The study was conducted in two locations in Senegal: the village of Ndioukhane and the village of Bandafassi. The experimental work spanned 12 months, from August 2017 to July 2018. Ndioukhane (14˚42'13.05"N, 16˚50'54.88"W) is situated approximately 12 km from the city of Thiès and 70 km from Dakar, the capital of Senegal. Located in the rural commune of Notto Diobass, the village has a predominantly young population of about 837 inhabitants. It lies within a Sudano-Sahelian zone characterized by a long dry season. Water supply is supported by shallow wells connected to the local water table. Most homes are traditional dwellings with cement walls.

The Experimental station, situated about 400m from the outskirts of Ndioukhane and 500 meters from the nearest market gardens, was built in 2010 by the Laboratoire d'Ecologie Vectorielle et Parasitaire at the University Cheikh Anta Diop (UCAD). It follows the standard experimental hut design used throughout West Africa [10] [11] . The station comprises ten experimental huts arranged on two groups: eight on one side and two on the opposite. Each hut measures 2.64 m in length, 2 m in width, and 2 m in height and is constructed with cement bricks. The distance between two huts is approximately 5.3 m. The walls and floors are either cement or mud (6 and 4 huts, respectively). Roofs are made of corrugated iron, topped with false ceilings of plywood. Each hut features four entry slits to facilitate mosquito entry while preventing their escape. A fully screened veranda trap is located on the fourth wall to capture exophilic mosquitoes ( Figure 1 ).

Bandafassi, the second study site, is located about 15 km from the city of Kédougou and 701 km from Dakar, within the Sudano-Guinean savannah zone. The region borders Tambacounda to the west and north, Mali to the east, and Guinea to the south. The local climate is characterized by seven months of dry harmattan winds (October to April), followed by a rainy season that begins in May, peaks in June and July, and lasts until October or November. Annual rainfall averages around 1300 mm, supporting rich vegetation and enabling intense agricultural and livestock activities. The local economy largely depends on cotton farming and animal husbandry, both favored by the area’s environmental conditions.

Malaria vectors in this region are primarily of the An. gambiae complex, with An. gambiae, An. funestus and An. nili being the most common species [12] . These vectors exhibit marked resistance to pyrethroids, likely due to the widespread and prolonged use of pesticides such as cypermethrin, acetamiprid and lambda-cyhalothrin in horticulture and cotton cultivation [13] , as well as the long-standing use of pyrethroid-treated bed nets distributed in the region [14] .

The study evaluated SumiShield^® 50WG (Sumitomo Chemical), a wettable granule formulation containing 50% clothianidin and 50% inert ingredients. The product was applied at a target dose of 300 mg a.i./m². Packaged in 150 g sachets, it was diluted in 10 liters of water to treat 250 m², corresponding to an application volume of 40 ml/m².

The efficacy of SumiShield^® 50WG was evaluated between August 2017 to July 2018. At the experimental station, eight huts were treated (three with cement walls and three with mud walls), while two huts (one cement, one mud) served as untreated controls. In the village of Ndioukhane, five cement-walled rooms were included: three were treated and two served as controls. In Bandafassi, seven rooms (four cement and three mud), were treated, while two rooms (one cement, one mud) were used as untreated controls. Insecticide application was carried out using Hudson Xpert® 67422 AD compression sprayers (WHO specification No. WHO/VBC/89.9) in accordance with WHO guidelines [15] . Waste management and disposal WHO-recommended procedures.

1) Susceptible strain

A laboratory-maintained strain of An. coluzzii was used, previously employed in several studies [2] [16] . It was originally colonized in 2008 from Essos (3˚53'12''N, 11˚32'32''E), Yaoundé, Cameroon, and has been maintained under standard insectary conditions at 25˚C ± 2˚C and 80% ± 10% relative humidity.

2) Resistant strains

Three An. gambiae s.l. strains were collected from the field in December 2017 from Pikine (14˚42'33N, 17˚23'59W), Ndioukhane (14˚42'13,05N, 16˚50'54,88W) and Kedougou (12˚33′28″N, 12˚10′27″W). Larvae were reared to adulthood under controlled conditions (25˚C ± 2˚C and 80% ± 10% RH) and maintained with a daily supply of a sugar-water solution.

