Characteristics of VectoBac® WG spray, a Bacillus thuringiensis israelensis (Bti) strain AM65-62 water-dispersible granule formulation, from a backpack sprayer Stihl® SR420 to achieve effective dengue vector control in operational programs

The productive outdoor larval habitats of dengue vectors are widespread, cryptic and hard-to-reach. It is a challenge to larvicide such habitats using traditional manual sprayers. A wide-area treatment with Bacillus thuringiensis israelensis (Bti) microdroplets dispersed from a motorized backpack sprayer into such habitats is a proven method to suppress the vector population and interrupt disease transmission. This paper provides the answers to the constant inquiries by program operators on the characteristics of a Bti wide-area spray from a motorized backpack sprayer. The Bti spray mix can be either 125g or 250g in 10 liters water with a walking speed of 1m in 6 secs or 1m in 3 secs, respectively, to achieve the recommended dose of 400g/ha-500g/ha. The spray dispersed at a flowrate of 567mL-650mL through the No. 2 sprayer orifice, reached a 16m swath. A 90%-100% larval mortality was obtained in containers that were left outdoors for 7 to 14 days post spray under a shade, exposed to ambient temperature and sunlight. Higher Bti doses, at 2x and 3x to the recommended dose, achieved 90%-100% mortality in containers left outdoors for more than 14 days, but higher doses are not recommended in areas with persistent rainfall.