Globally, insecticides are a part of major management solutions against various insect vectors; thus, determining susceptibility to commonly used insecticides is crucial for the selection of appropriate and effective treatment [27, 28]. OPs, including chlorpyrifos, malathion, fenitrothion, pirimiphos-methyl, and diazinon, are acetylcholinesterase inhibitors, whereas pyrethroids, including alpha-cypermethrin, cypermethrin, deltamethrin, bifenthrin, and cyfluthrin, are sodium channel modulators . These conventional insecticides are the most commonly used adulticides for controlling mosquitoes and other medically important insect pests globally, including in Saudi Arabia [27, 30–32]. Therefore, conventional insecticide resistance has arisen and been reported in Cx. pipiens and Aedes in Saudi Arabia [30, 33, 34].
In the present study, susceptibility/low resistance to chlorpyrifos, malathion, fenitrothion, pirimiphos-methyl, and diazinon was observed in Cx. quinquefasciatus populations from the study regions. Field-evolved resistance to OPs has been well documented in Cx. quinquefasciatus [1, 7, 20, 35], Cx. pipiens [30, 36], Aedes albopictus [37, 38], and Ae. aegypti .
In this study, susceptibility/low to moderate resistance to alpha-cypermethrin, cypermethrin, deltamethrin, bifenthrin, and cyfluthrin was observed in Cx. quinquefasciatus populations from the study regions. Resistance to pyrethroid insecticides has been observed for many insect vectors, including Cx. quinquefasciatus [7, 32, 40], Ae. aegypti and Ae. albopictus [28, 37, 41–44], Cx pipiens , Anopheles gambiae Giles [45, 46], and An. stephensi Liston . The OP and pyrethroid resistance in these insect vectors may be due to increased metabolic enzyme activities [37, 48, 49] and target site insensitivity due to Kdr mutations, L1014F, F1534C, and I1532T on VGSC, V1016I, F1534C on 11S6 domain, and G119S on the acetylcholinesterase 1 gene [32, 43, 50–52].
In the Riyadh region, one that experiences a severe desert climate, OPs and pyrethroids are applied once a month during the periods March–May and September–November to control different household insect vectors, e.g., mosquitoes, in human dwellings. This low level of exposure may explain the detected susceptibility/low resistance to OPs and pyrethroids in the Cx. quinquefasciatus field populations. However, it appears that resistance to deltamethrin (pyrethroid) is increasing in the Al-Ghanemiya and Al-Masfa populations, where moderate RRs of 7.07 and 4.94, respectively, were observed. Thus, deltamethrin should be used carefully and correctly at these locations to prolong efficacy.
Interestingly, besides this finding of susceptibility/low resistance in Cx. quinquefasciatus populations to conventional insecticides, Hafez and Abbas (Unpublished data) found significant and higher levels of resistance in the same field populations toward new classes of insecticides, including insect growth regulators. Since the field populations in this study were collected from locations adjacent to farms or parks, their findings could be due to pesticide laws and regulations that have restricted or prevented the use of many conventional insecticides over the last two decades but permitted their replacement by new insecticides in the plant protection programs . This may have led to accidental exposure to the new insecticides, giving rise to a selection pressure in these Cx. quinquefasciatus populations over the last two decades, and this could be the reason for these unexpected findings.
In conclusion, the susceptibility/low resistance of Cx. quinquefasciatus adults to OP and pyrethroid insecticides suggests that they still have good potency against the adults of this species in Riyadh, Saudi Arabia. A strategic program must be instituted to control Cx. quinquefasciatus to sustain the efficacy of other OP and pyrethroid insecticides before resistance develops. Periodic monitoring must be continued to detect any increase in resistance to these conventional insecticides. Additionally, cultural practices, such as the removal of larval habitats and the use of biological control agents , must be used to manage larvae of Cx. quinquefasciatus to minimize overreliance on chemical insecticides. Our findings serve as a reference point for future monitoring efforts of Cx. quinquefasciatus insecticide susceptibility.