RAISING ACTIVITY OF BACILLUS-THURINGIENSIS VAR ISRAELENSIS AGAINST ANOPHELES-STEPHENSI LARVAE BY ENCAPSULATION IN TETRAHYMENA-PYRIFORMIS (HYMENOSTOMATIDA, TETRAHYMENIDAE)

Toxicity of Bacillus thuringiensis var. israelensis (B.t.i.) against s urface-feeding mosquito larvae of Anopheles stephensi was enhanced by encapsulation in the protozoan Tetrahymena pyriformis. In the laborato ry, larvae died about 8 times faster when exposed to protozoan cells f illed with B.t.i. than when exposed to the same concentrations of B.t. i. alone. Best larvicidal activities were achieved with ratios of 1:20 0-1:500 T. pyriformis cells to B.t.i. spores. The concentration of B.t .i. needed to kill 50% of exposed populations was 4-fold lower with T. pyriformis than with B.t.i. alone in 100 ml-test cups. Toxicity enhan cement is very likely a consequence of concentrating B.t.i. insecticid al crystal proteins in T. pyriformis cells and floating them to the wa ter surface in the larval feeding zone. Reduction in the exposure time of B.t.i. to unfavorable field conditions, as a result of the decreas e in larval mortality time, might improve the persistence of this biol ogical control agent in nature.