Dominant fitness costs of resistance to fipronil in Musca domestica Linnaeus (Diptera: Muscidae)

- Compared to an UNSEL strain, the Fipronil-selected strain was 181.94-fold resistant to fipronil.
- Resistance to fipronil was unstable after five generations without selection pressure.
- Fipronil-selected strain had lower fecundity and hatchability and prolonged larval duration.
- Intrinsic rate of natural increase and biotic potential was lower for the fipronil-selected strain.
- Relative fitness of fipronil-selected strain was 0.13 compared to an unselected strain.

House fly, Musca domestica L., (Diptera: Muscidae) a common pest of poultry, has developed resistance to the commonly used insecticide fipronil. The life history traits were examined in the fipronil-selected (Fipro-SEL), susceptible counterpart (UNSEL), and their hybrid progeny strains in order to design an effective resistant management strategy. Compared to the UNSEL strain, the Fipro-SEL was 181.94-fold resistant to fipronil. This resistance was unstable after five generations without selection. The Fipro-SEL had a significantly longer larval duration, lower pupal weight, lower fecundity, lower hatchability, lower number of next generation larvae, lower intrinsic rate of population increase and lower biotic potential than the UNSEL strain. Most fitness parameters of the hybrid progeny were similar and significantly lower than that in the UNSEL strain, suggesting autosomal and dominant fitness costs. Compared to the UNSEL strain, relative the fitness of the Fipro-SEL, Hybrid1 and Hybrid2 was 0.13, 0.33 and 0.30, respectively. Fipronil resistance resulted in high fitness costs and these fitness costs were dominant and autosomal in the Fipro-SEL strain of M. domestica. Rotation of fipronil with other insecticides having no cross resistance should be useful for delaying the development of resistance in M. domestica.