LAUSR

Simple Summary The common house fly, Musca domestica L., is a major carrier of serious diseases in humans and livestock. The common house fly has developed resistance to many insecticides used against it. In the present study, resistance to alpha-cypermethrin increased from 46.4-fold to 474.2-fold in alpha-cypermethrin-selected (Alpha-Sel) females and 41.0-fold to 253.2-fold in Alpha-Sel males, when compared with an alpha-cypermethrin-unselected strain (Alpha-Unsel). However, alpha-cypermethrin resistance was unstable when a field population was reared without exposure for 24 generations. The realized heritability (h2) of alpha-cypermethrin resistance was 0.17 and 0.18 for males and females, respectively, in G1–G24. The Alpha-Sel strain revealed low cross-resistance (CR) to two pyrethroids and five organophosphates and moderate CR to bifenthrin (15.5-fold), deltamethrin (28.4-fold), or cyfluthrin (16.8-fold). The results of instability of resistance trait, low h2, and lack of CR associated with alpha-cypermethrin resistance will provide an opportunity to stakeholders and entomologists to plan better and more effective insect pest and vector management programs in Saudi Arabia. Abstract Musca domestica L., the common house fly, is a cosmopolitan carrier of human and livestock disease pathogens. The species exhibits resistance to many insecticides; therefore, effective M. domestica insecticide resistance management programs are required worldwide. In the present study, the development of alpha-cypermethrin resistance, realized heritability (h2), instability of resistance trait (DR), and cross-resistance (CR) was investigated in an alpha-cypermethrin-selected M. domestica strain (Alpha-Sel) across 24 generations (Gs). Compared with an alpha-cypermethrin-unselected strain (Alpha-Unsel), resistance to alpha-cypermethrin increased from 46.4-fold (G5) to 474.2-fold (G24) in Alpha-Sel females and 41.0-fold (G5) to 253.2-fold (G24) in Alpha-Sel males. Alpha-cypermethrin resistance declined by between –0.10 (G5) and –0.05 (G24) in both M. domestica sexes without insecticide exposure for 24 generations. The h2 of alpha-cypermethrin resistance was 0.17 and 0.18 for males and females, respectively, in G1–G24. With selection intensities of 10–90%, the G values required for a tenfold increase in the LC50 of alpha-cypermethrin were 6.3–53.7, 4.1–33.8, and 3.0–24.7, given h2 values of 0.17, 0.27, and 0.37, respectively, and a constant slope of 2.1 for males and h2 values of 0.18, 0.28, and 0.38, respectively, and a constant slope of 2.0 for females. Compared with Alpha-Unsel, Alpha-Sel M. domestica exhibited moderate CR to bifenthrin (15.5-fold), deltamethrin (28.4-fold), and cyfluthrin (16.8-fold), low CR to two pyrethroids and five organophosphates, and no CR to insect growth regulators. The instability of resistance trait, low h2, and absent or low CR associated with alpha-cypermethrin resistance in M. domestica indicate resistance could be managed with rotational use of the insecticide.