Montana State University Extension’s Urban IPM program documents five insect pests in the state, but only two of them actually feed on onion. The rest show up in yards and fields for reasons that have nothing to do with your onion bed. Knowing which is which saves you from spraying something that was never going to eat your crop in the first place.
Which pests attack onion in Montana
Montana State University Extension, Urban IPM lists five insects as documented pests in the state: Brown marmorated stink bug, Grasshoppers, Miller moths, Social wasps, and Thrips. That is the full list this service tracks here, and it does not stretch further just because a neighboring state’s extension office names something else.
Sorting that list against an onion bed narrows it fast. Only two of the five actually feed on onion tissue: Thrips and Grasshoppers. The other three are on this list for other crops or other reasons entirely.
Brown marmorated stink bug feeds by piercing fruit and seed pods, tomatoes, peppers, and tree fruit are its usual targets. Onion foliage and bulbs give it nothing worth piercing, so a stink bug walking through your onion row is passing through, not feeding.
Social wasps are predators and scavengers, not plant feeders. They hunt other insects and clean up dead ones. A wasp near your onions is more likely eating a thrips than eating your crop, which matters when you get to the “what to do” question later.
Miller moths, the adult stage of the army cutworm, are nectar feeders on the wing and famous for showing up in porch lights and window sills during their spring migration. They are not chewing onion leaves. Their larval stage does damage other crops earlier in the season, but that is a different insect at a different time, and it is not what this list is flagging for gardens.
That leaves thrips and grasshoppers as the two worth watching. Thrips are the classic onion pest almost everywhere onion is grown, tiny, slender insects that rasp the leaf surface and feed on the cell contents underneath. Grasshoppers are opportunistic chewers, less picky about what they eat, and they will take a pass through an onion bed when the surrounding grass dries out and their preferred forage thins.
The short version: five names on the extension list, two of them actually eat onion, and the other three are documented pests for entirely different reasons.
When each one shows up, and what it does first
Onion doesn’t run on the calendar the way a lot of garden crops do. It runs on day length. The plant spends its early weeks building leaves, then switches to building a bulb once daylight stretches long enough, and in Montana the longest day of the year brings roughly 15 hours and 32 minutes of daylight. That shift from leaf-growth to bulb-growth is also roughly when both pests ramp up, because it lines up with the hottest, driest stretch of the growing season.
Thrips show first as fine, silvery-white streaks or blotches on the leaf surface, sometimes with tiny dark specks of frass mixed in. The damage looks almost like the leaf has been scraped rather than chewed, because that’s exactly what’s happening: thrips rasp the surface open and feed on what leaks out. Numbers build through the warm, dry weeks as onion leaves reach full size and the plant is putting its energy into the bulb.
Grasshoppers show up differently. Their damage looks like ragged notches and holes along the leaf margins, chewed rather than scraped, and it tends to get worse as surrounding rangeland and pasture grasses cure out in the heat. When their usual forage turns brown and tough, a garden full of green onion leaves looks a lot more appealing, and pressure often climbs toward late summer as a result.
Neither pest arrives on a fixed date you can circle on a calendar, because the trigger is day length and dryness, not the month itself. A grower near the state’s northern edge and one near the southern edge are working with slightly different day-length curves, since the 15-hour-32-minute figure belongs to one reference point and not to every corner of the state equally. Watching the plant’s own growth stage, leaf development moving into bulb swell, tells you more than the date on the wall does.
Telling the damage apart from everything else
An onion bed can look ragged for reasons that have nothing to do with insects, and treating a nutrient problem like a pest problem wastes both time and product. The single most useful habit is simple: look for the insect itself before assuming one caused the damage. If you can’t find thrips or grasshoppers on or near the plant, the cause is probably something else.
| What you see | Likely cause | What tells them apart |
|---|---|---|
| Silvery streaking, tiny black specks on leaves | Thrips feeding | Insects visible in leaf folds with a hand lens; worse in hot, dry stretches |
| Ragged notches or holes along leaf edges | Grasshopper feeding | Insects visible on or near the plant during the day, often worse near dry field edges |
| Even yellowing, tips dying back uniformly | Nutrient deficiency or drought stress | No insects present, and damage is spread evenly rather than patchy |
| Soft, water-soaked or mushy bulb | Rot or disease | Smell and texture change, no chewing or scraping marks |
| Bent, bruised, or broken leaves | Wind, hail, or foot traffic | Timing matches a specific weather event, not a spreading pattern |
None of that replaces an actual look at the plant. If the symptom doesn’t fit cleanly into one of these buckets, or if it’s spreading in a way that doesn’t match either pest’s pattern, that’s the point to send a sample or a clear photo to your local extension office rather than guessing further.
What actually works here, and what is a waste of time
Timing beats product on a growing season this short. Row covers or fine insect netting installed before the plants emerge or right at transplanting keep both grasshoppers and thrips off the crop during the weeks they’d otherwise be settling in, and removing that barrier too early undoes the benefit. Once either pest is already established on the leaf, a physical barrier does far less.
Hand-picking grasshoppers isn’t realistic once populations build, they move in from surrounding grass faster than one gardener can keep up with. Watching the edges of the garden closest to dry pasture or vacant lots, and acting early while numbers are still low, does more than trying to catch up later in the season.
One thing that backfires: treating the whole bed with a broad-spectrum approach out of frustration. Social wasps are predators, not pests, on this crop, and they take a share of thrips and other small insects along the way. Wiping out everything in the bed removes that natural check and can let thrips numbers rebound faster than before.
Beyond that, this site doesn’t name specific products or active ingredients, and for good reason: what’s labeled and legal to use on onion changes by product and by year, and only your state’s extension service tracks that current list. If a physical barrier and early monitoring aren’t enough, that’s the next call to make, not a guess at what to spray.
Where to check this for your own county
Montana State University Extension, Urban IPM is the source behind the pest list on this page, and it documents these five insects at the state level, not county by county. What that program tracks statewide and what’s actually chewing on a specific onion row can be two different things, since grasshopper pressure near dry rangeland looks nothing like grasshopper pressure in an irrigated valley garden a few counties over.
The daylight figures used here belong to one reference station, and Montana spans enough latitude that a garden near the state’s northern border sees a noticeably longer summer day than one near its southern edge. That difference shifts exactly when onion plants switch from making leaves to making bulbs, and it shifts the pest pressure that tracks along with that switch.
Contact your county Extension office before treating anything you’re not sure about. They know what’s actually been reported in your area this season, not just what’s documented for the state as a whole, and they can look at an actual sample instead of a description of one.