Short answer. Shake a plant and a puff of tiny white flies lifts off: whiteflies. They are less common on cannabis than mites or thrips (Grower practice), but they breed steadily indoors, drip honeydew and can carry broad mites between plants [11]. Greenhouse whitefly needs about 381 degree-days above 8.3 °C per generation, roughly 23 days at 25 °C [2]; silverleaf whitefly grows fastest at 25–27 °C [4]. Monitor with yellow cards, work out which species you have, and release the matching parasitoid or predator early in veg. Keep sprays to veg.
Whitefly numbers at a glance
| Parameter | Greenhouse whitefly (T. vaporariorum) | Silverleaf whitefly (B. tabaci, MEAM1 / "B") |
|---|---|---|
| Adult length | ~1.5 mm | ~1.5 mm |
| Lower developmental threshold | 8.3 °C [2] | 12.5 °C on eggplant [4]; 11.1 °C cotton, 13.2 °C cantaloupe [5] |
| Degree-days egg to adult | 380.7 DD (sum of stages) [2] | 312.5 DD cotton, 250 DD cantaloupe [5] |
| Egg to adult, 25 °C | ~23 d (model from [2]) | ~21–23 d (models from [5]) |
| Egg to adult, 30 °C | ~18 d (model from [2]) | 14 d on eggplant [4] |
| Egg to adult, 15 °C | ~57 d (model from [2]) | 105 d on eggplant [4] |
| Upper limit | Longevity 5.9 d and lowest r at 26 °C on bean [3] | No development at 35 °C on cotton or cantaloupe [5]; r 0.073 at 35 °C on eggplant [4] |
| Eggs per female | Up to ~400 [10]; 8.6–127 on bean cultivars [3] | 324 at 20 °C, 22 at 35 °C [4]; up to 280 at 18 °C on cotton [7] |
| Adult female longevity | 22.6 d at 19 °C, 5.9 d at 26 °C (bean) [3] | 44 d at 20 °C, 10 d at 35 °C [4] |
| Sex ratio | ~54% female (average across hosts) [3] | no value verified here |
| r (intrinsic rate of increase) | 0.035–0.072 /day on bean [3] | 0.192 /day at 25–27 °C on eggplant [4] |
| Doubling time | 9.6–20 d (poor bean hosts) | 3.6 d |
| Best parasitoid | Encarsia formosa [8] | Eretmocerus eremicus [8] |
| Published action threshold for cannabis | none | none |
What do whiteflies look like on cannabis?
Whiteflies on cannabis are tiny, about 1.5 mm, with four white wings dusted in mealy wax; they rest on leaf undersides and fly up when the plant is disturbed [1]. They are Hemiptera, related to aphids, not true flies [1].
Nymphs. The first instar, the "crawler", moves for a few hours and then settles [1]. Later nymphs are flat, oval, immobile scales with tiny legs [1]. The fourth instar, often called the "pupa", thickens and shows red eyes, and is the easiest stage to identify [1].
Greenhouse whitefly (Trialeurodes vaporariorum). Fourth-instar nymphs carry very long waxy filaments and a marginal fringe [1]. Adults hold their wings flat over the body (Supported).
Silverleaf or sweetpotato whitefly (Bemisia tabaci). Fourth-instar nymphs lack the filaments and fringe [1]. Adults hold their wings slightly tilted, so the yellow body shows [1]. B. tabaci is a complex of look-alike species; the greenhouse ones are usually called MEAM1 (formerly "B") and MED (formerly "Q")being reviewed.
Exit holes. A healthy adult leaves a T-shaped slit in the empty nymph skin; a parasitoid leaves a round or oval hole [1].
Parasitised nymphs. Encarsia-parasitised greenhouse whitefly nymphs turn black [8]. Eretmocerus-parasitised silverleaf whitefly nymphs turn light brown [8]. Colour alone is not proof, because some healthy nymphs are dark [1].
What damage do whiteflies do to cannabis?
Whiteflies damage cannabis by sucking phloem sap and dripping honeydew, which turns leaves sticky and grows black sooty mould [1]. Large nymph populations cause yellowing, drying or leaf drop; adults alone rarely cause much damage [1].
