Vol.:(0123456789) Parasitology Research (2025) 124:17 https://doi.org/10.1007/s00436-025-08457-5 RESEARCH Trends in insecticide resistance in natural populations of Culex quinquefasciatus and its impact on mosquito fitness in Dschang, West Cameroon Yacouba Poumachu1,2,7 · Michel Lontsi‑Demano2,8 · Jehan Zeb3,9,10 · Joel Djoufounna2 · Borel Djiappi‑Tchamen1,2 · Abeer Hashem4 · Reem Atalla Alajmi5 · Christelle Layelle Mochie2 · Herman Parfait Awono‑Ambene1 · Christophe Antonio‑Nkondjio1 · Timoléon Tchuinkam2 · Elsayed Fathi Abd‑Allah6 Received: 19 August 2024 / Accepted: 15 January 2025 / Published online: 31 January 2025 © The Author(s) 2025 Abstract Specific information about the dispersion of Culex quinquefasciatus from Dschang in western Cameroon is scarce, and evidence-based interventions are needed. Common use of larvicides and adulticides conduct to the development of vectors resistance which can lead to deep biological changes, including fitness costs. We assessed the profile of insecticide resistance in field populations of Cx. quiquefasciatus and its potential fitness cost in a lineage selected for deltamethrin and permethrin resistance in the laboratory for two generations. The resistance intensity of the Cx. quinquefasciatus population was moder- ated when the population was exposed to deltamethrin at 10 × . Preexposure to PBO led to the restoration of full susceptibility to both deltamethrin and permethrin. Compared with that of the control group, female fecundity rates, egg hatchability, and pupation rates were significantly lower in the insecticide exposed groups. Larval development time and adults emergence rates were comparable between insecticide-exposed groups and the control. Insecticide-exposed adults lived longer than control adults did. Our findings suggest that the mechanisms selected for pyrethroid resistance are associated with negative impacts on different life-trait parameters and support the hypothesis that insecticide resistance is related to a high fitness cost. Keywords  Culicide · Vector control · Insecticide classes · PBO · Fitness Section Editor: Alessia Cappelli. * Yacouba Poumachu poumachuyacouba@gmail.com * Jehan Zeb zebjehan2012@gmail.com 1 Institut for Research of Yaoundé, (IRY), Laboratory for Health Research of Central African Organization for Endemic Disease Control, P.O. Box 288, Yaounde, Cameroon 2 Vector Borne Diseases Laboratory of the Research Unit of Biology and Applied Ecology, (VBID‑ RUBEA), Department of Animal Biology, Faculty of Sciences of the University of Dschang, P.O. Box 067, Dschang, Cameroon 3 Centre for Immunology and Infection (C2i), Hong Kong Science and Technology Parks Corporation, Hong Kong SAR, China 4 Botany and Microbiology Department, College of Sciences, King Saud University, P.O. Box. 2460, 11451 Riyadh, Saudi Arabia 5 Zoology Department, College of Sciences, King Saud University, P.O. Box. 2460, 11451 Riyadh, Saudi Arabia 6 Plant Production Department, College of Food and Agricultural Sciences, King Saud University, P.O. Box. 2460, 11451 Riyadh, Saudi Arabia 7 Royal Society of Tropical Medecine and Hygiene, Northumberland House, 303‑306 High Holborn, London, UK 8 AgroEcoHealth Unit, International Institute of Tropical Agriculture, (IITA), 08 Tri‑Postal, P.O Box 0932, Cotonou, Benin 9 School of Public Health, University of Hong Kong, Hong Kong SAR, China 10 Government Degree College Samarbagh Dir Lower, Higher Education Archives & Libraries Department, Peshawar, Pakistan http://crossmark.crossref.org/dialog/?doi=10.1007/s00436-025-08457-5&domain=pdf Parasitology Research (2025) 124:1717  Page 2 of 13 Introduction More than 17% of the global burden of infectious threat is caused by vector-borne diseases (VBDs), which increase the levels of morbidity and mortality in tropical and sub- tropical areas (WHO 2023). The parasites of these dis- eases, which frequently affect the sub-Saharan African population, are transmitted through the bite of vectors known as Anopheles mosquito species for malaria (WHO 2013; Abdalla 2014), Aedes species for dengue, Zika, yel- low fever, chikungunya (Powell 2018; Alberto 2023), and Culex quinquefasciatus for lymphatic filariasis caused by Wuchereria bancrofti (Lardeux 1995) as well as Rift Val- ley fever virus in the western part of Africa (Madani 2003; Turell 2007) and potentially West Nile virus and Japanese encephalitis around the world (Yan-Jan 2015). Chemical control approaches based on insecticide use remain the standard strategy for both public health responses to disease outbreaks and general indoor vector pest control (Ligsay 2021; Loe 2023). Culex quinquefas- ciatus control in tropical countries has been based pri- marily on the use of neurotoxic insecticides belonging to the organochlorine, organophosphate, and pyrethroid classes (Centre Métérologique secondaire de Dschang 2004; Leong 2014). This species is, however, adapting to these insecticide-based control strategies, as their pre- ferred larval breeding sites are sewers or other wastewater collection facilities (Rodriquez 1993) contaminated with insecticide residues and a wide range of natural toxins. In recent decades, mosquito control strategies focused on insecticides have increasingly failed because of the resistance of mosquitoes to available insecticides. The natural toxins present in Cx. quinquefasciatus breeding sites may exert selective pressure, leading to insecticide resistance in this species (Corbel 2007; Jones 2012). This phenomenon may be associated with multiple resistance mechanisms observed in mosquitoes, including a decrease in their cuticular absorption rate to environmental sub- stances, the