Short answer. Silvery, papery patches on leaves sprinkled with tiny black dots are thrips feeding scars and droppings. Western flower thrips (Frankliniella occidentalis) needs about 268 degree-days above 7.9 °C to go from egg to adult, which is about 16 days at 25 °C (77 °F) [2]. Its population can grow by 12–23% a day in laboratory life tables [4][5][6]. Eggs sit inside leaf tissue and most pupae sit in the medium, so leaf sprays alone keep failing. Hit both levels: predatory mites up top from early veg, soil predators or nematodes below, blue sticky cards to track the trend, and clean stock so they never get in.
Thrips numbers at a glance
| Parameter | Value | Conditions and source |
|---|---|---|
| Adult length | ~1.2–1.5 mm | Onion and western flower thrips [8] |
| Egg to adult, 25 °C | 15.9 days | WFT, chrysanthemum leaf discs, lab [2] |
| Egg to adult, 20 / 30 °C | 21.6 / 11.8 days | Same study [2] |
| Lower developmental threshold | 7.9 °C (egg 9.9 °C) | Linear model, 10–35 °C [2]; 9.4 °C on cucumber [3]; 8.9 °C on lima bean [7] |
| Degree-days per generation (egg to adult) | 268 DD above 7.9 °C | [2]; 199–233 DD on lima bean [7] |
| Upper limit | 96% larval mortality at 35 °C | [2] |
| Lifetime eggs per female | ~102 | Bisexual females, purple cabbage, 25 °C [4] |
| Adult longevity, 25 °C | females 17.8 d, males 9.8 d | [4] |
| Pre-oviposition period | ~1.8 days after adult emergence | 25 °C [4] |
| Sex ratio | ~71% female (mated mothers) | [4]; 57–59% female at 27.3 °C [5] |
| r (intrinsic rate of increase) | 0.23 /day | 25 °C, purple cabbage [4] |
| r | 0.18 /day | 27.3 °C constant [5] |
| r | 0.12 /day | 26 °C, bean pods [6] |
| Doubling time | 3.0–5.7 days | ln 2 / r, from the three r values above |
| Diapause | none | WFT overwinters active [2] |
| Share pupating in the medium | ~98% | French bean pots [10] |
| Published action threshold for cannabis | none | See scouting section |
What do thrips look like and which species attack cannabis?
Thrips on cannabis are slender insects about 1.2–1.5 mm long with pale yellow wingless larvae and light-brown to dark winged adults [8]. Two species turn up most: onion thrips (Thrips tabaci) and western flower thrips (Frankliniella occidentalis). Colorado State University has confirmed onion thrips in multiple indoor cannabis samples and calls it the primary indoor species; western flower thrips is likely present on some indoor crops too [8].
Onion thrips is the main confirmed indoor cannabis thrips in Canada as well: a trial in a commercial indoor facility found it proliferating on the crop [9].
Western flower thrips is the classic greenhouse thrips and the species with the best life-table data, so most numbers in this article come from it. Treat them as a close guide for onion thrips, not a substitute (Supported). Onion thrips populations are often reported to be mostly female and to reproduce without matingbeing reviewed.
Echinothrips americanus is a third greenhouse thrips worth knowing. It is dark brown to black, about 1.6 mm, with orange bands between the abdominal segments and white patches at the base of the wings [13]. All its life stages stay on the leaves, usually along the midvein, and it pupates on the leaf rather than in the medium [13]. No published record on cannabis was found; it matters because nematodes and soil predators do nothing against it.
What does thrips damage look like on cannabis?
Thrips damage on cannabis is a silvery or bronze, papery scar, often along veins, sprinkled with black fecal specks [1][8]. Thrips pierce surface cells and suck out the contents, which leaves pale stippled patches that later turn brown [1].
Thrips feed more on the upper leaf surface than spider mites do [8]. Feeding on expanding leaves can distort them [8]. In flower, thrips can hide in buds, and damage often shows before you see the insect [1].
Indoors, where natural enemies are absent, high populations build and are sustained, so scars accumulate into extensive leaf scarring [8]. In a Canadian indoor trial, plants started with five onion thrips yielded less fresh flower than plants started with none [9].
