Four pests do most of the damage to onions in New Mexico: cutworms and wireworms in the soil, thrips on the leaves, and onion maggots in cool, wet spells. Each one shows up at a different stage of the crop. Thrips are the one that costs the most bulb size. They arrive while the plant is still building the leaves it will turn into a bulb, so their timing matters more than their numbers.
The short answer for New Mexico
Albuquerque sits at 35.04 degrees north and gets 14 h 22 of daylight on the longest day of the year. That number sets the whole pest calendar. Day length, not warmth, tells an onion to stop making leaves and start swelling a bulb. A short-day variety begins bulbing at about 12 hours, according to Clemson Cooperative Extension. Here the days cross that line in spring, well before summer heat, so the leaf-building window is short. Whatever the plant has grown by then becomes the bulb. Anything that eats leaf before that moment shrinks the harvest, and nothing grown afterward makes up the difference.
Early season: the pests in the soil
Cutworms and wireworms work in the first weeks after transplanting. Cutworms are caterpillars that feed at night and chew a seedling off at the soil line, so a row can look fine on Monday and have gaps by Wednesday. Wireworms are slender, hard-bodied beetle larvae that bore into the base of the plant and the young bulb. They are most common where sod or a weedy field was recently turned under. If a bed was grass last year, expect them.
Onion maggots belong to this early stretch as well, but they favor cool, damp springs, so they trouble northern mountain gardens more than the warm, dry valleys. The larvae tunnel into the base of the bulb, and the plant wilts and collapses. Pull one and you will find it hollowed out.
Mid-season: the pest on the leaves
Thrips are the big one. They are slivers about the size of a comma, and they hide in the tight fold where the leaves meet. They rasp the surface and leave silvery streaks, and populations climb in warm, dry weather, exactly what New Mexico does best. A heavy infestation in the weeks before bulbing trims the green factory that feeds the bulb. The damage rarely kills a plant, but it quietly lowers the harvest. Grasshoppers, which are common across the state in some years, can strip tops in the same stretch.
These are the broad patterns. This page does not publish specific trap counts or spray-date thresholds, because those vary by county and by season. Your local extension office does track them.
What will not work here
The most expensive mistake has nothing to do with insects. It is planting a long-day variety. Those onions wait for 14 to 16 hours of daylight before they bulb. Albuquerque touches 14 h 22 only around the solstice, and by then the heat has arrived and the plant has spent months as nothing but leaves. Gardeners get a lush green row, a thin neck where the bulb should be, and then rot as the tops fall over in summer. Leaves like that are also a perfect thrips habitat, because the pests get a full season of tender growth to feed on.
Varieties that are poor fits for a garden near 35 degrees north:
- Walla Walla types: bred for the long summer days of the northern states, so they stay green and bulb late or poorly here.
- Ailsa Craig and similar exhibition onions: long-day plants that need far more daylight than the state delivers, and they rot before they size up.
- Northern storage onions sold as “winter keepers”: long-day by design, which means leaves all season, no real bulb, then rot.
- Any packet labeled for Canada, the Pacific Northwest or the northern tier: the label is the clue that it wants 14 to 16 hours of light.
Timing errors do the same kind of harm. Planting late does not push the harvest back. It shrinks it. Bulbing is triggered by the calendar, so a transplant set out three weeks behind its neighbors reaches the trigger with three weeks less leaf, and the bulb comes out smaller. Thrips damage works by the same rule. A plant stripped of foliage in the weeks before bulbing is a late plant in effect.
Planting without checking the soil history does not work either. Putting onions where sod was just turned over invites wireworms. For the calendar side of this, see When to Plant Onions in New Mexico.
