Orius insidiosus
Minute pirate bug- Rate
- 1–2 /m² per release; up to 10 /m² on hot spots
- Temp
- Best above 20 °C; 18–28 °C
- Best for
- Wild ally in the field; releases under glass
Pest
Onion thrips
Thrips tabaci
The short answer
A slender, pale-yellow to light-brown thrips about 1/20 inch (1.3 mm) long. It hides in the tight inner leaves of onions, leeks and garlic, and between the head leaves of cabbage. Its feeding leaves silvery streaks and white blotches, and heavy attacks cause tip dieback and smaller bulbs. It also spreads iris yellow spot virus in onions. In a Quebec field trial, bought predators did not give control. The tools that work are overhead watering, straw mulch, siting away from grain and alfalfa, and, on farms, well-timed sprays.
hero macro · adults and larvae in an onion leaf fold
Thrips tabaci adults and pale larvae among silvery feeding scars at the base of an onion leaf. Photo: pending.
Identification
Adults are about 1/20 inch (1.3 mm) long, slender, and pale yellow to light brown. UMass gives 1/16 inch and says they range from yellow to black. They have two pairs of narrow wings fringed with long hairs. Larvae are wingless, paler and the same shape. Western flower thrips is similar and often turns up on the same plants. Telling the two apart takes about 400× magnification: onion thrips has seven antennal segments and lacks the long hairs on the thorax.
Look where it hides, not on the open leaf. On onions it packs into the leaf folds and the protected inner leaves near the bulb. On cabbage, feeding is concentrated on the third to sixth leaves in from the outside, which is where the head loses value. The damage is silvery streaking dotted with black specks of excrement. When it merges into whitish blotches, growers call it 'blast'.
Easily confused with
- Western flower thripsAlso a pestOnly magnification separates them. Western flower thrips has long hairs on the front of the thorax and eight antennal segments; onion thrips lacks the hairs and has seven.
- Predatory thripsBeneficial — leave itSixspotted thrips has three dark blotches on each forewing; banded-wing thrips is black with black-banded white wings. Predatory thrips are seldom seen in large numbers.
- Minute pirate bug nymphsBeneficial — leave itPirate bug nymphs are wingless, teardrop-shaped and yellow-orange to brown, darkening with each moult, and up to about 1/8 in (3 mm) long.
Damage symptoms
Adults and larvae both damage the plant. They rasp surface cells and suck out the contents, which cuts leaf area and gives disease a way in. In onions the damage matters most during early bulbing, and it is severe on green onions, which are sold on looks. In cabbage the injury is inside the head, where sprays cannot reach. In one Slovenian trial, trimming off damaged leaves cost 23–26% of the head weight.
- Silvery lines and white patches on onion and leek leaves, flecked with black specks
- A silvery cast across the whole planting in a severe attack
- Leaf-tip dieback, and curled or twisted leaves
- Smaller bulbs and lower yields; more purple blotch where leaves are damaged
- Discoloured, rough-textured patches on cabbage leaves, often deep inside the head
Lifecycle & degree days
Adults spend the winter in crop remains, alfalfa, wheat, weeds and greenhouses, and move into onions when nearby grain dries down or alfalfa is cut. Eggs are laid inside the leaf. The larvae feed on the leaves and then drop to the soil to pupate. Development is fast in warm weather, so numbers can climb quickly in midsummer. There are both female-only and sexual strains, and in laboratory studies they differed in how well they did on onion and on cabbage.
- Egg to adult
- As little as 2 weeks in warm weather
- Generations / yr
- At least 2 in the Northeast; thrips in general up to ~8
- Pupates
- Usually in the soil
- Overwinters
- Adults, in crop debris, alfalfa, wheat and weeds
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Host crops
Onions and their relatives are the main crop. UMass also lists cabbage, broccoli and cauliflower, and cabbage is where it does the most hidden damage. Some cabbage varieties suffer far less injury than others. In New York trials, a few had negligible damage whether or not they were sprayed. The RHS and Koppert add many greenhouse and garden hosts, including cucumber, tomato, pepper, rose, carnation, chrysanthemum, begonia, cyclamen and dahlia. On dahlia, feeding at the shoot tips can stunt the plant badly.