Honeydew attracts ants, which protect whiteflies from natural enemies [1].
Broad mites hitch rides on whitefly adults [11]. In a clone room with both, whiteflies can move the mite into clean plants (kb-pests-broad-russet-mites).
Honeydew in flower sticks to trichomes and supports mould. That alone justifies control before the flip (Grower practice).
Differentials. Aphids have tail pipes (cornicles), cluster on stems and new growth and rarely fly up in a cloud (kb-pests-aphids). Fungus gnats are dark, long-legged and hover over the medium (kb-pests-fungus-gnats).
What is the whitefly life cycle in numbers?
The whitefly life cycle runs egg, crawler, two more flat nymphal instars, a fourth instar ("pupa"), then the winged adult [1]. For greenhouse whitefly, egg to adult takes about 23 days at 25 °C and 33 days at 20 °C by a degree-day model [2], and 3–4 weeks under favourable greenhouse conditions [10].
Greenhouse whitefly by temperature
Osborne measured greenhouse whitefly development above a threshold of 8.3 °C: 122.9 degree-days for the egg, 63.9 for the first instar, 38.4 for the second, 40.1 for the third and 115.4 for the fourth, about 381 in total [2]. Converted to days at constant temperature:
| Temperature | Egg | Instars 1–3 | Instar 4 ("pupa") | Egg to adult |
|---|---|---|---|---|
| 15 °C (59 °F) | 18.3 | 21.3 | 17.2 | 56.8 |
| 20 °C (68 °F) | 10.5 | 12.2 | 9.9 | 32.5 |
| 25 °C (77 °F) | 7.4 | 8.5 | 6.9 | 22.8 |
| 30 °C (86 °F) | 5.7 | 6.6 | 5.3 | 17.5 |
These are linear-model values; near the upper limit real development is slower than the model says. Colorado State University gives eggs hatching in 5–7 days and a non-feeding pupal stage of almost a week [10], which matches the 25 °C row.
Greenhouse whitefly dislikes heat. On a bean cultivar at about 75% RH, adult longevity fell from 22.6 days at 19 °C to 5.9 days at 26 °C, and r peaked at 22 °C (0.061 per day) and fell to 0.035 at 26 °C [3]. Female output was 8.6, 32.6 and 33.3 eggs at 19, 22 and 26 °C on that cultivar, but 127.2 at 19 °C on a better cultivar [3]. Host plant changed r by more than half [3]. Colorado State University gives up to 400 eggs per female over up to two months on favourable hosts [10].
Silverleaf whitefly by temperature
Wang and Tsai reared silverleaf whitefly (Bemisia argentifolii, now B. tabaci MEAM1) on eggplant at six constant temperatures [4].
| Temperature | Egg to adult | Survival egg to adult | Female lifespan | Eggs per female | r (per day) |
|---|---|---|---|---|---|
| 15 °C | 105 d | 40% | – | – | – |
| 20 °C | – | – | 44 d | 324 | – |
| 25 °C | – | 89% | – | – | 0.192 |
| 27 °C | – | – | – | – | 0.192 |
| 30 °C | 14 d | – | – | – | – |
| 35 °C | prolonged | 37% | 10 d | 22 | 0.073 |
Dashes mean the value is not in the abstract; the shared threshold for all immature stages was 12.5 °C [4]. Mean generation time fell from 46 days at 20 °C to 18 days at 30 °C, and the best range for population growth was 20–30 °C [4].
On cotton and cantaloupe from 20 to 32 °C, egg to adult took 14.6–37.9 days, with thresholds of 11.1 °C (cotton, 312.5 degree-days) and 13.2 °C (cantaloupe, 250 degree-days) [5]. No whitefly developed at 35 °C or on pepper [5]. Fecundity at 30 °C was 117–158 eggs per female on cotton and cantaloupe but 2–41 on pepper [5]. On cotton, females laid up to 280 eggs at 18 °C and only 5 at 37.2 °C [7]. Wagner's 31-temperature cotton study gives the full development-rate model [6].
No whitefly life table on cannabis has been published. The pepper result is a warning: host suitability can swing from excellent to almost nil [5].
