The recent development of highly selective acaricides for spider mite control has made it necessary for the economic entomologist to know the particular species he is dealing with. This report, a guide to the spider mites known to occur on deciduous fruit trees in the United States and Canada, is intended to aid him in making determinations in the field and in the laboratory. Included are a taxonomic key to the species, a field guide, and instructions for sampling, slide preparation, and recognition of diagnostic characters. Of special interest are three new combinations, one new species, four new synonyms, and new information on color forms and seasonal phases of Tetranychus bimaculatus.
