Influence of Larval Growth and Habitat Shading on Retreatment Frequencies of Biolarvicides against Dominant Malaria Vectors in Tanzania

Abstract We investigated the efficacy of two biolarvicides, VectoBac® and VectoMax®, against Anopheles funestus and Anopheles arabiensis, in shaded and unshaded semi-natural habitats; and explored how their varied larval development durations might influence retreatment intervals. Semi-natural aquatic habitats were created for field-collected An. funestus and An. arabiensis larvae in sun-exposed and shaded locations and treated with VectoBac® or VectoMax® or left untreated as controls. The larval development was assessed in semi-natural and natural habitats by recording daily densities of larval and pupal stages. Both biolarvicides reduced densities of early and late instar larvae of both mosquito species by over 98% within 72 hours. Efficacy lasted one week in sun-exposed environments regardless of dose but remained above 50% for two weeks in shaded habitats. On average, An. funestus larvae spent up to two weeks before pupating (13.2[10.4–16.0] days in semi-natural and 10.0[6.6–13.5] days in natural habitats), while An. arabiensis pupated after slightly over one week (8.2 [5.8–10.6] days in semi-natural and 8.32 [5.0-11.6] days in natural habitats). This data suggests that An. funestus might need a minimum of biweekly retreatments, while An. arabiensis requires a minimum of weekly treatments. Moreover, sun-exposed habitats may need more frequent applications than shaded habitats.