Short answer. If new growth is twisted, glossy and stunted (broad mite), or leaves bronze, go brittle and curl their edges upward from the bottom of the plant (russet mite), and your pH and feed check out, get a 30–60x loupe or microscope on the leaves. Both mites are near invisible, both spread on hands, clones and air, and once they are in flowering plants your options are mostly removal and harvest timing. Prevention is the real treatment.
What's happening
Broad mite (Polyphagotarsonemus latus, family Tarsonemidae) feeds on the youngest tissue and injects a toxin-like saliva that twists, hardens and distorts leaves; terminal buds can die, and the damage can persist after the mites are gone [1]. It attacks many crops.
Hemp russet mite (Aculops cannabicola, family Eriophyidae) feeds only on Cannabis [3]. Feeding bronzes ("russets") leaves, shrinks them, makes them brittle and curls edges upward [5]. CSU calls it one of the most injurious arthropods in hemp [4]. In heavy infestations mites move into flowers. Eriophyid mites can also transmit plant viruses [7].
Lifecycle. - Broad mite: females lay 30–75 eggs over 8–13 days; eggs hatch in 2–3 days; egg-to-adult can take about one week. Warm (21–27 °C / 70–80 °F) and humid (80–90% RH) conditions favour it [1]. - Russet mite: the whole cycle can finish in about a week in warm, humid conditions; outdoors it overwinters in crop debris, indoors it persists year-round on a steady food supply [4].
How to recognise it
| Sign | Broad mite | Hemp russet mite |
|---|---|---|
| Where first | Newest growth, shoot tips | Lower and inner leaves, moving up |
| Leaf shape | Twisted, cupped, leaf edges pucker downward, thick and brittle [1] | Edges curl upward, leaves smaller, brittle [5] |
| Colour | Glossy, then bronze on lower surface [1] | Bronze/russet, dull, later yellow |
| In flower | Distorted bud leaves, stalled buds | Mites on pistils and bracts, dusty look, poor resinbeing reviewed |
| Mite visibility | Not visible at 10x; 20x+ with good light; microscope preferred [1] | Not visible at 10x; 30x+being reviewed |
| Key ID | Pale, white stripe on back; eggs covered in whitish bumps like rows of diamonds [1][2] | Carrot-shaped, two pairs of legs at the front end [3][4] |
Differentials: broad-mite damage mimics calcium or boron deficiency, herbicide injury and cold injury [1]. Russet damage mimics magnesium or potassium problems, heat stress and spider mites without webbing. Viroid/virus infections (stunting, brittle stems) can also overlap: see kb-diseases-hop-latent-viroid-and-viruses.
Numbers that matter
| Parameter | Value | Notes |
|---|---|---|
| Broad mite size | ~0.2 mm | being reviewed |
| Russet mite size | ~0.17–0.2 mm | being reviewed |
| Magnification to see | 20x minimum; 40–60x or microscope better | [1] |
| Broad mite egg-to-adult | ~1 week | [1] |
| Broad mite favoured climate | 21–27 °C (70–80 °F), 80–90% RH | [1] |
| Russet mite lifecycle | ~1 week, warm and humid | [4] |
| Action threshold | none established | Any confirmed find is significant (Grower practice) |
What to check first
- pH and root zone. Twisted new growth is also what calcium uptake failure looks like. Check input and runoff pH/EC and root health first (kb-diagnosis-yellow-new-growth).
- Look properly. 40–60x clip-on lens or a cheap USB microscope on several affected leaves, both surfaces, and petioles. Check the newest growth for broad mite, mid and lower leaves for russet.
- Map affected plants and their history. One clone source? One mother? Common in both.
- Ask a lab. Many extension plant diagnostic labs will ID mites from a sample.
How to fix it
Registrations for pesticides on cannabis differ by jurisdiction; many are not legal on cannabis. Check local regulations and follow the label. Little research exists on effective chemical or biological controls for russet mite [6].
Cultural
- Clean stock is the main defence. Bring in seeds where possible, or clones only from a trusted source, and quarantine all new plants 2+ weeks with microscope checks.
- Keep mother plants under the strictest watch; they carry mites into every cycle.
- Remove and destroy infested plants; CSU recommends removing infested plants in the field [4]. Indoors, bag them before carrying them out.
- Between cycles: remove all plant debris, clean surfaces, and leave the room plant-free for a period (Grower practice)being reviewed.
Physical
- Work clean rooms before infested ones; change gloves and clothing. Broad mites ride on hands, clothing and even whitefly adults [1].
- Hot-water treatment of cuttings has been studied against mites in other crops (risk of killing the cutting)being reviewed.
Biological
- Preventive predatory mites: Neoseiulus cucumeris, Neoseiulus californicus and Amblyseius swirskii are used against broad mite; they must go in before populations build [1]. These generalists also eat pollen and thrips larvae. Confirm rates with the supplier.
- Russet mite: predatory mites such as Amblyseius andersoni and Neoseiulus fallacis are used by growers (Grower practice)being reviewed. A Kentucky field report found the fungus Hirsutella thompsonii killing large numbers of russet mites and suggested it as a possible tool (early research).
Least-toxic options
- Horticultural oils (highly refined mineral and seed oils) have been used against hemp russet mite [3].
- Insecticidal soaps (potassium salts of fatty acids): contact only; repeat coverage needed. None of these work systemically, so thorough coverage and repeats are needed [8].
- Sulfur: active against eriophyid mites in other crops; harms predatory mites; never combine with or apply near oil sprays.
What not to do in flower
- Do not spray oils, soaps or sulfur on buds.
- Do not reach for unregistered miticides; residues fail testing and the use may be illegal.
- Do not try to "spray through" a russet infestation in mid-late flower; isolate the plants, remove the worst, and consider an early harvest.
- Do not take clones from an infested plant.
How to confirm it worked
- New growth emerges flat and normal coloured. Distorted leaves stay distorted; judge only new growth.
- Repeat microscope checks weekly for at least 3 weeks; no live mites or eggs.
- Run the next cycle from clean stock and check at 2 and 4 weeks.
Common mistakes
- Chasing a "calcium deficiency" with ever more Cal-Mag.
- Looking with a 10x lens and declaring the plant clean.
- Keeping the infested mother "because she's the keeper".
- Spraying once and assuming eggs died too.
Evidence
- Broad mite biology and damage: Established (greenhouse extension consensus) [1][2].
- Hemp russet mite host specificity, damage and short lifecycle: Established in extension literature [3][4][5].
- Chemical and biological control efficacy for russet mite: Contested/limited; little research [6].
- Preventive predatory mites for broad mite: Supported in other greenhouse crops [1]; extrapolated to cannabis.
