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

## TL;DR

A new test was developed to evaluate the effectiveness of Sylando® 240SC, a pro-insecticide, for indoor residual spraying, showing it performs similarly to SumiShield® 50WG over 12 months.

## Contribution

The study introduces the I-ACT mini-hut assay for evaluating pro-insecticides and demonstrates Sylando® 240SC's non-inferiority to SumiShield® 50WG.

## Key findings

- Sylando® 240SC showed non-inferior mosquito mortality compared to SumiShield® 50WG at 72 and 168 hours.
- The I-ACT mini-hut assay is a practical tool for evaluating pro-insecticides under controlled conditions.
- Both insecticides had higher effects on malaria vectors than on non-malaria vectors like Aedes aegypti.

## Abstract

Alternative classes of insecticides for indoor residual spray (IRS) are needed for insecticide resistance management. Chlorfenapyr (Sylando® 240SC) is a pro-insecticide and requires mosquitoes to be active to metabolize the parent molecule into its insecticidal metabolite tralopyril, a process that may take longer than other insecticidal modes of action. To evaluate its residual activity, a new laboratory assay was developed using a miniature experimental hut with a rabbit as a host inside the Ifakara Ambient Chamber Test (I-ACT). This study’s aim was to measure the residual efficacy of Sylando® 240SC using malaria and dengue vectors. The entomological efficacy of Sylando® 240SC was assessed against SumiShield® 50WG over 12 months on mud, wood, and concrete surfaces using twelve mini-huts. Each substrate had two huts for Sylando® 240SC, one for SumiShield® 50WG and one negative control (water). The study evaluated the non-inferiority of Sylando® 240SC against three malaria vectors (pyrethroid-susceptible An. gambiae and -resistant An. arabiensis and An. funestus) and their efficacy on non-malaria vectors (Aedes aegypti and Cx. quinquefasciatus). Sylando® 240SC demonstrated comparable performance to SumiShield® 50WG IRS, making it a potential candidate for indoor residual spraying (IRS) in vector control. Additionally, a new bioassay has been developed for evaluating pro-insecticides.

New insecticides for indoor residual spraying (IRS) are being developed to manage resistance. Chlorfenapyr (Sylando® 240SC), a pro-insecticide, is metabolized by active mosquitoes into the toxic metabolite tralopyril. This mode of action requires adapted “free flying” bioassays (because chlorfenapyr is converted to its toxic metabolite tralopyril when mosquitoes are metabolically active). A miniature-experimental hut (MEH) assay was developed within the Ifakara Ambient Chamber Test (I-ACT) with a rabbit as a 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 with untreated control. Five replicates of 20 mosquitoes per strain (malaria vectors: pyrethroid-susceptible Anopheles gambiae and -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-effect logistic regression with binomial errors and log link function assessed non-inferiority with a 7% margin on mosquito mortality as the primary outcome for malaria vectors. 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 on mortality measured 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 additional 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.