Laboratory efficacy of Bactivec ® and Griselesf ® biolarvicides used for large-scale larviciding in Tanzania

Tegemeo Gavana^1,2,3*Denis Kailembo^2,3Jane Johnson Machange⁴Venance Michael⁴Johnson Kyeba Swai^2,3,4Olukayode Odufuwa^2,3,4Frank Tenywa^2,3,4Charles Dismas Mwalimu⁵Jubilate Bernard Minja⁵Samwel Lazaro⁵Best Yoram⁶Stella Kajange⁶Elizabeth Kasagama²Noela Kisoka²Emmanuel Mbuba^2,3,4Prosper Pius Chaki¹Christian Lengeler^2,3Sarah Moore^2,3,4,7

• ¹Environmental Health and Ecological Sciences, Ifakara Health Institute, Dar es Salaam, Tanzania
• ²Swiss Tropical and Public Health Institute (Swiss TPH), Basel, Basel-Landschaft, Switzerland
• ³University of Basel, Basel, Switzerland
• ⁴Vector Control Product Testing Unit, Environmental Health and Ecological Science, Ifakara Health Institute, Bagamoyo, Tanzania
• ⁵National Malaria Control Programme, Ministry of Health, Dodoma, Tanzania
• ⁶President Office, Regional Administration and Local Government, Government City – Mtumba TAMISEMI street,, Dodoma, Tanzania
• ⁷Nelson Mandela African Institution of Science and Technology, Arusha, Arusha, Tanzania

Background: From 2022 to 2024, a project piloting large-scale larviciding in Tanzania was implemented in Tanga Region. The project used in-country manufactured biolarvicides, Bactivec® and Griselesf®. This study independently assessed the efficacy of both biolarvicides to ensure that they represented a good option for scaling up. Methodology: Laboratory-based dose-response experiments were performed using Bactivec® and Griselesf® against laboratory-reared early third instar larvae of five different mosquito species. Larvae were exposed to various larvicide concentrations. VectoBac® served as a positive control, and distilled water as a negative control. Twelve replicates per concentration, with 25 larvae per replicate, were tested. Larval mortality was recorded at 24 and 48 hours after exposure to Bactivec® and Griselesf®, respectively. Probit regression analysis was used to determine the lethal concentration (LC₅₀ and LC₉₀) values. Results: Bactivec® demonstrated similar LC50 values across all species, ranging from 0.0122 mg/mL (95% CI: 0.0114 - 0.0130) for An. gambiae s.s. to 0.0155 mg/mL (95% CI: 0.0147 - 0.0163) for An. funestus. LC90 varied slightly, with An. arabiensis being the most susceptible at 0.0217 mg/mL (95% CI: 0.0202 - 0.0233), and An. gambiae s.s. the least at 0.0330 mg/mL (95% CI: 0.0298 - 0.0363). Griselesf® showed greater variation, with LC50 ranging from 0.0130 mg/mL (95% CI: 0.0124 - 0.0136) for An. gambiae s.s. and 0.0212 mg/mL (95% CI: 0.0199 - 0.0226) for An. arabiensis. Similarly, the LC90 for Griselesf® also varied, being the lowest for An. gambiae s.s., 0.0235 mg/mL (95% CI: 0.0219 - 0.0252) and the highest for An. arabiensis, 0.0609 mg/mL (95% CI: 0.0548 - 0.0669). Ae. aegypti was not susceptible to Griselesf® at the concentrations tested. The LC90 observed in this study were below the maximum application rates recommended by the manufacturer but exceeded the minimum application rates for both larvicides.Conclusion: The Tanzanian-made Bactivec® and Griselesf® demonstrated efficacy against multiple species of mosquitoes, when applied according to the manufacturer’s recommendations except for Ae. aegypti, which was not susceptible to Griselesf®. Field applications should use maximum application rates. The availability of these efficacious biolarvicides within the country will enhance both the feasibility and sustainability of the scale-up efforts.