Pesticide resistance is a genetic change in a pest or pathogen population that lets it survive a product that used to work. It is selected, not created: every application kills susceptible individuals and leaves the few resistant ones to breed. Pests with short generations and huge populations, such as two-spotted spider mites, western flower thrips, Bemisia whiteflies and powdery mildew, develop resistance fastest.
In practice
Growers slow resistance by rotating products with different modes of action rather than different brand names, since many products share an active ingredient. Industry groups classify actives by mode of action: the Insecticide Resistance Action Committee (IRAC) for insecticides and miticides, and the Fungicide Resistance Action Committee (FRAC) for fungicides. Other habits: use full label rates, do not spray when scouting says it is unnecessary, and lean on biological and cultural controls, which pests cannot easily evolve around. "It stopped working" after a few applications usually means poor coverage, wrong identification or resistance.
Don't confuse with
- Plant disease resistance: a crop trait, such as mildew-resistance genes.
- Tolerance: a plant coping with damage without losing yield.
Tetranychus urticae, the commonest cannabis mite indoors: a 0.5 mm sap-feeder that stipples leaves pale and spins webbing, breeding fastest in hot, dry rooms.Slender insects about 1-2 mm long, chiefly western flower thrips (Frankliniella occidentalis), that rasp leaf surfaces, leaving silvery streaks dotted with black frass.Small white-winged sap-sucking insects of the family Aleyrodidae, mainly Trialeurodes vaporariorum and Bemisia tabaci, that fly up when disturbed and coat leaves in honeydew.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.Pest and disease prevention by changing how the crop is grown, such as humidity, airflow, spacing, watering and cultivar choice, so pests and pathogens struggle to establish.A gene is a stretch of DNA that carries the instructions for one functional product, usually a protein such as an enzyme, and is the basic unit of inheritance.Contact products act only where the spray touches the pest; systemic products are absorbed and move inside the plant, which raises residue concerns in cannabis.Integrated pest management: preventing and controlling pests by combining monitoring, prevention, cultural, biological and, as a last resort, chemical tools against set action thresholds.
