Plastics play a pervasive role across various facets of human existence, from food packaging and household items to medicine and electronics (Andrady, 2015). Nevertheless, their extensive utilization, affordability, and ineffective waste disposal practices have led to their release into the environment. Under these conditions, they can break down into microplastics (MPs) ranging from 1 µm to 5 mm and even smaller particles (nanoplastics, NPs) that are less than 1 µm (Barnes et al., 2009; Gigault et al., 2018). Although MPs are found in nearly all environments, from soil and groundwater to the atmosphere, surface water is one of the primary repositories of MPs on Earth (Ricciardi et al., 2021; Rochman and Hoellein, 2020). Numerous aquatic animals ingest MPs, mistaking them for food particles or consuming them passively, resulting in the entry of plastics into food chains (Lehel and Murphy, 2021; Li et al., 2021; Pironti et al., 2021). Until recently, the bioaccumulation of MPs by aquatic organisms has primarily been recognized in marine species. Currently, MPs are found in freshwater organisms at nearly all trophic levels (Gallitelli et al., 2021; Miloloža et al., 2020; Parker et al., 2021), and evidence of their accumulation and biomagnification in freshwater food webs has been reported (Krause et al., 2021; Szymańska and Obolewski, 2020).

Mosquitoes (Diptera: Culicidae) are blood-sucking insects and vectors of diseases in wild animals and humans, such as yellow fever, dengue, chikungunya, and Zika viruses (Bhatt et al., 2013; Clements, 2012; Dobson and Foufopoulos, 2001). Given the ecological role of mosquitoes and their life cycle bridging aquatic and terrestrial environments, they represent a crucial but underexplored vector through which MPs could influence broader ecological dynamics and potentially human health, warranting focused investigation of their interactions with MPs during their larval stages. Despite their ecological impact, studies on MPs in mosquito larvae in their natural freshwater habitats remain scarce. In Iran, the available literature primarily focuses on mosquito fauna, larval habitats, and ecological factors in various regions (Azari Hamidian et al., 2011; Nikookar et al., 2016). An interesting contradiction is that while most papers discuss natural breeding sites, Doosti et al. (2021) mention artificial habitats, such as discarded tires, as breeding grounds for Culex pipiens in southern Iran (Doosti et al., 2021). Even globally, only Griffin et al. (2023) have studied the link between MPs and mosquitoes and found that polyethylene microplastics increased early instar larval mortality in Aedes albopictus and Culex quinquefasciatus, with different effects observed at low concentrations between the two species (Griffin et al., 2023).

Mosquitoes can affect the circulation of MPs and NPs in the environment by transmitting fine particles through the transmission networks of vector pathogens. Additionally, mosquitoes are invertebrates that undergo their early stages in water but their adult stages on land, enabling them to transport MPs between aquatic and terrestrial environments. This has also been observed in Culex pipiens (Dadd, 1971). and Ae. aegypti, Anopheles albimanus, Anopheles quadrimaculatus, and Culex quinquefasciatus mosquito filter-feeding larvae ingested latex microspheres (Aly, 1988). Recently, evidence of the ontogenic transmission of MPs in Culex mosquitoes has been reported. Culex mosquito larvae have ingested fluorescent polystyrene beads. The 2 µm particles were readily transferred between life stages that utilize distinct habitats: from the larval to pupal stage and then to the adult terrestrial stage (Al-Jaibachi et al., 2018, 2019). Quantifying such transport would be highly beneficial for modeling the global cycling processes of MPs.

