Investigation of Larval Susceptibility and the First Evidence of Larval Resistance to Spinosad in the House Fly, Musca domestica L. (Diptera: Muscidae)

Spinosad is a naturally derived insecticide obtained from a soil-dwelling bacterium and is widely used against various agricultural and public-health pests. Although resistance to spinosad has been reported in several pest groups-including thrips, fruit flies, beetles, lepidopterans, and even mosquitoes-no study to date has evaluated its resistance status in the house fly, <i>Musca domestica</i> L. (Diptera: Muscidae). The present study provides the first field-based assessment of spinosad resistance in larvae of <i>M. domestica</i>, based on field-collected populations from Türkiye, and offers the first baseline data on larval susceptibility for this species. House fly larvae collected from seven provinces were exposed to a series of spinosad doses, and LD₅₀ values were calculated using probit analysis. According to the results, LD₅₀ values showed variability of up to approximately 18-fold, ranging from 0.002 g a.i./m² in the Adana population to 0.036 g a.i./m² in the Şanlıurfa population. The corresponding resistance ratio (RR₅₀) values calculated based on the most susceptible population (Adana) ranged from 1.0 to 18.0, indicating low-to-moderate variation in baseline tolerance among the tested field populations. Despite these differences at low doses, the minimum operational dose of 0.25 g a.i./m² consistently produced 97-100% emergence inhibition in all populations. Overall, no high or very high resistance levels were detected across the tested populations. These findings indicate that, despite detectable differences in baseline tolerance, spinosad remains a highly effective larvicide for house fly management. The establishment of these baseline data is essential for future resistance-monitoring programs, and continued surveillance is strongly recommended to detect early shifts in susceptibility before operational resistance emerges.