Labortary evaluation of Bacillus sphaericus recycling in mosquito larvae

ABSTRACT

After ingestion by Culex pipiens and Anopheles pharoensis 4th instar larvae, the spores of Bacillus sphaericus strain faiyoum rapidly germinated inside live mosquito midgut. Bacterial counts and electron microscopic observations on intoxicated larvae revealed that the number of viable spores rapidly decreased during the first 12 hours with a maximum between 12 and 24 hours. In cadavers, the number of heat-resistant spores quickly increased between the first and second day post-feeding. After one week, the number of spores inside the dead larvae reached approximately 20 times the number of ingested spores for both mosquito species [4 x 165 spores/larva]. Ultrathin sections of recycled spores showed the presence of a crystalline inclusion identical to that initially present in spores before ingestion. Bioassay on Cx. pipiens 4th instar larvae showed a similar toxicity between the in vivo recycled spores [LC50 = 1.1 +/- 0.3 x 105 spores/ml after 24 hours exposure] and culture-medium-grown spores of B. Sphaericus strain faiyoum [LC50 = 1.7 +/- 105 spores/ml]