Reproduction. Whiteflies are haplodiploid: unmated females lay sons, mated females lay both sexesbeing reviewed. Greenhouse whitefly averages about 54% females across hosts [3].
Why do temperature and humidity matter for whiteflies?
Temperature sets whitefly speed and also picks the species that wins. Greenhouse whitefly does best near 22 °C and lives and breeds less at 26 °C [3]; silverleaf whitefly grows fastest at 25–27 °C and still develops at 30–32 °C [4][5].
Grow-room set-points, converted to generation time (degree-day models, constant temperature):
| Room | Set-point | Greenhouse whitefly [2] | Silverleaf, cotton model [5] | Encarsia formosa [2] |
|---|---|---|---|---|
| Cool | 18 °C (64 °F) | 39 d | 45 d | 36 d (adults barely fly) |
| Late flower | 22 °C (72 °F) | 28 d | 29 d | 20 d |
| Flower | 24 °C (75 °F) | 24 d | 24 d | 17 d |
| Veg | 26 °C (79 °F) | 22 d | 21 d | 14 d |
| Warm veg | 28 °C (82 °F) | 19 d | 19 d | 12 d |
The Encarsia crossover. Encarsia formosa develops above 12.7 °C and needs 188.9 degree-days; greenhouse whitefly develops above 8.3 °C and needs about 381 [2]. Solving the two models, they run at the same speed at about 17 °C. Below that, the whitefly out-breeds its parasitoid; above it, the wasp gets ahead. That is why Encarsia programs fail in cold rooms, together with the fact that Encarsia adults do not fly below about 18 °C (65 °F) [8].
Humidity. No published effect of RH on whitefly growth rate in the grow-room range was found. RH matters through the natural enemies: Encarsia works best at 50–80% RH [8], A. swirskii at about 70% [8], and fungal biopesticides need moisture at the leaf surfacebeing reviewed.
Light. Encarsia searches for hosts in light, and growers report poor performance under low lightbeing reviewed. Sachets and cards belong in shaded lower canopy to avoid drying [8].
Grow-room translation. Veg at 24–28 °C and 60–70% RH (about 0.9–1.5 kPa VPD) suits Encarsia, Eretmocerus and A. swirskii. Flower at 45–55% RH, and 40–50% late, is drier than the predators like (kb-term-vpd).
How fast can a whitefly population grow? A worked example
Ten silverleaf whiteflies can become about 560 in three weeks at 25–27 °C if nothing eats them, using r = 0.192 per day [4]. The method is N(t) = N0 × e^(r × t), with N0 = 10 and t in days.
| Scenario | r (per day) | Doubling time | Day 14 | Day 21 | Day 28 | Day 42 |
|---|---|---|---|---|---|---|
| Silverleaf, 25–27 °C, eggplant [4] | 0.192 | 3.6 d | 147 | 564 | 2,162 | ~31,800 |
| Silverleaf, 35 °C, eggplant [4] | 0.073 | 9.5 d | 28 | 46 | 77 | 215 |
| Greenhouse WF, 19 °C, good bean cultivar [3] | 0.072 | 9.6 d | 27 | 45 | 75 | 206 |
| Greenhouse WF, 22 °C, poor bean cultivar [3] | 0.061 | 11.4 d | 23 | 36 | 55 | 130 |
| Greenhouse WF, 26 °C, poor bean cultivar [3] | 0.035 | 19.8 d | 16 | 21 | 27 | 43 |
This is the unchecked upper bound: no enemies, unlimited food, stable age structure.
The greenhouse whitefly rows come from bean cultivars that suit it poorly [3]. On favourable hosts such as eggplant or tomato its r is higherbeing reviewed. Treat the silverleaf row as the planning case until cannabis data exist.
Practical reading: whiteflies are slower than thrips or spider mites, which gives parasitoids time, but a silverleaf infestation at 26 °C still goes from "a few on a card" to "honeydew everywhere" in about a month.
Where do whiteflies hide, how do they spread and how do they get in?
Whiteflies enter cannabis rooms on plants: clones, houseplants, vegetable transplants and bedding plants. Greenhouse whitefly has more than 250 ornamental and vegetable hosts [10].