limit of their contact with insecticides by avoiding areas with high concentrations, and an increase in their capacity to expel insecticides. Mosquitoes also sometimes resist insecticides by increasing the production of metabolic enzymes for detoxification or reducing the degree of sympatry between target sites and insecticide molecules (Victoria 2023). Repeated exposure increases resistance intensity, so progressively higher concentra- tions are required to have the same effect on mosquito populations, and insecticide classes eventually lose their effectiveness. Alternation of insecticide classes is strongly recommended to sustain insecticide-based mosquito con- trol strategies (Becker 2010; Liu 2015; Moyes 2017). Therefore, the application of a complementary tool for larval source management has become urgent because it focuses on targeting the juvenile lifecycle (Walker 2007; Tusting 2013). The larvicide approach seems to be a complementary part of a mosquito management program. Larvicidal action can intragenerationally lower the mass impact of proficient adults and transgenerationally impact the suppression of the mosquito population because killed larvae cannot yield off- spring (Meier 2022). Once exposed to the natural setting, insecticides can partially affect larvae, and the fitness of sur- vivors can be impacted (Evans 2020; Stoks 2022), as in the case of Aedes aegypti (Culicidae: Culicinae), whose larvae develop slowly after exposure to sublethal doses of mala- thion, and adults become more susceptible to Sindbis virus (Muturi 2011). Culex quinquefasciatus larvae exposed to a sublethal dose of Cinnamomum verum oil produced fecund adults that laid fewer eggs and unviable offspring (Benelli 2018). Additionally, exposure of Anopheles gambiae (Culi- cidae: Anophelinae) larvae to a sublethal dose of the mono- molecular surface film Aquatain Mosquito formulation led to reduced adult fecundity (Mbare 2009). Conversely, owing to these undesirable effects, sublethal exposure of Aedes aegypti larvae to Spinosad leads to overfecund adults (Anto- nio 2009), indicating that exposure to a low dose of nocive substances may be beneficial to some organisms, known as hormesis (Guedes 2017; Cutler 2013). These fluctuations in fitness following sublethal larvicidal exposure can alter mosquito density and disease transmission (Moller 2014; Shaw 2019; Evans 2020). In Cameroon, many studies have reported the susceptibil- ity status of mosquitoes originating from different lowland ecological habitats (Michael 2021), but little or no informa- tion is available concerning the status of mosquitoes from highland areas in the western region. Mosquito control in the country focuses on the use of long-lasting insecticide nets (LLINs). The impact of LLIN distribution is affected both by the abundance of nuisance biting culicids and the level of insecticide resistance in these mosquitoes. These two param- eters are affected by the knowledge of local communities regarding vector-borne diseases and their behavioral pat- terns. Mosquitoes from highland areas are known to be less aggressive, which explains the low adherence of local popu- lations to the use of LLINs. This low insecticide pressure on mosquitoes may be compensated for by the abundant use of insecticide spray bombs. Studies in Cameroon tend to focus on Anopheles species, and little information is available on Cx. quinquefasciatus, which is a relatively resilient mos- quito that causes significant nuisance in the lives of many people in this region. Therefore, in this study, we intend to improve the understanding of the level of insecticide resistance of field-derived adults of Cx. quinquefasciatus from three mostly overcrowded areas in Dschang, namely, Parasitology Research (2025) 124:17 Page 3 of 13  17 Foréké, Paidground, and Foto, and its potential implications for usual vector control tools yields and the health of local inhabitants. Material and methods Material collection Dschang (5°20–5°28’ N; 10°3–10°6’ E) is the head adminis- trative of the Menoua Division on the outskirts of the west- ern Cameroon Highlands (Fig. 1). This savannah landscape within the Guinea‒Congolese bioclimatic domain lies along the Cameroon Volcanic line and displays great variation in its topography and climate: the main dry season (Novem- ber to mid-March), the short rainy season (mid‒March to May), the short dry season (June to July), and the main rainy season (August to October) (WHO 2013; Timoléon 2022). This area is, on average, 1,382 m above sea level, receives an average annual rainfall of 1,500 mm/year, and is surrounded by the Lac River, leading to many small temporary ponds that serve as breeding sites for mosquitoes. The tempera- ture (T°C) (ranging from 17.8 to 22.2 degrees Celsius) and relative humidity (RH) (ranging from 64.3 to 97%) were measured between January and December 2004 (Centre Métérologique secondaire de Dschang, 2004). Addition- ally, this region spans 1,380 km2, and in a given census, the population was counted as 372,000 individuals, leading to a population density of 270 individuals per square kilometer (BUCREP 2010). The rural economy in Dschang is surrounded by mostly agricultural practices, which may be the source of mos- quito breeding sites and is dependent on the production of tobacco and grain cereals, including maize and vegeta- bles, by large and small landowners. Several lakes impact regional public health as a result of contamination with household waste. Habitat is diverse (dry, semihard, and hard soils), and poor urbanization increases the frequency of numerous forms of mosquito breeding sites (Mayi 2020). On