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, but they breed steadily indoors, drip honeydew, and can carry broad mites between plants. Monitor with yellow cards, work out which species you have, and release the matching parasitoid wasp early. Keep sprays to veg.
What's happening
Adults and nymphs suck phloem sap and excrete honeydew that leads to sticky leaves and black sooty mould; ants may farm them [1]. Two species matter indoors: greenhouse whitefly (Trialeurodes vaporariorum) and silverleaf/sweetpotato whitefly (Bemisia tabaci) [2]. Broad mites can hitch rides on adult whiteflies [6].
Lifecycle. Eggs on leaf undersides → mobile first-instar "crawler" → three further flat, oval, immobile nymphal stages → winged adult [1]. Egg to adult takes roughly 3–4 weeks at greenhouse temperatures (temperature-dependent)being reviewed.
How to recognise it
- Adults ~1.5 mm, white wings, resting on leaf undersides; fly up when disturbed.
- Nymphs: flat, translucent-to-pale scales on undersides; pupae are thicker with visible red eyes.
- Honeydew and sooty mould on leaves below.
- Greenhouse whitefly holds wings flat and overlapping, almost triangular; silverleaf holds them tent-like with a gap showing the yellow body (lens ID)being reviewed.
- Parasitised greenhouse-whitefly pupae turn black (Encarsia) or yellow-brown (Eretmocerus)being reviewed. Encarsia exit holes are round; healthy adults leave T-shaped slits [1].
Numbers that matter
| Parameter | Value | Notes |
|---|---|---|
| Adult length | ~1.5 mm | |
| Encarsia formosa best range | 21–27 °C (70–80 °F), 50–80% RH | Adults stop flying below ~18 °C (65 °F) [2] |
| Eretmocerus eremicus | performs better than Encarsia at higher temperatures | [2] |
| Encarsia host | greenhouse whitefly only | Eretmocerus also attacks silverleaf [2] |
| Release goal | continue until ~80% of the population is parasitised | [2] |
| Action threshold | none established for cannabis | Grower practice: first adults on cards |
What to check first
- Confirm with a lens; separate from aphids (no tail pipes, fly readily).
- Yellow sticky cards near the canopy, sticky side facing plants, out of direct sun; replace or clean regularly. Whiteflies do not fly far, so you need several [1].
- Identify species (lab or lens).
- Find the source: houseplants, vegetable transplants, outdoor plants near intakes.
How to fix it
Registrations differ by jurisdiction; check local regulations and the label. No products or rates here.
Cultural
- Quarantine incoming plants; keep vegetable seedlings and houseplants out of the grow room.
- Remove weeds and other host plants near intakes.
- Phase out residual insecticides well before releasing parasitoids [2].
Physical
- Yellow sticky cards (monitoring and minor trapping) [1].
- Vacuuming adults off plants early in the morning, when they are sluggish (Grower practice).
- Insect screening on intakes where feasible.
Biological
- Encarsia formosa: parasitoid of greenhouse whitefly; best at 21–27 °C and 50–80% RH; very sensitive to pesticide residues on plants and surfaces [2].
- Eretmocerus eremicus / E. mundus: better in warmer rooms and against silverleaf whitefly; do not release in direct sunlight [2][3]. Mixtures of Encarsia and Eretmocerus are commonly sold [2].
- Amblyseius swirskii: predatory mite that eats whitefly eggs and young nymphs and also thrips larvaebeing reviewed.
- Generalist predators: lacewings, minute pirate bugs and some lady beetles prey on whiteflies [1].
- Entomopathogenic fungi (Isaria fumosorosea, Beauveria bassiana): listed in extension guidance [4]; need humidity.
Least-toxic options
- Insecticidal soaps and horticultural oils: contact; target undersides; repeat at short intervals because eggs and pupae are tolerant.
- Avoid broad-spectrum residuals that kill parasitoids for weeks [2].
What not to do in flower
- Do not spray buds.
- 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.
- In flower, rely on parasitoids and predators established in veg.
How to confirm it worked
- Card counts fall over 3–4 weeks.
- Most pupae on lower leaves are parasitised (dark or exit holes round).
- New leaves stay free of honeydew.
Common mistakes
- Releasing Encarsia against silverleaf whitefly; it does not parasitise it [2].
- Releasing parasitoids into a room with fresh pesticide residues.
- Relying on cards for control instead of monitoring.
