Comparative Toxicity of Avermectin B1 to the Predator Metaseiulus occidentalis (Nesbitt) (Acari: Phytoseiidae) and the Spider Mites Tetranychus urticae Koch and Panonychus ulmi (Koch) (Acari: Tetranychidae)

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Abstract
The relative toxicity of avermectin B1 was assessed in the laboratory, using adult females of Tetranychus uritcae Koch, Panonychus ulmi (Koch), and Metaseiulus occidentalis (Nesbitt). Avermectin B1 was less toxic to M. occidentalis than to the spider mites. However, at the proposed field rates (4, 8, and 16 ppm), few M. occidentalis survived and fecundity was reduced. Larvae of M. occidentalis were more susceptible to avermectin B1 at 0.1, 4, 8, and 16 ppm, than adult females. Eggs of M. occidentalis hatched on leaf discs containing residues of avermectin B1 applied at field rates, but none of the larvae survived to the nymphal stage. Larvae exposed to avermectin B1 at 0.001, 0.01, and 0.1 ppm matured into adults and deposited eggs, although at 0.1 ppm survival, activity, and developmental rates were reduced. The differential toxicity of avermectin B1 to this predator could be of practical importance in managing spider mite populations in the field, particularly if refuges exist or if resistant predator strains can be developed.