Evaluation Of The Potential Of Bacillus Thuringensis Ver. Israelensis De Barjac As A Mirobial Control Agent Of Target Mosquito Species In Kenya

Abstract

The efficacy of Bacillus thuringiensis serotype israeliensis (B.t.i) de BarLac as a larvicide of mosquitoes was evaluated both under laboratory and field conditions. In the laboratory, a wettable powder of B.t.i. Lot Rl5378 prepared by Laboratory Ro~er Bellon S.A. was bioassayed against larvae of Aedes aegypti. Anopheles arabiensis, Culex pipiens fatigans and Ano heles gambiae. Preliminary bioassays showed that the percenta~e mortality of all the four mosquito species decreased with a lowering of the concentration of B.t.i. A determination of the effect of water type on the larvicidal activities of B.t.i. indicated that there was no difference in the mortality of mosquito larvae in both lake and tap water. but that there was a decrease of larvicidal activity of B.t.i in pond water for all the mosquito species. An increase in the concentration of pond-water sediment resulted n a corresponding decrease in efficacy of B.t.i. A determination of the extent of settling of B.t.i. in still water. showed that B.Li. settles quickly if left aside for a short period of even 1 hour . Thus, there was a 30 fold difference in activity of the suspensions of B.t.i. from the top and bottom layer of a 14.2 em column within the first 2 hours of bioassay, although after 24 hours the mortality caused by both samples was the same. The presence of food and dead-B.t.i.-infected larvae increased the ortality rate against all the mosquito species tested. The activity of B.t.i. was unaffected by salinity, and pH within the tested limits, but an exposure of B.t.i. to UV-radiation for a period of 24 hours depleted the activity of B.t.i. Two B.t.i. formulations were used for small scale field trails against the 4 mosquito species. A Teknar formulation, (Tok ir Lot 41681), and Vectohac (As Lot No. 75 - Ol8-BA), both aqueous suspensions, Rave excellent results when applied at three dosage levels of 0.125. 0.25 and 0.5 kR/ha although the chemical check - Coopex have 100% ortality. Larval mortality of Ae. aegypti and Cx. fatigone has not significantly different, but was significantly different from that of An arabiensis and An. gambiae i.e the anopheline larvae were less susceptible to B.t.i. than Aedes and Culex larvae. The laboratory and field data, obtained indicate that B.t.i. is a very promising candidate microbial insecticide'for the control of larval mosquitoes.