Hiding places. Eggs, nymphs and pupae sit on leaf undersides [1]. Adults gather on the youngest leaves, where females lay, so the oldest nymphs are on lower, older leaves (Supported).
Dispersal. Adults fly, but not far; you need several cards to detect them [1]. Air movement and people carry them around a room (Grower practice). Adults carry broad mites [11].
Outdoors. In warm climates greenhouse and silverleaf whiteflies breed year-round outdoors [1], so intakes near gardens and weeds are a route in.
Quarantine. Isolate new plants for 1–2 weeks [10]. Given 23 days egg to adult at 25 °C [2], three weeks of isolation with weekly underside checks is the safer choice (Grower practice).
Starve-out. A host-free period of 2–3 weeks starves an established infestation, weeds included [10].
How do I scout for whiteflies?
Scout for whiteflies weekly by turning leaves, shaking plants and reading yellow sticky cards. Record counts, so you see the trend rather than a single snapshot.
Leaf checks. Turn 3–5 upper and lower leaves per plant on 10% of plants, or all plants in a small room (Grower practice). Young upper leaves show adults and eggs; lower leaves show older nymphs and parasitised pupae.
Lens. 10x shows adults and nymphs; 20x shows the waxy filaments that separate the species [1]. Biotype ID needs a lab.
Yellow sticky cards. Hang cards sticky side toward plants, out of direct light, level with the infestation [1]. Colorado State University uses 1 card per 1,000 ft² (~93 m²), replaced weekly, for greenhouses [10]; UC IPM suggests one trap per one or two plants in a home setting [1]. In small rooms, use about 1 card per 1–2 m² (Grower practice).
Cards and parasitoids. Yellow cards trap Encarsia too. Take cards down before release and put them back 3–4 days later [8].
Thresholds. No action threshold has been published for whiteflies on cannabis. Practical rule: first adults on cards plus nymphs on leaves starts the program (Grower practice).
How do I control whiteflies by life stage?
Whitefly control works stage by stage. Contact sprays and predators hit adults, crawlers and young nymphs; eggs and the fourth instar tolerate most contact sprays; parasitoids lay in the nymphs.
| Stage | Where | What reaches it |
|---|---|---|
| Egg | Undersides of young leaves | A. swirskii, Delphastus [8]; contact sprays poorly |
| Crawler, instars 1–2 | Undersides | A. swirskii, Delphastus, soaps and oils, fungi [8] |
| Instars 3–4 | Undersides, older leaves | Encarsia (prefers instars 3–4), Eretmocerus, fungi [8] |
| Adult | Young leaves, flying | Yellow cards, vacuuming, fungi, Delphastus, soaps [1][8] |
Re-application interval. Survivors of a contact spray are eggs and fourth instars. The next spray must land before the hatchlings from surviving eggs reach the tolerant fourth instar, so the interval must be shorter than the egg stage plus the crawler window. Run the program at least one full generation.
| Room temperature | Egg stage [2] | Max interval | Program length | Applications |
|---|---|---|---|---|
| 20 °C | 10.5 d | ~9–10 d | ≥ 33 d | 4–5 |
| 25 °C | 7.4 d | ~6–7 d | ≥ 23 d | 4–5 |
| 30 °C | 5.7 d | ~5 d | ≥ 18 d | 4–5 |
These intervals come from the greenhouse whitefly model [2]. Colorado State University recommends repeating oils, soaps and neem every 7–10 days until eggs and nymphs are gone [10], which fits the 20–25 °C rows. Use soaps and oils only when plants are not drought-stressed and below about 32 °C (90 °F) [1].
Resistance. Both species have long histories of insecticide resistance, and B. tabaci MED is noted for itbeing reviewed. Rotate by IRAC group, never by brand, and change group after each generation-length window. Registrations on cannabis vary by jurisdiction: follow the label and local law. No products or rates are given here.
Residuals vs parasitoids. Encarsia is very sensitive to pesticide residues, including long-residual products [8]. Phase residual sprays out well before release.
Which biological controls work against whiteflies?