Bioassays were conducted using the susceptible strain An. coluzzii at all the three sites, beginning seven days after treatment and continuing monthly until 12 months post-application (August 2017 to July 2018). WHO cone tests were performed according to established protocols [17] . In each treated and untreated hut or room, cones were affixed to all interior walls. Ten unfed female mosquitoes (3 - 5 days old) were introduced into each cone and exposed for 30 minutes. After exposure, mosquitoes were transferred to labeled holding cups stored and maintained in a controlled environment (25˚C ± 2˚C and 80% ± 10% relative humidity). They were provided with a 10% sugar solution throughout the observation period. Knockdown were recorded at 30 and 60 minutes post-exposure, and mortality was assessed at 24, 48, 72 and 96 hours. Additionally, to assess efficacy on resistant strains, the same protocol was used to test field-collected An. gambiae s.l. from Pikine, Ndioukhane, and Kedougou five months post-treatment.

Treatment efficacy was evaluated by calculating knockdown and mortality rates. knockdown rates were expressed as the proportion of mosquitoes knocked down at 30 and 60 minutes post-exposure relative to number tested. Mortality rates were calculated as the percentage of dead mosquitoes at each time point (24, 48, 72 and 96 hours) out of the total exposed. A residual efficacy threshold of 80% was used in line with Who guidelines [17] . When control mortality ranged between 5% and 20%, mortality rates were corrected using Abbott’s formula [17] . Statistical analyses and visualizations were performed using R software (version 3.6.1).

Bioassay results using the susceptible An. coluzzii strain conducted from August 2017 to July 2018 at the three sites (Bandafassi, Ndioukhane and the experimental station), showed that knockdown rates at 60 minutes (KD60mn) were consistently higher than those at 30 minutes (KD30mn) on both cement and mud substrates ( Figure 2 , Figure 3 ). In Bandafassi, where the highest mortality rates were recorded, 24-hour mortality generally exceeded the 80% efficacy threshold, except during month 4 on the mud substrate. Specifically, in cement-walled rooms in the village, SumiShield^® demonstrated sustained efficacy throughout the study period, with 24-hour mortality consistently above 80% ( Figure 2 ).

At the experimental station, efficacy on cement walls persisted during the first five months, with 24-hour mortality consistently above 80%. However, a gradual decline was observed thereafter, with the lowest rate recorded in month 10 (67.7%). On the mud susstrate, mortality rates after 24 hours remained above 80% from day 7 to month 9 post-treatment, but fluctuated thereafter. The lowest mortality rate after 24-hour mortality on the mud support was recorded at month 10 61.6% ( Figure 2 ).

In Ndioukhane, on the cement substrate, sustained efficacy was observed for the first five months, followed by a decline in month 6. Although efficacy briefly rebounded in month 7, the product lost its effectiveness between months 8 and 11. The lowest 24-hour mortality rate was recorded in month 10 post-treatment, at 40.1% ( Figure 3 ).

Delayed mortality rates, assessed 96 hours post-exposure, consistently exceeded the 80% efficacy threshold at all three sites throughout the evaluation period. The lowest delayed mortality was recorded on cement walls in Ndioukhane, at 98.1% in month 10 post-treatment ( Figure 2 and Figure 3 ).

Efficacy tests conducted five months post-treatment in the experimental huts revealed knockdown rates at both 30 minutes (KD30mn) and 60 minutes (KD60mn), with values consistently failing below the 80% threshold ( Figure 4 ). However, 24-hour mortality rates remained high. On mud walls, the Bandafassi strain exhibited

the highest 24-hour rate (97.5%), followed by the Ndioukhane strain (96.6%) and the Dakar strain (95.8%). On cement surfaces, the Ndioukhane strain showed a mortality rate of 96.6%, while the Dakar and Bandafassi strains recorded rates of 90.8% and 88.3%, respectively.

Tests conducted with the same strains in the village of Ndioukhane yielded similar trends to those observed in the experimental huts. The lowest 24-hour mortality rate was recorded for the Ndioukhane strain (43.4%), whereas the Dakar and Kédougou strains showed mortality rates of 86.6% and 85.6%, respectively. Despite the variability in initial knockdown rates, all delayed mortality rates at 96 hours post-exposure consistently exceeded the 80 % efficacy threshold ( Figure 5 ).