No thrips-transmitted virus (tospovirus) is known to infect cannabis [8]. The damage is direct feeding.
Differentials. Spider mites leave round pale stipples, no black specks, and webbing at high numbers. Russet and broad mites bronze or twist growth without scars (kb-pests-broad-russet-mites). Nutrient spotting has no specks and no insects (kb-diagnosis-rust-spots).
What is the thrips life cycle in numbers?
The western flower thrips life cycle runs egg (inserted into leaf tissue), two feeding larval instars, two non-feeding stages (propupa and pupa), then the winged adult [1][15]. At 25 °C the whole run takes about 16 days in a lab study on chrysanthemum leaf discs at 18 h light [2].
Development time by temperature (days, mean). McDonald, Bale and Walters reared western flower thrips on chrysanthemum leaf discs at six constant temperatures [2]. Values below are read from their Table 1.
| Temperature | Egg | Larvae (L1+L2) | Propupa | Pupa | Egg to adult |
|---|---|---|---|---|---|
| 10 °C (50 °F) | 31.3 | 59 | 9.4 | 22.1 | 118.6 |
| 15 °C (59 °F) | 9.2 | 17.0 | 3.9 | 7.8 | 37.1 |
| 20 °C (68 °F) | 6.7 | 9.4 | 1.6 | 3.8 | 21.6 |
| 25 °C (77 °F) | 4.1 | 7.8 | 1.3 | 2.8 | 15.9 |
| 30 °C (86 °F) | 3.1 | 6.3 | 1.1 | 1.9 | 11.8 |
| 35 °C (95 °F) | 2.5 | 96% died | – | – | only 4 adults |
Thresholds and degree-days. Development rate rose linearly with temperature: rate (1/days) = 0.0037 × T − 0.029 [2]. That gives 268 degree-days above 7.9 °C from egg to adult [2]. Stage thresholds were 9.9 °C for eggs, 6.1 °C for larvae, 5.2 °C for propupae and 9.2 °C for pupae [2]. Other studies put the egg-to-adult threshold at 9.4 °C on cucumber [3] and 8.9 °C on lima bean, with 199–233 degree-days [7]. Larvae develop best near 31 °C [2].
Adults. At 25 °C on purple cabbage, mated females lived 17.8 days, males 9.8 days, and females started laying about 1.8 days after emerging [4]. At 28 °C females live about 26 days, some up to 5 weeks [15]. Adult longevity is shortest at 30 °C and longest at 15 °C on cucumber [3].
Reproduction. Mated females laid about 102 eggs each at 25 °C, about 71% of them daughters [4]. Other hosts give fewer: 58.7 first instars per female at 27.3 °C [5] and 35 eggs per female on bean pods at 26 °C [6]. Food quality matters as much as temperature [2].
Population growth. The intrinsic rate of increase was 0.23 per day at 25 °C on cabbage [4], 0.18 per day at 27.3 °C [5] and 0.12 per day at 26 °C on bean pods [6]. On cucumber, r_m peaked at 30 °C, and at 15 °C the population was nearly stable [3]. No cannabis-specific life table has been published.
How does thrips reproduction work, and why does it matter?
Western flower thrips is arrhenotokous: unmated females lay unfertilised eggs that all become males, and mated females produce mostly daughters [4]. This is haplodiploidy: males are haploid.
A single unmated female can still found a colony. Virgin females first produce sons, then mate with those sons and start producing daughters [4]. One escaped thrips on a clone is enough.
Haploid males carry one copy of each gene, so a resistance allele is fully expressed in them and selected fast. This is a standard explanation for quick resistance in thrips and mites (Supported; see kb-term-arrhenotoky and kb-term-pesticide-resistance).
Why do temperature and humidity matter for thrips?
Temperature sets the speed of every thrips generation: from about 22 days at 20 °C to 12 days at 30 °C in lab rearing [2]. Growth peaks near 30 °C and collapses near 35 °C, where 96% of larvae died [2]. Western flower thrips has no diapause and simply slows in the cold [2].
