Acarodomatia are small tufts of hair or invaginations in the leaf surface and are frequently inhabited by several taxa of non-plant-feeding mites. For many years, ecologists have hypothesized that these structures represent a mutualistic association between mites and plants where the mites benefit the plant by reducing densities of phytophagous arthropods and epiphytic microorganisms, and domatia benefit the mite by providing protection from stressful environmental conditions, other predaceous arthropods, or both. We tested these hypothesized benefits of domatia to domatia-inhabiting mites in laboratory and growth chamber experiments. In separate experiments we examined whether domatia on the wild grape, Vitis riparia, provided protection against drying humidity conditions or predaceous arthropods to two species of beneficial mite: the mycophagous species Orthotydeus lambi, and the predaceous species Amblyseius andersoni. For both taxa of beneficial mite, domatia significantly increased mite survivorship in the presence of the predatory bug, Orius insidiosus and the coccinellids Coccinella septempunctata and Harmonia varigata. There was no evidence for a protective effect of domatia with a third species of predatory arthropod, lacewing larvae Chrysoperla rufilabris. In contrast, there was no evidence for either species of beneficial mite that domatia provided any protection against low humidity. Thus in this system the primary mechanism by which domatia benefit beneficial mites is by protecting these organisms from other predatory arthropods on the leaf surface.

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Predaceous arthropods are frequently more abundant on plants with leaves that are pubescent or bear domatia than on plants with glabrous leaves. We explored the hypothesis that for some predatory mites this is because pubescence affords protection from intraguild predation. In laboratory experiments, we tested whether apple leaf pubescence protected Typhlodromus pyri eggs from predation by western flower thrips, Frankliniella occidentalis. To investigate the effect of pubescence further, we added cotton fibers to trichome-free leaves. We also determined whether webbing produced by Tetranychus urticae protected Phytoseiulus persimilis eggs from predation by F. occidentalis. Predation by thrips on T. pyri eggs oviposited on field-collected pubescent "Erwin Bauer" apple leaves was significantly less than on glabrous "Crittenden" apple leaves. Phytoseiid eggs oviposited in the cotton fibers were preyed upon significantly less than those on the trichome-free bean disk. Increasing the cotton fiber density from 5 to 20 fibers only slightly further reduced predation by thrips on T. pyri eggs. Thrips fed upon significantly fewer P. persimilis eggs oviposited in Te. urticae webbing than eggs oviposited on a surface that differed only in the absence of Te. urticae web. We conclude that a complex leaf topography reduces intensity of intraguild predation in this system.