Evaluation of a long-lasting microbial larvicide against Culex quinquefasciatus and Aedes aegypti under laboratory and a semi-field trial

Abstract

Background Microbial larvicides containing both Lysinibacillus sphaericus and Bacillus thuringiensis svar. israelensis (Bti) insecticidal crystals can display advantages for mosquito control. This includes a broader action against larvae that are refractory to the Binary (Bin) toxin from Lysinibacillus sphaericus, as Bin-resistant Culex quinquefasciatus and Aedes aegypti larvae, that often co-habit urban areas of endemic countries for arboviruses. Our major goal was to assess the toxicity of a L. sphaericus/Bti larvicide (Vectomax FG™) to Cx. quinquefasciatus (susceptible CqS and Bin-resistant CqR) and Ae. aegypti (Aae) and to determine its persistence in the breeding sites with those larvae. Methods The toxicity of L. sphaericus/Bti to larvae was performed using bioassays, the persistence was evaluated in simulate field trials carried out under the shade and testing two label concentrations for twelve weeks. A laboratory strain SREC was established with CqS and CqR larvae and kept for four generations to evaluate the ability of the L. sphaericus /Bti to eliminate resistant larvae. Results The L. sphaericus/Bti showed toxicity to larvae from all strains with a decreasing pattern for CqS (LC₅₀ = 0.006, LC₉₀ = 0.030), CqR (LC₅₀ = 0.009, LC₉₀ = 0.069) and Aae (LC₅₀ = 0.042, LC₉₀ = 0.086). In a simulated field trial, the larvicide showed a persistence of six and eight weeks, controlling larvae from all strains, in containers with 100 L of water, using 2 g or 4 g per containers, respectively. The treatment of SREC larvae with L. sphaericus/Bti showed its capacity to eliminate the Bin-resistant larvae using suitable concentrations to target those larvae. Conclusion Our results showed the high efficacy and persistence of the L. sphaericus/Bti larvicide to control Cx. quinquefasciatus and Ae. aegypti that might cohabit breeding sites. These findings demonstrated that such larvicides can be an effective tool for controlling those species in urban areas with a low potential for selecting resistance.