Grow-room set-points, converted to generation time. Using 268 degree-days above 7.9 °C [2], at constant temperature:
| Room | Set-point | Days egg to adult |
|---|---|---|
| Veg, warm | 28 °C (82 °F) | 13.3 |
| Veg, typical | 26 °C (79 °F) | 14.8 |
| Flower | 24 °C (75 °F) | 16.6 |
| Late flower | 22 °C (72 °F) | 19.0 |
| Cool room | 18 °C (64 °F) | 26.5 |
These are linear-model estimates; real rooms with a cooler night run a little slower. Calculate with your own day/night average.
Humidity. No strong published effect of RH on western flower thrips growth rate was found for the grow-room range. RH matters mainly through the predators. Neoseiulus cucumeris reproduces best above 65% RH (ideally 75%), and its eggs shrivel and die at low RH [12]. A. swirskii wants about 70% RH [12]. Entomopathogenic fungi also need moisture at the leaf surfacebeing reviewed.
VPD trade-off. A veg room at 24–28 °C and 60–70% RH (roughly 0.9–1.5 kPa air VPD) suits predatory mites. A flower room at 45–55% RH, and 40–50% in late flower, does not. That is one more reason to build predator numbers in veg (kb-term-vpd).
Day length. Short days matter for the predators, not the thrips: some Orius insidiosus strains and some N. cucumeris strains enter diapause under short days [8][11].
How fast can a thrips population grow? A worked example
Ten western flower thrips can become about 1,200 in three weeks at 25 °C if nothing eats them, using r = 0.23 per day [4]. The method is the exponential model N(t) = N0 × e^(r × t), with N0 = 10 founding thrips and t in days.
| Scenario | r (per day) | Doubling time | Day 14 | Day 21 | Day 28 | Day 42 |
|---|---|---|---|---|---|---|
| 25 °C, cabbage [4] | 0.23 | 3.0 d | 249 | 1,239 | 6,177 | ~154,000 |
| 27.3 °C [5] | 0.18 | 3.8 d | 126 | 446 | 1,580 | ~19,900 |
| 26 °C, bean pods [6] | 0.12 | 5.7 d | 54 | 127 | 296 | 1,611 |
| 15 °C, cucumber [3] | ~0 | – | ~10 | ~10 | ~10 | ~10 |
This is the unchecked upper bound: no predators, unlimited food, a stable age structure. Real rooms grow slower once leaves are damaged, predators arrive or numbers crowd.
Read the table for what it shows about timing. At r = 0.12–0.23, the gap between "a few on one card" and "every leaf scarred" is two to four weeks. A clone brought in at the start of veg can be a crop-wide problem by the flip.
Host plant changes r by a factor of two [4][6]. Cannabis has no published value, so treat the range as the planning band.
Where do thrips hide, how do they spread and how do they get in?
Thrips enter a cannabis room mainly on clones and other live plants, and through vents and openings in the warm season [8]. Winged adults move between plants and are small enough to pass through tiny gaps; thrips-proof screening must be very fine mesh [8].
Hiding places. Eggs are inside leaf tissue [1]. Larvae feed on leaves, often along veins. Prepupae and pupae of most species drop to the medium or leaf litter, or wedge into crevices [1]; on French beans about 98% of western flower thrips left the plant to pupate in the soil [10]. In flower, thrips shelter inside buds [1]. Echinothrips stays on the leaf for its whole life [13].
Vectors. Clones, cuttings and potted plants [8]. Cut flowers and houseplants. Weeds near intakes and in the room, which support thrips [8]. Clothing after time in a garden (Grower practice).
Starve-out. Colorado State University advises a host-free period of at least 3–4 weeks, weeds included, to let residual thrips die out between crops [8].
Quarantine. Hold new plants in a separate area for at least a month, inspect them regularly and hang traps before they enter a thrips-free room [8]. One month covers about two generations at 25 °C, which is long enough for eggs and pupae that came in to show up.
How do I scout for thrips?
Scout for thrips weekly with a lens, a tap test and sticky cards, and record counts so you can see the trend. A rising card count week on week, or fresh scars on new leaves, is the signal to act.
Lens. A 10x lens shows adults and larvae; 20x helps tell thrips larvae from mites and see the black specks. Species ID (onion vs western flower vs Echinothrips) needs 40–60x or a lab.
Tap test. Tap a branch or bud over white paper or a dark card; count larvae and adults (Grower practice).