the basis of the intensity of mosquito nuisance, unreglemented use of agricultural pesticides, and the eco- logical setting, an exploratory cross-sectional survey was conducted during the dry season from April to June 2021 at three randomly selected sites in the Dschang division: Foréké (5° 26° 09° Nord, 10° 03° 18° Est) in the south part, Foto (5° 27° 59° Nord, 10° 04° 05° Est) in the north and Paidground (5° 34° 34° Nord, 10° 01° 25° Est) in the center part (Fig. 1). These sides present some distinctive ecological aspects and were chosen to obtain an estimate of heterogeneity on a broader scale. Fig. 1   Map showing the locations of differents sampling sites for Culex species in Dschang Parasitology Research (2025) 124:1717  Page 4 of 13 Paidground areas are characterized by sparsely populated neighborhoods with high-standard dwellings, high vegeta- tion coverage, a piped water supply, and regular garbage collection. The houses generally have large and shaded peri- domestic environments, whereas Foréké shows unplanned urbanization and a high population density. Foto is described as an unplanned urbanization area recovering from high pop- ulation density. The mosquito populations of these three sites represent the Dschang population, with some particularities, including biological, chemical and insecticide resistance, related to different aspects of the ecological environment and dispersal distance. The sampled streets were character- ized by low vegetation coverage, an insufficient piped water supply and irregular waste collection. The dipping method and hand pipette technique were used during the series of larval and pupal collections of Culex quinquefasciatus and other species from a range of polluted drains to represent the diversity of mosquito popu- lations. To have genetic diversity, mosquitoes were collected from severals preferred larvals habitats to avoid having sib- lings in the tested sample. At least 30 drains were screened once per week for larvae and pupae collection in each study area. Larvae and pupae were then kept in separate labelled bottles and specifically codified per locality. The samples were transported to the Vector-Borne Disease Laboratory of the Research Unit of Biology and Applied Ecology (VBID- RUBEA). Larvae collected from all previous sites were sieved according to their stage of development. To avoid having a high proportion of siblings in the same test sam- ple, larvae were pooled and reared together in uncrowded trays irrespective of their origin into adulthood. They were maintained with spring water and fed each third day with TetraMin baby fish food (Tetra GmbH, Herrenteich 78. D 49324 Melle, 30 g/66 ml, Ref: 295629) under controlled temperature (25 °C ± 5 °C and humidity (~ 75% ± 10%) con- ditions, with a 12 h day/night cycle) until pupation (Tchu- inkam 2011). Once pupated, all individuals were transferred to standard 30 cm3 cubic insect cages (Megaview Science Education Services Co., Ltd., Taiwan). Emerged F1 adults were accumulated in cages as they emerged from pupae and allowed to feed ad libitum with 10% sucrose on cotton wool. A subsample from an adult cage was selected for subsequent bioassay experiments. Emerged adults of Culex quinquefasciatus were identified, sorted, and isolated from other species using morphologi- cal keys (Edwards 1941) prior to bioassays. The susceptible F20 (Filial Twenty) reference Culex strain S-lab (sensitive laboratory) and the susceptible An. gambiae Kisumu strain were obtained from the laboratory of OCEAC (Central Afri- can Organization for Endemic Disease Control) in Yaounde, Cameroon. They were used to verify the efficacy of impreg- nated papers before bioassays. These colonies were bred continuously under laboratory-controlled conditions at the Malaria Research Unit. The S-lab G20 colony was physio- logically susceptible to deltamethrin (Abdou 2021), whereas Kisumu was susceptible to pyrethroids (Luma 2008) accord- ing to a WHO adult bioassay (WHO 2016). Four diagnostic doses of insecticides including 0.25% permethrin, 0.0025% deltamethrin, 5% malathion, and 0.10% propoxur were used. These insecticides have low toxicity to human health and have been recommended by the WHO. Each insecticide- impregnated paper was prepared and provided by the WHO collaborating center, University Sains, Malaysia, and kepted between 3 and 5 °C in a cool refrigerator.One hour prior to bioassays, box with impregnated papers were taken out of refrigerator. Tests with Kisumu and S-lab strains were run to confirm the efficacy of these impregnated papers. Bioassays studies Insecticide selection was performed only with freshly emerged and unfed female adults, 3–5 days old staved dur- ing 6 h prior to test in uncrowed cage. Mosquitoes were sub- jected to first-use impregnated papers for three tests series. Four batches of 20–25 F0 field-derived Culex quiquefascia- tus populations were exposed via the WHO tube bioassay for 1 h to 3 h at 27 ± 2 °C and 75 ± 10% relative humidity (Cyrille 2018). For the control, two replicates containing 20–25 emerged, and unfed female mosquitoes were used and exposed to control papers. The number of Knocked down was recorded 1 h after exposure to insecticide, then mosqui- toes were moved back into WHO holding tubes with unim- pregnated papers and provided with cotton wool saturated with a 10% sucrose solution for supplemental 24 h duration prior to mortalities scoring. Metabolic detoxification, in which insecticides are metab- olized by enzymes, including cytochrome P450s, hydrolases, and glutathione-S-transferases (GSTs), to become more polar and less toxic, is one of the major mechanisms