A predator:prey ratio compares predator numbers with pest numbers on the same leaves or plants, and is the practical test of whether a biological control release is winning. Biocontrol guides for spider mites commonly give ratios in the region of one Phytoseiulus persimilis to 10-20 spider mites as the point where control usually follows; supplier technical sheets give similar figures (exact ratios vary by source).
In practice
The ratio only works with the climate in the predator's window. A 1:10 ratio can still fail above about 30 C or below 60% RH for persimilis, because the pest's r_m then outruns the predator. Count both on the same marked leaves weekly and record them in your grow journal; a rising predator share means the predators are winning.
Don't confuse with
- Release rate: predators per m2 or per plant, set before a ratio can be measured.
- Action threshold: the pest level that triggers a step, with or without predators.
Using living natural enemies, such as predators, parasitoids, nematodes and microbes, to suppress pests and diseases instead of, or before, chemical sprays.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.A specialist predatory mite that eats almost only Tetranychus spider mites, eggs to adults; fast-acting on hot spots but starves once prey runs out.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.The pest or disease level, set in advance, at which a grower takes a specific control action because waiting longer would cost more than acting.Mites, mostly of the family Phytoseiidae, released as biological control to eat pest mites, thrips larvae, whitefly eggs or fungus gnat larvae.