Sticky cards. Blue cards attract thrips better than yellow and make them easier to see; yellow also works and catches other pests [8]. Thin adhesive makes thrips easier to count [8]. Hang cards just above the canopy and raise them as plants grow (Grower practice). Change or read them weekly. Cards catch only flying adults, never larvae or pupae [8].
Card density. Greenhouse guidance uses about 1 card per 1,000 ft² (~93 m²) for monitoring whiteflies, which is far too sparse for a tent (kb-pests-whiteflies). In small rooms, growers use about 1 card per 1–2 m² plus one by each door and intake (Grower practice).
Echinothrips. Adults are sedentary and weak fliers, so they may not show on cards; green cards are reported to catch them better than yellow or blue [13]. Look along the leaf midvein instead.
Thresholds. No action threshold has been published for thrips on cannabis. The Canadian trial found that five founding onion thrips per plant reduced yield [9]. A workable rule: in veg, any confirmed thrips starts the program; in flower, act on rising counts, not on a single card (Grower practice).
How do I control thrips by life stage?
Thrips control works stage by stage because each stage hides in a different place. Contact sprays and foliar predators hit larvae and adults; soil predators and nematodes hit propupae and pupae; nothing practical hits eggs inside the leaf.
| Stage | Where it is | What reaches it |
|---|---|---|
| Egg | Inside leaf tissue | Nothing practical; wait for hatch |
| Larva 1 | Leaf surface | N. cucumeris, A. swirskii, Orius, contact sprays |
| Larva 2 | Leaf surface | Orius, contact sprays; too big for predatory mites [12] |
| Propupa, pupa | Medium, leaf litter (WFT, onion thrips) or leaf (Echinothrips) | Stratiolaelaps, Dalotia, Steinernema feltiae [12]; foliage-only for Echinothrips [13] |
| Adult | Leaves, buds, flying | Orius, sticky cards, contact sprays |
Re-application interval. Match it to your temperature. A contact spray kills exposed larvae and adults. Survivors are eggs in the leaf, which hatch within about 4 days at 25 °C, and pupae in the medium, which emerge within about 4 days and lay about 2 days later [2][4]. So the next spray must land while those hatchlings are still larvae: the interval must be shorter than the larval period. The whole program must run at least one full generation plus the pre-oviposition period.
| Room temperature | Larval period [2] | Max spray interval | Program length | Applications |
|---|---|---|---|---|
| 20 °C | 9.4 d | ~7 d | ≥ 24 d | 4 (days 0, 7, 14, 21) |
| 25 °C | 7.8 d | ~5 d | ≥ 18 d | 4 (days 0, 5, 10, 15) |
| 30 °C | 6.3 d | ~4 d | ≥ 14 d | 4 (days 0, 4, 8, 12) |
This schedule is a calculation from the life table, not a label. The label interval and local law come first.
Resistance. Western flower thrips is notorious for resistance [2][12]. A greenhouse strain from Almería, Spain, was 356,547-fold resistant to spinosad, an IRAC Group 5 spinosyn, compared with a susceptible lab strain [14]. Haploid males and 2–3-week generations make resistance fast.
Mode-of-action rotation. Rotate by IRAC group, never by brand, and change group after each generation-length window, not after each spray. Products, rates and registrations for cannabis vary by jurisdiction: follow the label and local law. This article names classes only.
Least-toxic contact options in veg. Insecticidal soaps and horticultural oils kill only what they touch, so coverage of leaf undersides and buds-to-be matters (kb-term-insecticidal-soap, kb-term-horticultural-oil). Fungal biopesticides such as Beauveria bassiana act on contact and are compatible with many predators (kb-term-beauveria-bassiana).
Which biological controls work against thrips?
