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.

Laboratory tests determined the relative toxicity of 11 rose house pesticides to a non-diapausing, carbaryl- and organophosphorus-resist-ant strain of M. occidentalis and T. urticae. At normal label rates, acephate was the only one rated highly toxic to this predator strain. Carbaryl, pirimicarb, endosulfan, hexakis, dienochlor, benomyl, triforine, parinol, piperalin, and oxycarboxin were rated as having low toxicity to M. occidentalis. Hexakis and benomyl were toxic to T. urticae, endosulfan was moderately toxic, and all others exhibited low toxicity.

The non-diapausing strain of M. occidentalis remained reproductive all year under simulated rose house conditions, whereas the diapausing strain did not. Biological attributes of the non-diapausing strain were sufficiently good to conclude this strain could be effective all year.

Metaseiulus occidentalis was evaluated for its ability to regulate T. urticae on small noncommercial rose house plots during 2 years. Carbaryl, pirimicarb, triforine, oxycarboxin, hexakis, and dienochlor were successfully used for pest and disease control without disrupting the predators, confirming the laboratory data. Metaseiulus occidentalis is unlikely to provide complete biological control of T. urticae on greenhouse roses, however. Acaricidal control may be needed (using reduced rates) while M. occidentalis establishes, and following annual pruning. Large-scale commercial trials should be considered with the non-diapausing pesticide-resistant strain.