Short answer. Silvery, papery patches on leaves sprinkled with tiny black dots are thrips feeding scars and droppings. Thrips spend part of their life in the growing medium, so spraying leaves alone keeps failing. Hit both levels: predatory mites (and pirate bugs in veg) up top, soil predators or nematodes below, sticky cards to track progress, and clean stock to stop reinfestation.
What's happening
Thrips are slender insects about 1–2 mm long. They rasp the leaf surface and suck out cell contents, which leaves silvered, stippled or scabby scars with black fecal specks [1]. Western flower thrips (Frankliniella occidentalis) is the classic greenhouse species; onion thrips and others also occurbeing reviewed. Thrips can also vector tospoviruses in other cropsbeing reviewed.
Lifecycle. Egg (inserted into leaf tissue), two feeding larval stages, then two non-feeding stages (prepupa, pupa), then adult. Most species drop to the medium or leaf litter to pupate. In warm weather egg to adult can take as little as about 2 weeks, and there can be up to ~8 generations a year outdoors [1]. Indoors generations continue year-round.
How to recognise it
- Leaves: silver or bronze patches, often along veins, with black specks; damaged areas later turn brown and papery [1].
- Insects: shake a leaf over white paper: tiny elongated yellow-white larvae, darker winged adults that flick away.
- Cards: adults land on blue or yellow sticky cards [4].
- Where: fan leaves first, any height; in flower they can hide in buds where damage shows before you see the insect [1].
Differentials: spider mites (round stipples, no black specks, webbing), russet mites (bronzing without scars), nutrient spotting (no specks, no insects). See kb-diagnosis-rust-spots.
Numbers that matter
| Parameter | Value | Notes |
|---|---|---|
| Adult length | ~1–2 mm | |
| Egg to adult | as little as ~2 weeks in warm weather | [1] |
| Generations | up to ~8/year outdoors; continuous indoors | [1] |
| Orius effective range | 20–29 °C (68–85 °F) | [2] |
| N. cucumeris target | first-instar larvae only | So release preventively [2] |
| Action threshold | none established for cannabis | Grower practice: rising card counts week on week, or scars on new leaves |
What to check first
- Confirm with a lens and paper shake. Black specks plus slender insects = thrips.
- Hang cards. 1 card per few plants at canopy height; count weekly to set your own baseline (Grower practice).
- Check the medium. Wet, organic-rich tops support pupae and also fungus gnats.
- Check the source: new clones, cut flowers, outdoor plants near intakes.
How to fix it
Pesticide registrations on cannabis differ by jurisdiction. Check local regulations and follow the label; no products or rates are given here.
Cultural
- Quarantine and inspect all incoming plants.
- Remove weeds and other plants near the grow; they host thrips.
- Screen intakes where feasible; clean up leaf litter where pupae shelter.
Physical
- Blue or yellow sticky cards for monitoring and modest mass trapping [4].
- A top layer of dry, coarse material on pots may reduce pupation success (Grower practice)being reviewed.
Biological
- Neoseiulus (Amblyseius) cucumeris: generalist predatory mite; eats first-instar larvae; release preventively, sachets or loose [2][3].
- Amblyseius swirskii: costs more, performs better in warm, long-day conditions [3]; also eats whitefly eggs and crawlers.
- Orius spp. (minute pirate bugs): eat larvae and adults; most effective at 20–29 °C (68–85 °F); release early morning or evening [2]. Orius will also eat predatory mites [2]. Some Orius insidiosus populations enter reproductive diapause under short days, so they may establish poorly under 12/12 (non-diapausing strains are sold)being reviewed.
- Below the canopy: soil-dwelling predatory mites (Stratiolaelaps scimitus), the rove beetle Dalotia coriaria and entomopathogenic nematodes target pupae in the medium [2].
- Entomopathogenic fungi (Beauveria bassiana): contact; need humidity; compatible with many predators.
Least-toxic options
- Insecticidal soaps and horticultural oils: contact only; thrips hide, so coverage and repeats matter.
- Spinosyn class (spinosad): effective in other crops and judged compatible with some biocontrols in a greenhouse cucumber study [5], but western flower thrips have developed resistance to it in many regions; never rely on a single classbeing reviewed. Check registration for cannabis where you are.
What not to do in flower
- Do not spray buds with soaps, oils or anything with residue.
- Do not raise RH to help fungi in late flower.
- Do not keep stacking predators into late flower expecting a rescue; set them up in veg so the population is low before buds form.
How to confirm it worked
- Card counts fall over 2–4 weeks.
- New leaves stay unscarred. Old scars stay; judge new growth.
- Predators visible on leaves (fast, pear-shaped mites; small black-and-white bugs).
Common mistakes
- Treating only foliage and ignoring pupae in the medium.
- Releasing N. cucumeris after adults are everywhere; it does not eat adults [2].
- Relying on one spray class until resistance shows up.
- Bringing in clones without inspection.
Evidence
- Lifecycle, pupation in media and damage signs: Established [1].
- Predatory mites and Orius against western flower thrips: Established in greenhouse ornamentals and vegetables [2][3]; cannabis-specific trials sparse (Supported by extrapolation).
- Short-day diapause in some Orius: Supported in entomology literature; cannabis-specific data lackingbeing reviewed.
