Predatory mites Amblydromalus limonicus and Amblyseius herbicolus (Acari: Phytoseiidae) as potential biocontrol agents of Eotetranychus sexmaculatus (Acari: Tetranychidae) in avocado: examining predation on different prey life stages

Abstract

The six-spotted spider mite, Eotetranychus sexmaculatus (Riley) (Acari: Tetranychidae), is a severe pest of avocado trees, causing excessive leaf drop and reduced yield. Two predators found to be sympatric with E. sexmaculatus in surveys, Amblydromalus limonicus (Garman & McGregor) and Amblyseius herbicolus (Chant) (Acari: Phytoseiidae), may serve as potential biological control agents. We examined their potential in 2 laboratory experiments. (i) In no-choice tests, each predator was presented with 40 eggs, 40 larvae, 40 nymphs, or 20 female adults of E. sexmaculatus, and predation and oviposition rates (per day) were measured. (ii) In choice tests, predators were given a mixture of 10 eggs, 10 larvae, 10 nymphs, and 10 female adults, and the same parameters were measured, but with prey stage preference also assessed using Manly's preference index. Our results showed both A. limonicus and A. herbicolus fed and reproduced on all stages of E. sexmaculatus. Significantly higher predation rates were observed for A. limonicus on E. sexmaculatus than A. herbicolus. Both predator species preferred earlier life stages of E. sexmaculatus, with the preference more pronounced in A. herbicolus than in A. limonicus. In both choice and no-choice experiments, the oviposition rate (one criterion for determining whether artificially released biological control agents can use the nutrients of the new prey to reproduce) was higher in A. limonicus than A. herbicolus. This study is the first to report on the biological control potential of these two predatory mite species for use against E. sexmaculatus in avocado plantations.

Keywords: Manly’s preference index; biological control; interspecies interaction; non-invasive; six-spotted spider mite.

Fig. 1.

Experiment 1 (no-choice) egg treatment group data showing the proportion of prey eggs partially consumed by Amblydromalus limonicus and Amblyseius herbicolus out of all prey eggs consumed over a 24-h period. Asterisks denote the significant differences between two predator species (Wilcoxon rank-sum test: **P < 0.01). The “cloud” represents the kernel density estimation (KDE), showing the smoothed distribution of the data. The width of the “cloud” reflects the density of the data (the wider the clouds, the more possible data point estimates, or vice versa).

Fig. 2.

The number (Mean ± SEM) of eggs laid by Amblydromalus limonicus and Amblyseius herbicolus in the arena during no-choice (left) and choice (right) tests. Asterisks denote significant differences in oviposition rate within the same prey life stage between two species of predatory mites (Wilcoxon rank-sum test: ** P < 0.01; * P < 0.05). ^ab Means ART ANOVA pairwise comparisons in oviposition rate within the same species of predatory mites, different letters denote significant differences.

Fig. 3.

Choice test (Experiment 2): Manly’s preference index curves for adult females of Amblydromalus limonicus and Amblyseius herbicolus consuming different life stages of the prey Eotetranychus sexmaculatus. Bars show mean ± SEM values.