involved in the development of insecticide resistance. Piperonyl butox- ide (PBO), S,S,S,-tributylphosphorotrithioate (DEF), and diethyl maleate (DEM) are inhibitors of P450s, hydrolases, and GSTs, respectively, and are frequently used as insecti- cide synergists in assessing the metabolic mechanisms that may be involved in the detoxification of insecticides and in the development of insecticide resistance. Synergistic assays can be used to identify the detoxification enzyme that leads to resistance to a specific insecticide and restore the suscep- tibility. In order to asses the synergistic mechanism in the Cx. quinquefasciatus population, 600 adult mosquitoes were exposed to diagnostic doses of permethrin and deltamethrin in two series of experiments via standard WHO susceptibil- ity tests. Oxidase activities were investigated by preexposing mosquitoes to WHO papers treated with piperonyl butox- ide (PBO) (4%) for 1 h before being directly transferred to different WHO tubes containing insecticide-impregnated Parasitology Research (2025) 124:17 Page 5 of 13  17 papers with different doses of permethrin and deltamethrin for an additional 1-h duration (WHO 2013). Three days after the bioassays, survivals female adults mosquito were divided into two parts. The individuals from the first part were used for adult longevity assessment while those from the second part were allowed the opportunity to feed on a chicken that was fixed in the mosquito cage weekly during one hour for three consecutive days for eggs release. This procedure was approved by the local research ethics committee (ethical clearance N° 2020/22/836/CE/ CNERSH/SP). The chicken was obtained from the animal facility of VBID-RUBEA, University of Dschang, Cam- eroon. Three days after a blood meal, adults were main- tained at 25 ± 5 °C and 75 ± 10% relative humidity and allowed to oviposit in a cup containing deionized natural water and chicken faeces at the bottom of the cage to simu- late contamination, for several nights. Two days later, the oviposition cups were removed from the cage, and the eggs were introduced into transparent plastic trays containing natural deionized water. After the hatching process, the lar- vae were monitored as indicated above until pupation. The recovered pupae were transferred into new acrylic cages for adult emergence, and the resulting adults were maintained and allowed to consume blood as described above for egg release. Fitness assessment After insecticides selection, several fitness parameters were assessed. The same measurements were simultaneously carried out between the insecticide-exposed groups and the insecticide free exposed group (control). Eggs were allowed to hatch in 1 L transparent plastic trays. Two days later, approximately 300 larvae from each insecticide-exposed group and 300 larvae from the insecti- cide free exposed group (control) were incubated and reared in 1L of dechlorinated water. These larvae were fed wich 0.02 mg of TetraMin baby food/larvae/daily until the pupa- tion stage. The day before the experiment, the pupae from each insecticide-exposed group were first mechanically separated into male and female according to their mor- phological differences of sex apparatus[18] and then kept in new separate acrylic cages. A subsample of 100 emerged individuals (50 virgin females and 50 virgin males) from each insecticide-exposed group was selected and placed in another new acrylic cage. They were maintained and allowed to continuously feed on a 10% sucrose solution. Four days later, the females were offered a chicken blood meal as described above for egg oviposition. The total number of eggs release by females (fecun- dity) per group of insecticide-exposed individuals dur- ing a single gonotrophic cycle was counted and recorded under a stereomicroscope using a manual counter. To assess the impact of insecticides exposure on the egg hatching rate (fertility), a subset of 100 eggs per group of insecticide-exposed individuals was randomly chosen and kepted in small plastic pans (10.5 × 10 × 6.5 cm3) con- taining 0.5 L of dechlorinated water. Three days later, third instar larvae were collected for the experiments. Egg fertility (fertility of female) was assessed as the propor- tion of eggs that hatched (number of L1 larvae observed) and the number of eggs previously introduced to each pan per group of insecticide. The fertility of the females in each insecticide-exposed group was compared to that of the control females. Experiments were repeated thrice for each biological cage of the insecticide-exposed group (50 virgin females and 50 virgin males) and the insecti- cide free exposed group (50 virgin females and 50 virgin males). Eggs from insecticide exposed group and control were immersed in dechlorinated water for 24 h to induce hatch- ing. Three independent experimental samples of 300 F1 L1 larvae each (a total of 900 F1 L1 larvae) per group were carefully placed in plastic pans containing 1 L of dechlorinated water and reared as indicated above. For the experiment, three additional control groups were main- tained under the same conditions. At no point was there an exchange of individuals between any of the groups. Pupae were removed from the pans, scored into individu- als and transferred to cages each day as they developed to assess the emergence process. Pupae were checked for mortality 48 h and 72 h later. Dead adults were removed during emergence, and their numbers were recorded as dead pupae (unsurviving pupae). Three days later, the prevalence of survival pupae (as the difference between the total number of mosquitoes that emerged from recov- ered pupae