A Baseline Quantitative Index for Ranking the Predation Capacity of Predatory Mites Against Tetranychus urticae

Choosing among predatory mites for controlling the two-spotted spider mite, Tetranychus urticae, still relies largely on empirical rules. We propose the Predation Potential Ranking Index (PPRI), a laboratory-derived index: the ratio of the predator’s to the prey’s intrinsic rate of increase (rm) multiplied by the predator’s daily prey consumption. We applied it to Phytoseiulus persimilis and Neoseiulus californicus at 20, 25 and 30 °C, using age-stage, two-sex life tables and daily predation assays. Mean rm across temperatures was 0.179 d−1 for T. urticae, 0.204 d−1 for P. persimilis and 0.226 d−1 for N. californicus. The predator-to-prey ratio ranged from 1.01 to 1.49, and its 95% CI included 1 at 30 °C for both predators. Mean daily consumption by adult females was 12.45 and 9.36 prey. PPRI was 14.44 (95% CI [13.68–15.20]) for P. persimilis and 12.05 ([11.31–12.80]) for N. californicus; the difference (2.39, [1.46–3.32]) was supported at 20 °C (2.86, [1.25–4.47]) and 30 °C (3.64, [2.13–5.15]) but not at 25 °C (0.67, [−1.07–2.41]), and arose from consumption, not from the rm ratio. PPRI assumes non-limiting prey and constant conditions and ignores density dependence. It is a screening and ranking tool, not a release-rate calculator, and requires greenhouse and field validation.