Molecular evidence of Rdl mutations linked to Fipronil resistance in urban populations of Blattella germanica in Iran

Background The German cockroach is a major urban pest and a persistent public health threat, largely due to its ability to rapidly develop resistance to commonly used insecticides. Fipronil, a phenylpyrazole insecticide widely incorporated into bait formulations, is increasingly compromised by resistance in field populations. This study evaluated the susceptibility of four urban B. germanica populations from Iran to fipronil and investigated the presence of Rdl gene mutations associated with phenylpyrazole resistance.Results Topical bioassays revealed substantial variation in susceptibility, with resistance ratios ranging from 1.77 to 13.35 across populations, indicating heterogeneous exposure histories and potential genetic differentiation. Sequencing of a 245-bp fragment of the Rdl gene revealed no occurrences of the well-known A302S mutation; however, the A299S and A301S mutations, previously linked to fipronil resistance, was detected in two populations exhibiting the highest levels of resistance. Haplotype reconstruction of 21 individuals identified 17 haplotypes, suggesting considerable genetic diversity that may reflect multiple evolutionary origins of Rdl variants and ongoing selective pressure. However, larger sample sizes would be required to confirm this pattern.Conclusion Field populations of Blattella germanica exhibited variable susceptibility to fipronil, ranging from susceptible to moderately resistant. Molecular analysis did not detect any established target-site resistance mutations in A302S, although A299S and A310S substitution were identified in two populations and require further investigation. These findings suggest that fipronil remains an effective control agent in many urban environments; however, reduced susceptibility in some populations underscores the importance of ongoing resistance monitoring and integrated pest management strategies to mitigate future resistance development.