Biological control of thrips on cannabis uses foliar predatory mites against first-instar larvae, Orius bugs against larvae and adults, and soil predators or nematodes against pupae [8][12]. All of them work best released preventively in veg. Release rates below are generic ranges from biocontrol supplier technical sheets; check the sheet for the strain you buy.
| Agent | Stage eaten | Temperature | RH | Notes |
|---|---|---|---|---|
| Neoseiulus cucumeris | L1 larvae only | 24–29 °C (75–85 °F) optimum | > 65%, ideally 75% | Egg to adult 8 d at 25 °C, 11 d at 20 °C; adults live ~3 weeks [12]; some strains diapause in short days [8] |
| Amblyseius swirskii | L1 larvae; whitefly eggs and crawlers | 21–27 °C (70–80 °F) | ~70% | Outperforms cucumeris in warm, long-day conditions; costs more [12] |
| Orius insidiosus | All mobile stages | 20–29 °C (68–85 °F) | – | Needs pollen; up to 8–10 weeks to establish [12]; short-day diapause [11] |
| Stratiolaelaps scimitus | Pupae in medium; fungus gnat larvae | – | Moist medium | Single preventive release at planting [12] |
| Dalotia coriaria | Pupae in medium | – | – | Rove beetle; also fungus gnats [12] |
| Steinernema feltiae | Pupae in medium | – | Moist medium | Drench, then weekly; useless against Echinothrips [12][13] |
Neoseiulus cucumeris
N. cucumeris eats only first-instar thrips larvae; second instars are too big [12]. Second instars still feed less with mites around, about 30% of the time in one Canadian study, so damage drops [12].
Release N. cucumeris at planting or at the start of veg, before thrips are found [12]. Slow-release sachets keep mites emerging for 4–6 weeks [12]. Hang sachets inside the canopy, not in bright light: sun heats them, dries them and kills the predator eggs [12].
Supplier technical sheets commonly give 50–100 mites per m² per release preventively, repeated every 2 weeks, and up to 100–250 per m² on hot spots (biocontrol supplier technical sheet).
Some N. cucumeris strains go dormant under short days [8]. Ask for a non-diapausing strain before the flip to 12/12.
Amblyseius swirskii
A. swirskii is the warm-room choice. In winter short days on chrysanthemum, swirskii and cucumeris controlled thrips equally, so cucumeris is the economical choice for cool rooms [12]. In summer, with heat, strong light and long days, swirskii ate and laid more [12]. It also eats whitefly eggs and crawlers, broad mites and spider mites (kb-term-amblyseius-swirskii).
Orius insidiosus and short-day diapause
Orius insidiosus, the insidious flower bug, eats all mobile thrips stages [12]. Females lay about 129 eggs over a ~35-day adult life, and egg to adult takes at least 20 days under optimum conditions [16]. It needs pollen when prey is scarce and can take 8–10 weeks to establish [12]. Release it in early morning or evening, away from bright light [12].
Diapause under 12/12. In a laboratory culture at 20 °C, nearly 100% of Orius insidiosus females entered reproductive diapause at 10 h light, 56.8% at 12 h, 29.7% at 13 h and 5.3% at 14 h; the critical photoperiod fell between 12 and 13 h [11]. A cannabis flower room at 12/12 sits right on that line. Strains differ by origin, and non-diapausing strains are sold. Use Orius in veg (18/6) and treat flower as a bonus.
Sinsemilla cannabis makes little pollen, so Orius has little alternative food on a seedless crop (Grower practice).
Soil predators and nematodes against pupae
In French bean pots, 5 Hypoaspis miles per pot caused at least 44.9% mortality of soil-dwelling thrips stages and 10 caused 61%, while 20 H. aculeifer caused 80.5% [10]. The commercial "H. miles" is now Stratiolaelaps scimitus (kb-term-stratiolaelaps-scimitus).
Steinernema feltiae nematodes drenched into the medium also hit thrips pupae; start with a drench, then spray or drench weekly [12]. Apply early or late, keep lights off for at least 2 hours after application, and keep the medium moist (kb-term-steinernema-feltiae) [12].
Compatibility
Orius also eats predatory mites [12]. Most residual insecticides kill predators for weeks; check side-effect tables and phase out residual sprays before release. Soaps and oils are less harmful once dry but still kill predators they hit (Supported).
How can I use the environment as a weapon against thrips?
The environment helps against thrips mainly by favouring the predators, not by stopping the thrips. You cannot cool a cannabis room enough to halt thrips without hurting the plant: they still develop at 15 °C [2].
