A genetic selection program to develop a carbaryl-resistant strain of Metaseiulus occidentalis, the predator of the spider mite, has been successful. This is believed to be the first time that a genetically improved biological control agent has been proved effective in the field.

Appreciation is expressed to Dennis Culver of ICI Americas, Inc., Visalia, and Earl Surber of Blackwell Farms, Blackwell's Corner, for their generous cooperation and to Wilbur Reil and Les Barclay for their logistical support. This project was supported by the Almond Board of California, California Agricultural Experiment Station Project No. 3522-H, and a National Science Foundation Predoctoral Fellowship to the first author.