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, Tetranychus urticae. In lab life tables its population roughly doubles every 2.3 days at 30 °C (86 °F) and every 4.3 days at 20 °C (68 °F) [9], so a warm room turns a handful of mites into thousands in three weeks. Find them early, take the heat and the driest air out of veg, release predatory mites while numbers are low, and time any contact spray to egg hatch. Sprays stay out of flower.
Spider mite numbers at a glance
| Parameter (T. urticae) | Value | Conditions and source |
|---|---|---|
| Adult female size | ~0.4-0.5 mm | Visible as a dot; confirm at 10-20x [4][5] |
| Egg-to-adult, 15.5 °C | 25.8-29.0 days (females) | Cotton cotyledons, lab [7] |
| Egg-to-adult, 20 °C | 15.5 days (females) | Apple leaves, lab [9] |
| Egg-to-adult, 23.8 °C | ~10.5-10.7 days | Cotton, lab [7] |
| Egg-to-adult, 29.4 °C | 6.1-6.7 days | Cotton, lab [7] |
| Egg-to-adult, 35 °C | 6.5 days (females) | Apple, lab [9] |
| Egg-to-adult, 37 °C | 5.3 days | Eggplant, lab [10] |
| Lower developmental threshold | 8.4-13.8 °C (47-57 °F) | Population and host vary [9][10][11] |
| Degree-days, egg-to-adult | 80.5-172 DD | Eggplant 80.5 [10]; peach 136 [11]; apple 172 [9] |
| Upper threshold | No published value found | Development still completes at 37 °C [10] |
| Lifetime eggs per female | 64-141 (8-40 on peach) | 103 at 23.8 °C, 64 at 29.4 °C (cotton) [7]; 141 at 27 °C (eggplant) [10]; peach [11] |
| Eggs per female per day | ~2-5 | 4.6 at 25 °C, apple [9]; 2-3 on peach [11] |
| Female adult longevity | ~4-15 days in lab | 14.7 d at 23.8 °C, 9.7 d at 29.4 °C (cotton) [7]; 3.6-12.9 d (peach) [11] |
| Sex ratio | ~63-88% female | Peach [11]; about 3 females per male [4] |
| r_m (per day) | 0.08 (15 °C) to ~0.30-0.32 (30 °C) | [8][9] |
| Doubling time | ~8.7 d at 15 °C; ~2.2-2.3 d at 30 °C | ln2 / r_m, computed |
| Cannabis action threshold | None published | Act on first confirmed colony (Grower practice) [3][5] |
No life table for T. urticae on Cannabis sativa has been published that we could find. Every number above is from another host plant in the lab, and host plant changes the numbers a lot (see the peach-leaf caveat below).
What are spider mites doing to a cannabis plant?
Twospotted spider mites pierce cannabis leaf cells from the underside and suck out the contents, leaving a pale dot (stippling) on the upper surface for each emptied cell [1][3]. Thousands of dots bronze and yellow the leaf, photosynthesis falls, and leaves dry and drop [3][5][6].
Heavy infestations reduce flower growth, and webbing over colas is cosmetic damage that buyers notice [5][6]. Utah State rates the twospotted spider mite a high threat to indoor hemp and a moderate one outdoors [3]; Colorado State calls it a significant indoor pest that is rarely a problem in field hemp [6].
How do you recognise spider mites on cannabis?
Spider mites on cannabis show first as fine white-yellow stippling on the upper leaf surface, with mites, round eggs and cast skins on the underside [1][4]. Webbing comes later, when numbers are already large.
- Leaf tops: scattered pale stippling, often first near lamps, walls, fans and other hot, dry spots [4].
- Leaf undersides: at 10-20x, oval mites about 0.5 mm, yellow-green with two dark side blotches; round, clear to pale eggs [4][5].
- Shake test: tap a branch over white paper; moving specks that streak when smeared are mites [3][6].
- Heavy populations: silk at leaf tips and over colas, a dusty, dirty look, and mites massing at the top of the web [3][6].
- Orange-red females: in cool, short-day conditions, adult females may turn bright orange-red and stop feeding (diapause) [4][5].
