Short answer. If new growth is twisted, glossy, stiff and stalled while pH and feed check out, put a 40-60x scope on the shoot tips before you touch the nutrients. Broad mites (Polyphagotarsonemus latus) are about 0.2 mm long, finish a generation in about 4.4 days at 25 °C (77 °F) and can double their numbers every 2.2 days in the lab [1]. They arrive on clones, hands and whitefly adults. Prevention, early predatory mites and repeat treatments timed to the life cycle are the whole game. Hemp russet mite is a different animal; it has its own article and a summary below.
What is a broad mite and what does it do to cannabis?
A broad mite is Polyphagotarsonemus latus, a tarsonemid ("thread-footed") mite that feeds on the youngest leaves, shoot tips and buds of hundreds of crops [2][7]. It injects a toxic saliva as it feeds, and the plant responds with twisted, hardened, distorted growth out of all proportion to the number of mites [3]. Terminal buds can die.
The damage persists long after the mites are gone [3][4]. That is why "it still looks bad" tells you nothing about whether a treatment worked; judge only new growth.
Gerson's review found no evidence that broad mites transmit plant viruses [4]. The harm is the toxin and the feeding, not a disease.
How do you recognise broad mite damage on cannabis?
Broad mite damage on cannabis shows first at the top: newest leaves narrow, cup, twist and turn glossy or glassy, leaf edges pucker downward, and internodes shorten so tips look bunched [3][2]. Lower leaf surfaces may bronze. Damage often appears suddenly and is spotty across a room, because populations start on one plant or one clone line [3].
| Sign | What you see | Source |
|---|---|---|
| Where first | Newest growth, shoot tips, flower buds | [2][3] |
| Leaf shape | Narrow, twisted, cupped; edges pucker downward; thick and brittle | [3] |
| Colour | Glossy then bronze on lower surface; leaves may turn coppery or purplish | [2][3] |
| Growth habit | Shortened internodes, extra lateral buds breaking, stalled tips | [2] |
| Mites | Females ~0.2 mm, oval, pale amber to green with a faint pale stripe; males ~0.11 mm, faster | [2] |
| Eggs | ~0.08 mm, clear, elliptical, with 29-37 white tufts in rows ("rows of diamonds") | [2][3] |
| Magnification | Hard to see below 10x; 20x workable with practice; microscope preferred | [2][3][7] |
Differentials. Broad mite injury mimics calcium or boron deficiency, herbicide drift (2,4-D, glyphosate), cold injury and physiological disorders [3][7]. On cannabis add heat or light stress at the tips, genetic leaf mutations on a single cut, and viroid or virus infection: see kb-diseases-hop-latent-viroid-and-viruses.
How is broad mite different from cyclamen mite and russet mite?
Broad mite and cyclamen mite (Phytonemus pallidus) are both tarsonemids that feed in buds and distorted tissue, but broad mite favours warm, humid conditions while cyclamen mite does best in cool, humid ones [7]. Broad mite eggs carry rows of white tufts; cyclamen mite eggs are smooth, pale ovals about half the adult's length [7]. Broad mite bodies taper and are widest between the second pair of legs; cyclamen mite sides are nearly parallel [7].
Cyclamen mite on cannabis is rarely reported; most "cyclamen mite" calls on cannabis turn out to be broad mite or hemp russet mite. See cyclamen mite.
Hemp russet mite (Aculops cannabicola) is an eriophyid: carrot-shaped, only two pairs of legs at the front, feeds only on Cannabis, and starts low and inside the plant, bronzing leaves and curling edges upward [15]. Details below and in kb-pests-hemp-russet-mite.
Numbers that matter for broad mites
| Parameter | Value | Conditions and source |
|---|---|---|
| Female length | ~0.2 mm (0.008 in) | [2] |
| Male length | ~0.11 mm | [2] |
| Egg length | ~0.08 mm | [2] |
| Egg-to-adult at 25 °C | 4.4 days (females 4.5, males 4.1) | Grape leaf discs, 65 ± 10% RH, lab [1] |
| Lower developmental threshold, egg-adult | 9.71 °C | Grape, lab [1] |
| Thermal constant, egg-adult | 62.73 degree-days | Grape, lab [1] |
| Upper developmental threshold | 33-35 °C | Citrus, Jones & Brown 1983, as cited in [1] |
| Fecundity | 44.3 eggs/female at 25 °C | Grape, lab [1] |
| Eggs per day | about 5 | Extension summary [2] |
| Sex ratio | ~4 females : 1 male (mated females); unmated females lay only males | [2] |
| r_m peak | 0.31 per day at 25 °C (doubling ~2.2 days) | Grape, lab [1] |
| Favoured climate | 21-27 °C (70-80 °F), 80-90% RH | Extension [3] |
| Action threshold | None published for cannabis | Any confirmed find is significant (Grower practice) |
What is the broad mite life cycle in numbers?
