Three pests do most of the damage to Pennsylvania onions, and each one shows up on its own schedule. Root maggots hit young plants in cool, wet spring weather. Allium leafminer is a newer arrival that strikes in spring and again in fall. Onion thrips build through hot, dry summer weather, just as the bulbs are swelling. Match your scouting to those windows and you will catch most problems early. This Onions in Pennsylvania cluster page explains why the windows fall where they do.
The short answer for Pennsylvania
An onion does not decide when to bulb by checking the thermometer. It counts hours of light. Philadelphia sits at 39.87 degrees north (the coordinates of NOAA NCEI station USW00013739, the same station the climate normals come from), and on the longest day of the year it gets 14 h 50 of daylight. Clemson Cooperative Extension puts the bulbing trigger for short-day onions at about 12 hours. Long-day types wait for 14 to 16 hours. Philadelphia clears both thresholds, so a Pennsylvania garden can make a bulb from either group, as long as the plant has enough leaf by the time the trigger trips.
Pests ride on that calendar. Before bulbing, the plant is a stand of leaves, and that is the stage maggots and leafminers prefer. After the trigger, every leaf already on the plant becomes one ring of the bulb, so anything that chews, rasps or tunnels through foliage is taking bulb weight directly.
Spring: maggots and leafminers
Root maggots lay eggs at the base of seedlings and transplants in cool, damp weather. The larvae tunnel into the stem and bulb, and a plant that wilts and yellows in April or May is worth pulling to check. Allium leafminer adults lay eggs in the leaves, leaving neat rows of small white dots. The larvae then tunnel down into the plant. The adults fly again in autumn, so garlic, leeks and fall-planted onions are not safe just because spring went well.
Summer: thrips
Onion thrips are tiny, pale insects that hide in the folds where leaves meet. They rasp the surface, and the leaves turn silvery and then tip-burned. Populations climb in hot, dry weather and often peak while bulbs are sizing up. A stand that lost a third of its leaf area in July makes a smaller bulb in August, and nothing done later brings the size back.
What will not work here
Planting late does not delay the harvest, it shrinks it. Whatever size the plant has reached when day length triggers bulbing is the size the bulb will be. That one rule explains most of the failed fixes people try after pests hit, and the list below runs through the ones that do not hold up in Pennsylvania.
- Replanting in June after a maggot or thrips wipe-out. The days are already long, so a new transplant bulbs with only a few leaves and makes a bulb the size of a golf ball at best.
- Short-day varieties as a way to dodge summer thrips. They start bulbing at about 12 hours of light, which Pennsylvania passes early in the season, so they bulb small and early. Pick a group that fits the latitude instead, as laid out in The Best Onion Varieties for Pennsylvania.
- Covering onions with row cover over last year’s allium bed. Maggot and leafminer pupae overwinter in the soil, so the cover seals the next generation in with the crop.
- Spraying on a fixed calendar. Thrips shelter deep in leaf folds, and a spray applied before or after the population builds misses them. Weekly counts on the leaves tell you far more than a date on the wall.
- Leaving culls, leek tops and trimmings in the bed or compost pile. Pupae survive in them and emerge the next season.
- Waiting for wilting before checking. By the time a leaf collapses, the maggot has already reached the bulb.
What changes across Pennsylvania
A state is not one latitude. The figures above belong to Philadelphia, in the southeast corner, and Pennsylvania stretches several degrees north from there. The northern tier gets more daylight in summer and a later spring. Pest windows shift with it: maggot flights and leafminer activity come later in the spring there, and the fall flight can arrive while the garden is still full.
| Location | Latitude | Longest day | What it changes |
|---|---|---|---|
| Philadelphia (reference city) | 39.87 degrees north | 14 h 50 | Long-day and short-day types can both bulb; spring pests arrive early |
| Southern tier | Close to the reference; check yours | Close to 14 h 50 | Same trigger window, early spring planting |
| Northern tier | Higher than 39.87; not published here | Longer than 14 h 50 | Later planting, later maggot flight, short-day types stay small |
Gardeners in the far north should plan their dates from the When to Plant Onions in Pennsylvania calendar and then adjust for their own frost dates. The pest windows move with the planting date, so a transplant set out weeks later meets maggots at a different stage of growth. We do not publish pest-flight dates by county here, so your local extension office is the right place to confirm them.
Sweet now, or onions that last
The sharp bite of an onion comes from volatile sulfur compounds, and those same compounds protect the bulb in storage. University of Missouri Extension puts it plainly: “As a rule, ‘sweet’ onions do not store as well as the more pungent types.” A sweet variety carries fewer of them. It is better raw and gone by Christmas.
Day length lines up with that trade. Short-day types run sweeter and shorter-keeping, while long-day types are sharper and keep longer. Pennsylvania’s 14 h 50 allows either, so the choice is yours. Grow a few sweet ones for summer eating and a larger block of pungent ones for the pantry.
Pests feed into that decision. A bulb with a maggot tunnel has an open wound for rot to enter, and it will not last in storage however sharp the variety. Eat those first. Thrips-stunted onions often have thin necks and small bulbs, and they cure poorly, so they go in the kitchen bin rather than the cellar.
By March, a gardener who stored well has firm, papery bulbs left. Clemson Extension gives the ideal conditions as between 45 °F and 55 °F with 50 to 60 percent humidity (see its onion, leek, shallot and garlic factsheet). Onions want dry air, the opposite of carrots, so one cellar corner will not hold both well. Curing comes first: bulbs go into storage only once the necks are tight and the skins papery, never straight from the ground.
Checking this against your own garden
Nothing on this page should be read as a statewide rule. The numbers come from one station in Philadelphia, and your garden may differ. Here is what you can verify this week:
- Look up your garden’s latitude and compare it with 39.87 degrees north. The farther north you are, the more summer daylight you get.
- Ask your county extension office for your average last spring frost and first fall frost dates. We do not publish them here.
- Walk the rows once a week and look inside the leaf folds for pale, tiny thrips and silvery streaks.
- Check leaves for rows of small white dots, the sign of allium leafminer egg-laying, in spring and again in fall.
- Pull one wilted plant and look at the base for maggots.
- Write down where onions, garlic and leeks grew this year so next year’s bed goes somewhere else.
Common questions
What is the most common onion pest in Pennsylvania?
Onion thrips are the usual summer problem, especially in hot, dry weather. In spring, root maggots and allium leafminer cause the earliest damage.
When should I start scouting?
Start as soon as transplants or seedlings go in. Check the base of each plant and the leaf surface weekly through the season, and again in fall if you grow garlic or leeks.
Can I still use onions that have maggot damage?
Yes, if you cut out the damaged part and use them soon. Do not put them in storage, because the tunnels let rot in.
Do short-day onions escape pests?
No. They bulb at about 12 hours of daylight, which Pennsylvania passes early, so they finish small and early. That makes them a poor trade, not a pest solution.