# Predator:prey ratio

> The number of predators per pest at release or on scouting, used to judge whether biological control will catch up; spider mite guides often cite 1:10 to 1:20.

- Canonical: https://genetixbay.com/knowledge/glossary/predator-prey-ratio.html
- Also written: predator to prey ratio, predator-prey ratio, release ratio
- Glossary category: Pests and diseases · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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](https://genetixbay.com/knowledge/glossary/biological-control.md) release is winning. Biocontrol guides for [spider mites](https://genetixbay.com/knowledge/glossary/two-spotted-spider-mite.md) commonly give ratios in the region of one [Phytoseiulus persimilis](https://genetixbay.com/knowledge/glossary/phytoseiulus-persimilis.md) 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](https://genetixbay.com/knowledge/glossary/relative-humidity.md) 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.

## Related terms

[Predatory mite](https://genetixbay.com/knowledge/glossary/predatory-mite.md), [Phytoseiulus persimilis](https://genetixbay.com/knowledge/glossary/phytoseiulus-persimilis.md), [Biological control](https://genetixbay.com/knowledge/glossary/biological-control.md), [Action threshold](https://genetixbay.com/knowledge/glossary/action-threshold.md)

## Read more

- [Integrated pest management (IPM) for cannabis: a complete programme for home and small commercial growers](https://genetixbay.com/knowledge/pests/integrated-pest-management-program.md)
- [Temperature, humidity and pest population growth in cannabis: every major pest compared](https://genetixbay.com/knowledge/pests/temperature-humidity-and-pest-population-growth.md)
- [Spider mites on cannabis: life tables, ID and IPM](https://genetixbay.com/knowledge/pests/spider-mites.md)

## Sources

1. UConn IPM. Biological control of spider mites (2019 fact sheet). https://ipm.cahnr.uconn.edu/wp-content/uploads/sites/3216/2022/12/2019biologicalcontrolofspidermitesfinal2-1.pdf
2. UC IPM. Pest Notes: Spider Mites (Publication 7405). University of California ANR. https://ipm.ucanr.edu/PMG/PESTNOTES/pn7405.html