The broad mite life cycle runs egg, larva, quiescent "pupa" (often called nymph), adult, and at 25 °C (77 °F) the whole egg-to-adult run takes about 4.4 days on grape leaves [1]. The best life table with five temperatures comes from Ferreira and colleagues on grape (Vitis vinifera) leaf discs at 65 ± 10% RH and 12 h light [1]. No cannabis-specific life table has been published, so treat these as the closest lab proxy.
| Temp | Egg (d) | Larva (d) | Quiescent (d) | Egg-adult (d) | Eggs/female | Female longevity (d) | r_m (/day) | Doubling (d) | Generation time T (d) |
|---|---|---|---|---|---|---|---|---|---|
| 18 °C | 3.9 | 1.7 | 1.3 | 7.3 | 16.5 | 27.9 | 0.10 | 6.9 | 25.7 |
| 22 °C | 2.7 | 1.2 | 0.9 | 5.1 | no published value found | – | – | – | – |
| 25 °C | 2.3 | 0.9 | 0.7 | 4.4 | 44.3 | 15.8 | 0.31 | 2.2 | 10.8 |
| 28 °C | 1.6 | 0.8 | 0.5 | 3.3 | no published value found | – | – | – | – |
| 32 °C | 1.8 | 0.8 | 0.5 | 3.5 | 13.3 | 7.7 | 0.12 | 6.0 | 8.2 |
| 15 °C, 20 °C, 35 °C | no published value found |
All values from [1]; doubling time computed here as ln2 / r_m. Net reproductive rate R0 was 13.2 at 18 °C, 30.1 at 25 °C and 2.6 at 32 °C [1].
Pre-oviposition is very short. Females started laying 0.3 days after emerging at 25 °C, 1.1 days at 32 °C and 1.8 days at 18 °C [1]. Egg-laying peaked at 4.3 eggs on day 3 of adult life at 25 °C [1].
Other hosts give similar numbers. On lemon at 27.5 °C, immatures took 3.6-3.7 days; on cotton at 28.5 °C, about 4.1 days; on lime at 25 °C, 8.5 days with an r_m of 0.427 per day (as cited in [1]). Host plant clearly matters.
Adult males live 5-9 days; females lay 30-76 eggs over 8-13 days and then die (extension summary across hosts) [2]. The grape lab study found lower fecundity, a reminder that host and method shift every number [1].
How do broad mites reproduce and why does that matter for resistance?
Broad mites reproduce by arrhenotoky: unmated females lay eggs that become males, and mated females lay about four female eggs for every male egg [2]. One overlooked female can therefore found a colony by producing sons and mating with them.
Males do a strange job. Quiescent female larvae attract males, which pick them up with enlarged hind legs, carry them at right angles to their bodies to fresh foliage and mate the moment the female emerges [2]. The males' "frenzy" to carry females accounts for much of the dispersal within a plant [2].
Haplodiploidy, short generations and high r_m are the classic recipe for fast pesticide resistance: a surviving resistant female's sons carry her genes unmasked. Rotate modes of action and never rely on one product class.
Why do temperature and humidity matter so much for broad mites?
Temperature sets broad mite speed: egg-to-adult falls from 7.3 days at 18 °C to 3.3 days at 28 °C, and r_m peaks near 25 °C at 0.31 per day before dropping to 0.12 per day at 32 °C because fecundity and female lifespan collapse in the heat [1]. Above roughly 33-35 °C development stops (citrus data cited in [1]).
Degree-days. With a lower threshold of 9.71 °C and 62.73 degree-days per egg-to-adult, a room averaging 25 °C delivers 15.3 degree-days a day and a generation in about 4.1 days; at 22 °C about 5.1 days; at 18 °C about 7.6 days [1]. See degree-day and developmental threshold.
Humidity. Extension sources say 80-90% RH at 21-27 °C favours broad mites [3], and leaves "collected during humid conditions are more likely to have mites" [3]. The lab life table above was run at 65 ± 10% RH, so its r_m does not test dry rooms. Jones & Brown (1983) measured broad mite reproduction across constant temperature-humidity regimes (as cited in [1]) (egg sensitivity to low RH is widely repeated but no accessible primary value found)being reviewed.