Biological control of whiteflies on cannabis uses a parasitoid matched to the species plus a predator for eggs and crawlers: Encarsia formosa for greenhouse whitefly, Eretmocerus eremicus for silverleaf whitefly, and Amblyseius swirskii for both [8]. Release in veg, when whiteflies are first found.
| Agent | Target | Temperature | RH | Notes |
|---|---|---|---|---|
| Encarsia formosa | Greenhouse whitefly instars 3–4 | 21–27 °C (70–80 °F); no flight < 18 °C; survival falls > 30 °C | 50–80% | Threshold 12.7 °C, 188.9 DD [2]; 25 d at 21 °C [9]; pupae black [8] |
| Eretmocerus eremicus | Silverleaf whitefly (also greenhouse) | 25–29 °C (77–84 °F) | – | Desert origin; host-feeds; no direct sun [8] |
| Amblyseius swirskii | Eggs, crawlers; also thrips | ~21 °C and up | ~70% | Prefers eggs and first instars [8] |
| Delphastus (lady beetle) | All stages, prefers eggs and nymphs | 24–27 °C (75–80 °F); no flight < 13 °C | – | Avoids parasitised whiteflies [8] |
| Beauveria bassiana, Cordyceps (Isaria) fumosorosea | All stages | – | needs leaf-surface moisturebeing reviewed | Start when populations are low [8] |
Encarsia formosa
Encarsia formosa is a tiny wasp that lays in greenhouse whitefly nymphs, preferring the third and fourth instars [8]. Parasitised nymphs turn black and release a wasp through a round hole [1][8].
At 21 °C on greenhouse whitefly, development from egg to adult wasp took 25 days [9]. Females lay about 5 eggs a day and about 59 in a lifetime of about 12 days [9]. They also kill about 3 nymphs a day by host-feeding, about 95 over a lifetime [9].
Populations are all-female (thelytokous) because of Wolbachia bacteria [9]. Every wasp you release lays eggs.
Release weekly from first detection until 80–90% of the nymphs are parasitised [8]. Supplier and university schedules use either a regular low "dribble" of about 1 parasitised nymph per plant, or larger releases when whiteflies appear [9]. Banker plants have been used at 1 per 352 crop plants in tomato [9].
Encarsia also attacks silverleaf whitefly. Against B. argentifolii, under 2 wasps per plant per week was reported to work in some trials, while more than 3 failed in others [9]. Eretmocerus is still the usual first choice for silverleaf [8].
Eretmocerus eremicus
Eretmocerus eremicus comes from the deserts of southern California and Arizona and tolerates 25–29 °C (77–84 °F) [8]. It has been sold since the 1990s for Bemisia [8]. Females also kill nymphs by host-feeding [8]. Do not release it in direct light; replace yellow cards four days after release [8]. A quality check: hold a card in a shaded, screened container at room temperature for two weeks; about 95% of adults should emerge [8].
Amblyseius swirskii
A. swirskii eats whitefly eggs and first-instar nymphs, plus thrips larvae, broad mites and pollen [8]. It works best around 21 °C and 70% RH [8]. In poinsettia, A. swirskii combined with Eretmocerus suppressed whiteflies better than either alone [8]. On cannabis it does double duty against thrips (kb-pests-thrips).
Delphastus
Delphastus is a 1.3–1.4 mm dark lady beetle that eats all whitefly stages, preferring eggs and nymphs; optimum 24–27 °C, no flight below about 13 °C (55 °F) [8]. It avoids parasitised whiteflies, so it fits with wasps [8]. Growers use it on hot spots, because a beetle needs many prey (Grower practice).
Predatory mirid bugs: caution
Macrolophus and Dicyphus are omnivorous mirid bugs used against whiteflies in tomato. They also feed on plants when prey run short (no cannabis data found)being reviewed. Do not release them on cannabis without trial evidence.
Fungi
Beauveria bassiana and Cordyceps (formerly Isaria) fumosorosea infect whiteflies; infected nymphs turn brown or pink [8]. Start when populations are low [8]. They need humidity at the leaf surface to infectbeing reviewed. Do not raise room RH in flower for them (kb-term-entomopathogenic-fungi).
