Two insects show up on Iowa onions with any regularity: the common stalk borer, which tunnels into the stalk and leaves a single leaf wilting from the inside out, and grasshoppers, which chew ragged notches along the leaf margins once their numbers build later in the season. Everything else gardeners ask about, cucumber beetles, Japanese beetles, squash pests, feeds on other crops sharing the same bed, not on onions themselves.
Which pests attack onion in Iowa
Iowa State University Extension documents nine garden pests active across the state: common stalk borer, grasshoppers, iris borer (Macronoctua onusta), Japanese beetle (Popillia japonica), lily leaf beetle, spotted cucumber beetle, squash bug, squash vine borer, and striped cucumber beetle. That is the full documented list for the territory, and it is worth saying plainly: it is short, and it does not get padded out here with anything the service has not recorded.
Most of those nine have nothing to do with an onion row. Sorting them out matters, because a gardener who spots a beetle on their onion leaves and matches it to the wrong name ends up treating the wrong problem.
- Iris borer feeds inside iris rhizomes and nowhere else in the garden.
- Japanese beetle skeletonizes broad, soft leaves on roses, grapes, and linden trees; it largely ignores the narrow, waxy foliage of onions.
- Lily leaf beetle is a specialist on true lilies and does not wander onto alliums.
- Spotted cucumber beetle, squash bug, squash vine borer, and striped cucumber beetle are all tied to cucurbits, cucumbers, squash, melons, not to onions.
That leaves two. Common stalk borer larvae bore into the base of the stalk and hollow it out from within, which is why the damage often looks like the plant collapsed for no reason. Grasshoppers are generalist feeders that will strip onion leaf tissue from the outside when their preferred grasses and weeds dry up nearby. When you’re scouting, check the base of the stalk, not just the leaf tips, since that’s where the borer’s entry hole and frass show up first, while grasshopper damage stays visible on the leaf surface.
When each one shows up, and what it does first
Common stalk borer moths lay eggs in grassy field edges and roadside weeds the previous fall. The larvae hatch in spring and feed inside grass stems first, then move into nearby garden crops, including onions, once that grass starts drying down. By the time onion transplants have put on real growth, a borer that has already fed for weeks in the grass can tunnel into a stalk within days. The plant does not show an obvious external wound at first.
Grasshoppers follow a slower, more visible arc. Nymphs hatch in undisturbed grassy areas as the soil warms and feed there through the earlier part of the season, largely ignoring cultivated beds while wild food is still green and plentiful. As that food dries out later in summer, especially in a dry stretch, adults and late-stage nymphs move into gardens in numbers and begin working the edges of onion leaves. Damage tends to build gradually rather than appear overnight, and it gets worse the drier the season runs, since grasshoppers thrive when their competition for green forage disappears.
The practical difference for a gardener checking a bed is timing of the symptom, not the calendar month. The first sign of stalk borer is a single leaf going limp or bending over while the rest of the plant stays upright; the first sign of grasshopper feeding is a series of small notches eaten inward from the leaf edge, usually spread across several plants rather than isolated to one. Neither pest announces itself with a visible insect sitting on the leaf when you first notice trouble, which is part of why the damage gets misread.
Telling the damage apart from everything else
Onions fail for plenty of reasons that have nothing to do with an insect, and the two pests documented here can be confused with several of them. Nutrient shortages, uneven watering, wind damage, and bulb rot all produce symptoms that overlap with pest feeding at first glance.
| What you see | Likely cause | What tells them apart |
|---|---|---|
| Wilted or bent single leaf, small hole and sawdust-like frass near the base | Common stalk borer | Frass and an entry hole sit right at or below soil level; nearby leaves on the same plant stay green |
| Ragged notches chewed from the leaf edge, worse in a dry stretch | Grasshopper feeding | Damage runs from the outer edge inward, no boring, no frass, several plants affected at once |
| Even yellowing or pale striping across most of the bed | Nutrient shortage or uneven watering | Whole planting looks affected together, with no chew marks or holes anywhere |
| Browning tips while the base near the bulb stays firm | Wind whip, sunscald, or a dry spell | Worst on the tallest, most exposed leaves, not scattered randomly through the bed |
| Soft, water-soaked bulb with an intact-looking leaf surface | Bulb rot (disease, not an insect) | The problem starts underground, before the leaves show much of anything |
The single most useful habit here is simple: note whether the damage is confined to one leaf or spread evenly across the whole bed before deciding what caused it. A localized, single-plant symptom points toward an insect; a uniform pattern across every plant almost always points toward soil, water, or disease instead. When the pattern still isn’t clear, that’s a job for a diagnostic sample, not a guess.
What actually works here, and what is a waste of time
Prevention beats reaction with both of these pests, because once a stalk borer is inside the stem, nothing applied to the leaf surface reaches it, and once grasshopper numbers have already built up in a dry year, they keep arriving from surrounding fields faster than hand control can keep pace.
For stalk borer, the leverage point is the grass and weeds where the moths lay eggs the fall before. Mow or till grassy field edges and fence lines near the onion bed before the larvae move out of that grass and into garden crops, and that single step does more than any treatment applied after a stalk has already wilted. For grasshoppers, physical barriers such as row covers placed over young transplants before populations build keep the insects off the leaves entirely; hand removal works too, but only while numbers are still low, since a serious outbreak in a dry year usually outpaces what one gardener can pick off.
What doesn’t work: treating a stalk after it’s already wilted, since the borer causing the damage is safely inside the plant tissue by then. Broad treatment for grasshoppers late in a dry summer tends to disappoint for the same reason, adults keep moving in from untreated grass nearby faster than a single garden bed can be protected. This site does not recommend specific products or active ingredients; what’s labeled and legal to use in Iowa is a question for the state’s extension service, not a garden blog.
Where to check this for your own county
The list above comes from Iowa State University Extension, and it describes what’s documented across the state, not what’s necessarily present in any one backyard. Iowa spans real distance north to south, and pest pressure, along with grasshopper outbreaks tied to local dry spells, varies from one county to the next in ways a statewide list can’t capture.
The reference latitude behind the seasonal figures on this site, 41.53°N, sits in central Iowa. A garden well north or south of that line can see earlier or later activity from the same insects, since local weather and the condition of nearby grassland drive the timing more than a fixed calendar date does.
The next useful step is contacting your county extension office directly, either to confirm an identification with a photo or a sample, or to ask what’s currently being reported active in your part of the state. That local office, working under Iowa State University Extension, is the resource built to answer the question a statewide list can’t: what’s actually happening in the ground you’re standing on this week.