An action threshold is the level of pest or disease, measured by scouting, at which a defined control step is triggered. In field crops it is tied to the economic injury level, the point where damage costs more than control. For high-value cannabis, thresholds are set low because the tolerance for damage or contamination in flowers is close to zero.
In practice
Thresholds turn vague worry into a rule. Examples a grower might write down: more than 10 thrips per yellow card per week in veg triggers an extra predator release; any confirmed broad mite or russet mite on a clone triggers destruction of that batch; any powdery mildew colony in late veg triggers removal of affected leaves and an environment check. The numbers are grower-set, not universal; record counts weekly and adjust thresholds as you learn what your room tolerates. Stage matters: the same count that is tolerable in early veg can be unacceptable two weeks into flower, when options run out.
Don't confuse with
- Economic injury level: the damage break-even point; the action threshold sits below it to leave time to act.
- Zero tolerance: a threshold of one, used for pests such as russet mites and for pathogens such as HLVd.
- Spray schedule: action by calendar, not by count.
Slender insects about 1-2 mm long, chiefly western flower thrips (Frankliniella occidentalis), that rasp leaf surfaces, leaving silvery streaks dotted with black frass.Polyphagotarsonemus latus, a microscopic tarsonemid mite whose toxic feeding twists, glazes and stunts new growth; often mistaken for calcium deficiency or heat stress.Aculops cannabicola, a microscopic eriophyid mite specific to cannabis that causes upward leaf curl, bronzing and dull, greasy-looking trichomes.A fungal disease, on cannabis caused mainly by Golovinomyces ambrosiae, that grows as white powdery patches on leaves and buds and thrives in moderate humidity.HLVd, a small infectious RNA that spreads through sap and tools, causing stunting, brittle stems, small loose buds and reduced resin in cannabis; called dudding.Integrated pest management: preventing and controlling pests by combining monitoring, prevention, cultural, biological and, as a last resort, chemical tools against set action thresholds.Systematic, regular inspection of plants, traps and roots to detect pests and disease early, identify them correctly and record numbers for decisions.A coloured card coated in non-drying glue that catches flying insects, used mainly to monitor pest trends: yellow for gnats, whiteflies and aphids, blue for thrips.Releasing beneficial organisms before a pest is found, so a standing population is ready to catch the first arrivals rather than chase an outbreak.
