Short answer. Fine pale speckles on the upper leaf surface, tiny moving dots underneath, and later silk webbing: that is almost always the twospotted spider mite. A generation can finish in under a week when it is warm, so find them early, cool the room and raise humidity where you can, and release predatory mites while numbers are still low. Contact sprays belong in veg, not on flowers.
What's happening
Spider mites (Tetranychus urticae, the twospotted spider mite, is the usual one indoors) pierce leaf cells and suck out the contents. Each emptied cell shows as a pale dot (stippling). Enough dots and the leaf bronzes, yellows and dies; photosynthesis and yield fall with it [1][3].
Lifecycle. Egg, larva, two nymphal stages, adult [1]. Speed depends on temperature: roughly 40 days egg-to-adult at 13 °C (55 °F) versus about 5 days at 24 °C (75 °F) [6]being reviewed. UC IPM states a generation can finish in under a week under favourable temperature and food [1]. Hot, dry, dusty conditions favour them [1][3].
How to recognise it
- Leaf tops: fine white-yellow stippling, starting on scattered leaves, often mid-canopy or near hot spots.
- Leaf undersides: a hand lens (10–20x) shows oval mites about 0.5 mm, pale yellow-green with two dark blotches, plus round clear eggs and cast skins [1][4].
- Shake test: tap a branch over white paper; moving specks that streak when smeared are mites [3].
- Heavy populations: silk webbing at leaf tips and over colas, a "dirty" look, mites massing at the top of the web [3].
Do not confuse with: broad/russet mites (no stippling-and-web pattern, twisted or bronzed growth, mites invisible at 10x), thrips (silvery scars and black fecal specks), or nutrient spotting (no mites, no eggs). See kb-diagnosis-rust-spots.
Numbers that matter
| Parameter | Value | Notes |
|---|---|---|
| Adult size | ~0.5 mm | Visible as a dot; 10–20x lens to confirm [1] |
| Egg-to-adult | ~5 days at 24 °C (75 °F); ~40 days at 13 °C (55 °F) | being reviewed |
| Development threshold | ~11 °C (52 °F) | Pacific spider mite model; T. urticae similarbeing reviewed |
| Phytoseiulus persimilis RH need | ≥ ~60–70% RH | Immature predators fail to moult below ~70% RH [2] |
| P. persimilis heat limit | releases failed above ~32 °C (90 °F) | Greenhouse poplar trial (non-cannabis) |
| Predator feeding | 5–20 prey/day (adult P. persimilis) | [2] |
| Action threshold | none established for cannabis | Grower practice: act on first confirmed colonies |
What to check first
- Confirm the pest. Lens on leaf undersides; shake test. No mites, no eggs = not mites.
- Map the spread. Flag every infested plant. Hot spots near lamps, walls and fans are typical.
- Read the room. Leaf temperature, RH and VPD. Hot (>28 °C / 82 °F) and dry (<40% RH) rooms favour mites.
- Find the source. New clones, a nearby houseplant, a pet, your clothes after the garden.
- Check plant stress. Drought-stressed and dusty plants suffer more [3].
How to fix it
Registrations for pesticides on cannabis differ by country, state and province, and many products are not legal on cannabis at all. Always check local regulations and follow the label. This article names classes, not products or rates.
Cultural
- Quarantine new plants 1–2 weeks and inspect them before they enter the room.
- Avoid drought stress; keep foliage free of dust [3].
- Where the stage allows, keep temperatures moderate and RH toward the upper end of the safe range for the stage (in veg, 60–70% RH is often workable). Do not raise RH in late flower; bud rot costs more than mites (kb-diseases-botrytis-bud-rot).
- Remove and bag heavily infested leaves or whole plants; do not shake them through the room.
Physical
- A strong water spray on leaf undersides knocks down mites in veg (Grower practice).
- Wash hands and change clothes before moving between rooms; work clean rooms first.
- Sticky cards do not catch spider mites usefully; use a lens.
Biological
- Phytoseiulus persimilis: specialist; eats all mobile stages, 5–20 prey per day [2]. Needs RH around 70% and dies or starves above ~32 °C or when prey run out, so it often needs repeat releases [2][5]. Best on established hot spots.
- Neoseiulus californicus: generalist; tolerates warmer, drier conditions and survives on low prey numbers, so it suits preventive use (Supported)being reviewed.
- Other options: Feltiella acarisuga (predatory midge) and Stethorus punctillum (mite-eating lady beetle) are sold for hot spotsbeing reviewed.
- Release early, when mites are first seen [4]. Confirm rates with the supplier. Avoid residual pesticides before and after release.
Least-toxic options
- Insecticidal soaps (potassium salts of fatty acids): contact only; must hit the mites on leaf undersides; repeat needed because eggs survive [3].
- Horticultural oils: smother eggs and mites; avoid in heat, under lights, and on stressed plants.
- Entomopathogenic fungi (e.g. Isaria fumosorosea): listed in hemp extension notes [3]; need humidity to work.
- Sulfur: has miticidal activity, but it harms predatory mites and can burn foliage if mixed with or applied near oils. Keep it out of programmes that rely on beneficials.
What not to do in flower
- Do not spray soaps, oils or sulfur onto developing buds; residues end up in the product, and wet buds invite botrytis.
- Do not use systemic or residual miticides; most are not registered for cannabis and residues fail compliance testing in regulated markets.
- Do not jack RH in late flower to slow mites.
- In flower, rely on predators, removal of infested leaves, and accepting some damage; if the infestation is severe in late flower, harvest early rather than spray.
How to confirm it worked
- Re-check flagged plants with a lens every 3–4 days: fewer live mites and eggs, predators present (P. persimilis is bright orange-red, faster, no dark blotches [2]).
- New leaves emerge without stippling. Old stippling does not heal; judge by new growth.
- Two clean checks a week apart before you relax.
Common mistakes
- Waiting for webbing. By then the population is large.
- Treating only the visible plants. Mites walk and ride air currents.
- Releasing P. persimilis into a hot, dry room and wondering where they went.
- Rotating through sprays with no change to climate or hygiene.
- Mistaking broad or russet mite damage for spider mites and treating the wrong pest.
Evidence
- Lifecycle, damage and stippling/webbing signs: Established [1][3].
- Temperature dependence of development: Established in general entomology; the exact day counts here are from a non-cannabis extension source [6].
- Phytoseiulus persimilis as a top biocontrol agent and its humidity needs: Established in greenhouse crops [2][5]; cannabis-specific trials are few, so this is extrapolated (Supported).
- Cannabis action thresholds: none published; first-sighting action is Grower practice.
