RELATIVE TOXICITY OF BACILLUS-THURINGIENSIS VAR TENEBRIONIS TO THE 2-SPOTTED SPIDER-MITE (TETRANYCHUS-URTICAE KOCH) AND ITS PREDATOR METASEIULUS-OCCIDENTALIS (NESBITT) (ACARI, TETRANYCHIDAE AND PHYTOSEIIDAE)

Bacillus thuringiensis var. tenebrionis (Btte) wettable powders were tested in the laboratory to determine their relative toxicity to the two-spotted spider mite (Tetranychus urticae Koch) and its predator Metaseiulus (= Typhlodromus) occidentalis (Nesbitt). These microbial preparations, when tested at 0.1X, 0.5X, and 1.0X the recommended field rate of 0.9 kg ai/75.7 l/acre (2.0 lb ai/20.0 gal/ acre) caused little mortality to adult females of T. urticae but were considerably more toxic to M. occidentalis females at 48 h. Female T. urticae exhibited 90.0 +/- 14.2 % survival vs. 26.0 +/- 23.4 % survival for M. occidentalis at the field rate. Starvation of female predators for 24 h prior to treatment with Btte wettable powder significantly increased mortality. Neither predator nor spider mite eggs treated with Btte wettable powder at the field rate suffered a depression in hatch rate (98.0 % and 97.2 % hatch, respectively, of eggs treated when 24 h old). However, only 65.0 % of the resulting immature predators reached adulthood compared to 88.8 % of the immature spider mites. Female predators which survived treatment as eggs and developed on residues did not show a significant decrease in fecundity. Thus, under these laboratory conditions, Btte was moderately toxic to predator mites. Finally, treatment of M. occidentalis females with other Btte products (Mycogen M-ONE(R) and Sandoz Trident(R)) and with B.t. var. kurstaki (Dipel 2X) also resulted in significant mortality.