Do not confuse with: broad and russet mites (twisted or bronzed growth, mites invisible at 10x; see broad and russet mites and hemp russet mite), thrips (silvery scars and black fecal specks; see thrips), or nutrient spotting (no mites, no eggs; see rust spots). For green versus red spider mites, see two-spotted vs carmine mite.
What is the spider mite life cycle in numbers?
The twospotted spider mite passes through egg, six-legged larva, two eight-legged nymphal stages (protonymph and deutonymph) with resting stages between, and adult [1][5]. Egg-to-adult takes about 26 to 29 days at 15.5 °C and 6 to 7 days at 29.4 °C on cotton in the lab [7].
Development time by temperature
| Temperature | Egg-to-adult (days) | Host, lab | Source |
|---|---|---|---|
| 15.5 °C (60 °F) | 25.8-29.0 (females) | Cotton | [7] |
| 17 °C (63 °F) | 25.8 | Eggplant | [10] |
| 17 °C (63 °F) | 30.3 (females) | Peach | [11] |
| 20 °C (68 °F) | 15.5 (females) | Apple | [9] |
| 23.8 °C (75 °F) | ~10.5-10.7 | Cotton | [7] |
| 25 °C (77 °F) | ~10.4 (computed from 172.41 DD above 8.43 °C) | Apple | [9] |
| 29.4 °C (85 °F) | 6.1-6.7 (females) | Cotton | [7] |
| 35 °C (95 °F) | 6.5 (females) | Apple | [9] |
| 37 °C (99 °F) | 5.3 | Eggplant | [10] |
Males develop slightly faster than females [7][9]. On cotton, temperature, not host, dominated development time [7].
Thresholds and degree-days
The lower developmental threshold for twospotted spider mite ranges from about 8.4 to 13.8 °C depending on population and host [9][10][11]. Kasap estimated 10.78 °C and 57.8 degree-days (DD) for the egg, and 8.43 °C and 172.41 DD for egg-to-adult, on apple [9]. Kim and colleagues found 12.5 °C and 80.5 DD for females on eggplant [10]. Riahi and colleagues found 13.79 °C and 136.43 DD on peach, and no eggs hatched at 13 °C [11].
No published upper developmental threshold was found for T. urticae. Development still completed at 35 °C on apple [9] and 37 °C on eggplant [10], although egg hatch was poor at 37 °C [10].
Reproduction and life-table parameters
| Temperature | r_m (/day) | R0 (daughters/female) | Mean generation time T (days) | Host, source |
|---|---|---|---|---|
| 15 °C | 0.080 | - | - | Lab, [8] (host not stated in abstract) |
| 20 °C | 0.161 | 67.0 | 26.0 | Apple, [9] |
| 23.8 °C | 0.219 | 74.8 | 19.7 | Cotton, [7] |
| 25 °C | 0.231-0.243 | 92.2 | - | Apple cultivars, [9] |
| 25 °C | 0.144 | 16.9 | 19.3 | Peach, [11] |
| 27 °C | 0.213 | 12.1 | 11.7 | Peach, [11] |
| 29.4 °C | ~0.29 | - | - | Cotton, [7] (0.293-0.295 across three species) |
| 30 °C | 0.302 | 84.3 | - | Apple, [9] |
| 30 °C | 0.321 | - | - | Lab, [8] |
| 35 °C | - | 12.0 | 9.9 | Apple, [9] |
Doubling time is ln2 / r_m: about 8.7 days at 15 °C, 4.3 days at 20 °C, 3.2 days at 23.8 °C, 2.9 days at 25 °C (apple) and 2.2-2.3 days at 30 °C.
One study on eggplant reported r_m rising to 0.565 per day at 37 °C [10]. Treat that as an outlier tied to fast development on that host; on apple, net reproduction at 35 °C collapsed to 12 daughters per female [9].
The peach-leaf caveat. On peach leaves, the same species laid only 40 eggs per female at 25 °C and 8 at 30 °C, and r_m peaked at 0.213 at 27 °C [11]. Host plant quality can halve or triple the numbers. Cannabis has not been measured, so use the apple and cotton values as a working range and assume the worse case.
