Efficacy and persistence of Bacillus sphaericus strain 1593 against populations of Anopheles minimus and Anopheles dirus larvae in the field

Abstract

A flowable concentrated liquid formulation of Bacillus sphaericus strain 1593 was applied against Anopheles minimus larvae in slow running stream located in Ban Soke Noi, Dong Phayayen District. Nakorn Ratchadima Province at dosages of a laboratory-determined thousand times the LC(,50) value. The experiment was performed from January 24, 1987 to April 14, 1987. Mortality of the sentinel larvae and larval reduction in the field indicated that this formulation was an effective biocide against the larvae of An. minimus in this slow running stream with current rate between 1,228.5 cm(3)/min to 0.61 m(3)/min for at least nine weeks. Furthermore, B. sphaericus strain 1593 might be able to recycle and showed no noticeable adverse effects on the ecperimental aquatic organisms. Field trials of the persistence of B. sphaericus strain 1593 against Anopheles dirus larvae were conducted in gem pools located in orchards at Sri Phya, Thamai District, Chandhaburi Province, between July 30,1988 to October 10,1988. A flowable concentrated liquid formulation of B. sphaericus strain 1593 was applied to the test pools. Complete mortality (100%) of sentinel larvae of An. dirus was observed 48 h after exposure to B. sphaericus strain 1593 at dosages of a laboratorydetermined 5xLC(,50), 10xLC(,50) and 50xLC(,50). High mortality (92-100%) of sentinel larvae of An. dirus was detected after the first week of treatment with the 5xLC (,50) and 10xLC(,50) dosages. The mortality of sentinel larvae treated with these concentratios sharply declined to lower than 50% from the second week to the tenth week after exposure. Whereas the mortality of wentinel larvae treated with higher dosage equal to 50xLC(,50) was lower than 50% after first week of treatment onward.