Do leaf domatia mediate a plant-mite mutualism? An experimental test of the effects on predators and herbivores

1. Leaf domatia are tiny structures in leaf vein axils that are widespread among plant taxa and have been described to be typically inhabited by predatory and fungivorous mites. The mutualism hypothesis for the function of leaf domatia predicts that predatory and/or fungivorous mites benefit from having a favourable place to take refuge and reproduce and that plants benefit indirectly from reduced herbivory and/or pathogen attack. 2. The effect of leaf domatia on populations of predatory and herbivorous mites was examined for avocado, Persea americana. In separate experiments, domatia were added to leaves of a variety of avocado plants lacking domatia (Hass) and domatia were blocked on a domatia-bearing variety (Toro Canyon). 3. In two out of the five experiments conducted, domatia-bearing plants had significantly higher numbers of predatory mites compared with controls. Although herbivore numbers were consistently lower on plants with domatia than on plants without domatia, in no case did the presence of leaf domatia result in a statistically significant decrease in herbivorous mite populations. 4. These results suggest that domatia may frequently benefit predatory mites, however, indirect effects on herbivorous mites may not commonly exist or may be too difficult to detect.