Fecundity, longevity and sex ratio
Twospotted spider mite females laid 103 eggs each at 23.8 °C and 64 at 29.4 °C on cotton [7], and 141 at 27 °C on eggplant [10]. Extension sheets quote up to about 100-200 eggs per female [4][5].
Female adults lived 14.7 days at 23.8 °C and 9.7 days at 29.4 °C on cotton [7]. UConn gives about one month for a female in greenhouse conditions [4].
About 63-88% of offspring were female on peach [11], and UConn describes females as three times more abundant than males [4]. The first eggs a female lays are mostly males; later eggs are mostly females [11].
Why do temperature and humidity matter so much for spider mites?
Temperature sets the speed of the twospotted spider mite: r_m roughly doubles from about 0.16 per day at 20 °C to about 0.30 per day at 30 °C on apple [9]. Dry air adds to it: spider mites develop faster at low humidity [12][13], while their main predators need humid air.
Heat
Between the lower threshold (about 8-14 °C) and about 30 °C, every degree speeds twospotted spider mite development [7][9]. Above about 33-35 °C, net reproduction falls: 12 daughters per female at 35 °C versus 84 at 30 °C on apple [9], and 8 eggs per female at 30 °C on peach [11].
A cannabis room never gets hot enough to control mites without cooking the crop. Heat above about 30 °C hurts the plant and the predators before it hurts the mite.
Humidity
Low humidity favours twospotted spider mites. Boudreaux showed in 1958 that relative humidity affects spider mite egg-laying, hatching and survival [13], and later work confirmed spider mites develop faster at low than high humidity [12].
In greenhouse cucumber, spider mite densities were up to three times higher at about 55% RH than at about 80% RH [12]. UConn notes egg-to-adult in as little as 7 days above 29 °C (85 °F) at 30-50% RH [4].
High humidity favours the predators. Phytoseiulus persimilis eggs held at 27 °C hatched 7.5% at 40% RH versus 99.7% at 80% RH [14]. The humidity at which 50% of eggs die (LH50) is about 66-70% for both P. persimilis and Neoseiulus californicus [12].
The surprise: in the same greenhouse trials, both predators still controlled spider mites at an average of about 55% RH, probably because humidity rose at night and the leaf boundary layer is more humid than room air [12]. Short humid periods compensate for dry ones [12].
Diapause
Diapause is a hormonally controlled resting state. In twospotted spider mites, short days, cool temperatures and declining food make adult females stop feeding and laying, turn bright orange-red and hide in cracks [4][5]. NC State gives fewer than 10 hours of light or cooler temperatures as triggers [5].
On peach leaves at 12 h light, immature mites reared at 17 and 20 °C became winter-type (diapausing) females that laid no eggs; at 25 °C and above, none did [11]. One strain had a critical photoperiod of about 13.5 h at 18 °C [23].
That matters for cannabis. A 12/12 flower room is a short day for a spider mite. In a warm flower room (24-26 °C lights on), diapause is unlikely [11]. In a cool room or a cool dark period near 17-20 °C, some females may diapause, stop laying, and survive the clean-out in cracks and under trays (Supported, inferred from lab data, not tested on cannabis).
Diapause ability varies between populations and can be lost in tetranychid mites kept without winter [22]. Greenhouse and indoor populations may barely diapause at all; nobody has surveyed cannabis-room populations.
What the set-points mean in practice
| Room state | Mite r_m (approx., lab) | Predator outlook | Note |
|---|---|---|---|
| Veg, 24-28 °C, 60-70% RH | ~0.22-0.30 /day | P. persimilis and N. californicus both workable | Best window for biocontrol |
| Flower, 22-26 °C, 45-55% RH | ~0.20-0.25 /day | P. persimilis eggs at risk; N. californicus better | Night RH helps the predator |
| Late flower, 40-50% RH | ~0.20-0.25 /day | Predators struggle; dry air favours mites | Bud rot outranks mites; do not raise RH |
| Hot room, >30 °C, <45% RH | ~0.30 /day | P. persimilis falls behind | Worst case |
r_m values are read from the apple and cotton tables above [7][9]. See VPD explained and humidity by stage.