How can I use the environment as a weapon against whiteflies?
The environment helps against whiteflies by keeping the room in the parasitoid's favour, not by killing the pest. Encarsia gets ahead of greenhouse whitefly above about 17 °C and does best at 21–27 °C [2][8].
Greenhouse whitefly room: moderate and steady. Hold 22–26 °C. At 24 °C Encarsia needs about 17 days per generation against 24 for the whitefly [2]. Avoid nights below about 18 °C, where Encarsia adults stop flying [8].
Silverleaf whitefly room: Eretmocerus likes it warm. Silverleaf grows fastest at 25–27 °C [4], and Eretmocerus tolerates 25–29 °C [8]. Do not try to cook silverleaf out: at 35 °C it still grew at r = 0.073 per day on eggplant [4], and the crop suffers more than the pest.
Greenhouse whitefly and heat. At 26 °C on bean, greenhouse whitefly adults lived only 5.9 days [3]. A warm veg room at 26–28 °C slows it, but never trade that against heat stress or spider mites (kb-pests-spider-mites).
Flower trade-off. RH at 45–55%, and 40–50% late, protects buds from botrytis (kb-diseases-botrytis-bud-rot) and powdery mildew (kb-diseases-powdery-mildew). Parasitoids keep working in the drier room; fungal sprays do not.
What should I not do about whiteflies in flower?
Do not spray cannabis buds in flower. Residues stay in dense buds, wet buds invite botrytis, and most residual and systemic insecticides are not registered on cannabis.
- Do not use systemic insecticides. Not registered on cannabis in most places, and residues persist.
- Do not raise RH in late flower to help fungal biopesticides. Keep 40–50% RH.
- Do not hang dozens of fresh yellow cards right after a parasitoid release; wait 3–4 days [8].
- Do not let the room drop below about 18 °C at night if you rely on Encarsia [8].
- In flower, rely on parasitoids and predators established in veg. Vacuum adults from lower leaves early in the morning, when they are sluggish (Grower practice).
What are the common myths about whiteflies?
"Yellow cards will control whiteflies." Myth. Cards are for monitoring and detection [1]. Eggs and nymphs keep developing under the leaves.
"Encarsia works on any whitefly." Myth. Encarsia is the greenhouse whitefly specialist; it attacks silverleaf whitefly less reliably, and Eretmocerus is the usual choice there [8][9].
"Whiteflies love heat, so cool the room." Myth. Silverleaf whitefly does well in heat [4], but greenhouse whitefly breeds less at 26 °C [3]. Cooling below about 17–18 °C helps greenhouse whitefly more than Encarsia [2][8].
"One spray clears whiteflies." Myth. Eggs and fourth instars survive, and eggs hatch within about a week at 25 °C [2].
"Whiteflies are harmless apart from honeydew." Myth. Large nymph populations cause yellowing and leaf drop [1], and adults carry broad mites [11].
Common mistakes
- Releasing Encarsia against silverleaf whitefly and expecting greenhouse-whitefly results [8][9].
- Releasing Encarsia into a cool room or one with fresh pesticide residues [2][8].
- Relying on cards for control instead of monitoring [1].
- Spraying once, seeing adults vanish, and stopping before eggs hatch [2].
- Ignoring ants that farm honeydew and protect whiteflies [1].
- Keeping houseplants or vegetable seedlings in the grow room [10].
Evidence
- Biology, identification and honeydew damage: Established [1][10].
- Greenhouse whitefly and Encarsia degree-day models: Established, from one lab study [2]; linear models overestimate speed near the upper limit.
- Silverleaf whitefly temperature responses: Established on eggplant, cotton and cantaloupe [4][5][6][7].
- Greenhouse whitefly r on bean: Established for those cultivars [3]; low values reflect poor hosts.
- Encarsia and Eretmocerus biology and temperature windows: Established in greenhouse crops [8][9].
- Use on cannabis: Supported by extrapolation; no cannabis trials or life tables found.
Glossary terms used here
- Glossary: parasitoid, honeydew, degree-day, developmental threshold, intrinsic rate of increase, sticky trap, Amblyseius swirskii, biological control
