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

## TL;DR

This study finds that house fly larvae in Türkiye show some natural variation in resistance to spinosad, but the insecticide remains highly effective at recommended doses.

## Contribution

The first evidence of larval resistance to spinosad in house flies and the establishment of baseline susceptibility data in Türkiye.

## Key findings

- Spinosad resistance in house fly larvae varies up to 18-fold across different regions in Türkiye.
- The minimum operational dose of spinosad (0.25 g a.i./m2) inhibits 97–100% of fly development in all tested populations.
- No high or very high resistance levels were detected, supporting spinosad's continued effectiveness as a larvicide.

## Abstract

Spinosad is a biologically derived insecticide widely used against a broad range of pests that negatively affect human and animal health. The house fly, Musca domestica, is a major pest in livestock facilities, where it not only spreads disease-causing organisms among animals but also causes stress and irritation, leading to reduced milk and meat production and contamination of eggs and animal products. In this study, we evaluated whether house fly larvae collected from different regions of Türkiye show resistance to spinosad. Although some natural variation in sensitivity was observed among populations, the minimum operational dose resulted in almost complete inhibition of fly development in all tested populations. These findings provide the first baseline data on larval susceptibility of M. domestica to spinosad and support its potential role as an alternative larvicidal option where it is locally used, while emphasizing the importance of continued resistance monitoring.

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, Musca domestica L. (Diptera: Muscidae). The present study provides the first field-based assessment of spinosad resistance in larvae of M. domestica, 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.