Monitoring & action thresholds
Action threshold
1–3 thrips per leaf (onion)
This is the UMass figure for onions. It uses the lower end during rapid bulb swelling. UC IPM uses 30 thrips per plant at midseason, scaled to plant size, and says to treat green onions at the first sign of feeding. Our sources give no threshold for home gardens or for cabbage. For cabbage, New York trials point to resistant varieties as the foundation of control, and in a Slovenian trial the timing of a single spray made no significant difference.
Scout weekly, starting at the field edges and then across the planting. UC IPM checks at least five plants in each of four areas. Pull the leaves apart and count the thrips on the inner leaves near the bulb and under the folds with a hand lens. They move fast, so a click counter helps. On cabbage, peel back the outer leaves and look between the layers, where the damage concentrates.
Beneficial
Biological control
Wild natural enemies feed on onion thrips: minute pirate bugs, lacewing larvae, predatory thrips, predatory mites and hoverfly larvae. Broad-spectrum sprays such as pyrethroids knock them out. Buying predators for field onions has not worked so far. In a Quebec study, releases of Stratiolaelaps, Neoseiulus cucumeris and Amblyseius swirskii did not keep thrips below the economic threshold, and neither did Beauveria sprays. Flower strips beside the onions cut thrips numbers by up to 50% (Gagnon and colleagues, 2024). In laboratory tests, A. swirskii and Orius insidiosus ate more onion thrips than western flower thrips. Under glass, the thrips mite and pirate bugs are used against thrips in general, but outbreaks in Ontario greenhouses show that programmes built for western flower thrips have not controlled onion thrips well enough to prevent losses.
Amblyseius cucumeris
Thrips predatory mite- Rate
- 50–100 /m² per release; up to 350 /m² at peak pressure (Cornell)
- Temp
- 15–25 °C best (Koppert); UConn gives 75–85 °F and Cornell 25–35 °C; not effective above 30 °C (Koppert); humidity above 65–70%
- Best for
- Under glass only — failed in field onions
Cultural controls
For onions in the open, the things that work are cultural, and they are well documented.
- Water overhead where you can — UC IPM notes that sprinklers and rain suppress thrips
- Mulch onions with straw: in New York trials it delayed reaching the threshold by 7–14 days, and in lab tests it cut adult emergence by 54%
- Do not plant onions beside grain, alfalfa or clover — thrips move in when those are cut or dry down
- Clear crop debris and volunteer onions at the end of the season, and do not replant old sets
- Plant a strip of flowers alongside (Quebec trials: up to 50% fewer thrips and 25% higher yields)
- Choose cabbage varieties that are listed as thrips-tolerant
Chemical, last resort
Sprays barely reach thrips in cabbage heads or onion necks, and pyrethroids wipe out their predators. Spinosad, spinetoram and abamectin are toxic to bees: never spray plants in flower or let drift reach them.Check any product before it goes on. Open the compatibility matrix →
- SpinosadREI 4 h
- SpinetoramREI 4 h
- AzadirachtinREI 4 h
- Beauveria bassianaREI 4 h
- AbamectinREI 12 h
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UC IPM Pest Management Guidelines: Onion and Garlic — Thrips (Dara et al., updated 10/2018)
UC IPM Pest Notes 7429, Thrips (Bethke, Dreistadt & Varela, 05/2014)
UMass Extension Vegetable Program, Onion Thrips fact sheet (updated 2013)
RHS, Thrips advice page; Koppert, Onion thrips pest page
UC IPM Relative Toxicities in Strawberry (07/2018) and Floriculture (01/2022)
Gagnon et al. 2024, Insects 15 (biocontrol and flower strips for onion thrips, Quebec)
Larentzaki et al. 2008, J. Econ. Entomol. (straw mulch and onion thrips)
Shelton et al. 2008, J. Econ. Entomol.; Trdan et al. 2004 (onion thrips in cabbage)
Summerfield et al. 2024, Insects (lab predation on onion vs western flower thrips)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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