What changes across New Mexico
All the figures above come from Albuquerque, the reference city. New Mexico is a big state, and its far north and far south sit several degrees apart. The northern edge gets more summer daylight and the southern edge gets less. A gardener near the Colorado line may belong closer to the intermediate-day group, while one near the Mexican border is firmly in short-day territory. Elevation shifts the pest picture too: cool, damp mountain springs favor maggots, and hot, dry low-desert summers favor thrips.
| Location | Latitude | Longest day | What it changes |
|---|---|---|---|
| Albuquerque (reference city) | 35.04 degrees north | 14 h 22 | Short-day onions bulb in spring; thrips build in warm, dry weather |
| Far north of the state | Not published here; look up your own | Longer than Albuquerque | Planting dates run later; cool, damp springs favor maggots |
| Far south of the state | Not published here; look up your own | Shorter than Albuquerque | Planting runs earlier; bulbing starts sooner; heat favors thrips |
I have left the northern and southern figures out on purpose. The only verified numbers this page carries belong to Albuquerque, and guessing at others would be worse than leaving the cells open. The variety side of the question gets its own treatment in The Best Onion Varieties for New Mexico, and the overview sits at Onions in New Mexico.
Sweet now, or onions that last
Pest damage and flavor are tied together more than most gardeners expect. The bite of an onion comes from volatile sulfur compounds, and those same compounds protect the bulb in storage. A sweet variety carries fewer of them. University of Missouri Extension puts it plainly: “As a rule, ‘sweet’ onions do not store as well as the more pungent types.” Short-day types run sweeter and long-day types keep longer. A southern garden therefore produces onions to eat soon, and a northern one produces onions that feed you into March.
Here is what that means for your pantry. By March, a bin of sweet short-day onions from a New Mexico garden will mostly be gone, either eaten or composted. A bin of pungent keepers might still be firm. Thrips scarring and wireworm holes make the gap wider, because any wound is an entry point for rot. A damaged bulb goes first, whatever its type, so eat the nicked ones early.
Curing comes before any of this. Bulbs go into storage only once the necks are tight and the skins papery, never straight from the ground. The ideal is between 45 °F and 55 °F with 50 to 60 percent humidity, according to Clemson Cooperative Extension. That is dry air, the opposite of what carrots want, so one cellar cannot hold both well. Never count on a sweet onion to store. Plan to eat it first.
Checking this against your own garden
Albuquerque’s numbers are a starting point, not a verdict on your yard. A state-wide figure never applies to every garden in it, and pest pressure changes with elevation, irrigation and what grew in the bed last year. Your county extension office knows the local thrips and maggot calendar and can tell you what is moving this season. Here is what you can check this week:
- Look up your own latitude, especially if you are far north or far south of Albuquerque, and compare it with 35.04 degrees north.
- Find your average last spring frost date and plan transplants around it.
- Note what grew in each bed last year. Sod or heavy weeds mean wireworm risk.
- Check the packet label for a day-length group, and set aside anything marked long-day.
- Pull a few plants and look inside the leaf folds for thrips. Do it before bulbing starts.
- Call or visit the county extension office and ask which pests are active now.
Common questions
What is the worst onion pest in New Mexico?
Thrips usually cost gardeners the most, because they feed on the leaves during the weeks when the plant is deciding how big the bulb will be. Cutworms can wipe out seedlings faster, but thrips damage is the one that quietly shrinks a harvest.
When should I start watching for pests?
Start the day you transplant. Cutworms and wireworms hit in the first weeks, and thrips follow as the weather warms and dries. Your county extension office can tell you when they are showing up locally.
Do sweet onions attract more pests than pungent ones?
This page does not publish a verified comparison of pest preference. What is documented is that sweet onions carry fewer sulfur compounds and store worse. A nicked or scarred sweet bulb therefore spoils quickly, so eat damaged ones first.
Can I rescue a long-day variety I already planted?
Not reliably. At Albuquerque’s latitude the day reaches 14 h 22 only around the solstice, which is too late and too hot for a long-day onion to bulb properly. You can harvest the green tops for cooking. For next season, choose a short-day type and keep the long-day packets for friends who garden in the north.