Impact of Bacillus sphaericus exposure on Anopheles dirus's fecundity and resistance development

Mosquitoes are important vectors of many infectious diseases. Bacillus sphaericus (Bs) is an ideal larvicide and has attracted more and more attention, recently. However, the fundamental research of its application is very limited, especially on the subsequent impact of Bs exposure on mosquito's fecundity and resistance emergence. Through bioassay, LC50 and LC95 of Bs in killing Anopheles dirus larvae were determined as 9.793 ± 1.878 IU/L and 62.4 ± 6.438 IU/L at 48 h posttreatment, 7.128 ± 0.913 IU/L and 34.385 ± 12.547 IU/L at 72 h post treatment, respectively. After being treated with a sub-lethal dose of Bs, gravidity, oviposition, hatch, pupation, and eclosion of the surviving mosquitoes were counted and analyzed to elucidate the subsequent effects of Bs exposure on the reproductive capacity of A. dirus. The result interestingly showed that the exposure of Bs significantly reduced the oviposition ability of the surviving A. dirus, without effect on egg formation/gravidity, hatch, pupation, and eclosion. The surviving mosquitoes were also maintained routinely for generations to test the sustained effect of Bs exposure on the fecundity of the offsprings. After conventional breeding for generations, the capacity of egg laying totally recovered. To explore the rules of resistance development, bioassays were performed after treatment twice with a sub-lethal dose of Bs on two continuous generations of A. dirus larvae. The killing efficacies between the Bs treated group and control group were compared. The results showed that LC50 and LC95 increased by 4.35- and 7.37-folds after treatment with the sub-lethal dose of Bs on two consecutive generations, respectively. The results indicated that A. dirus was sensitive to Bs, which could reduce oviposition of the surviving A. dirus. The subsequent effect might help to further decrease the mosquito population. However, a sub-lethal dose of Bs exposure could easily cause resistance development. Our study provides a dose standard and reference for the rational use of Bs, which will be helpful for mosquito control.