Purdue Extension Entomology documents ten insect pests across Indiana’s vegetable gardens, but only one of them has any real business in an onion patch. That pest is the seedcorn maggot, and it does its damage underground, early in the season, before a gardener even sees a green shoot.
Which pests attack onion in Indiana
Purdue Extension Entomology lists ten species for Indiana’s home vegetable garden: Asparagus beetle, Brown marmorated stink bug, Cabbage looper, Colorado potato beetle, Mexican bean beetle, Seedcorn maggot, Spotted cucumber beetle, Squash bug, Squash vine borer, and Striped cucumber beetle. That’s the full roster the extension service tracks, and it isn’t padded with anything beyond it here.
Most of that list has nothing to do with onion. Asparagus beetle feeds only on asparagus ferns and spears, a plant family onion doesn’t belong to. Brown marmorated stink bug pierces fruiting vegetables and tree fruit, going after pods and fruit walls that onion simply doesn’t have. Cabbage looper is a caterpillar built to chew broad, flat brassica leaves like cabbage and kale, and it shows little interest in onion’s narrow, waxy foliage. Colorado potato beetle sticks to nightshades, potato, tomato, eggplant, none of which share a family with alliums. Mexican bean beetle skeletonizes bean and legume leaves and has no recorded pull toward onion at all.
Four more come from the same corner of the garden: Spotted cucumber beetle, Squash bug, Squash vine borer, and Striped cucumber beetle all target cucurbits, squash, cucumber, melon. They’re drawn to the sprawling vines and open flowers of that family, a growth habit that looks nothing like an onion’s upright, tubular leaves and underground bulb.
Seedcorn maggot is the only one of the ten with a documented tie to onion in Indiana. It’s not a leaf feeder or a flower feeder. It attacks germinating seeds and the roots of freshly emerged seedlings, and onion, planted from seed or from sets in cool spring soil, is exactly the kind of crop it targets. If a gardener wants a short answer to “what eats onion here,” this is it: one pest, working below the soil line, at the very start of the season.
When each one shows up, and what it does first
Onion in Indiana starts as seed or sets going into ground that’s still cool from winter, and that’s precisely when seedcorn maggot is looking for a meal. The adult fly lays eggs in soil that’s damp and rich in decaying organic matter, and the maggots that hatch tunnel straight to the nearest seed or root. Because they work underground, there’s no chewed leaf to spot and no visible insect on the plant to point a finger at.
The first sign a gardener usually sees isn’t an insect at all. It’s a gap in the row. Seed that should have sprouted doesn’t, or a seedling comes up, then yellows, wilts, and falls over within a few days of emerging. On a crop grown from sets rather than seed, the young shoot can simply collapse at the base while the bulb underneath turns soft and hollow. None of that looks like pest damage at first glance. It looks like bad seed, or a bad spring.
Because onion bulbing itself is driven by day length rather than temperature (Indiana’s reference latitude of 39.71 degrees north brings up to 14 hours and 49 minutes of daylight on the longest day, well past the roughly 12-hour threshold that triggers bulbing in short-day varieties), the plant’s actual bulb-forming stage comes much later than the maggot’s feeding window. Seedcorn maggot activity is tied to germination and early root establishment, not to the day-length trigger that shapes the bulb months afterward. That timing gap matters: a gardener who loses a row in the first weeks after planting is dealing with the maggot, not with anything related to bulb formation.
If less than half the expected seedlings emerge in a row, dig where the seed was planted before assuming the seed itself was bad. That’s the one habit that separates a maggot problem from a dozen other reasons a row can fail to fill in.
Telling the damage apart from everything else
Onion rows fail for reasons that have nothing to do with insects, and mixing those up leads to the wrong fix. Cold, waterlogged soil slows or blocks germination on its own. Damping-off, a fungal problem that thrives in the same cool, wet conditions the maggot likes, kills seedlings at the stem base without any insect involved. Nitrogen shortage shows up as pale, yellowing leaf tips across the whole bed rather than gaps in a row. And plain mechanical damage, a hoe that went too deep, foot traffic, an animal digging, can knock out seedlings that never had a pest anywhere near them.
| Symptom | Likely cause | What tells it apart |
|---|---|---|
| Seed never sprouts, gaps in the row | Cold, wet soil delaying germination, or seedcorn maggot feeding on the seed | Dig where the seed was placed; maggot damage leaves a hollowed, mushy seed or visible small white larvae |
| Seedling emerges, then wilts and collapses | Damping-off fungus, or maggot feeding on the root or stem base | Fungal collapse shows a dark, water-soaked stem with no larvae present when the soil is dug |
| Pale, yellowing leaf tips across the bed | Nitrogen deficiency or drought stress | Uniform pattern across the whole planting, not isolated gaps in a single row |
| Plants knocked over or torn | Mechanical damage from tools, foot traffic, or animals | Tool marks, disturbed soil, or footprints rather than any insect at the root |
The only reliable way to tell a maggot problem from everything else on this list is to dig, not to guess from the leaves. If digging turns up something unclear, that’s a sample worth bringing to a local extension office rather than a question to settle from a photo online.
What actually works here, and what is a waste of time
Seedcorn maggot is drawn to soil that’s cool, wet, and freshly loaded with decaying organic matter, which means the fix starts before the seed ever goes in the ground. Waiting until the soil has dried out and warmed a bit before planting reduces how attractive the bed is to egg-laying females. Working in fresh manure, cover crop residue, or other undecomposed organic matter right before planting does the opposite: it’s exactly the kind of rotting material that pulls flies in to lay eggs near the row.
A floating row cover set over the seeded row right after planting, not after damage shows up, keeps the adult fly from reaching the soil to lay eggs in the first place. That’s the one physical step that has a real shot at stopping the problem on a short growing season, because it works before the maggot ever exists rather than trying to undo feeding that’s already happened.
What doesn’t work is treating a row after gaps appear. By the time a seedling has wilted or a seed has failed to sprout, the maggot has already fed and moved on or finished its damage underground; there’s no fix for a seed that’s already hollow. Extra watering doesn’t revive a rotted seed piece either, and replanting into the same untreated, organically rich soil without changing the timing just invites the same result. Rotating away from a bed that recently had a cover crop or heavy manure turned under, and shifting planting to slightly warmer, drier soil, does more for onion in Indiana than any treatment applied after the fact.
Where to check this for your own county
Everything above comes from Purdue Extension Entomology’s statewide list, and a statewide list is a starting point, not a forecast for one backyard. Seedcorn maggot pressure shifts with local soil type, drainage, and what was growing in that bed the season before; a garden on heavy, poorly drained clay carries more risk than one on a well-drained raised bed, even a few miles apart.
Contact the local Purdue Extension office for the county before planting, not after gaps show up in the row. County offices can speak to conditions in that specific area, including whether seedcorn maggot has been an active issue locally that season, in a way a state-level list can’t.
Bringing a soil sample, a photo of the damage, or even the seed packet along helps the office give a faster, more specific answer. That one call, made before the seed goes in the ground, does more for an Indiana onion bed than any guess made from a website after the damage is already done.