Unstable fipronil resistance associated with fitness costs in fipronil-selected Aedes aegypti L.

Aedes aegypti L. is one of the most important insect vectors known to cause serious viral diseases in humans and in animals (National Institute of Health, 2020). It is found in the tropical and subtropical regions of the world (Organization, 2012). It is responsible for the dissemination of the dengue virus in humans (Brady and Hay, 2020) and most likely to transmit lumpy skin disease virus (LSDV) in animals (Chihota et al., 2001). A large number of cases of dengue fever are being reported annually worldwide (Farrar et al., 2007; Organization, 2022). Moreover, production losses (45–65%) due to lumpy skin disease virus were previously reported in cattle farming (Namazi and Khodakaram Tafti, 2021). Currently, lumpy skin disease may cause heavy economic loss to the livestock industry of Pakistan. Nowadays, lumpy skin disease is a major problem in Pakistan. The control of this potential vector is necessary to prevent the spread of these viral diseases.

There are different control measures, such as sterile insect techniques, endosymbionts (Wolbachia), destruction of habitats, and genetic manipulation, for the management of Ae. Aegypti but these measures are applied in specific areas of the world (Baldacchino et al., 2015). The use of insecticides is an effective, and easy method for the quick control of Ae. Aegypti (Ningsih and Ribal, 2019; Tikar et al., 2009). With continuous application of different insecticides, Ae. Aegypti has developed resistance against these insecticides. Previously, resistance to lambda-cyhalothrin, propoxur, deltamethrin and temephos has been observed in Ae. Aegypti (Ocampo et al., 2011; Valle et al., 2019).

Fipronil, a phenyl pyrazole insecticide, inhibits the flow of the chloride ion by interrupting the gamma amino butyric acid receptors in the insect's nervous system (Zhao et al., 2004). It is used against different sucking (Poché et al., 2013) and chewing insect pests (Ali et al., 1998; Asaro and Creighton, 2011). In despite of the adverse effects of fipronil on honeybees and beneficial insects (El Hassani et al., 2005; Pisa et al., 2015; Roat et al., 2013), it is being used in many countries including Pakistan for the control of agricultural and household pests (Sarmad et al., 2020; Wazir and Shad, 2022). However, fipronil resistance has been observed in different household pests. For example, a fipronil selected population of Musca domestica L. (Abbas et al., 2016a) and Ae. Aegypti (Sumra et al., 2021) showed a very high and high level of fipronil resistance, respectively.

Population dynamics, fitness costs, pest management techniques, selection pressure, and frequency of resistance alleles are involved in the onset of resistance against specific insecticides in insects. Among these, fitness costs linked with insecticide resistance is an important factor for resistance evolution resulting in a fitness disadvantage of the insect compared with its reference strain (Kliot and Ghanim, 2012). Previously, fitness costs linked with resistance to different insecticides in veterinary and medically important pests have been observed, like fitness costs of spiromesifen and fipronil resistance in M. domestica (Abbas et al., 2016b; Alam et al., 2020), temephos resistance in Ae. Aegypti (Khan and Akram, 2019), and dimethoate resistance in Culex quinquefasciatus Say (Alam et al., 2017). To the author's best knowledge there is no report on fitness costs of resistance to fipronil in Ae. Aegypti.

Resistance to chemicals is frequently unstable due to the presence of fitness costs in a chemical-free environment. Stability of insecticide resistance poses a threat for prolonging effectiveness of any insecticide. For designing insecticide resistance management techniques, knowledge of stability of resistance is very important (Banazeer et al., 2020). Several ecotoxicological studies have been reported about the stability of fipronil resistance in various insects. For instance, unstable fipronil resistance was reported in a fipronil-selected population of M. domestica L. when selection stress was removed for five generations (Abbas et al., 2016a). Resistance of fipronil was stable in a spiromesifen-treated population of M. domestica (Alam et al., 2020). However, stability of fipronil resistance has not been previously observed in Ae. Aegypti.

Assessment of fitness costs associated with fipronil resistance might be of benefit for planning effective techniques to manage resistance and to prevent the spread of resistance alleles (Abbas et al., 2016b). Therefore, a number of biological parameters of a Fipro-Sel Pop, Un-Sel Pop and reciprocal crosses were compared to evaluate fitness costs associated with fipronil resistance. Moreover, the stability of fipronil resistance was also evaluated in the Fipro-Sel Pop of Ae. Aegypti to determine whether the use of this insecticide should be stopped or rotated with other insecticides.

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