Grow-room set-points. A veg room at 24-28 °C and 60-70% RH sits right on the broad mite optimum. A flower room at 22-26 °C and 45-55% RH is drier but still within the mite's thermal sweet spot; temperature alone will not save you. See kb-pests-temperature-humidity-and-pest-population-growth and VPD.
No diapause is known in broad mites; they are tropical and subtropical and "generally don't survive outdoors" in cold winters, so northern indoor infestations come in on plants [3].
How fast can 10 broad mites become a problem?
Ten broad mites at 25 °C can become roughly 7,400 in three weeks if nothing stops them, compared with about 80 at 18 °C and about 110 at 32 °C. The method is N(t) = N0 × e^(r_m × t), using the grape-leaf r_m values of 0.1005, 0.3149 and 0.1159 per day at 18, 25 and 32 °C [1].
| Temp | r_m (/day) | Doubling (d) | Day 14 | Day 21 | Day 28 |
|---|---|---|---|---|---|
| 18 °C | 0.1005 | 6.9 | 41 | 83 | 167 |
| 25 °C | 0.3149 | 2.2 | 822 | 7,446 | 67,493 |
| 32 °C | 0.1159 | 6.0 | 51 | 114 | 257 |
| 25 °C, lime (cited in [1]) | 0.427 | 1.6 | 3,947 | 78,400 | 1,557,488 |
This is the unchecked exponential upper bound: no predators, unlimited young tissue, constant temperature. Real populations crash when tips die. The point stands: at room temperature, a clone carrying a handful of mites can be a visibly sick plant within three weeks.
Where do broad mites hide, how do they spread and how do they get in?
Broad mites hide where tissue is youngest and most folded: shoot tips, unfolding leaves, flower buds and the undersides of new leaves [2][3]. They crowd into crevices and buds, which shelters them from contact sprays [2].
Clones and plant material are the main way in. UConn notes the species generally does not survive outdoors in the US Northeast and "may be introduced on vegetatively propagated material" [3].
Hands, clothing and tools carry them between plants; UConn advises entering infested areas last [3].
Air currents and plant contact move them within a crop [3].
Insects. Broad mites ride whiteflies (Bemisia, Trialeurodes) and the green peach aphid Myzus persicae from plant to plant (Fan & Petitt 1998; Palevsky et al. 2001, as cited in [1] and [2]). A whitefly problem is therefore also a broad mite distribution network. See kb-pests-whiteflies and kb-pests-aphids.
Within the plant, males carrying quiescent females to new foliage drive much of the spread [2].
How do you scout for broad mites?
Scout for broad mites by examining the newest leaves and shoot tips under 40-60x at least weekly, and every incoming clone before it joins the room. Extension guidance says eggs can be seen at 20x with practice and good light, but a microscope is preferred [3].
- What to sample. The youngest unfolded leaves, shoot tips and lower surfaces; leaves collected during humid conditions are more likely to carry mites [3].
- Lens. 10x is not enough for confident ID; 20x minimum; 40-60x clip-on lens or a USB microscope is the practical choice (Grower practice, consistent with [2][3]).
- Frequency. Weekly in veg and mothers; every incoming plant during quarantine (Grower practice).
- Sticky cards do not catch broad mites usefully, but yellow cards tell you about whiteflies and aphids that can carry them.
- Action threshold. No published threshold exists for cannabis. Any confirmed broad mite on a mother or clone line is an action point (Grower practice). See action threshold.
- Lab ID. Extension plant diagnostic labs will identify mites from a sample.
Which broad mite controls hit which life stage?
Contact treatments (soaps, oils, sulfur) hit mobile larvae and adults they touch, but broad mite eggs and quiescent stages hidden in buds largely escape, so every contact program needs at least two or three timed applications. UConn advises high-volume and repeat applications, and translaminar products where labelled [3]. Cloyd notes contact miticides struggle because the mites sit in meristematic tissue [14].
Timing the repeat at your temperature. The second application must land after eggs that survived the first have hatched, and before females from those eggs start laying. From the grape life table [1]:
| Room temp | Egg hatch (d) | Egg-adult (d) | Pre-oviposition (d) | First new eggs at (d) | Repeat window |
|---|---|---|---|---|---|
| 18 °C | 3.9 | 7.3 | 1.8 | ~9.1 | day 4-8 |
| 22 °C | 2.7 | 5.1 | no published value (between 0.3 and 1.8) | ~5.5-7 | day 3-5 |
| 25 °C | 2.3 | 4.4 | 0.3 | ~4.7 | day 3-4 |
| 28-32 °C | 1.6-1.8 | 3.3-3.5 | ~1.1 at 32 °C | ~4.4-4.6 | day 2-4 |
Plan three applications in that rhythm, then re-inspect. This is a calculation from lab data, not a tested cannabis protocol (Grower practice informed by [1]).
