Onions face a surprisingly wide range of insect and mite pests, from tiny thrips that scar the leaves to maggots that destroy bulbs underground. The most damaging pests in North America are onion thrips, onion maggots, allium leafminers, and bulb mites. Knowing what each one looks like, when it strikes, and how to prevent it can mean the difference between a healthy harvest and a total loss.
Onion Thrips
Onion thrips are the most widespread onion pest across the United States and Canada. Adults are roughly 1/12 of an inch long, thin, and pale yellow to brown, with fringed wings. Nymphs look similar but smaller and wingless. Both stages feed by scraping leaf tissue and sucking the contents, leaving behind whitish blotches that often appear as silvery streaking. As the damage progresses, affected tissue dries out, yellows, and eventually browns and dies. The youngest, innermost leaves take the worst hit because thrips prefer tight spaces between leaves.
Beyond cosmetic damage, heavy thrips feeding leads to distorted and undersized bulbs, directly cutting your yield. Thrips also carry viruses, including Iris yellow spot virus, which can devastate an onion planting on its own. Females can reproduce without mating, and eggs hatch in just five to ten days, so populations explode quickly in warm weather.
Monitoring is straightforward: pull plants and count thrips. In early summer (June to early July), especially on susceptible varieties like red onions, the action threshold is 15 thrips per plant. By mid to late summer, more tolerant yellow and white varieties can handle up to 30 thrips per plant before intervention pays off. A standard scouting protocol is to check 5 plants each in 10 different spots across a field, totaling 50 plants per sample. Thrips overwinter in alfalfa fields, wheat stubble, weedy borders, and unharvested onion bulbs, so cleaning up these refuges in the off-season reduces the population that migrates into your crop the following year.
Onion Maggot
The onion maggot is the single most destructive insect pest of onions in Canada and the northeastern and north-central United States. The adult is a small gray fly that emerges from overwintering pupae in late April through May. After mating, females lay eggs in the soil right at the base of onion plants. The larvae that hatch burrow into roots and the lower bulb, feeding on below-ground tissue. Infested plants wilt, yellow, and often die outright. Heavy infestations can wipe out up to 100% of a planting if nothing is done.
There are three generations per year. The first adults typically appear around mid-May in the Northeast, with peak flight activity tracked by degree-day models (roughly 712 to 1,018 degree days, base 40°F). That first generation is usually the most damaging because it hits seedlings when they’re smallest and most vulnerable. The full life cycle, from egg to adult, takes anywhere from 37 to over 60 days depending on temperature. Larvae from the third generation pupate in the soil and overwinter there, ready to start the cycle again the following spring.
Because the pupae survive in the soil right where onions grew, crop rotation is essential. Avoid planting onions or other alliums in the same ground in consecutive years. Floating row covers are highly effective at keeping egg-laying flies off your plants, but timing matters: the cover must go on at planting, and you need to avoid covering ground where onions grew the previous year, since adult flies can emerge from pupae already in the soil beneath the cover.
Allium Leafminer
The allium leafminer is an invasive fly first detected in the U.S. in 2015 in Pennsylvania. It has since spread to New York, Maryland, and New Jersey, and its range continues to expand. Native to most of Europe, from Spain to Ukraine, this pest attacks onions, garlic, leeks, scallions, shallots, and chives.
Adult flies puncture leaves to feed, creating rows of small round holes that are one of the earliest visible signs of an infestation. Larvae then mine through the interior of the leaves and eventually move down to pupate near the leaf bases or inside the bulb. This mining causes young plant leaves to twist and curl, stunting growth, and can make mature crops unmarketable. The larval tunnels also open the door for bacterial rot, compounding the damage. In heavily affected areas, surveys have found 100% of plants infested with as many as 20 pupae per plant.
The allium leafminer has two generations per year: a spring generation whose pupae go dormant through summer, and a fall generation that overwinters as pupae. Floating row covers are the most reliable defense for home and small-scale growers, particularly during the spring and fall flight windows when adult flies are active.
Bulb Mites
Bulb mites attack onions both in the field and in storage, making them a pest you can carry right into your pantry or curing shed. There are two main types. Acarid mites are shiny, creamy-white, and bulb-shaped, roughly the size of a pinhead (0.5 to 1 mm). They cluster under the root plate of bulbs and in damaged tissue. Dry bulb mites and wheat curl mites are much smaller, about 0.25 mm, elongate and worm-like, visible only under magnification.
Direct feeding from either type produces sunken brown spots and streaks, and causes bulbs to dry out prematurely. More importantly, mites break through the outer protective layers of bulb tissue and create entry points for rot-causing fungi and bacteria. A mite infestation that looks minor can lead to widespread storage rot weeks later. The good news is that proper curing, the normal drying process before storage, can destroy light to moderate infestations. Ensuring bulbs are well dried and storing them in cool, dry conditions goes a long way. For seed stock, soaking sets in a solution of 2% soap and 2% mineral oil for 24 hours before planting has shown success in reducing mite populations carried into the next crop.
Black Cutworm
Black cutworms are generalist pests that feed on at least 49 plant species, and onions are among their targets. Unlike the other pests on this list, cutworms are caterpillars that hide in the soil during the day and emerge at night to chew through stems at or just below the soil line. A single cutworm can sever several seedlings in one night. Infestations tend to appear exceptionally early in spring, often catching growers off guard.
Cutworms are most dangerous to transplants and young seedlings. Larger onion plants with thicker necks are less vulnerable. Cardboard or plastic collars pushed an inch into the soil around transplants provide a physical barrier. Keeping the area around your planting free of weeds in the weeks before and after planting also helps, since cutworm moths prefer to lay eggs in weedy or grassy ground.
Prevention and Integrated Management
Most onion pest problems share a few common weak points you can exploit. Crop rotation breaks the life cycle of soil-dwelling pests like onion maggots and bulb mites. A minimum two-year gap before returning alliums to the same spot is a practical starting point, and three years is better if you have the space.
Floating row covers are one of the most effective tools available, especially for organic growers and home gardeners. The lightweight fabric lets in light, water, and air while physically blocking flies and moths from reaching plants. For pest exclusion, the edges must be completely buried in soil, with no gaps. Put covers in place at planting, not after you see damage. The key limitation: if you place row cover over soil that already contains overwintering pupae from last year’s infestation, you’ll trap emerging adults inside with your crop.
Sanitation matters more than many growers realize. Remove cull onions, crop debris, and volunteer alliums promptly after harvest. These leftovers serve as breeding sites for maggot flies and overwintering habitat for thrips. Keeping field borders mowed and weed-free reduces the reservoir of thrips migrating into your crop each spring.
When scouting indicates populations have reached action thresholds, targeted sprays timed to pest biology are far more effective than calendar-based applications. For thrips, that means treating when counts hit 15 per plant early in the season or 30 per plant later on, rather than spraying on a fixed weekly schedule. For onion maggot, targeting the first generation of adults in May, before eggs are laid, gives the best return. Degree-day tracking tools offered by many university extension programs can help you pinpoint these windows precisely for your location.