and the number of pupae that died prior to adult emergence) was calculated. Mortality at each devel- opmental stage (larvae) was checked out every 2 days until all non-insecticide-exposed individuals reached the pupal stage. The pupation rate was assessed as the ratio between the number of pupae recovered and the number of larvae introduced at the beginning of the experiment per group. Adults longevity tests of Culex quinquefasciatus were performed with a samples of 30 3–5-days-old F1 female from each insecticide-exposed group and control. Experi- ments were replicated thrice for a total of 90 individual from the permethrin-exposed group, 90 individuals from the deltamethrin-exposed group and 90 individuals from control. The were stored en masse in nine separate acrylic cubic cages. Upon eclosion, the adults were fed ad libitum with 10% sucrose solution and maintained at a controlled temperature and humidity (25 ± 5 °C; 75 ± 10% rh). Their survival (number of days that adult mosquitoes survived) was monitored. Mortalities were recorded daily until the last individual passed on. Parasitology Research (2025) 124:1717  Page 6 of 13 Data analysis All data collected were recorded in Microsoft Excel 2007 and transferred to SPSS v 22.0 (Statistical Package for the Social Sciences) for analysis. The resistance status of Culex quinquefasciatus was determined according to the WHO criteria (WHO 2013): (i) low resistance intensity (mean mortality < 98% at 1 × diagnostic dose and between 98 and 100% at 5 × dose); (ii) moderate resistance inten- sity (mortality < 98% at 5 × diagnostic dose and between 98 and 100% at 10 × diagnostic dose); and (iii) high resist- ance intensity (mortality < 98% at 10 × diagnostic dose). The normality of the data was confirmed via the Shap- iro‒Wilk test, with the P value set at 0.05. Means and standard errors were calculated via the means package in R. Comparisons were made between mortality rates with and without PBO preexposure via the Cochran–Man- tel–Haenszel test, whereas chi-square tests (Adejumo 2013; Elliot 2023) were used to evaluate female fecundity and egg fertility. One-way ANOVA (Analysis of variance) and Tukey’s post hoc tests were used to compare larval development time and adult longevity between insecticide exposed groups. Larval survival was evaluated via the Kruskal‒Wallis test. Student’s t test was used to assess the impact of insecticide exposure on Cx. quinquefascia- tus fitness. Result Bioassays results Mosquitoes’ resistance to usuals insecticides is now wide- spread, reducing control efforts and herbicides effects due to cross-resistance. Insecticide bioassays methodologies were used here to quantitatively determine the level of toxicity of insecticides and to evaluate the susceptibility or resist- ance level of mosquitoes to specific insecticides. All col- lection sites presented species diversity. At Foréké, Culex quinquefasciatus and Culex buttoni occurred in sympatry, whereas at Paidground, Culex tigripes was also found. At Foto, Cx. quinquefasciatus was observed more often than other species. An overall total of 1,500 field-derived adult females of Cx. quinquefasciatus were used in the bioassays for phenotypic resistance. The female Cx. quinquefasciatus from Dschang presented low mortality in response to per- methrin (3.0%) and deltamethrin (31%) but total mortalites in response to propoxur (100%) and malathion (100%) com- pared with that of the control females (0.05%). The S-lab reference strains of Cx. quinquefasciatus and the insecticide- susceptible strains were both susceptible to all insecticides, with mortalites rates above 99% compared to field strain (Table 1). High-intensity insecticide resistance was observed against permethrin and deltamethrin at all tested doses Table 1   Resistance profile to differents classes of insecticides in Culex quinquefasciatus from Dschang with information about the levels of resistance and susceptibility restoration to pyrethroids R resistant; S Susceptible; / = not applicable. Insecticides classes and lineage Insecticides concentrations (%) Number of mosqui- toes exposed Number of knoc- down (60 min) Mortalites rates (%) (24 h) Status Pyrethrinoïds Deltamethrin 0.025 300 18 31 R PBO + 0.025 200 93 100 0.125 300 38 75 0.25 200 48 100 Permethrin 0.25 300 2 3.0 R PBO + 0.25 200 75 78 1.25 240 38 59 2.5 200 70 72 Carbamates Propoxur 0.10 300 100 100 S Organochlorites Malathion 5 300 100 100 S Cx. Slab Deltamethrin 0.025 240 90 100 / permethrin 0.25 300 95 100 / Malathion 5 300 98 100 / Kisumu strain (An. gambaie) Deltamethrin 0.025 240 100 100 / Permethrin 0.25 300 100 100 / Malathion 5 300 100 100 / Control 00 135 0 0 / Parasitology Research (2025) 124:17 Page 7 of 13  17 (mortality < 98%), except for 10 × deltamethrin. The mean mortality rates for permethrin were 3.0% at 1 × ; 59% at 5 × ; and 72% at 10 × , whereas those for deltamethrin were 31% at 1 × ; 75% at 5 × and 100% at 10 × (Fig. 2). Globally, the Cx. quinquefasciatus population from Dschang was sig- nificantly resistant to permethrin compared to deltamethrin (P < 0.01). After synergistic pre-exposure assays, results revealed that population of Dschang develop susceptible to deltame- thrin (mean mortality rate of 100%) but still resistant to permethrin althougt mortalites have highly increased (mean mortality rate of 78%). Also, significant difference in mean mortalities was detected after exposure to PBO (OR: 1.41 (0.64–3.82), P > 0.05), sometime suspecting the develop- ment of differents metabolic resistance mechanisms to both insecticide in Dschang (Fig. 3). Fitness results There were fewer eggs laided by females Culex