Modes of action. Miticides labelled for broad mite in greenhouse ornamentals come from several IRAC groups, including 6 (avermectins), 13 (pyrrole uncoupler), 21A (mitochondrial complex I inhibitors) and 23 (tetronic/tetramic acid derivatives) [14]. Many have no cannabis registration. Rotate by class, never repeat one class back-to-back across generations, and follow the label and local law. See mode of action.
How do predatory mites control broad mites?
Predatory mites control broad mites only when released preventively, before populations build and before the tips are damaged [3]. The species with the best broad mite record are Amblyseius swirskii, Neoseiulus cucumeris and Neoseiulus californicus; all are generalists that also eat pollen and so can be released before broad mites are found [3].
| Predator | Temperature window | Humidity | Broad mite evidence |
|---|---|---|---|
| Amblyseius swirskii | Development 11.3-37.4 °C, optimum ~31.5 °C; population growth 15.5-37 °C, optimum ~30 °C; slow below 20 °C [8] | 50% of eggs fail to hatch at ~63% RH, the most drought-sensitive of six species tested [9] | Suppressed broad mites on sweet pepper [5]; up to ~18 broad mites/day and 1.2 eggs/day on a broad-mite diet; on pepper, control lagged two weeks after release [6] |
| Neoseiulus cucumeris | Lower development threshold ~10.3-11 °C on pollen diets [11] | Needs high RH for egg hatchbeing reviewed | As effective as A. swirskii on begonia, cleared broad mites within a week [6]; used on pepper (Weintraub et al. 2003, cited in [6]) |
| Neoseiulus californicus | Prefers 10-33 °C (50-91 °F) [10] | Tolerates 40-80% RH, prefers the upper end [10] | Develops and reproduces on broad and cyclamen mites [10]; used against broad mite on pepper in Florida and Israel [6] |
| Amblyseius andersoni | Eggs did not hatch at 16 °C (one Chinese population); fecundity peaked at 25 °C (45.7 eggs/female) [12] | Low-RH sensitivebeing reviewed | Little published broad mite data; more often used against russet and spider mites |
Release rates. Published trials used 25 predators per plant twice on begonia and 60 then 100 per plant on sweet pepper [6]. For cannabis, follow the biocontrol supplier technical sheet; no cannabis-specific rate has been published.
Predator:prey and lag. On pepper, broad mite suppression took two weeks after the first A. swirskii release [6]. With prey doubling every 2.2 days at 25 °C, a two-week lag means curative releases arrive too late; this is why predators go in at transplant (Grower practice informed by [1][6]).
Compatibility. Most miticides, sulfur and oils harm predatory mites. Leave the label's re-entry interval for beneficials before releasing, and never spray sulfur on a predator program.
Trichomes. On tomato, A. swirskii established mainly on leaves where glandular trichomes had collapsed (russet mite study, kb-pests-hemp-russet-mite). Cannabis is also glandular and sticky; expect predators to work better in veg than on resinous flowers (Grower practice).
How can the environment be used against broad mites?
Use the environment to favour predators rather than to "dry out" broad mites. The predatory mites that eat broad mites need humid air for their eggs (A. swirskii 50% hatch failure at ~63% RH) [9], while broad mites are favoured by warm, humid rooms [3]. There is no setting that helps one and hurts the other cleanly.
- Veg and mothers: 24-26 °C and 65-70% RH supports A. swirskii and N. cucumeris egg hatch. It also suits broad mites, so the predators must already be there (Grower practice).
- Cooler mother rooms at 20-22 °C slow broad mites (egg-adult 5.1 days at 22 °C versus 4.4 at 25 °C) [1] but also slow A. swirskii, which grows slowly below 20 °C [8]. N. cucumeris and N. californicus cope better with cool rooms [10][11].
- Flower: drop RH to 45-55%, then 40-50% in late flower, for botrytis and powdery mildew control. That will not stop broad mites, and it shortens predator egg survival. Accept it; bud rot costs more than a flower-stage mite fight.