How fast can a spider mite population grow? A worked example
Ten female twospotted spider mites growing without limits for 21 days become about 290 females at 20 °C and about 5,700 at 30 °C, using r_m of 0.161 and 0.302 per day from apple-leaf life tables [9]. Warm rooms do not add mites; they multiply them.
Method. Unchecked exponential growth: N(t) = N0 × e^(r_m × t). N0 = 10 adult females, t in days. r_m counts female offspring per female per day, so N is females; add about a third more for males.
| Temperature (r_m, source) | Day 7 | Day 14 | Day 21 | Day 28 | Doubling time |
|---|---|---|---|---|---|
| 15 °C (0.080, [8]) | 18 | 31 | 54 | 94 | 8.7 d |
| 20 °C (0.161, [9]) | 31 | 95 | 294 | 907 | 4.3 d |
| 23.8 °C (0.219, [7]) | 46 | 215 | 994 | 4,604 | 3.2 d |
| 25 °C (0.243, [9]) | 55 | 300 | 1,645 | 9,014 | 2.9 d |
| 30 °C (0.302, [9]) | 83 | 686 | 5,679 | 47,032 | 2.3 d |
| 30 °C (0.321, [8]) | 95 | 895 | 8,464 | 80,064 | 2.2 d |
| 30 °C on peach (0.108, [11]) | 21 | 45 | 97 | 206 | 6.4 d |
Reading it. At 30 °C, three weeks gives about 19 times more mites than at 20 °C (5,679 vs 294). One week at 30 °C multiplies the population about 8-fold; at 20 °C about 3-fold.
Caveats. This is the unchecked upper bound: no predators, no sprays, unlimited leaf, constant temperature. Real populations crash when leaves are exhausted. The peach row shows how much host plant matters [11]. No cannabis r_m has been published.
Where do spider mites hide, how do they spread and how do they get in?
Twospotted spider mites get into cannabis rooms mainly on infested clones and cuttings, then spread on hands, clothing and tools, along touching leaves, and through the air [4][5][20][21]. They hide on leaf undersides, under webbing, and as diapausing females in cracks [4].
- Clones and cuttings: the main door. Quarantine and inspect every incoming plant [4] (Grower practice; established in greenhouse IPM).
- People: mites ride on workers and clothing; scout and work infested areas at the end of the day [4].
- Weeds and outdoor plants: weeds are a source; control them inside and around the facility [4][6]. The species feeds on hundreds of plants [5].
- Dust: dust on webbing makes refuges from predators; field hemp infestations are often worse near dusty drive paths [5][1].
- Walking: mites walk between plants whose leaves touch [4].
- Air: mated females climb to the top of the plant and take off in wind in a "take-off posture", forelegs raised [21]. Lower leaf surfaces facing down are favoured take-off sites [21].
- Silk balls: under food shortage and high density, mites gather at the plant apex and form collective silk balls (141 to 2,162 mites, mean about 794; 98.9% immatures) that can be carried by wind or animals [20].
- Hiding places: leaf undersides near the midrib, under webs, in cracks, under trays and in debris (diapausing females) [4][6].
Indoors, mites can stay active year-round [6]. Outdoors, females overwinter in leaf litter and under bark [6][1].
How should I scout for spider mites?
Scout cannabis for twospotted spider mites at least weekly by turning leaves over with a 10-20x hand lens, starting in hot, dry spots and on plants that showed mites before [4][5]. Sticky cards are useless for spider mites; use a lens and a shake test.
- Frequency: weekly random inspections are the extension baseline [4]; twice weekly in warm rooms is sensible (Grower practice). At 30 °C a population can double in about 2.3 days [9].
- Where: hot spots near lamps, walls and fans; plants not overhead-watered; susceptible cultivars [4]. Check mid-canopy and lower leaves where stippling starts.
- Lens: 10x minimum [5]; 14x was used in strawberry scouting [15]; 10-20x shows all stages [4]. Use 40-60x (a clip-on scope) to tell spider mite eggs from predator eggs, which are larger and oval [4][2].
- Shake test: tap stems over white paper or a cloth tray and count what falls [3][6].
- Indicator plants: tag infested plants and re-check them to judge control [4].
- Sampling scale: strawberry growers in Florida sample 100 leaflets weekly per field block [15]; in a room, aim for at least 10-20 leaves per plant group (Grower practice).