quinquefas- ciatus surviving insecticide exposure than that of the control. A total of 1,900 eggs and 2,600 eggs were obtained from permethrin and deltamethrin exposing individuals compared with 4,751 eggs obtained from the free insecticide exposed individuals over a single gonotrophic cycle (Chi-square test; P < 0.001). The average number of 13.0 eggs/female in the permethrin group and 18 eggs/female in the deltamethrin group were significantly lower than the 31.0 eggs/female in the control group (Chi-square test, P < 0.001) (Fig. 4). These results indicate that sublethal insecticide exposure may lead to reduce capacity of females to produce progenies. The fecundity of females in the two insecticide-exposed groups was not significantly different (chi-square test, P = 0.611). Compared with that of the control group, the fertility of the eggs laided by the permethrin-exposed Culex quinque- fasciatus females was significantly lower. Compared to 231 larvae from the control females group (average hatch- ing rate of 75%), 108 larvae in the permethrin-exposed females group (average hatching rate of 37%) were released (P < 0.05). Controversely, there was no significant difference between the fertility of the eggs from the control females group and that of the deltamethrin-exposed females group, which released 191 larvae (average hatching rate of 64%) (P = 0.511) (Fig. 5). The time needed to complete larval development was similar between each group of insecticide-exposed indi- viduals, taking approximately 13 and 15 days from L1 until pupation for the progenies issue from permethrin and deltamethrin-exposed adults, respectively, compared to 12 days for the progenies issues from control adults. To standardize these experiments, 900 L4 larvae were sub- sampled from each insecticide-exposed individuals group to assess the pupation rate. From the 900 L4 larvae of Fig. 2   Levels of resistance of Culex quinquefasciatus from Dschang to differents pyrethroids. The intensity of resistance can be deter- mined by the mortality rate in response to distinct concentrations of deltamethrin and permethrin. The error bars represent 95% confi- dence intervals Fig. 3   Mortalities levels of Culex quinquefasciatus in Dschang in response to differents concentrations of traditional insecticides when pre-exposed to PBO. The error bars represent 95% confidence intervals Parasitology Research (2025) 124:1717  Page 8 of 13 the permethrin-exposed individuals, 183 pupae (20.21%) were collected, and 173 adults (93.93%) emerged. Simi- larly, 900 L4 larvae were chosen from the pan containing the deltamethrin-exposed individuals during the rearing process to assess their pupation rate. This led to the col- lection of 167 pupae (18.55%) and 106 emerging adults (63.47%). For the control group, 549 adults (75.78%) emerged from 724 pupae (80.31%) after rearing 900 L4 larvae. The pupal survival rate was significantly reduced to 18.55% for deltamethrin-exposed individuals and to 20.21% for permethrin-exposed individuals compared with 80.31% for the control idividuals (F 2.6 = 1.06; P = 0.432) (Fig. 6). The emergence rate of Culex quinquefasciatus significantly differ between the deltamethrin-exposed indi- viduals (63.47%) and the permethrin-exposed individuals (93.93%) (chi-square test, P < 0.001). Compared with that of the control individuals (75.78%), the emergence rate of the deltamethrin-exposed individuals did not significantly differ from that of the permethrin-exposed individuals. Larval mortality was significantly greater in the insec- ticide-treated individuals (ranging from 34 to 81%; mean: 56.5% for the permethrin-exposed individuals and from 29 to 72%, mean: 48% for the deltamethrin-exposed individu- als) than in the control individuals (ranging from 0 to 9%; mean: 5.63%) (Fisher’s exact test, P < 0.0001) (Fig. 7). There was no significant difference in larval mortalities between the two insecticide-exposed groups (Fisher’s exact test, P = 0.621) (Fig. 7). A trend toward longevity reduction in the insecticide- exposed groups was observed. From day 6 of the experi- ments, continuous and daily deaths were observed in each insecticide-exposed individuals, until day 29 for the permethrin-exposed individuals, day 24 for the deltame- thrin-exposed individuals and day 47 for the control indi- viduals (Fig. 7). This progression subsequently stabilized until the end of the study. Compared with those of the females in the control, the survival rates of the females in the insecticide-treated Culex quinquefasciatus were Fig. 4   Effects of pyrethroids exposure on the number of eggs laided by female Culex quinquefasciatus. The error bars represent 95% confidence intervals. The presence of an asterix indicates that there was a significant reduction in the average number of eggs laided by insecticide-exposed females compared with the control (*** P < 0.001) Fig. 5   Effects of pyrethroids insecticides on eggs hatchabil- ity rates in survivals females of Culex quinquefasciatus. The error bars repersent 95% confidence intervals. The pres- ence of an asterisk indictaes that there was a significant reduction in the average number of eggs laided compared with that of the control (* P < 0.05); ns: no significant difference compared with that of the controls Parasitology Research (2025) 124:17 Page 9 of 13  17 significantly different (log-rank test, X2 = 8.1; ddl = 2; P = 0.02). These results indicate that the observed dif- ferences in survival probabilities between the treatment groups are unlikely to have occurred by chance. These findings suggest that the insecticide treatments had a greater effect on reducing the mortality rate