Abstract:
Background: Chemical-based malaria vector control interventions are threatened by the
development of insecticide resistance and changes in the behavior of the vectors, and
thus require the development of alternative control methods. Bacterial-based larvicides
have the potential to target both insecticide resistant and outdoor-biting mosquitoes and
are safe to use in the environment. However, the currently available microbial larvicide
formulations have a short duration of activity requiring frequent re-applications which
increase the cost of control interventions. This study was designed to evaluate the
efficacy and duration of activity of two long-lasting formulations of Bacillus
thuringiensis var. israelensis (Bti) and Bacillus sphaericus (Bs) (LL3 and FourStar®)
under field conditions in western Kenya highlands.
Methods: Three sites were selected for this study in the highlands of western Kenya. In
each site, one hundred anopheline larval habitats were selected and assigned to one of
three arms: (i) LL3; (ii) FourStar®; and (iii) untreated control larval habitats. Four types
of larval habitats were surveyed: abandoned gold mines, drainage canals, fish ponds and
non-fish ponds. The habitats were sampled for mosquito larvae by using a standard
dipping technique and collected larvae were recorded according to the larval stages of the
different Anopheles species. The larvicides were applied at manufacturers' recommended
dosage of 1 briquette per 100 square feet. Both treatment and control habitats were
sampled for mosquito larvae immediately before treatment (day 0), and then at 24 hours,
3 days and weekly post-treatment for 5 months.
Results: Overall larval density in treatment habitats was significantly reduced after
application of the two microbial larvicides as compared to the control habitats. Postintervention reduction in anopheline larval density by LL3 was 65, 71 and 84% for 1 day,
2 weeks and 4 weeks, respectively. FourStar® reduced anopheline larval density by 60,
66 and 80% for 1 day, 2 weeks and 4 weeks, respectively. Comparisons between the
treatments reveal that LL3 and FourStar® were similar in efficacy. A higher reduction in
Anopheles larval density was observed in the abandoned goldmines, while drainage
canals had the lowest reduction.
Conclusions: Both LL3 and FourStar® long-lasting microbial larvicides were effective in
reducing immature stages of An. gambiae complex and An. funestus group species, with
significant reductions lasting for three months post-application.