Straight answer: not much, at least not by insect standards. University of Missouri Extension / MU IPM tracks a set of garden pests statewide, and none of them are onion specialists. Most onion trouble in Missouri traces back to disease, drought stress, or storage rot rather than chewing insects. The one bug worth watching is spider mites, and only during hot, dry stretches once the crop is established. Everything else documented for this state feeds on a different plant entirely.
Which pests attack onion in Missouri
University of Missouri Extension / MU IPM documents ten pests for gardens across the state: Bagworms, Brown marmorated stink bug, Cucumber beetles, Hunting billbug, Japanese beetle (Popillia japonica), Spider mites, Squash bug (Anasa tristis), Squash vine borer (Melittia cucurbitae), Tobacco hornworm, and Tomato hornworm. That’s the documented list for this territory, and it’s short by design, not by omission.
Run it against an onion bed and most of it falls away immediately. Bagworms feed on conifers and a handful of woody shrubs, not on strap-shaped leaves in a vegetable row. Hunting billbugs chew on turfgrass crowns and roots, a lawn problem, not a garden one. Squash bugs and squash vine borers are cucurbit specialists, tied to squash, pumpkin, and melon vines by both mouthpart and habit. Cucumber beetles follow the same cucurbit family. Tobacco hornworm and tomato hornworm are both built to strip nightshades, tomato, pepper, potato, eggplant, and have no reason to touch an allium. Brown marmorated stink bug is a broad feeder on fruit and some vegetables, but it targets fruiting tissue, not onion foliage or bulbs, so it doesn’t show up in onion damage reports. Japanese beetles skeletonize leaves on roses, grapes, and linden more than anything in a vegetable bed, and onion foliage isn’t on their preferred list.
That leaves one name from the ten: spider mites. They’re generalist feeders, capable of moving onto almost anything under the right conditions, and onion foliage under heat and drought stress is exactly the kind of stressed, dusty tissue they favor. That’s the entire documented insect risk for onion in this state, one generalist pulling double duty rather than a dedicated onion pest. It also means most of what gets blamed on “bugs” in a Missouri onion patch turns out to be something else once you look closer, which the next two sections cover.
When each one shows up, and what it does first
Spider mites don’t follow a fixed date on a calendar. They follow weather, specifically stretches of heat combined with dry soil, and that pattern tends to build several weeks after transplanting, once onion tops have put on enough growth to hold a colony. Cool, wet spring weeks keep them in check; a run of dry, hot days changes that fast because their reproduction speeds up sharply in heat while their fungal predators slow down in dry air.
The first sign isn’t a visible bug. It’s stippling, tiny pale or yellowish specks scattered across the leaf blade where mites have punctured cells to feed. Leave it unchecked and the stippling spreads into a dull, bronzed or silvery cast across the whole leaf, sometimes with fine webbing where the leaf meets the neck of the plant. Onion leaves are already narrow and upright, so damage that starts near the base is easy to miss until it’s climbed halfway up the blade.
Because none of the other nine pests on the documented list target onion, there isn’t a second timeline to track here, no staggered wave of a different insect a month later. That’s unusual compared to crops like tomato or squash, where several pests overlap through the season. For onion, the single practical habit that decides how much damage spider mites do is checking leaf undersides during any dry spell that runs past a week or two, before stippling turns into visible bronzing. Catching it at the speckled stage is the difference between a plant that recovers and one that’s already lost photosynthetic leaf area heading into bulb sizing, which is when the plant needs that leaf surface most.
Bulb-sizing onions rely on their tops to feed the bulb below, so leaf damage that shows up during that stretch costs more than the same damage earlier in the season. That timing detail matters more than the exact week it happens, since the trigger is weather, not the date.
Telling the damage apart from everything else
Onion beds throw off a lot of symptoms that get blamed on insects when the real cause is something else entirely. Sorting them out starts with looking at where the damage sits and whether anything is actually feeding.
| Symptom | Likely cause | What distinguishes it |
|---|---|---|
| Pale stippling, later bronzing, on leaf blades | Spider mites | Fine webbing near the leaf base under magnification; worse after dry heat |
| Yellowing tips, no stippling pattern | Nitrogen deficiency or uneven watering | Affects whole plants evenly across the bed, not scattered individual leaves |
| Water-soaked or slimy patches, foul smell | Bacterial or fungal rot | Soft, wet tissue rather than dry, stippled tissue; often starts at the neck |
| Torn or shredded leaf tips | Wind, hail, or cultivation tool contact | Damage follows a straight or jagged mechanical line, not a feeding pattern |
| Purplish lesions with concentric rings | Purple blotch or similar foliar disease | Lesions have defined rings and expand along the leaf regardless of dry weather |
The distinguishing detail worth remembering is this: true insect feeding on onion in Missouri almost always comes with fine webbing or stippling concentrated on individual leaves, while nutrient and water problems spread evenly across the whole planting and disease shows up as wet or ringed lesions rather than dry speckling. None of that replaces an actual look at the plant. A local extension agent or a diagnostic lab can confirm what a written description can’t, and that’s the right next step once the symptom doesn’t match cleanly to one of these categories.
What actually works here, and what is a waste of time
Because spider mites thrive on stressed, dusty foliage, the most effective step is also the simplest one: keep onion beds evenly watered through hot, dry stretches so the plants never sit under drought stress, which is the exact condition that lets mite populations build. A hard spray of water directed at leaf undersides can knock down an early colony mechanically, without any product involved, and it’s often enough if caught at the stippling stage rather than after bronzing sets in.
What’s a waste of time is treating an onion bed for anything else on the documented Missouri pest list. Traps or sprays aimed at Japanese beetles, squash bugs, or hornworms have no effect on a crop those insects don’t feed on, and applying a broad-spectrum treatment “just in case” tends to do more harm than good, since it can wipe out the small predatory mites and insects that normally keep spider mite numbers down on their own. Rotating in cucurbits or tomatoes near an onion bed doesn’t change onion pest pressure either, since the two problems don’t overlap.
Beyond watering, avoiding overcrowded rows helps, since good airflow between plants keeps humidity and dust levels more even and makes it easier to spot early stippling before it spreads. This site doesn’t recommend specific pesticide products or active ingredients, because what’s labeled for use on onion changes and only a state extension service tracks current registrations accurately.
Where to check this for your own county
Statewide documentation from University of Missouri Extension / MU IPM tells you what’s been recorded across Missouri as a whole, not what’s active in a specific backyard. Pest pressure shifts county to county and even bed to bed, driven by local weather, nearby host plants, and how a particular garden is managed.
The county extension office is the right place to confirm a suspected problem, since staff there can point to what’s currently being reported nearby and can often look at a sample or a clear photo directly. Reach out to the local county extension office before applying any treatment, especially if the symptom doesn’t line up cleanly with the stippling-and-webbing pattern spider mites leave behind. That single call sorts out far more misdiagnosed onion problems than guessing from a list ever will.