than did the control. The longevity of the permethrin-exposed individuals did not significantly differ from that of the deltamethrin-exposed individuals (log-rank test, X2 = 1.4; ddl = 2; P = 0.5). The observed mean survival time was 27 days (CI = 18.564–35.636) for the control, 28 days (CI = 19.324–38.476) for the permethrin-exposed individ- uals and 30 days (CI = 20.406–40.594) for the deltame- thrin-exposed individuals. The median survival time did not significantly differ between the permethrin-exposed individuals and the deltamethrin-exposed individuals (log-rank test, X2 = 1.405; P = 0.655) (Fig. 8). Discussion The development of complementary strategies to fight Culex quinquefasciatus is fundamental given the ubiquity of increasing of resistance to pyrethroid insecticides. In Dschang, neurotoxic insecticides are currently used to control Cx. quinquefasciatus and other vectors. This phe- nomenon has led to a rapid increase in mosquito resist- ance to insecticides, hindering the usual approach of con- trolling vectors, and failure has been shown in regions where resistance has been reported in target insects (Jones 2012). Although studies have shown the impact of these compounds on mosquitoes, resistance dynamics are still lacking. We evaluated the impact of insecticide exposure on Cx. quinquefasciatus field population from Dschang, which were selected for resistance using diagnostic doses Fig. 6   Comparaison of the survival rates and emergence rates of pyrethroid-exposed individuals and control strains of Culex quinquefasciatus. The error bars represent 95% confidence intervals. The pres- ence of an asterisk indicates that there was a significant reduction in the average number of pupation rates compared with that of the control (* P < 0.05; ** P < 0.01). ns: no significant difference compared with that of the control Fig. 7   Effects of insecticide exposure on the mortalities of Culex quinquefasciatus larvae. The error bars represent 95% confidence intervals. The pres- ence of an asterisk indicates that there was a significant reduction in the average mortality rate compared with that of the con- trol (***P < 0.001) Parasitology Research (2025) 124:1717  Page 10 of 13 of permethrin, deltamethrin, propoxur and malathion. We subsequently assessed several key fitness parameters of permethrin and deltamethrin selected mosquitoes com- pared with those of the control (insecticides free expo- sure). The objective is to fill a knowledge gap related to insecticide resistance and dissemination, which was recently highlighted by the Public Health Minister as important for the future control of vector-borne diseases. We noted that field-derived adult Culex. quinquefas- ciatus in Dschang were highly resistant to pyrethroids (deltamethrin, permethrin) but fully susceptible to carba- mates (propoxur) and organochlorites (malathion). This observed resistance phenomenon may sometime be a con- sequence of presence of kdr mutations (1014F/S/C), and higher expression of CYP450, CCE genes and glutathione S-transferase for pyrethroids or to mutations in the gene coding for an insensitive acetylcholinesterase (G119S, F290V, F331W) causing various levels of resistance to carbamates. Cx. quinquefasciatus adults were highly resistant to diagnostic doses of deltamethrin (31%), per- methrin (3.0%). These status increased with doses greater than 5 × the diagnostic dose, leading to mean mortalities of 75% at the 5 × dose and 100% at the 10 × dose for the deltamethrin-exposed individuals and 59% at the 5 × dose and 72% at the 10 × dose for the permethrin-exposed indi- viduals. This may be the result of a series of increasing deleterious effects on both development and reproduction in the course of pyrethroid selection, with a consequent reduction in general fitness (Ademir 2020). It has been shown that female mosquitoes of Cx. quinquefasciatus used to lay eggs at breeding sites surrounding urban set- tings in Dschang, where insecticide and pollutant residues or waste products accumulate in the soil during the rainy season and ultimately contaminate these breeding sites (Etang 2016). Larvae may therefore undergo selection pressure from these chemical compounds and promote the selection of specific genetic mutations in resistant adults against insecticide classes commonly used in public health (Antonio-Nkondjio 2011; 2014) as reported in Cotonou by Akogbéto (2010), who surligned that Anopheles gam- biae from from agricultural area exhibited high resistance to pyrethroids. In contrast, it has been shown that some fungicides or xenobiotic larvicides used for crops can syn- ergistically promote the efficacy of insecticides targeting mosquitoes (Adjeumo 2013). The high levels of pyrethroid resistance against perme- thrin and deltamethrin detected in the Culex quinquefascia- tus population collected from Dschang indicate the prob- able presence of target-site resistance. The overexpression of mixed-function oxidases (MFOs) is known as an important way to develop resistance mechanisms in Cx. quinquefas- ciatus mosquito population of Dschang, as proven by sig- nificantly greater mortality rates after PBO preexposure (Fig. 2). Synergists (PBOs) act by inhibiting the detoxifi- cation of metabolic enzymes through oxidases leading to an increase in the sensitivity of mosquitoes to insecticides. A comparison of fitness between insecticide-exposed and control individuals (unexposed) revealed several differences. For example, blood feeding success was high in the labora- tory strain, which was a consequence of the development of a good egg laying rate, compared with that of fresh field individuals. This is because