Effects of larval exposure to sublethal doses of Bacillus thuringiensis var. israelensis on body size, oviposition and survival of adult Anopheles coluzzii mosquitoes.

BACKGROUND

Application of the larvicide Bacillus thuringiensis var. israelensis (Bti) is a viable complementary strategy for malaria control. Efficacy of Bti is dose-dependent. There is a knowledge gap on the effects of larval exposure to sublethal Bti doses on emerging adult mosquitoes. The present study examined the effect of larval exposure to sublethal doses of Bti on the survival, body size and oviposition rate in adult Anopheles coluzzii.

METHODS

Third-instar An. coluzzii larvae were exposed to control and sublethal Bti concentrations at LC, LC and LC for 48 h. Surviving larvae were reared to adults under standard colony conditions. Thirty randomly selected females from each treatment were placed in separate cages and allowed to blood feed. Twenty-five gravid females from the blood-feeding cages were randomly selected and transferred into new cages where they were provided with oviposition cups. Numbers of eggs laid in each cage and mortality of all adult mosquitoes were recorded daily. Wing lengths were measured of 570 mosquitoes as a proxy for body size.

RESULTS

Exposure to LCBti doses for 48 h as third-instar larvae reduced longevity of adult An. coluzzii mosquitoes. Time to death was 2.58 times shorter in females exposed to LCBti when compared to the control females. Estimated mortality hazard rates were also higher in females exposed to the LC and LC treatments, but these differences were not statistically significant. The females exposed to LC concentrations had 12% longer wings than the control group (P < 0.01). No differences in oviposition rate of the gravid females were observed between the treatments.

CONCLUSIONS

Exposure of An. coluzzii larvae to sublethal Bti doses reduces longevity of resultant adults and is associated with larger adult size and unclear effect on oviposition. These findings suggest that anopheline larval exposure to sublethal Bti doses, though not recommended, could reduce vectorial capacity for malaria vector populations by increasing mortality of resultant adults.