THE SPIDER MITES, Tetranychus pacificus McGregor and Eotetranychus willamettei (Ewing) have increased to abundance in vineyards since World War II. In at least some cases it appears that organic pesticides have caused an imbalance in the occurrence of spider mites and their natural enemies, particularly by inhibiting action by Metaseiulus occidentalis (Nesbitt) —a predatory mite. Chemicals may cause imbalance in vineyards by differential kill of predators and prey, by conferring an advantage to the prey (by stimulating reproduction), or by a combination of the two.