The Ifakara Ambient Chamber Test (I-ACT) for Indoor Residual Sprays Evaluation: A Non-Inferiority Testing of Sylando^® 240SC and SumiShield^® 50WG

New insecticides for indoor residual spraying (IRS) are being developed to manage resistance. Chlorfenapyr (Sylando® 240SC), a pro-insecticide, is metabolised by active mosquitoes into the toxic metabolite tralopyril. This mode of action requires adapted “free flying” bioassays. A miniature-experimental hut (MEH) assay was developed within the Ifakara Ambient Chamber Test (I-ACT) with a rabbit blood host to measure residual efficacy under controlled conditions. Sylando® 240SC was compared with SumiShield® 50WG (clothianidin) for 12-month residual efficacy against malaria and arbovirus vectors. Residual activity was assessed on mud, wood and concrete with two huts per substrate treated with Sylando® 240SC, one with SumiShield® 50WG, and one untreated control. Five replicates of 20 mosquitoes per strain (malaria vectors: pyrethroid-susceptible Anopheles gambiae, resistant An. arabiensis and An. funestus; culicines Aedes aegypti and Culex quinquefasciatus) were exposed overnight at one-week post-spraying and monthly thereafter. Multivariable mixed-effects logistic regression assessed non-inferiority with a 7% margin. Both products induced delayed mortality, with higher effects on malaria vectors than culicines. Across all substrates and malaria species combined over the full 12 months of observation, Sylando® 240SC was non-inferior to SumiShield® 50WG at 72 h (76% vs. 67%, OR=0.86, 95% CI: 0.77–0.97) and 168 h (89% vs. 82%, OR=0.74, 95% CI: 0.63–0.87). Sylando® 240SC performed comparably to SumiShield® 50WG, supporting its use as an IRS option. The new I-ACT mini-experimental-hut assay provides a practical tool for evaluating pro-insecticides. The importance of free flight evaluation methods for pro-insecticides is discussed.