SIMULATED FIELD STUDIES WITH 3 FORMULATIONS OF BACILLUS-THURINGIENSIS VAR ISRAELENSIS AND BACILLUS-SPHAERICUS AGAINST LARVAE OF MANSONIA-BONNEAE (DIPTERA, CULICIDAE) IN SARAWAK, MALAYSIA

The susceptibility levels of Mansonia bonneae Edwards larvae to Bacillus thuringiensis israelensis (B.t.i.) SkeetalR, B.t.i. BactimosR briquettes and Bacillus sphaericus RB80 were evaluated under field conditions. Satisfactory control of larvae was achieved during 24 h—48 h post-treatment. At 72 h after treatment with a 48 h exposure period, over 90% reduction was achieved by SkeetalR at both applied dosages and by B. sphaericus RB80 at the higher applied dosage. BactimosR briquettes achieved less control at both applied dosages, due to slow, sustained release of B.t.i. spores. SkeetalR and B. sphaericus RB80 released their toxic ingredients as early as 2 h after treatment, after which the spore counts (CFU/ml) increased steadily in the middle and bottom water layers and reached maximum level at 72 h post-treatment. However, SkeetalR displayed very little residual activity as evidenced by the drastic decline of spore counts as well as larval reduction at one week post-treatment. By contrast, BactimosR briquette and B. sphaericus RB80 prolonged their residual toxicity throughout the four week period. The peaks of spore counts in the middle and bottom water layers for SkeetalR and B. sphaericus RB80 preceded the corresponding peaks for larval reduction rates by 24 h and 48 h and 1 week to 2 weeks, respectively. The spore counts of BactimosR briquette at the high applied dosage fluctuated closely with the reduction rates of larvae throughout the four week period. © 1990, Cambridge University Press. All rights reserved.