the laboratory strain is more adaptable to laboratory conditions because of its long-term colonization (Victor 2020), in contrast to the fresh field strain, which acclimatizes. This breeding condition leads to a decrease in the egg-laying ability of females. Fig. 8   Kaplan Meier survival curves for Culex quinquefascia- tus females following expo- sure to 0.25% permethrin and 0.025% deltamethrin compared with that in the control group Parasitology Research (2025) 124:17 Page 11 of 13  17 Female fecundity and egg hatchability were also founded to be reduced in insecticide-exposed females of Culex quinquefasciatus compare to control females, likely due to the sublethal effects of insecticides on their phenotype wich negatively impact they ability, contrary of the laboratory strain to feed on chicken blood. Briegel (1993) demonstrated that egg hatchability increases with successive blood meals in An. gambiae, increasing up to 50% for a gonotrophic cycle. Moreover, permethrin can destroy embryos in eggs before they are released or may induce premature oviposi- tion in females. Recent observations revealed that the fecun- dity and egg hatchability of Anopheles gambiae were lower in the insecticide-exposed individuals than the unexposed laboratory one (Thomson 2020). Compared with the insec- ticide-exposed strain, the laboratory strain also presented a high eggs hatching rate and viability. Egg hatchability was also reduced in the field-derived strains, which is likely associated with their resistant phenotype. This difference in behaviour may due to their differents background or cost of resistance. Compared with those of the control mosquitoes, the larval development times of the insecticide-exposed indi- viduals were shorter, as observed for An. gambiae (Thom- son 2020). These findings indicate that insecticides may increase the mortality of offspring throughout larval devel- opment and reduction of the pupation process. A longer larval development period could lead to a greater level of exposure to pollutants and physicochemical parameters at the breeding sites and greater vulnerability to natural predators. A shorter development time is likely to accel- erate the emergence of adults, leading to an increase in vector density, which is an important parameter of vector capacity. This hypothesis is excluded here, as the density of larvae per pan was 300 F1 L1 larvae/L and was carefully controlled. A longer development time probably reflects the influence of insecticide resistance or exposure on the general metabolism of mosquitoes. Pupation rates and adult emergence rates were not significantly different between the two groups, sug- gesting that the effects of insecticide exposure or resist- ance mechanisms primarily affect larvae. This could be explained by the fact that the pupae do not feed, limit- ing the impact of exogenous factors on their evolution rhythm. The lack of difference at this stage, notably for the exposed colonies, could be the consequence of the reduction in the number of surviving larvae that grow until pupation followed by adult emergence. Neverthe- less, the decrease in general fitness observed in different selected strains suggests the accumulation of deleterious effects due to pleiotropy of the resistant alleles or to a hitchhiking effect. In conclusion, our results indicate that pyrethroid insec- ticides are effective public health coumpounds against field populations of Culex quinquefasciatus in Dschang. How- ever, long terh selection with permethrin and deltamethrin led to the development of resistance in these mosquitoes, despite associated fitness costs. This highlights the impor- tance of restricting and controlling the handling of insecti- cides when designing future control programs for vectors of medical importance. Supplementary Information  The online version contains supplemen- tary material available at https://​doi.​org/​10.​1007/​s00436-​025-​08457-5. Authors’ contributions  Conceptualization: Yacouba Poumachu; Meth- odology: Yacouba Poumachu, Michel Lontsi-Demano, Joel Djoufanna, Borel Djiappi-Tchamen, Christelle Layelle Mochie; Formal analysis and investigation: Yacouba Poumachu; Writing-original draft prepara- tion: Yacouba Poumachu; Writing–review and editing: Michel Lontsi- Demano, Joel Djoufanna, Borel Djiappi-Tchanmen, Jehan Zeb, Her- man Parfait Awono-Ambene, Christophe Antonio-Nkondjio, Timoléon Tchuinkam, Abeer Hashem, Reem Atalla Alajmi, Elsayed Fathi Abd_ Allah; Funding acquiqition: Jehan Zeb, Abeer Hashem, Elsayed Fathi Abd_Allah; Resource: Zeb Jehan, Abeer Hashem, Reem Atalla Ala- jmi, Elsayed Fathi Abd_Allah; Supervision: Yacouba Poumachu. All authors read and approved the final manuscript. Funding  The authors would like to sincerely thank the Researchers Supporting Project Number (RSP2025R356), King Saud University, Riyadh, Saudi Arabia. Data availability  The authors declare that the data supporting the find- ings of this study are available within the paper. Declarations  Ethics approval  All procedures performed in the study were in accord- ance with the local research ethics committee Statement for Use of Animals in Research. The ethical principles established by the Regional Ethics Committee for Human Health Research and Use of Laboratory Animals were followed. The research protocol was approved by the Ethics Committee on Animal Use (ethical clearance N° 2020/22/836/ CE/CNERSH/SP). Consent for publication  Not applicable. Consent to participate  All authors confirm their participation in the study. Competing interests  The authors declare no competing interests. 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