- Do not run veg above ~30 °C to "cook" mites. r_m does fall at 32 °C [1], but the plant suffers too and other pests (spider mites) thrive in heat.
What should you not do about broad mites in flower?
Do not spray oils, soaps or sulfur onto flowering buds; residues and wet buds invite botrytis and can fail testing. Do not reach for unregistered miticides; the residue can fail compliance testing and the use may be illegal. Follow the label and local law; registrations on cannabis vary by jurisdiction.
- Do not try to "spray through" broad mites in mid to late flower; the mites sit inside buds where sprays miss.
- Do not keep an infested mother "because she's the keeper"; every clone carries the problem.
- Do not take clones from infested plants, even "clean-looking" tips.
- Do isolate the plant, remove the worst, consider an early harvest, and restart from clean stock.
What are the common myths about broad mites?
The common broad mite myths are that a 10x loupe is enough, that twisted tips mean calcium deficiency, that clones are the only route in, and that low humidity or one hot dip will clear them.
- "A 10x loupe will show broad mites." Myth. Females are ~0.2 mm and eggs ~0.08 mm; extension calls for 20x or more and prefers a microscope [2][3].
- "Twisted new growth is calcium deficiency." Myth as a default. Broad mite injury mimics calcium and boron deficiency [3]; check pH, root zone and then the scope.
- "Broad mites only come in on clones." Myth. Clones are the main route, but hands, clothing, air and whitefly or aphid adults also carry them [1][3].
- "Hot water dips clean cuttings." Contested for cannabis. 43-49 °C for 15 minutes is documented for broad mite on plant material in other crops [2]. On hemp cuttings, 41-43 °C (106-109 °F) for 10-15 minutes reduced russet mites but did not eliminate them, with no rooting damage [13]. No cannabis broad-mite protocol exists.
- "Drop the humidity and they die." Contested. High RH favours broad mites [3], but no study shows low RH eliminates them, and low RH kills predator eggs [9].
- "Damaged leaves recover after treatment." Myth. Distortion persists after the mites are dead [3][4].
Hemp russet mite: the short version
Hemp russet mite (Aculops cannabicola, family Eriophyidae) feeds only on Cannabis and is described by Colorado State as one of the most injurious arthropods in industrial hemp [15]. It bronzes, shrinks and stiffens leaves, curls edges upward, starts low and inside the plant and moves up into buds as numbers rise [15]. It needs at least 15-20x to glimpse and about 55x to count [15] (kb-pests-hemp-russet-mite).
- Life cycle: about one week in favourable conditions [15]; no published A. cannabicola life table. The tomato russet mite (Aculops lycopersici) is the closest proxy: egg-to-adult 5.5 days and r_m 0.286 per day at 25 °C on tomato leaves.
- Spread: wind from leaf edges, clones and cuttings, people and tools [15].
- Control: clean mothers and seed starts, quarantine, removal of infested plants, oils in veg; predatory mites gave mixed results in field hemp [13].
The full life table, worked growth example, scouting method and control program are in kb-pests-hemp-russet-mite.
How do you confirm broad mite control worked?
Confirm broad mite control by watching new growth and repeating microscope checks. New leaves should emerge flat and normal-coloured within one to two weeks; old distorted leaves stay distorted [3].
- Re-inspect the same tips at 40-60x weekly for at least three weeks; no live mites, no fresh eggs.
- Three weeks covers four to five broad mite generations at 25 °C [1].
- Run the next cycle from clean stock and check again at two and four weeks.
Common mistakes
- Chasing a "calcium deficiency" with ever more Cal-Mag.
- Looking with a 10x lens and declaring the plant clean.
- Spraying once, or twice a week apart; at 25 °C the second spray lands after new eggs are already laid.
- Releasing predators after damage shows; there is a two-week lag [6].
- Ignoring whiteflies and aphids that ferry broad mites [1].
- Keeping the infested mother.
Evidence
- Broad mite biology, damage, and male carrying behaviour: Established (extension consensus and lab studies) [2][3][4].
- Temperature-dependent development and r_m: Established for grape and other hosts [1]; extrapolated to cannabis (no cannabis life table).
- Humidity preference (80-90% RH): Supported by extension [3]; egg sensitivity to low RH: Contested/unverified.
- Phoresy on whiteflies and aphids: Established (cited in [1][2]).
- Hot water treatment: Supported in other crops [2]; Grower practice/untested for cannabis broad mite.
- Preventive predatory mites: Supported in pepper and begonia [5][6]; extrapolated to cannabis.
