Louisiana’s climate favors only two of the ten pests the state’s extension service tracks on vegetable crops as real threats to onion. Fall armyworm chews foliage from the top down, usually in warm weather. Vegetable weevil works the other way: it damages bulbs and leaves during the cooler months, when most other garden pests have gone quiet. Knowing which is active tells you what to check first.
Which pests attack onion in Louisiana
The LSU AgCenter documents ten vegetable pests for this territory: Cowpea curculio, Crape myrtle bark scale (Acanthococcus lagerstroemiae), Fall armyworm, Leaf-footed bug, Pepper weevil, Pickleworm, Squash bug, Squash vine borer, Tomato pinworm, and Vegetable weevil. Most of that list has nothing to do with a row of onions.
Cowpea curculio targets peas and cowpeas, not alliums. Crape myrtle bark scale is a tree pest, feeding on bark, and has no business in a vegetable bed at all. Pepper weevil is locked onto pepper fruit and won’t touch an onion bulb. Pickleworm, squash bug, and squash vine borer are cucurbit specialists, built around squash, cucumber, and melon vines, and they simply don’t recognize an onion as food. Tomato pinworm mines tomato leaves and fruit exclusively. Leaf-footed bug feeds mainly on fruiting vegetables like tomatoes and peppers by piercing developing fruit, a feeding style that doesn’t apply to a bulb crop with no fruit to puncture.
That leaves two species actually worth watching in an onion patch: fall armyworm and vegetable weevil. Fall armyworm is a generalist caterpillar with a wide host range that includes onion foliage, especially when populations build up after feeding on nearby corn, grass, or other crops. Vegetable weevil is more specific to this crop group, feeding on onion, carrot, turnip, and related plants both as an adult and as a larva. Check the base of the leaves and the soil line weekly once transplants are established, since both pests leave their earliest clues there before damage becomes obvious from a distance.
Two documented pests is a short list, and that’s simply what the LSU AgCenter records for this crop in this territory. It doesn’t mean an onion bed is risk-free, just that these are the two with a track record here worth planning around.
When each one shows up, and what it does first
Fall armyworm activity tracks warm weather and tends to build through the growing season as moths move in from nearby fields and lay eggs in clusters on leaves. The first sign isn’t usually a caterpillar in plain sight. It’s small, ragged windows chewed into the leaf surface, followed by larger sections stripped down to the tougher leaf veins. Young larvae feed in groups near where the eggs were laid, so damage often starts in a cluster of plants rather than spread evenly across a row. As larvae grow, they scatter and feeding gets heavier, sometimes chewing all the way through a leaf rather than skeletonizing it.
Vegetable weevil runs on a different clock. It’s active during the cooler part of the year, when onion foliage is still tender and low to the ground. Adults are the ones that chew notches into leaf edges, working at night and hiding in soil or debris during the day, which is why a gardener often finds the damage before ever spotting the insect. Larvae are the more damaging stage: they feed on the developing bulb itself, tunneling into the outer layers and sometimes into the base of the leaves where they meet the bulb. Look for notched, ragged leaf margins low on the plant as the earliest field sign, well before any bulb damage is visible above ground.
Because these two pests operate on opposite temperature preferences, an onion planting rarely faces both at full strength at the same time. A cool-season flush of vegetable weevil damage on young transplants is a different problem than a late-season fall armyworm outbreak on mature foliage, even though both can leave a patch looking chewed and thin. Matching the symptom to the season narrows the guesswork considerably before any closer inspection is needed.
Telling the damage apart from everything else
Chewed or ragged onion foliage doesn’t always mean an insect is responsible. Louisiana’s heat, humidity, and clay-heavy soils in many parishes produce their own set of stress symptoms that mimic pest feeding closely enough to fool a quick glance.
| Symptom | Likely cause | What separates it from a pest |
|---|---|---|
| Yellowing, thin leaf tips | Nutrient deficiency or waterlogged soil | No chewed edges or tunneling; pattern spreads evenly across the bed rather than in patches |
| Wilted, collapsed leaves in hot weather | Drought stress | Leaves recover somewhat after watering; no feeding marks present |
| Discolored streaks or rotted patches on bulb | Disease such as pink root or bulb rot | Damage starts inside the bulb tissue, not as surface notching from the outside |
| Bent, broken, or crushed leaves | Mechanical damage from wind, tools, or foot traffic | Clean breaks or bruising rather than irregular chewed margins |
| Notched leaf edges, low on the plant | Vegetable weevil | Feeding concentrated near soil line, often at night, worse in cool months |
| Ragged windows or stripped leaf sections | Fall armyworm | Clustered damage on nearby plants, worse in warm weather, larvae sometimes visible |
No home gardener needs to nail the exact diagnosis alone. The one detail that tells the story is where the damage starts, at the soil line versus scattered across the leaf blade, since that single clue does more to separate insect feeding from environmental stress than any amount of guessing. When the cause isn’t obvious, a local extension office can look at a sample or a clear photo and settle it.
What actually works here, and what is a waste of time
Timing beats almost everything else in Louisiana’s onion patches, because both documented pests have narrow windows when they’re actually active. A floating row cover installed right at transplanting keeps egg-laying moths and adult weevils off young plants during their most vulnerable stage, then can be removed once bulbs are sizing up and less attractive to feeding. That single step does more for a small home planting than most reactive measures applied after damage is already visible.
Removing crop debris and weedy growth around the bed matters more than it sounds like it should. Vegetable weevil larvae and adults both shelter in plant litter and loose soil during the day, so a clean bed edge removes a hiding spot rather than just tidying up appearance. Handpicking is realistic for a small home row, especially for fall armyworm larvae once they’ve been spotted clustered on a few plants, since removing them physically stops the group before it disperses and spreads feeding across the whole bed.
What wastes time is treating symptoms without confirming the cause first. Applying anything to leaves that are actually showing nutrient stress or drought wilt won’t touch the real problem, and it delays the fix that would. Confirm feeding damage at the soil line before treating anything, and skip guessing at products or rates. Because pesticide products, active ingredients, and legal use rates vary by label and change over time, a parish extension office is the only reliable source for what’s actually approved for use on onion in Louisiana right now.
Where to check this for your own county
Everything above comes from what the LSU AgCenter documents at the state level, and state-level documentation doesn’t guarantee what’s actually present in one particular garden. Pest pressure shifts parish to parish, and even yard to yard within the same neighborhood, depending on nearby crops, soil, and microclimate.
The most direct next step is a call or visit to the parish extension office covering the specific location of the garden. Bring a sample or a clear photo showing exactly where the damage starts on the plant, since that detail is what separates a fast answer from a guess. Extension staff can also confirm what’s currently approved for use on onion in that parish, since recommendations shift as products and labels change.
A documented pest list at the state level is a starting point, not a diagnosis. It narrows the possibilities to two species worth checking for on onion here, but only a look at the actual plant, ideally by someone local, settles what’s really happening in a specific bed.