New Insular Record of the Giant Water Bug, Lethocerus patruelis (Hemiptera: Belostomatidae), from the Northeastern Aegean

Abstract

Lethocerus patruelis (Stål, 1854) is a large aquatic hemipteran and the only European representative of the family Belostomatidae. Commonly known as the giant water bug, this species was historically restricted to the Balkans, Anatolia, and parts of the Middle East, but has exhibited a marked westward and northward range expansion in recent decades. In this study, we report the first confirmed occurrence of L. patruelis on Lesvos Island, in the northeastern Aegean Sea, based on a direct observation made within a wastewater treatment facility. The individual was identified in situ using diagnostic morphological traits and photographed without disturbance. This finding extends the known insular distribution of the species and underscores its capacity to exploit anthropogenically modified aquatic systems. Given the island’s rich mosaic of natural and artificial wetland habitats—including over 200 mapped sites—Lesvos may offer suitable conditions for the establishment of local populations. This record highlights the need for targeted surveys and long-term monitoring across under-sampled insular landscapes.

The giant water bug Lethocerus patruelis (Stål, 1854) is a large aquatic hemipteran from the family Belostomatidae and the sole representative of this family recorded in Europe. Belostomatids are globally distributed across tropical and subtropical regions and are ecologically significant components of freshwater ecosystems due to their role as apex invertebrate and small vertebrate predators. L. patruelis displays a broad distribution, ranging from the Balkans and Anatolia to parts of the Middle East, including Israel, Syria, and Saudi Arabia, with further extensions into the Oriental region, notably Afghanistan, India, and Nepal [1,2]. In recent decades, the species has shown a clear northward and westward expansion [1,3,4], reaching as far as Italy [5,6,7], suggesting enhanced dispersal mechanisms, colonization potential, and ecological plasticity [4,5].

Ecologically, L. patruelis is typically associated with lentic and slow-flowing freshwater systems [8,9], including ponds, marshes, oxbow lakes, irrigation reservoirs, paddy fields, and wastewater lagoons. It exhibits a complex life history comprising an aquatic developmental stage and a reproductive phase involving the deposition of eggs on emergent vegetation. A distinctive trait of the species is its male-mediated parental care, whereby the male guards and maintains egg clutches, a strategy that significantly enhances embryonic survival [10,11]. Its predatory repertoire includes amphibians, fish, aquatic insects, and even small terrestrial organisms [12,13], positioning it as a key regulator of freshwater food webs [12]. Furthermore, due to its sensitivity to pollution, habitat degradation, and changes in hydrological regimes, L. patruelis has been proposed as a valuable bioindicator species, particularly in anthropogenically influenced or transitional wetland systems [13].

In the Mediterranean region, and particularly across the eastern Mediterranean, L. patruelis has been increasingly documented on islands, often in proximity to the Anatolian mainland. Confirmed occurrences from Rhodes, Lemnos, and Thassos indicate the establishment of isolated island populations beyond the species’ previously recognized continental distribution [8,14,15]. This pattern reflects a broader dispersal trend across Mediterranean countries, potentially driven by moderate climatic shifts and the species’ strong ecological plasticity [6,7,16]. Observations from climatically transitional and human-modified landscapes suggest that both environmental change and anthropogenic pressures are actively shaping its range dynamics [3]. Dispersal may occur either through active flight or via passive, human-assisted transport mechanisms such as irrigation networks, construction materials, and marine vessels [3,4].

Despite the species’ increasing regional presence, substantial knowledge gaps remain regarding its distribution across much of the Aegean archipelago. In particular, several islands that host structurally suitable freshwater habitats remain under-surveyed or entirely unsampled. Lesvos Island—located in the northeastern Aegean Sea and characterized by a complex mosaic of wetlands, seasonal ponds, pasture ponds, agroecosystems, and semi-natural aquatic systems—represents one such area [17]. Although its proximity to mainland Anatolia and adjacent islands with confirmed L. patruelis populations makes its colonization plausible, no verified record has been published to date.

Here, we report a new insular occurrence of Lethocerus patruelis on Lesvos Island, based on a direct field observation conducted within a managed wastewater treatment facility embedded in a semi-natural landscape mosaic. The observation was made in the context of our broader research on biodiversity across natural and anthropogenically modified wetland systems on the island [17].

On 13 September 2021, during routine biodiversity monitoring, a member of our team (A.K.) documented an insect within the operational boundaries of the municipal wastewater treatment plant (WWTP) near the town of Eressos, in western Lesvos (Figure 1). The WWTP consists of an equalization tank, an anoxic tank, an aerobic bioreactor, and a sedimentation tank, as well as a chlorination tank for the treated effluent. The facility also comprises a series of unsaturated and saturated vertical subsurface flow constructed wetlands, an anaerobic bioreactor, and concrete drainage elements.

Although engineered for wastewater management, the site maintains hydrological continuity throughout the year and supports microhabitats that structurally resemble semi-natural wetlands. Adjacent to the facility lies a small, intermittently flowing stream, embedded within a broader landscape dominated by olive groves, scattered shrub vegetation, and fragmented riparian zones.