- Records: note leaf counts, location and room temperature, so you can see the curve bend.
Action thresholds. No action threshold has been published for spider mites on hemp or cannabis [5]. USU advises acting at the first sign of damage or when low numbers are found [3]. In strawberry, the release rate is scaled to the share of leaflets infested (1-5% low, 6-10% moderate) [15]. In cannabis, treat first confirmed colonies (Grower practice).
How do you control spider mites by life stage?
Twospotted spider mite eggs survive most contact sprays, so any soap or oil programme must re-spray after the surviving eggs hatch and before the new larvae mature and lay [4][6]. That interval depends on room temperature.
Which tactic hits which stage
| Stage | Contact soap | Horticultural oil | Mite growth inhibitors (IRAC 10A/10B) | Predatory mites |
|---|---|---|---|---|
| Egg | Poor | Some (smothering) | Yes (ovicidal/larvicidal classes) | P. persimilis eats eggs |
| Larva, nymph | Yes, on contact | Yes | Yes | Yes |
| Adult | Yes, on contact | Yes | Weak on adults | Yes |
| Diapausing female in cracks | No | No | No | Soil predators near walls (Grower practice) [4] |
Soaps and oils need full coverage of leaf undersides, high volume and repeat applications because they work only on contact and do little to eggs [5][6].
Timing the re-spray to your temperature
Using Kasap's apple-leaf thermal constants [9] (egg: 57.8 DD above 10.78 °C; egg-to-adult: 172.41 DD above 8.43 °C) and a pre-oviposition period of about 1.3-1.6 days [11]:
| Room temperature | Egg hatch (days) | Egg to adult (days) | Re-spray interval |
|---|---|---|---|
| 20 °C (68 °F) | ~6.3 | ~14.9 | every 6-7 days |
| 24 °C (75 °F) | ~4.4 | ~11.1 | every 4-5 days |
| 25 °C (77 °F) | ~4.1 | ~10.4 | every 4 days |
| 28 °C (82 °F) | ~3.4 | ~8.8 | every 3-4 days |
| 30 °C (86 °F) | ~3.0 | ~8.0 | every 3 days |
The logic. Spray 1 kills mobiles. Eggs laid just before spray 1 hatch within the egg period; spray 2 at that interval kills the hatchlings before they become adults. Spray 3 catches survivors that coverage missed. On a faster host (eggplant, 80.5 DD above 12.5 °C [10]) egg-to-adult at 25 °C is only about 6.4 days, so err short.
Three applications at the egg-hatch interval is the usual pattern (Grower practice, consistent with extension advice to repeat [1][3][6]). Always follow the label and local law.
Resistance
Twospotted spider mites evolve resistance to acaricides fast because of a short life cycle, abundant progeny and arrhenotoky [18]. Arrhenotoky is a form of haplodiploidy: unfertilised eggs become haploid males, fertilised eggs become females [4][7].
Haploid males carry one copy of each gene, so a resistance mutation is expressed and selected immediately [4]. The genome also shows expanded families of detoxification genes linked to polyphagy and pesticide tolerance [19].
Repeated miticide use selects resistant populations [6]. Mites often flare after broad-spectrum insecticides that kill predators [1].
Rotate by class. Acaricide modes of action in the IRAC scheme include groups 6, 10A, 10B, 12, 13, 20, 21A, 23 and 25; sulfur is in group UN [24]. Never use the same group on consecutive mite generations. Very few of these are registered on cannabis anywhere; registrations differ by country, state and province. Follow the label and local law.
Which biological controls work against spider mites?
Phytoseiulus persimilis, a specialist predatory mite, is the main biological control for twospotted spider mite hot spots, with Neoseiulus californicus as the slower, hardier preventive partner [2][4][12]. Release early, when mites are first seen [4].
