Domatia are small invaginations and hair tufts usually found at vein junctions on the undersides of leaves in many woody dicots. Domatia of 32 plant species (of worldwide origin from 18 families) growing in California, Hawaii, and Costa Rica were examined for mites. Domatia of 31 of 32 (97%) of these plants contained mites, and 24 of 32 (75%) contained mite eggs. Mites were found within the domatia of 48% of the sampled leaves. The domatia of 26 of 31 (84%) plants had mite species considered beneficial (primarily in the families Phytoseiidae and Tydeidae, but also Bdellidae, Cheyletidae and Stigmaeidae), while 6 of 31 (19%) had mite species considered harmful (Tenuipalpidae and Eriophyidae). Based on these findings (and in part upon Lundström's 1887 domatia theory), we hypothesize the existence of a widespread facultative mutualism between plants with leaf domatia and beneficial mites: leaf domatia serve as shelters and nurseries for mites which in turn eat phytophagous arthropods and pathogens using the plants. This proposed mutualism could be of importance to agriculture since domatia are known to occur in some crop plants, including coffee, grape, and walnut.

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Seneciojacobaea, a poisonous weed from Eurasia, was brought under successful biological control in the Ft. Bragg, California area by 1976, through the combined action of the defoliating cinnabar moth (Tyriajacobaeae) and a root feeding flea beetle (Longitarsusjacobaeae). In 1987, 4 previously infested Ft. Bragg sites (3 sites where control had been documented and another unstudied site) were examined. Senecio jacobaea densities at these sites were 0.0, 0.0, 0.01 and 0.18 plants/m 2, indicating both continued and improved control of the weed. The flea beetle and the cinnabar moth both persist at the sites, despite very low numbers of S.jacobaea plants. The control of S. jacobaea has resulted in the return of near natural vegetation at the 2 coastal prairie sites and regained productivity at the 2 pasture sites.