Introduction

The German cockroach (Jones, Goven, Froger, Bantz, & Raymond) is a prevalent urban pest and poses substantial indoor public health risks (Vargo et al., 2021). Cockroaches contribute to the spread of respiratory, fungal, and bacterial diseases through their faeces, body fragments, and shed exoskeletons (Brenner and Kramer, 2019). Infestations can lead to significant economic impacts, including food contamination, increased medical expenses due to allergen exposure, and expenses related to regulatory compliance (Hassan Nasirian, 2019; Wang et al., 2021).

Effective management of cockroach populations contributes substantially to the mitigation of related public health risks (McPherson et al., 2021). Chemical interventions remain the primary and most effective strategy for controlling German cockroach populations (Brenner and Kramer, 2019). Pest management professionals (PMPs) rely heavily on residual insecticide sprays and baits, which constitute the main control tools for these vectors (C.-Y. Lee and Wang, 2021b).

The German cockroach reproduces rapidly, producing three to four generations per year, which accelerates adaptive responses to insecticidal pressure (Rust et al., 1995). Nevertheless, extensive and prolonged use of insecticides, such as pyrethroids, neonicotinoids, essential oils, and indoxacarb, has been employed to control German cockroaches (González-Morales et al., 2022) (Hu et al., 2020; C.-Y. Lee and Wang, 2021a; Phillips et al., 2010; Scharf and Gondhalekar, 2021). As a consequence, field populations have developed resistance across multiple chemical groups-organochlorines, organophosphates, carbamates, and pyrethroids-contributing to observed control failures (Enayati and Motevali, 2007; Kakeh-Khani et al., 2020; Rust et al., 1995; Zhu et al., 2016).

Fipronil, a relatively new insecticide, has been commonly used for three decades to control German cockroach infestations (Castilhos et al., 2019; Kaakeh et al., 1997; Singh et al., 2021). As a phenylpyrazole insecticide, fipronil disrupts the nervous system and behavior of insects by blocking GABA- and glutamate-gated chloride channels, leading to uncontrolled neural excitation and eventual death (D. Buckingham et al., 2017). Over the past decades, there has been a significant increase in global reports of fipronil resistance among B. germanica populations (González-Morales et al., 2022; Kassiri et al., 2020; Khoobdel et al., 2022b; H Nasirian et al., 2006a; Wu and Appel, 2017).

Rdl gene (resistance to dieldrin) mutations have been repeatedly associated with target-site resistance to cyclodiene and phenylpyrazole insecticides in various pest species (Liu et al., 2020).

In the German cockroach, the most widely reported substitution is A302S, which confers high resistance to dieldrin and fipronil. Interestingly, analogous amino-acid changes at nearby positions such as A296S, A299S and A301S have been identified in other insect taxa, including Anopheles mosquitoes, Musca domestica, Nilaparvata lugens, and D. melanogaster (Garrood et al., 2017; Liu et al., 2020; Ozoe et al., 2015; Remnant et al., 2014), where they have also been implicated in GABA receptor-mediated resistance.

Given the limited data on Rdl variation in B. germanica, this study investigates the genetic diversity and haplotype structure of the Rdl gene in populations collected from different urban environments and reports a previously uncharacterized substitution, A299S and A301S. This resistance is largely attributed to mutations in the Rdl gene, which encodes a subunit of the GABA-gated chloride channel, thereby reducing target-site sensitivity to fipronil (Ang et al., 2013; González-Morales et al., 2022; Li et al., 2021). These findings underscore the importance of ongoing molecular surveillance of Rdl mutations in cockroach populations to inform evidence-based resistance management programs.