Microplastics can be directly or indirectly ingested by aquatic organisms at various feeding levels and transferred along aquatic food chains, thereby affecting their activities. Additionally, microplastics can adsorb various environmental chemical pollutants and release toxic plastic additives, thereby serving as reservoirs and sources. Specifically for mosquitoes, MPs may affect larval development and survival by altering the nutritional quality of the aquatic environment. Mosquito larvae depend on organic matter and microorganisms for food; therefore, microplastics can disrupt feeding patterns or reduce nutrient availability (Barreaux et al., 2016; Carvajal-Lago et al., 2021; Girard et al., 2021). Nutritional stress during larval stages can affect adult mosquito traits such as body size, longevity, and vector competence, which are vital for disease transmission (Barreaux et al., 2016; Carvajal-Lago et al., 2021). Microplastics may also affect the microbial communities in larval habitats. Because microorganisms are crucial for mosquito development and oviposition site selection (Girard et al., 2021), changes in microbial composition due to microplastics could indirectly affect mosquito populations and distribution. However, understanding the true potential risks of microplastics and related chemical pollutants (such as hydrophobic organic pollutants, heavy metals, and plastic additives) to various organisms, particularly mosquitoes, requires separate, specific, and thorough investigation.

Finally, Shiraz presented a series of characteristics that made it an interesting case study. Climate and environmental factors play crucial roles in mosquito abundance and transmission of contaminants or diseases. Areas with favorable climatic conditions for mosquito breeding and survival, such as moderate temperatures, adequate rainfall, and suitable vegetation, are critical (Ferraguti et al., 2024). The Mediterranean climate of Shiraz provides interesting insights into mosquito ecology and disease dynamics. Furthermore, urbanization and land-use changes can significantly affect mosquito populations. A region undergoing rapid urbanization, such as Shiraz, offers opportunities to study how these changes affect mosquito habitats and disease-transmission patterns (Wang et al., 2020). Hence, it is a valuable location for studying the complex interactions between climate, urbanization, and mosquitoes, even before introducing the issue of MPs. Shiraz has often experienced surges of vector-borne diseases such as malaria, dengue, or West Nile virus (Gheibi et al., 2023; Soltan-Alinejad and Soltani, 2021), making it a valuable location for studying the complex interactions between climate, urbanization, mosquitoes, and disease transmission (Zhang et al., 2024), even before introducing the issue of MPs.

Based on a literature review, no studies have been conducted on the isolation of microplastics from mosquito populations in Iran, and only one has been conducted so far in total. This study provides the first detailed investigation of microplastics in mosquito larvae in Iran, contributing important baseline data to the growing global understanding of microplastics pollution. By documenting the presence of microplastics in freshwater mosquito habitats, which are key components of both local and global food webs, our findings highlight the need for continued attention to this emerging environmental issue. While focusing on a specific region, this study offers insights that may inform broader efforts to understand and mitigate the effects of microplastics on ecosystems worldwide. These results underscore the importance of further research into the pathways through which microplastics interact with terrestrial and aquatic ecosystems.

A total of 721 mosquito larvae were collected from six locations in Shiraz (Fars Province), southern Iran, between April and December 2023. From the collected samples, 524 mosquito larvae (3rd and 4th instars) were morphologically identified using valid taxonomic keys.

From these samples, we identified five species, four from the genus Culex and one from the genus Culiseta, across all the studied locations. Among the collected mosquitoes, the most abundant species (eudominate) were Cx. pipiens, which accounted for 46.4% of the total number of mosquitoes collected. Cs. alaskaensis was the second most abundant species (dominant species), accounting for 22.3% of the total number of mosquitoes collected. Cx. pusillus (18.9%) and Cx. vagans (12.2%) were the dominant species and comprised a smaller percentage of the total number of mosquitoes collected. Furthermore, Cx. bitaeniorhynchus was a subrecedent species with a frequency of 0.2% of all specimens. A summary of the findings is presented in Table 2.

The highest number of observed species in one community was recorded in the Khoshk River, with four of the five species being collected. Dasht Arjan was the second, where three of the five captured species were found. The highest geographical distribution in the study area was associated with Cx. pipiens, which were found at five locations. Cx. vagans and Cs. alaskaensis were present in two and three of the six studied regions, respectively. The lowest distribution was attributed to Cx. bitaeniorhynchus, which was only identified in the Khoshk river. Details of their abundance and the dominant species (in parentheses) are presented in Fig. 2.

