Short answer
Treat cannabis spider mites in this order: isolate the plant, remove leaves with stippling or webbing, spray leaf undersides with water, then apply insecticidal soap or horticultural oil. For an infestation with webbing, add predatory mites. In flower, use water and beneficial insects. Do not apply oil or soap to buds.
Confirm the pest
Two-spotted spider mite (Tetranychus urticae) measures 0.4 to 0.5 mm. Adults have two spots. Eggs sit on leaf undersides. Signs: stippling on upper leaf surfaces, webbing on stems and bud sites. Use a 10x loupe. Check 10 leaves per plant. Russet mites are smaller and show no webbing; their treatment differs.
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Life cycle and conditions
At 80 F (27 C), egg to adult takes 8 to 12 days. Eggs hatch in 3 to 5 days. A female lays 100 or more eggs. Adults live 2 to 4 weeks. Air at 80 F and 30% humidity raises reproduction. Hold humidity at 50% or above and temperature at 80 F or below. Check again at day 5 and day 10.
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Immediate steps
- Move the plant to a separate room. Check plants within 1 m.
- Remove and bag leaves with webbing or 10 or more mites per leaf.
- Spray leaf undersides with water. Repeat every 3 days.
- Clean tools with 70% isopropyl alcohol between plants.
Products and rates
- Insecticidal soap: mix per label, spray leaf undersides to runoff, repeat every 5 to 7 days for 3 rounds. Test on 2 leaves first. No residual.
- Horticultural oil: 1% to 2% per label. Coats eggs and adults. Do not apply above 90 F or in flower.
- Neem oil: azadirachtin products at label rate. Repeat every 7 days. Do not apply in flower.
- Predatory mites: Phytoseiulus persimilis, release 20 per square meter. Do not tank-mix with soap or oil. Stop broad-spectrum sprays 2 weeks before release.
- Abamectin (IRAC group 6) and bifenazate (IRAC group 20): rotate by group. Check state registration and label.
Flower stage limits
Soap and oil leave residue on buds. Use water sprays and predatory mites. Remove plants with webbing at bud sites. Discard trim. No product clears an infestation; the goal is to hold mites below the damage threshold until harvest.
Prevention
Quarantine new clones for 14 days. Filter intake air. Sanitize tools and trays. Scout 10 leaves per plant each week. Keep humidity at 50% or above. Remove plant debris from the floor.