Identifying and controlling spider mites on cannabis
Fine stippling on the upper leaf surface and webbing on the underside reveal spider mites. Here's how to spot the infestation early and stop the explosive reproduction with staggered IPM.
Pathogen
Tetranychus urticae and others
Key symptom
Stippling on top, webbing below
Risk climate
Warm (27–30 °C), dry
Generation time
In heat ~5–7 days
Beneficial insect
Phytoseiulus persimilis
Key points
- The leading symptom is tiny light stippling on the upper leaf surface; on the underside the mites sit with fine webbing.
- At 27–30 °C and low relative humidity, the population doubles within days — early detection with a magnifying glass is critical.
- Control it in stages: change the climate (cooler, more humid), reduce mechanically, introduce predatory mites, rotate active ingredients if needed — never spray during late flowering.
Note
- Never spray the buds during late flowering — residues and moisture ruin the harvest and promote botrytis.
- Spider mites develop resistance quickly; a monotherapy makes the infestation worse in the long run.
Definition and classification
Spider mites are not insects, but arachnids (< 0.5 mm). They suck out cell contents, which creates the typical light-colored dots.
Cannabis is particularly susceptible in warm, dry indoor setups; an unnoticed initial infestation can jeopardize an entire harvest.
Biology and life cycle
Larvae hatch from eggs, then nymphs, then adult mites; at 30 °C a cycle takes only ~5–7 days, at 20 °C considerably longer.
A single female lays over a hundred eggs. This exponential reproduction explains why a seemingly small infestation escalates within a week.
Damage pattern and symptoms by severity
Stage 1: Isolated light-colored speckles on leaf upper sides, individual moving dots visible on the underside with a magnifying glass.
Stage 2: Widespread stippling, leaves appear bronze-colored/silvery, first fine webbing in leaf axils.
Stage 3: Dense webbing over leaves and buds, leaf drop, drastic loss of vitality; buds become unsellable.
Checklist
- Check leaf undersides with a magnifying glass (20–60×)
- Hold white paper under shaken leaves — moving dots?
- Webbing on shoots and leaf axils?
Diagnosis — differentiation
Stippling + webbing on the underside → spider mites (not thrips: their damage is silvery-streaky with small black fecal specks, without webbing).
Nutrient speckling is static and often follows a deficiency pattern; mite damage spreads actively and shows moving organisms.
A magnifying glass is essential: eggs (round, pearly spheres) and moving mites on the underside confirm the diagnosis.
Control — staged IPM
1. Isolate immediately: separate infested plants, tighten sanitation (clothing, tools).
2. Adjust the climate: spider mites hate cooler, more humid air — lower the temperature, raise the relative humidity.
3. Reduce mechanically: remove heavily infested leaves, shower/spray off the undersides.
4. Biological: release predatory mites (Phytoseiulus persimilis, Amblyseius) while the population has not yet exploded.
5. Use active ingredients only during the vegetative stage/early flowering and on a rotation basis — resistance develops quickly with monotherapy.
Checklist
- Isolate infested plants
- Switch the climate to mite-hostile conditions
- Introduce predatory mites early
- Rotate active ingredients, never in late flowering
Prevention
Consistently quarantine new plants/clones (at least 1 week) and inspect them with a magnifying glass.
Don't run the climate hot and dry long-term; stable relative humidity makes reproduction harder.
Make regular scouting of the leaf undersides a routine — weekly, with a magnifying glass.
Environmental factors and interactions
High temperature and low VPD/dry air massively accelerate reproduction — climate control is the most effective long-term prevention.
Stress (drought, nutrient deficiency) makes plants more attractive and more susceptible to infestation.
During flowering, dead, mite-infested leaves also increase the risk of mold (botrytis).
Common mistakes
Acting only once webbing is visible — by then the population has already exploded.
Using only one product repeatedly → fast resistance.
Spraying the leaf tops but skipping the undersides (where the mites sit).
Advanced considerations
Preventively establishing predatory mites (a 'bank') during the vegetative stage is significantly more effective than curative applications during heavy infestation.
Resistance management: deliberately rotate active-ingredient classes (IRAC groups) and never treat two consecutive generations with the same mode of action.
Frequently asked questions
How do I tell spider mites apart from thrips?
Can I still do something during flowering?
Sources
→ Full register- 1Open ↗
Biology and Management of the Twospotted Spider Mite (Tetranychus urticae)
Annual Review of Entomology · 2013
- 2Open ↗
Integrated Pest Management of Arthropod Pests on Cannabis
Insects (MDPI) · 2020
- 3Open ↗
Pathogens and Molds Affecting Production and Quality of Cannabis sativa
Frontiers in Plant Science · 2019
Related articles
Growing & Harvest
Integrated pest prevention in the grow
How monitoring, hygiene, and early detection prevent losses without blindly resorting to chemicals or panic.
Diagnosis & Troubleshooting
Identifying and controlling thrips on cannabis
Silvery streaks with tiny black fecal specks are the key symptom. Here's how you distinguish thrips from spider mites and control the flying larvae with blue sticky traps and beneficial insects.
Diagnosis & Troubleshooting
Identifying and controlling fungus gnats on cannabis
Small black flies over the substrate and glossy larvae in moist soil are the leading symptoms. Here's how to fight them at the root of the problem: through your watering regime.
Diagnosis & Troubleshooting
Recognizing and preventing bud rot (Botrytis) in cannabis
Gray, cottony rot inside the bud with wilted individual leaflets is the warning sign. Bud rot is not curable — this climate strategy prevents it before it breaks out.