Semi-field evaluation of efficacy and residual activity of a microencapsulated pyriproxyfen formulation on Anopheles arabiensis emergence inhibition

Background: The integration of larviciding as a supplementary tool for malaria vector control requires effective larvicide formulations that are specifically suited to the target vector species, environmentally friendly, and cost-effective to reflect the needs of resource-limited settings in sub-Saharan Africa. This study evaluated the sublethal effect and its residual activity of two microencapsulated pyriproxyfen formulations containing 33% and 50% active ingredient (AI) against Anopheles arabiensis larvae exposed to sublethal doses.

Materials and Methods: Sublethal effects on fitness parameters, including fecundity and wing length, were evaluated at concentrations that resulted in 20%, 50%, and 70% emergence inhibition, obtained from a dose-response curve. In three replicates across all three concentrations, fecundity was assessed in 25 adult females, while wing length was measured in 33 adult males and 33 adult females. Residual activity was assessed by exposing 200 3^rd-instar larvae of An. arabiensis to each formulation at 0.06 mg/L 33% AI and 0.09 mg/L 50% AI in 5 replicates of artificial habitats. Once all mosquitoes had either emerged or died, a new batch of larvae was introduced every 2 weeks for 6 months.

Results: Females emerged from concentrations inhibiting 70% of larval emergence exhibited significantly reduced fecundity; 88% for 33% AI [RR = 0.12, 95% CI: 0.11, 0.13, p < 0.001] and 85% for 50% AI [RR = 0.14, 95% CI: 0.07, 0.32, p < 0.001]. Adults emerged from sublethal concentrations had reduced body size, decreasing as concentration increased. Residual activity on emergence inhibition was 14.4% [13.2%, 15.6%], and 23.1% [18.7%, 27.5%], relative to 23% and 14% for 33% and 50% AI, respectively in the 6th month. Control mortality was consistently below 10% during the entire evaluation period.

Conclusion: These findings demonstrate the efficaciousness and long-lasting potential of microencapsulated pyriproxyfen formulations to target An. arabiensis, and highlight its consideration for application in larviciding programmes.