Biological control of spider mites is difficult in California greenhouses. Reliable supplies of high quality predators are lacking, available natural enemies are susceptible to pesticides, and predators are thought to be unable to regulate spider mites in sufficiently low densities to prevent economic loss, especially on ornamental plants. This paper evaluates the performance of genetically improved strains of a phytoseiid, Metaseiulus occidentalis (Nesbitt), that possess traits that could provide effective, permanent control of Tetranychus urticae Koch on roses.
