Susceptibility of Anopheles gambiae s.l. and Residual Efficacy of Klypson 500WG and 2GARD-WP Under Operational IRS Conditions in Ethiopia

Abstract Background Insecticide susceptibility and residual activity determine the effectiveness of indoor residual spraying (IRS) for malaria control. Clothianidin-based insecticides have shown prolonged efficacy in experimental huts and limited field trials, but evidence on Klypson 500WG and 2GARD-WP is lacking. In this study, we evaluated the susceptibility of Anopheles gambiae s.l. to deltamethrin, pirimiphos-methyl, and clothianidin, and assessed the residual efficacy of Klypson 500WG and 2GARD-WP alongside SumiShield 50WG in Ethiopia. Methods The susceptibility of wild Anopheles gambiae s.l. was tested against clothianidin, pirimiphos-methyl, and deltamethrin following WHO protocols. The residual efficacy of Klypson 500WG, 2GARD-WP, and SumiShield 50WG was assessed monthly on mud and painted surfaces using cone bioassays against An. arabiensis , with mortality recorded between 24 to 120 hours after exposure. Results were interpreted based on WHO thresholds. Logistic regression models determined the effective residual lifespan of the insecticides, while mixed-effect logistic regression identified predictors of IRS performance. Results Anopheles gambiae s.l. populations were fully susceptible to clothianidin in most sites, though slightly reduced mortality in Dilla (93.2%) and Werkamba (97.3%) suggests the need for continued vigilance. Resistance to deltamethrin was evident in Mekhoni (77.3%) and Werkamba (64.3%), while pirimiphos-methyl achieved complete mortality in both sites. Klypson 500WG and 2GARD-WP maintained ≥ 80% mortality for 7–10 months on mud walls and up to 10–11 months on painted walls at 120-hour holding time, but only for 2–6 months at shorter holding times (24–72 hours). Regression analysis indicated significantly lower mortality odds for Klypson 500WG and 2GARD-WP compared to SumiShield 50WG, though differences were moderate. Conclusion Clothianidin remains effective for An. gambiae s.l. control across Ethiopian sites, with reduced mortality in Dilla and Werkamba, suggesting possible early resistance and highlighting the need for continuous monitoring to preserve its effectiveness. Retained susceptibility to pirimiphos-methyl highlights that organophosphates may remain viable alternatives, while deltamethrin resistance highlights potential limitations for ITNs' performance. Klypson 500WG and 2GARD-WP demonstrated prolonged residual efficacy under field conditions, with performance broadly comparable to SumiShield 50WG. These findings provide operational evidence on Klypson 500WG and 2GARD-WP and underscore the importance of continued monitoring to guide IRS policy and resistance management.