Nevertheless, and more importantly, microplastics were found in all species and locations studied (note: herein, we define a microplastic as a piece of a polymer that remained intact after the digestion and washing process and could be detected and its size measured via microscopy). Figure 2 shows the percentage of microplastics found per region and the dominance of the sampled and identified species. The Dasht Arjan sampling point (33%) was followed by the point in the Farzanegan district (26%) and the Deh Shikh and Faculty of Health points, with slight differences (19 − 16%).

Figure 3 shows the number of microplastics detected by color at various sampling locations. White microplastics were the most common, accounting for 52.81% of all detections across all locations. Black microplastics accounted for 25.84% and were found in five of the six locations. Red microplastics were the least frequent (4.49%) and were only detected in Cx. pipiens from the Farzanegan District.

Figure 4 illustrates the mean size of microplastics by color at various sampling locations. The largest microplastic was the blue microplastic detected in the Cs. alaskaensis from the Koshk River with a mean size of 500 µm. The smallest was a black microplastic found in the Cx. pipiens in Deh Shikh with a mean size of 75 µm.

Figure 5 presents a detailed visual representation of the types of microplastics, and other particles found in mosquito larvae. These images, captured with a stereomicroscope, facilitate a closer examination of the particles in the larval environment, allowing for the differentiation of the various colored plastics.

Table 3 provides a comprehensive overview of the various types of microplastics, along with other identified particles present in mosquito larvae. This data was organized according to mosquito species and sampling locations, allowing for a clear comparison and analysis of the findings across diverse environmental contexts.

Among all the particles identified in our study, cotton was the most prevalent, found in all mosquito species and in nearly all locations, except in the Farzanegan district. Polyalkenes were the most frequently detected microplastics in the mosquito larvae (three sites and two species). This finding highlights the prevalence of polyalkenes as significant contaminants present within the sampled mosquito populations. Polyethylene was the most frequently detected microplastic in the Mianrud and Farzanegan districts and was exclusively collected from Cx. pipiens. Another microplastic detected was polypropylene, which was found exclusively in Cx. pusillus at the Faculty of Health Campus.

Our study is the first to document the pervasive presence of microplastics in mosquito larvae across various locales in Iran and one of the few worldwide, revealing a concerning facet of environmental pollution that bridges aquatic and terrestrial ecosystems. The ubiquitous detection of microplastics, predominantly polyethylene, in all examined mosquito species underscores the extensive contamination of freshwater habitats and highlights the potential role of mosquitoes in the bioaccumulation and biomagnification of these pollutants. Particularly alarming is the finding that polyethylene, a widely utilized and disposed polymer, constitutes a sizable proportion of the microplastics detected, hinting at broader issues related to waste management and environmental stewardship.

The predominance of white and black microplastics observed in our study not only reflects the sources and pathways of microplastic pollution, but also raises questions about the visibility, persistence, and ecological impacts of diverse types of microplastics. The variations in microplastic color and size detected across locations further emphasize the need for a deeper understanding of how environmental factors influence the distribution and fate of microplastics.

Beyond immediate ecological concerns, our findings have profound implications for public health given the role of mosquitoes as vectors of disease transmission. The potential of microplastics to function as vectors for pathogens or to influence mosquito vector competence presents an urgent area for future research, wherein the intersection of microplastic pollution, vector biology, and disease transmission dynamics must be thoroughly investigated.

Looking ahead, it is imperative that further multidisciplinary research efforts are aimed at studying the complex interactions between microplastic pollution, mosquito ecology, and public health and to explore the long-term ecological and health impacts of microplastic contamination in freshwater systems. Finally, our study highlights the critical need for enhanced waste management strategies and policies aimed at reducing the plastic pollution at its source. Only concerted global efforts will mitigate the far-reaching consequences of microplastic pollution and safeguard both environmental and human well-being.