What are thrips and how do you spot them in your grow?
Thrips are insects of the order Thysanoptera. The species most commonly found on indoor cannabis is Frankliniella occidentalis (Pergande, 1895), known as the western flower thrips. Thrips tabaci (onion thrips) may also appear, causing similar damage and coexisting with F. occidentalis in enclosed spaces.
- Adults are 1–2 mm long with a slender body and two pairs of fringed wings. Females range from yellow-orange to nearly black depending on temperature and diet; males are lighter.
- Larvae are translucent white or yellow, wingless, with bright red eyes — invisible to the naked eye.
- Characteristic damage: silvery or bronze stippling where cell contents have been sucked out, accompanied by scattered black specks of excrement. This silver-plus-black combination is the quickest diagnostic sign.
- On young leaves: curling and distortion from feeding on apical buds.
- In flower: damage to bracts and pistils, smaller buds. A study in indoor cannabis (McCune et al., 2021) documented that an initial infestation of just 5 thrips per plant reduced fresh yield by 30.7% and dropped THC from 19.34% to 17.6%.
- Always inspect with a hand lens (10–20×) on the underside of young leaves and inside flowers — larvae hide in folds and between trichomes.
Under what environmental conditions do thrips appear?
Frankliniella occidentalis thrives in warm, relatively dry environments — conditions that largely overlap with what many growers maintain, especially in vegetative.
- Optimal temperature: 20–25 °C. At 20 °C the full cycle (egg to adult) takes about 15–20 days; at 30 °C it shortens to roughly 11 days. At 15 °C development slows dramatically.
- Under optimal conditions the population can double in approximately 4 days.
- Relative humidity: they prefer low to moderate RH (40–60%). Above 70% RH their reproduction slows somewhat and naturally occurring entomopathogenic fungi such as Beauveria bassiana are more favored.
- Indoors there is no winter dormancy — the cycle runs continuously all year.
- Eggs are deposited INSIDE plant tissue (females use a saw-like ovipositor to pierce leaves and petals), making them invisible and impervious to contact treatments.
- Prepupae and pupae drop to the substrate and complete development there — that is why foliar-only control is never enough.
- Areas with poor air circulation become hotspots: thrips are weak fliers and air currents hinder their movement between plants.
How are thrips controlled? Stepped IPM from least to most disruptive
Controlling thrips requires attacking two phases at the same time: the aerial phase (larvae and adults on the plant) and the soil phase (prepupae and pupae in the substrate). Ignoring either one allows the cycle to replenish itself.
Step 1 — Prevention and monitoring (no chemicals, always active)
- Quarantine all new plant material (clones, mother plants) in a separate space for 7–10 days before introducing them to the grow. Eggs inside plant tissue are invisible.
- BLUE sticky chromatic traps from day one: thrips are attracted to blue (yellow traps are for whitefly and fungus gnats). One trap per 10–15 m². They monitor and provide partial capture — they do not replace active control.
- Daily inspection with a hand lens (10–20×) on young leaves and flowers.
- Remove plant debris from the space (fallen leaves, trimmings) where pupae can develop.
- Maintain constant airflow with oscillating fans — this impedes adult movement.
- Indoors: check and replace HEPA filters regularly.
Step 2 — Cultural control
- Physically remove heavily infested leaves and destroy them (do not compost — pupae survive).
- Outdoors: remove alternative host plants (flowering weeds) around the perimeter of the grow.
- Do not overwater: compacted, waterlogged substrate does not stop pupae, but loose, well-aerated substrate makes it easier for soil predators to work.
Step 3 — Biological control (the smartest option, especially in flower)
Biocontrol in cannabis has a specific challenge: dense trichomes hinder predator movement, and some secondary metabolites can affect their survival (Lemay et al., 2022). For this reason it is best to combine agents that target the aerial phase with others that target the soil.
- AERIAL PHASE — Amblyseius cucumeris (phytoseiid mite): the most widely used in indoor cannabis. It feeds on first- and second-instar larvae. Apply preventively in slow-release sachets before the pest is detected. Works best between 20–28 °C and at RH >60%.
- AERIAL PHASE — Amblyseius swirskii: more generalist than cucumeris (also controls whitefly) and tolerates higher temperatures. A good choice in hot summer conditions.
- AERIAL PHASE — Orius spp. (pirate bug): highly effective generalist predator against adults and larvae. One of the few that actively enters flowers. IMPORTANT LIMITATION: several Orius species enter diapause at 12-hour photoperiods, which can reduce their effectiveness right when cannabis is in flower (12/12). Best used mainly in vegetative or in the first weeks of flowering.
- SOIL PHASE — Stratiolaelaps scimitus (= Hypoaspis miles): soil-dwelling predatory mite that attacks thrips prepupae and pupae in the substrate. Apply via watering from the start of the grow.
- SOIL PHASE — Steinernema feltiae (entomopathogenic nematode): effective against mature larvae and pupae in the substrate. Apply via irrigation when substrate temperature is between 10–25 °C.
- SOIL PHASE — Atheta coriaria (rove beetle): complements pupal control in the soil.
- FUNGI — Beauveria bassiana: entomopathogenic fungus that infects adults and larvae. Spray preferably at night or with lights off. Requires RH >60% to germinate. Compatible with biological control agents when applied correctly and at the right times.
Step 4 — Potassium soap and oils (vegetative; with restrictions in flower)
- Potassium soap at 1–2%: acts on contact, killing larvae and adults it directly touches. Does NOT penetrate eggs or pupae. Requires repeat application every 5–7 days. In flower, avoid spraying directly on buds — it can affect trichomes and the final aromatic profile.
- Neem oil (azadirachtin): acts as a repellent and molting disruptor. Effective in vegetative. Use with great caution in early flower; in late flower (last 4–6 weeks) AVOID — the odor persists in the buds. Do not apply under direct light.
- Diatomaceous earth: abrasive powder applied to the substrate and surfaces; kills larvae and insects that move through it. No toxic residues, but do not overuse as it can affect substrate structure.
Step 5 — Chemical treatments: last resort and ONLY in vegetative
Synthetic pesticides should not be used on cannabis in flower. The risk is twofold: residues in the bud (the product is consumed, not washed) and destruction of the predator ecosystem you have built. If biological control alone was insufficient by late flower, the most responsible decision is to consider whether an early harvest is appropriate.
- Spinosad (vegetative only): derived from bacterial fermentation. Acts on thrips larvae and adults. Low mammalian toxicity. Rotate the mode of action between applications: F. occidentalis develops resistance rapidly.
- Natural pyrethrins from Chrysanthemum (vegetative only): fast action, approximately one-week pre-harvest interval.
- NEVER in flower: organophosphates, neonicotinoids, permethrin, or synthetic pyrethroids.
Best practices: summary by growth stage
- Vegetative: blue traps from day one + preventive Amblyseius cucumeris + Stratiolaelaps scimitus in the substrate. If infestation occurs: potassium soap + predator reinforcement.
- Early flower (weeks 1–3): Orius spp. while the photoperiod allows it + Amblyseius swirskii + Steinernema feltiae in the substrate. Avoid chemicals.
- Late flower (week 4 onward): predators only. Beauveria bassiana with caution. Potassium soap as a last resort — NEVER directly on buds. No synthetic chemicals.
- Harvest and cleanup: thoroughly clean the entire space between cycles — surfaces, corners, trays. Pupae survive in cracks and restart the cycle in the next grow.