Predator windows
| Predator | Type | Temperature | RH | Notes |
|---|---|---|---|---|
| Phytoseiulus persimilis | Specialist mite | Optimum above 20 °C; falls behind above ~30 °C (86 °F) [4] | Eggs shrivel below ~60%; optimum >75% [4] | Egg-to-adult 25 d at 15 °C, 5 d at 30 °C [14]; eats 5-20 prey/day [2]; no diapause [4] |
| Neoseiulus californicus | Generalist mite | Active 8-35 °C (46-95 °F) [4]; develops 10-33 °C [15] | 40-80% [4] | ~3 eggs/day, 10-12 d life cycle at 25-27 °C [15]; survives on pollen and other mites [4][15] |
| Neoseiulus fallacis | Generalist mite | 9-29 °C (48-85 °F) [4] | >50%, dense canopy [4] | Also eats cyclamen mites [4] |
| Amblyseius andersoni | Generalist mite | 6-40 °C (42-104 °F) [4] | No value given | Cool-tolerant; also eats broad and eriophyid mites [4] |
| Galendromus occidentalis | Mite | 10-46 °C (50-115 °F) [4] | 30-60% [4] | Drier, hotter rooms [4] |
| Feltiella acarisuga | Predatory midge | Egg-to-adult 10 d at 27 °C, 34 d at 15 °C [4] | 60-95%; optimum 80% [4] | Larvae feed; adults fly to hot spots [4] |
| Stethorus punctillum | Lady beetle | 16-32 °C (61-90 °F) [4] | No value given | Flies to colonies; use with predatory mites [4] |
Ranges are from extension fact sheets for greenhouse crops, not cannabis trials.
Phytoseiulus persimilis: strengths and limits
P. persimilis feeds on all twospotted spider mite stages and cannot survive on anything else, so it starves or leaves when prey run out [2][4]. Expect to re-release when mites return [2].
Its development is faster than the spider mite's at most temperatures: about 5 days at 30 °C, 9 days at 20 °C and 25 days at 15 °C [4][14]. Cornell reports it reproduces faster than spider mites above 28 °C [14], yet UConn reports it cannot keep up above 30 °C (86 °F) [4]. The likely reason is humidity: hot rooms are usually dry rooms, and dry air kills its eggs [4][14]. Treat 28-30 °C as the edge.
It spreads best where leaves of neighbouring plants touch; dispersal drops by about 70% when plants barely touch [14][2]. Tag hot spots and release onto them [4].
Release rates and ratios
- Ratio: UC IPM gives about one predator per 10 spider mites as a guideline [1]. An initial ratio of about 1:10 N. californicus to twospotted spider mites gave season-long control in Florida strawberry [16].
- Combined releases: on greenhouse cut roses, simultaneous releases of about 26 P. persimilis and 24 N. californicus per m² controlled an infestation of 23.9 mites per leaf through the season; either species alone failed [17].
- Per plant: strawberry guidance is one N. californicus per plant for low to moderate infestations, avoiding releases below 7.2 °C (45 °F) or above 29.5 °C (85 °F) [15].
- Per area: biocontrol supplier technical sheets typically give about 5-25 P. persimilis per m² (0.5-2 per ft²) for light to heavy infestations, more on hot spots. Confirm rates with the supplier [4].
- Establishment: colonies should shrink within 2-3 weeks of a P. persimilis release [4]. Look for shrivelled, sucked-dry spider mites [4].
No release rate has been published for cannabis. These are greenhouse-crop figures.
Compatibility with sprays
Sulfur harms predatory mites; keep it out of programmes that rely on them (Supported). Avoid broad-spectrum and persistent pesticides before and after release [2]. Check supplier side-effect databases for any product you plan to use [4].
How can the grow-room environment be used against spider mites?
Use the environment to slow twospotted spider mites and help predators in veg: keep leaf temperature below about 28 °C (82 °F) and RH at 60-70%, with humid nights [12][4]. In flower, bud-rot risk caps humidity, so lean on predators that tolerate drier air.
- Veg (24-28 °C, 60-70% RH): the best biocontrol window. Release P. persimilis on hot spots and N. californicus preventively. Moderate the hottest corners, because r_m at 25 °C is still about 0.23-0.24 on apple [9].
- Kill the hot spots: spider mites start near lamps, walls and stagnant corners [4]. Even air mixing (see airflow) removes the hot, dry microclimates that breed them.
- Night humidity: short humid periods compensate predator eggs for dry days [12]. A slightly higher night RH in veg is a cheap advantage.