Veg: run humid, moderate. Hold 24–26 °C and 60–70% RH. At 25 °C a thrips generation takes 16 days [2], while N. cucumeris develops in 8 days [12], so the predator gets two generations for each thrips generation. RH above 65% keeps predator eggs alive [12].
Avoid hot spots. At 28–30 °C a thrips generation takes 12–13 days [2]. Lamps close to the canopy and dead-air corners create these spots.
Flower: accept a drier room. RH at 45–55%, and 40–50% in late flower, protects buds from botrytis (kb-diseases-botrytis-bud-rot) and powdery mildew (kb-diseases-powdery-mildew). Bud rot costs more than thrips. Predators carry on inside the canopy, where leaf-surface humidity is higher than room air (Supported).
Late flower: run a little cooler. Dropping from 26 °C to 22 °C stretches a generation from 15 to 19 days [2]. That buys time; it does not fix anything.
Medium surface. Pupae need a medium to pupate in. A dry, coarse top layer may reduce pupation success (Grower practice; no published value found for cannabis media).
What should I not do about thrips in flower?
Do not spray cannabis buds in flower. Soaps, oils and other residues stay in dense buds, wet buds invite botrytis, and most residual insecticides are not registered on cannabis.
- Do not raise RH in late flower to help predators or fungal biopesticides. Keep 40–50% RH.
- Do not start predators in week 5 of flower and expect a rescue. N. cucumeris and A. swirskii do not eat adults or second instars [12]. Build numbers in veg.
- Do not count on Orius in 12/12 without a non-diapausing strain [11].
- Do not drench nematodes into a medium you are drying back for ripening; they need moisture [12].
- If a late infestation is heavy, remove scarred fan leaves, keep cards up, and plan an early, clean harvest rather than spraying.
What are the common myths about thrips?
"Sticky cards control thrips." Myth. Cards catch only flying adults [8]. Eggs, larvae and pupae never touch them. Mass trapping with many cards can slow populations, but cards are a monitoring tool [8].
"Weekly leaf sprays will wipe out thrips." Myth. Eggs are inside leaf tissue [1] and about 98% of western flower thrips pupated in the soil in a bean study [10].
"Thrips will give cannabis a virus." Myth (for now). Thrips spread tospoviruses in other crops, but none are known to infect cannabis [8].
"Pirate bugs work just as well in flower." Myth. About 57% of females of one lab strain entered diapause at 12:12 [11].
"A cold room kills thrips." Myth. Western flower thrips develops above about 8 °C and overwinters active [2].
"Predatory mites can rescue a heavy infestation." Myth. N. cucumeris and A. swirskii eat only first-instar larvae, not second instars or adults [12]. They prevent; they do not rescue.
Common mistakes
- Treating only the foliage and ignoring pupae in the medium [10][12].
- Releasing N. cucumeris after adults are everywhere; it does not eat adults [12].
- Spraying at a fixed 7–10-day interval in a 28 °C room, where larvae can finish in about 6–7 days [2].
- Rotating brands instead of IRAC groups, then wondering why resistance builds [14].
- Releasing Orius under 12/12 and calling it a failure of biocontrol [11].
- Bringing in clones without a month of quarantine [8].
- Hanging sachets in direct light, which bakes the mites [12].
Evidence
- Life cycle, pupation in media and damage signs: Established [1][8][15].
- Development times, thresholds and degree-days of western flower thrips: Established in lab studies on chrysanthemum, cucumber and lima bean [2][3][7]; no cannabis data.
- Population growth rates: Established for the hosts tested (r = 0.12–0.23 per day) [4][5][6]; cannabis value unknown.
- Onion thrips as the main indoor cannabis species and its yield effect: Supported by Colorado extension and one Canadian trial [8][9].
- Predatory mites, Orius and soil predators against western flower thrips: Established in greenhouse crops [10][12]; cannabis trials sparse (Supported by extrapolation).
- Short-day diapause in Orius insidiosus: Established for the strain studied [11]; strain-dependent.
- Spinosad resistance: Established in Spanish greenhouse populations [14]; local status varies.
Glossary terms used here
- Glossary: degree-day, developmental threshold, intrinsic rate of increase, doubling time, diapause, arrhenotoky, sticky trap, Neoseiulus cucumeris, Orius
