High incidences of red, itching, and painful welts on people in the midwestern United States led to the discovery of a European species of mite, Pyemotes herfsi (Oudemans) (Acari: Pyemotidae), preying on gall-making midge larvae on oak leaves. The mites' great reproductive potential, small size, and high capacity for dispersal by wind make them difficult to control or avoid.