- Water and dust: water-stressed plants are more susceptible, and dust triggers outbreaks [1][5]. Water properly and keep surfaces clean.
- Flower (22-26 °C, 45-55% RH): N. californicus copes better than P. persimilis with drier air [4][12]. Do not chase predator humidity at the expense of bud rot (botrytis, powdery mildew).
- Late flower (40-50% RH): accept that conditions favour the mite. Your defence is a clean start, not climate.
- Cool flower rooms: dark periods near 17-20 °C at 12/12 may induce diapause in some females [11], which then hide and survive between runs. Clean cracks and trays between cycles.
- Between runs: a fully emptied, cleaned room with no green plant material breaks the cycle (Grower practice).
What should you not do about spider mites in flower?
Do not spray soaps, oils, sulfur or miticides onto developing cannabis buds: residues end up in the product, wet buds invite botrytis, and most miticides are not registered on cannabis [1][3]. In flower, rely on predators, leaf removal and acceptance of some damage.
- No residual or systemic miticides in flower; residues fail compliance testing in regulated markets.
- No oils above about 32 °C (90 °F) or on water-stressed plants; no sulfur within 30 days of an oil [1].
- Do not raise RH in late flower to slow mites; bud rot costs more (kb-diseases-botrytis-bud-rot).
- Do not shake or move heavily infested plants through clean rooms. Bag them in place.
- Do not release P. persimilis into a hot, dry late-flower room and expect miracles [4].
- If a late-flower infestation is severe, harvest early rather than spray (Grower practice).
What are the common myths about spider mites?
The most common spider mite myth is that humidity alone will control them; it only slows them, by up to about three times on cucumber [12].
- "Spider mites cannot survive high humidity." Myth. High RH slows them (up to 3 times lower densities at about 80% vs 55% RH [12]) but does not stop them.
- "A weekly spray matches the life cycle." Myth. Egg hatch takes about 3 days at 30 °C but about 6 days at 20 °C [9]. Time sprays to temperature.
- "Red or orange mites are predators." Myth. Diapausing females and the red form of T. urticae are also orange-red [4][22]. Predators have no dark side spots and run fast.
- "Spider mites love heat." Established, with a ceiling: r_m roughly doubles from 20 to 30 °C; net reproduction falls above about 33-35 °C [9][11].
- "P. persimilis works in any room." Myth. Its eggs fail in dry air [14] and it falls behind above about 30 °C [4].
- "Neem or one heavy spray wipes them out." Myth. Contact products miss eggs; plan three timed applications [5][6].
- "Mites come from nowhere." Myth. They arrive on clones, people, weeds and air [4][21].
Common mistakes
- Waiting for webbing; by then the population is large [3].
- Spraying on a fixed weekly calendar instead of the egg-hatch interval for your temperature.
- Treating only the plants with visible damage; mites walk and fly [4][21].
- Releasing P. persimilis into a hot, dry room, then blaming the supplier [4].
- Rotating products within the same IRAC group [24].
- Mistaking orange diapausing spider mites for predators, or the reverse [4].
- Mistaking broad or russet mite damage for spider mites (kb-pests-broad-russet-mites).
Evidence
- Life cycle, stippling and webbing: Established [1][3][4][5].
- Temperature-dependent development and r_m: Established in multiple lab life tables on cotton, apple, eggplant and peach [7][8][9][10][11]; not measured on cannabis.
- Low humidity favouring spider mites: Established [12][13].
- P. persimilis humidity needs and heat limit: Established in greenhouse crops [2][4][14]; the exact heat cut-off conflicts between sources.
- Diapause at 12/12 and cool temperatures in cannabis rooms: Supported, inferred from lab data [11][23].
- Re-spray intervals: computed from thermal constants [9]; the three-spray pattern is Grower practice.
- Release rates and ratios: Supported in strawberry and roses [16][17]; supplier rates are guidance; no cannabis trials.
- Cannabis action thresholds: none published [5]; first-sighting action is Grower practice.
Glossary terms used here
- Glossary: intrinsic rate of increase, degree-day, diapause, arrhenotoky, predator:prey ratio, Phytoseiulus persimilis, Neoseiulus californicus, VPD
