Resistance selection and biological performance of Aedes aegypti following laboratory spinosad exposure

Aedes aegypti is the primary vector of arboviruses of major public health importance which are spreading worldwide. Spinosad has been an important compound adopted for Ae. aegypti management, with selective larvicidal action based on the metabolites produced by the bacterium Saccharopolyspora spinosa . The expanding use of spinosad in mosquito control programs highlights the need for monitoring the potential development of resistance. This study evaluated the effects of continuous laboratory selection of an Ae. aegypti field-derived colony with spinosad, focusing on resistance development, cross-resistance patterns, and biological performance. Ae. aegypti field sample from Recife city, Brazil, was used to establish the RecCam colony and the RecSpi strain. This strain was subjected to artificial selection with spinosad under controlled laboratory conditions for 20 generations, while RecCam was used as a reference colony for the study. Larvae susceptibility to spinosad and to other three larvicides, not used in selection process, was determined by in vivo assays. The biological fitness of RecSpi individuals was evaluated and compared to RecCam based on blood-feeding performance, fecundity, fertility, pre-imaginal development, adult production, and longevity assays. The baseline susceptibility of the RecCam larvae to spinosad (LC₅₀ = 0.222 mg/L and LC₉₀ = 0.383 mg/L) was similar to another local reference colony (RecLab), while the Rockefeller colony exhibited the highest susceptibility (LC₅₀ = 0.160 mg/L, LC₉₀ = 0.265 mg/L). After 20 generations of spinosad selection, RecSpi larvae displayed a 9.7- and 8.5-fold resistance ratio at LC₅₀ (2.146 mg/L) and LC₉₀, (3.240 mg/L), compared to the RecCam reference, respectively. Notably, no cross-resistance was observed against Bacillus thuringiensis svar. israelensis (Bti), a combined larvicide based on Lysinibacillus sphaericus /Bti, or temephos. The RecSpi individuals displayed reduced biological performance characterized by prolonged larval development, reduced adult emergence, decreased adult longevity, and lowered blood intake compared to the RecCam strain. Continuous selection with spinosad promoted resistance in Ae. aegypti RecSpi strain accompanied by a reduced biological performance. The absence of cross-resistance to commonly used larvicides underscores the unique spinosad mode of action and supports its use in rotation strategies. To maintain the efficacy of spinosad, resistance management approaches including rotational use and longitudinal susceptibility monitoring are necessary.