What are spider mites and how do you identify them?
Tetranychus urticae Koch (Acari: Tetranychidae) is the most widespread phytophagous mite in indoor cannabis. It's not an insect — it has eight legs and measures 0.3–0.5 mm, so the first visible sign is the damage, not the mite itself. Colonies establish on the undersides of leaves, where they pierce cells and suck out the contents, leaving a stippled or bronzed texture on the upper surface. In heavy infestations the characteristic webbing appears — the feature that gives them their common name.
- Leaf undersides: tiny white or yellow dots (feeding damage), progressing to bronze patches.
- Tips and midribs: start your inspection there, where airflow is lowest.
- Fine webbing: on bud sites and between petioles — a sign of an established colony.
- Visible mites: at 10× magnification you can spot adults (orange-red or yellow-green with two dark spots) and the spherical, translucent eggs on the leaf underside.
Why does hot, dry air accelerate their lifecycle so much?
Temperature and relative humidity are the two biggest drivers of T. urticae development speed. Kasap (2004, Phytoparasitica, DOI: 10.1007/BF02980863) documented that egg-to-adult development time drops sharply as temperatures rise: at 20°C (68°F) it can exceed 14 days; at 27–30°C (80–86°F) it compresses to 7–9 days; at 35°C (95°F) it falls below 7 days. When that heat combines with RH under 40% — common in poorly regulated indoors during late flower — the full cycle can complete in 5–7 days.
A cycle that short means a single egg-laying female can have reproductive offspring in under a week. With multiple overlapping generations, populations grow exponentially before the damage becomes obvious to the naked eye.
What environmental conditions slow their development?
Environmental control won't eliminate an established infestation, but it does brake their reproductive rate and reduces pressure on the plant while other interventions are applied. The logic is simple: keeping VPD within the correct range for each growth stage automatically avoids the conditions that favor T. urticae.
- Temperature: keep below 27°C (80°F) whenever possible, especially during the light cycle. Above that threshold the reproductive cycle accelerates non-linearly.
- Relative humidity: never let RH drop below 40% at any point in the cycle. Late flower targets RH 40–50% to prevent Botrytis — that same floor protects against mite population explosions.
- Air circulation: active airflow across the canopy breaks up stagnant hot-and-dry microclimates where T. urticae first establishes.
- Nighttime temps: warm nights (above 24°C / 75°F) with low RH are just as favorable for mites as the light period. Don't neglect environmental control when the lights are off.
How do you run a systematic inspection before the problem escalates?
Spider mites are already spreading before the damage shows to the naked eye. A weekly inspection with magnification cuts your reaction time and prevents the infestation from reaching the webbing stage.
- Check the undersides of the oldest leaves and the lower third of the canopy first — that's where initial colonies establish.
- Use a 10× loupe or a pocket magnifier; your phone in macro mode can also catch eggs.
- Gently shake a leaf over white paper — if small specks move, you have mites.
- Mark plants showing early symptoms (bronze stippling) and track their progression week to week.
- During hot periods or if your indoor lacks precise temperature control, increase inspection frequency to twice a week.
What integrated pest management (IPM) strategies work best?
Integrated pest management combines preventive, cultural, and — when necessary — biological or conventional measures depending on the stage of the grow. Order matters: least disruptive interventions first.
- Quarantine new material: any plant, cutting, or substrate entering the space should be inspected before being introduced to the main crop.
- Cultural control: manually remove leaves with visible early-stage infestations and bag them before pulling them out — don't shake mites across the rest of the canopy.
- Biological control: predatory mites like Phytoseiulus persimilis or Neoseiulus californicus are the most effective IPM option in veg and early flower; their efficiency drops above 30°C (86°F).
- Horticultural oils and insecticidal soap: contact-acting, require thorough coverage of the leaf underside, low residual toxicity. Avoid in late flower to protect trichome quality.
- Conventional miticides: last resort, only in veg or very early flower if the infestation is already established. Mandatory rotation between modes of action to prevent resistance buildup.
What do you do when spider mites show up in late flower?
This is the toughest scenario because chemical intervention options are severely limited once the buds are developed. The focus has to shift to containing the spread, not eradication.
- Prioritize environmental control: dropping temps and keeping RH above 45% is the most immediate action and leaves zero residue.
- Remove the most damaged leaves from the lower third — they contribute negligible photosynthesis at this point and are the primary reservoir for the colony.
- If you apply anything, stick to insecticidal soap or oils on the fan leaves only — never directly on the buds.
- Assess whether the infestation is localized (a few plants) or widespread: if localized, physically isolating affected plants can slow dispersal.
