Susceptibility of Anopheles stephensi SDA500 strain to common insecticides and efficacy of glazed tile bioassay for resistance characterization

Research on the urban malaria vector Anopheles stephensi has intensified in recent years following its rapid spread throughout the Horn of Africa and beyond. In addition to behavioural and ecological traits which may limit the efficacy of control efforts, insecticide resistance is a notable problem in invasive An. stephensi populations. The most frequently used laboratory reference strain for An. stephensi is SDA500 originally colonized from Pakistan; though considered insecticide susceptible, quantitative demonstration of this crucial assumption is lacking. We characterized the susceptibility status of SDA500 against multiple insecticide classes used for adult and larval control using the standard WHO techniques for larval bioassays and two alternatives for adults: bottle bioassays and glazed tile bioassays. SDA500 showed full susceptibility against all insecticides tested, and via dose-response assays, we provide the first comprehensive LC₅₀ dataset for a strain of An. stephensi, filling a key knowledge gap and providing an important resource for all future studies of resistance in this important malaria vector. Whilst tile and bottle adult bioassays produced broadly comparable results for both SDA500 and additional laboratory strains, differences were found when testing neonicotinoids and butenolides, which require the addition of the compound MERO® for effectiveness. Nevertheless, the glazed tile bioassay represents a much higher throughput and less resource-intensive technique than bottle bioassays for simultaneous screening of multiple insecticides.