early season: seedlings and sets
Cutworms and onion maggots strike first, while tops are thin. A cutworm chews a seedling off at the soil line, usually overnight, and the row simply has gaps by morning. Onion maggots are the larvae of a small fly that lays eggs at the base of the plant. The larvae tunnel into the young stem and bulb, the center leaf yellows, and the whole plant wilts. Wireworms, the hard, shiny, wire-like larvae of click beetles, are a special problem in beds cut from sod, where they feed on roots and bore into bulbs.
Midseason: leaves and bulbing
Thrips are slender insects, barely a tenth of an inch long, that rasp the leaves and leave silvery streaks. They are mostly a hot, dry weather problem, so outdoor onions in a short northern summer usually see less of them than a greenhouse or high tunnel does. The cold summers do not make onions pest-free, though. They shift the pressure toward the soil pests and the maggot, which are happy in cool, damp ground.
After harvest: the store
The last pest is not an insect at all. Bulbs that went into storage with wet necks, or with a maggot tunnel already opened, rot from the inside. Clemson Extension puts ideal storage at 45 °F to 55 °F and 50 to 60 percent humidity (Clemson Cooperative Extension, Home & Garden Information Center). Cure first. Necks tight and skins papery, never straight from the ground.
One honest limit: no territory-wide survey of onion pest timing in the Northwest Territories is published here, so this page gives you the sequence, not a list of dates. For the planting side of the schedule, see When to Plant Onions in Northwest Territories, and for the broader picture, Onions in Northwest Territories.
What will not work here
At 62.46 degrees north, no onion group fails to bulb. Long-day types, the ones that wait for 14 to 16 hours, get far more than they need with 19 h 30 on the longest day. The usual northern warning (leaves all season, no bulb, then rot) belongs to the south, where days never stretch that far. Here the risk runs the other way, and it feeds straight into the pest problem.
Short-day onions start bulbing at about 12 hours of daylight. In a northern spring the days pass that mark quickly, so a short-day plant stops making leaves when it is still small. The result is a tiny bulb on a short stalk. A maggot tunnel that a big, long-day bulb could shrug off can ruin a small one outright, and a plant with few leaves has nothing left to regrow with after a thrips attack or a cutworm bite.
These are the types to leave out of a Yellowknife-latitude garden:
- Short-day storage onions such as Texas Early Grano types: they trigger at about 12 hours, bulb far too early, and finish small.
- Granex-type sweet onions (the Vidalia-style group): bred for southern winters, they bulb on short days and give you a thin, soft bulb that stores poorly.
- Any onion sold only as “overwintering” or “fall-planted”: the plan assumes a mild winter, and northern daylight turns on the bulbing signal early.
Planting late does not rescue any of this. A late planting does not push the harvest back. It shrinks it, because whatever size the plant has reached on the day the length of daylight triggers bulbing is the size of the bulb. A seedling set in the ground two weeks behind schedule meets the same day length with two weeks less growth, and it also spends its tender stage in the window when cutworms and the first maggot flies are active.
Several pest tactics also fail in practice. Waiting until you see yellow tops to act is the worst of them, because by the time the center leaf wilts, the maggot is already inside the bulb. Spraying for thrips before you have found any is wasted effort, since a cool outdoor bed is not where they thrive. And leaving culls and bolted onions in the bed over winter is an open invitation to the next generation of maggots. Pull them and throw them away from the garden. Do not compost them in the same plot.
To pick varieties that match the daylight, see The Best Onion Varieties for Northwest Territories.
What changes across Northwest Territories
The Northwest Territories is not one latitude. Its southern border follows the 60th parallel, and its mainland coast reaches past the Arctic Circle at places like Inuvik and Tuktoyaktuk. The 62.46 degrees north figure comes from a single reference city, Yellowknife, using the station YELLOWKNIFE A. A gardener in Fort Smith or Hay River sits closer to the southern border, and a gardener in the Mackenzie Delta lives in a different light regime altogether.
| Location | Latitude | Day length, longest day | What it changes |
|---|---|---|---|
| Southern border | About 60 degrees north | Shorter than Yellowknife’s 19 h 30; not calculated here | Still far past the 14 to 16 hours long-day types need; the thaw comes earlier, so soil pests and the first maggot flies show up sooner |
| Yellowknife (reference city) | 62.46 degrees north | 19 h 30 | The baseline for this page; every onion type passes its bulbing trigger |
| North of the Arctic Circle | Above about 66.5 degrees north | The sun does not set around the June solstice | Daylight is no longer the limit; the short thaw and cold soil are, so growth stage and pest timing compress |
For pests, the practical effect is that the far south warms sooner and has the longer growing window, so it also has more time for maggot and thrips populations to build. The far north has less season to get bulbs big, which makes each lost seedling count for more. How many weeks apart these two gardens plant is not a figure published here, and it varies with the year. Treat any advice that gives you one date for the whole territory with suspicion.
Checking this against your own garden
The figures above describe Yellowknife, not your plot. A few checks you can finish this week will tell you how much of this page applies to you:
- Find your own latitude. If you are far from Yellowknife, look up your community’s coordinates, since a few degrees north or south changes the daylight you get.
- Record your local last-frost and first-frost dates. This page holds no frost date for the territory, so take them from your own garden notes or a local source.
- Dig a test spot in your bed. If the bed was cut from sod, look for wireworms before you plant.
- Walk the rows at dusk after planting. Missing seedlings cut off at the soil line point to cutworms.
- Look at the base of any wilting plant. White larvae in the bulb or stem mean onion maggots, and the neighbors will follow.
- Measure your storage space. Check that you can hold 45 °F to 55 °F and 50 to 60 percent humidity, away from any root cellar that is damp.
The Northwest Territories has no county extension offices, so the place to take photos of pests or damaged bulbs is the territorial government’s agriculture staff or a local gardening group. Anyone who has grown onions in your community for a few seasons can tell you which year the maggots came and when.
Common questions
Do onion maggots live in the Northwest Territories?
The onion maggot is a cool-climate pest, and cold summers do not rule it out. No territory-wide survey figures are published here, so check with local growers about which communities have had trouble.
Does the long summer daylight make pests worse?
Daylight drives the plant’s bulbing, not the insects’ numbers. What it does is set the point where bulb size is fixed, so small, late or damaged plants lose the most.
Should I cover onions with row cover?
A cover set over the bed at planting keeps egg-laying flies off the base of the plants. It only works if the soil under it is free of last year’s pupae, so rotate the bed to a different crop and clear all culls first.
Can I store onions that have a little maggot damage?
Eat them first. A tunnel opens the bulb to rot, and a rotting bulb can spoil the ones next to it. Store only cured, sound bulbs, and keep them dry, at the 45 °F to 55 °F range Clemson Extension gives.