Two insects do most of the damage to onions in Utah: onion thrips, which chew the leaves, and onion maggots, which tunnel into the base. Both work during the leaf-making months, the stretch when the plant is building the factory that will fill its bulb. The timing is set by the clock more than the calendar. Damage done before the bulb starts to swell costs the most.
The short answer for Utah
The reference city for this page is Salt Lake City, at 40.78 degrees north. On the longest day of the year it gets 14 h 56 of daylight. That number explains why pest timing matters so much here. Day length, not warmth, tells an onion to stop making leaves and start making a bulb, and a long-day variety waits for 14 to 16 hours before it begins. Salt Lake City clears that bar, so a long-day onion will bulb, but only late in the season. That leaves a long stretch of leaf growth when insects can feed.
Thrips are tiny, slender insects that hide in the fold where the leaves meet and rasp the surface. The telltale sign is silvery or whitish streaks, and heavy feeding leaves leaves that look scorched. Onion maggots come from a small fly that lays eggs near the base of young plants in spring. The larvae tunnel into the stem and bulb, and a seedling that wilts and pulls out of the ground with almost no root is a classic case.
The bulb is only as big as the plant is when the day length triggers bulbing. Every leaf thrips destroy in June is a leaf that cannot feed the bulb in August. A maggot strike in April removes the whole plant. Exact dates for each pest in each Utah valley are not published here, and they shift with elevation and with how warm the spring runs. Your county extension office tracks them.
For a planting schedule that keeps the leaf season as short and productive as possible, see When to Plant Onions in Utah. For the broader picture, start with Onions in Utah.
What will not work here
Planting the wrong day-length group quietly makes every pest problem worse. A short-day variety starts bulbing at about 12 hours of daylight, which in Utah arrives early in the year. The plant has barely grown its leaves when the switch flips, so it makes a small bulb. Thrips that would have taken a fraction of a big plant’s leaf area now take a large share of a small one, and the harvest shrinks twice over.
Late planting does not delay the harvest. It shrinks it. Whatever size the plant has reached when day length triggers bulbing is the size of the bulb, so onions set out behind schedule bulb on time and come out small. These approaches fail in Utah:
- Short-day sweet types bred for the Deep South (Granex and Grano types): they bulb at about 12 hours of daylight, far too early at 40.78 degrees north, and the bulbs stay small.
- Onions set out late in spring: they reach the bulbing trigger at seedling size, and a thrips outbreak on a small plant does far more harm than on a large one.
- Planting onions in the same bed year after year: maggot pupae and other soil pests overwinter near the crop they fed on and wait for the next one.
- Spraying on a fixed schedule without checking the leaves: thrips hide in the leaf folds, and a calendar spray with no scouting is a guess.
- Putting uncured bulbs into storage: damaged or wet bulbs rot, and rot spreads through the whole bin.
Choosing the right group is the first defense, because a plant that bulbs on schedule and at full size can absorb some feeding. The variety side of this is covered in The Best Onion Varieties for Utah.
What changes across Utah
The figures above belong to Salt Lake City. Utah runs several degrees of latitude from the Idaho line to the Arizona line, and this page does not publish a figure for either edge. The figures are a starting point, not a verdict for every garden in the state.
| Location | Latitude | Longest day | What it changes |
|---|---|---|---|
| Salt Lake City (reference) | 40.78 degrees north | 14 h 56 | Long-day onions bulb here. Leaf season is long. |
| Far northern Utah | Higher than the reference; check your own | Longer than 14 h 56 | More summer daylight. Long-day onions are the clear fit. |
| Far southern Utah | Lower than the reference; check your own | Shorter than 14 h 56 | Less daylight, so a reader near the edge may belong closer to the neighboring group. Insects tend to start earlier in warmer ground. |
Latitude is only part of it. Elevation changes how early the soil warms, and the soil sets the pace for maggot flies and for thrips. A garden in a high valley and one in a low basin at the same latitude will not see the same pest calendar. The day length is the same at both, but one garden may get its plants into the ground two weeks before the other.
Sweet now, or onions that last
Insect damage raises a storage problem, and storage is where the sweet-versus-sharp trade-off comes in. The bite of an onion comes from volatile sulfur compounds, and those same compounds help protect the bulb while it sits in storage. A sweet variety has 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.”
That lines up with day length. Short-day types run sweeter and long-day types keep longer. A southern garden gives onions to eat soon, and a northern one gives onions that can carry a kitchen into March. No sweet onion stores well, and promising one would be a mistake.
Now add the pests. A bulb that a maggot has tunneled into, or one scarred at the neck, will not keep, however sharp it is. Use those first. The sound bulbs are your storage crop, and they go in only after curing, when the necks are tight and the skins are papery. Clemson Cooperative Extension gives the target: “Ideal conditions are between 45 °F and 55 °F, and 50 to 60 percent humidity.” Onions want it dry, the opposite of carrots and other roots, so one cellar cannot hold both well.
In March, a pantry stocked with pungent long-day bulbs still has onions in it. One stocked with sweet types is usually empty, or down to soft bulbs you should have eaten in fall.
Checking this against your own garden
This page gives one city’s numbers. Your garden may sit well north or south of Salt Lake City, at a different elevation, with a different spring. Verify these this week:
- Look up your own latitude and compare it with 40.78 degrees north. If you are far from Salt Lake City, your daylight differs from 14 h 56.
- Find your local last spring frost date. This page does not publish one, and a statewide figure would be wrong for many gardens.
- Call or visit your county extension office and ask when onion maggot flies and thrips typically show up where you live.
- Check the label or tag on your plants or sets to confirm the variety is a long-day type.
- Note where onions grew last year and pick a different bed.
- Look at the leaf folds on a few plants for silvery streaks, and pull any wilted seedling to check its base.
Common questions
What is the most common onion pest in Utah?
Thrips and onion maggots are the two to watch. Thrips feed on leaves through the season, while maggots strike young plants in spring. This page does not publish statewide damage figures, so your county extension office is the best source for local pressure.
Do thrips kill onions?
They rarely kill a plant outright. They shrink the leaf area, and since the leaves feed the bulb, heavy feeding before bulbing means smaller bulbs.
How do I know it is onion maggot damage?
Look for a seedling that wilts and lifts out of the soil with little root. Cut the base open and you may find small, pale larvae in the tissue.
Can I store onions that insects damaged?
Do not count on it. Damaged bulbs rot, and rot spreads to sound ones. Eat them first, and cure and store only the clean bulbs at 45 °F to 55 °F and 50 to 60 percent humidity.