What does a spider mite infestation look like?
Early-stage spider mite pressure is easy to miss. The first sign is typically fine stippling on the upper leaf surface: small, pale dots where mites have punctured the cell wall and consumed the contents. Under a loupe or jeweler's loop, mites and eggs are visible on the underside of affected leaves. As pressure builds, stippling becomes more extensive, leaves yellow and bronze, and webbing becomes visible in the canopy.
By the time webbing is visible without magnification, the infestation is already well established. Regular scouting with a loupe — underside of leaves, particularly lower and mid-canopy — is the only way to catch pressure before it spreads.
Why spider mites are a recurring problem in cannabis facilities
Spider mites do not appear at random. They enter a facility through clones, propagation material, workers' clothing, or HVAC intake. Once established, they persist because the conditions most cannabis facilities maintain — warm temperatures, moderate humidity, dense canopy — are close to optimal for mite reproduction.
Facilities that eradicate an infestation and do not address the entry vectors see recurrence within one to two cycles. The most common re-entry points:
Incoming clones and mother plants. Spider mites are frequently introduced through propagation material from outside the facility. A quarantine protocol for incoming genetics — inspection, isolation, and treatment before introduction to the main facility — is the standard prevention measure for operations sourcing clones externally.
Reused containers and media. Mite eggs survive on container surfaces and in media residue. Thorough surface sanitation between cycles, including the underside of trays and the interior faces of containers, is part of the control protocol.
Temperature and humidity drift. Spider mites thrive above 80°F and below 40% RH. Facilities that allow temperature or humidity to drift at night or during HVAC cycles create windows for population growth. Tight environmental control is a mite management tool.
Spider mites and secondary fungal pressure
The relationship between spider mite damage and fungal pressure is not coincidental. Mite feeding punctures the leaf epidermis at hundreds of points per leaf. Each wound is a potential entry point for Botrytis cinerea, which infects through mechanical damage as well as through open stomata and senescing tissue. Powdery mildew similarly colonizes stressed tissue more readily than healthy tissue.
Facilities managing active spider mite pressure are simultaneously managing elevated fungal risk, even if no visible mold has appeared. A surface sanitation program that addresses Botrytis entry points — benches, clip surfaces, canopy debris — should run alongside any mite control effort, not sequentially.