The red star indicates the locality of the wastewater treatment facility near Eressos, western Lesvos, where a single adult individual was recorded on 13 September 2021. Land cover classifications highlight key vegetation types and land uses across the island, including olive groves, broadleaved woodlands, transitional vegetation, wetlands, and cultivated areas. The facility is situated within a mosaic of agricultural and semi-natural habitats, adjacent to an ephemeral stream. Inset: geographic position of Lesvos within southeastern Europe and the northeastern Aegean Sea.

The insect was resting motionlessly on a shaded concrete surface bordering one of the treatment basins (Figure 2). It was fully exposed and stationary, allowing for close-range visual assessment and photographic documentation. To avoid unnecessary disturbance, we refrained from handling the specimen. Identification was performed in situ, based on external morphological traits—including color and patterns, body shape, dorsal coloration, and the raptorial foreleg structure—using the key of Novoselsky et al. [18]. Cianferoni & Nardi [3] highlight the morphological similarities of L. patruelis and the African closely related species, L. cordofanus Mayr, 1853. As posternal keel is variable, the aedeagus is the character that allows certain identification at the species level. The estimated total body length, measured against a photographic scale, was approximately 75.2 mm. Based on these features, the habitat where the specimen was recorded, the known distribution of L. patruelis and L. cordofanus, and the fact that Lesvos is adjacent to regions that are within range of the species, we unequivocally identified the specimen as Lethocerus patruelis (Stål, 1854), the only European representative of the family Belostomatidae [8]. The sex could not be determined, as the observation was non-invasive and no manipulation occurred.

The confirmed occurrence of Lethocerus patruelis on Lesvos Island constitutes a new insular record within the species’ known distribution in the eastern Mediterranean. This first documented occurrence prompts consideration as to whether the species represents a previously overlooked resident population, a recent introduction, or an instance of ongoing range expansion. Although Lesvos lies in close proximity to the Anatolian mainland and to other Aegean islands with verified records [8,14,15,19], this is the first formal documentation of its presence on the island. The species’ detection within a wastewater treatment facility surrounded by semi-natural agroecosystems and ephemeral aquatic features underscores its capacity to exploit both natural and anthropogenically modified environments.

The continued westward and insular expansion of L. patruelis is likely driven by a combination of climatic and anthropogenic forces. Climatic shifts in the eastern Mediterranean may be creating novel ecological opportunities in regions previously unsuitable for the species, while land-use changes—particularly those related to water management and agriculture—are generating artificial habitats that mimic the structural features of natural wetlands [3,9]. In this context, the species’ colonization potential appears closely tied to its ecological plasticity and its ability to exploit disturbed or transitional environments.

On Lesvos, habitat suitability is not limited to natural freshwater systems. In total, the island hosts more than 400 wetland-type sites, ≥50% of which are from artificial water bodies known locally as giolia—seasonal livestock ponds typically carved into soft sediment or bedrock [17]. The presence of such a diverse and widespread wetland network greatly enhances the likelihood that dispersing individuals may encounter habitats suitable for survival and reproduction. Moreover, the species’ reproductive ecology, which includes aquatic nymphal development and terrestrial oviposition on emergent vegetation, is particularly well suited to patchy and seasonal freshwater systems. Males engage in egg guarding behavior, which increases hatching success even under fluctuating hydrological conditions [10], potentially conferring resilience to stochastic environmental pressures.

Dispersal pathways remain uncertain, but both active and passive mechanisms are plausible. While the species is capable of flight, passive transport via human activity—such as the unintentional transfer of individuals through agricultural shipments, water infrastructure, or maritime routes—cannot be ruled out. Tourist ferries and cruise vessels have been proposed as potential vectors for occasional long-distance dispersal events, especially between mainland ports and nearby islands [3,4]. Such mechanisms could explain colonization patterns in the Aegean archipelago, where the proximity of Lesvos to the Anatolian coast and to other insular populations increases the likelihood of cross-island movement.

Although only a single individual was recorded, the observation site exhibits year-round water availability, vegetative structure, and adjacent habitat heterogeneity—conditions that collectively satisfy the ecological requirements of L. patruelis. Whether this record represents an isolated dispersal event or the early stages of colonization remains unclear. However, the availability of numerous artificial and semi-natural wetland habitats across the island strongly suggests that Lesvos could support a stable population if propagule pressure persists. Moreover, the sustainable managed agroecosystem on the island can create corridors, enhancing the connectivity of potential habitats that otherwise would be fragmented [20]. This agroecosystem consists mostly of semi-natural olive groves that have been found to host a diverse arthropod ecosystem, which can also provide stable prey for L. patruelis [20].

Beyond its biogeographic relevance, L. patruelis provides important ecological services as a top invertebrate predator in Mediterranean freshwater systems. At the same time, its persistence is threatened by various anthropogenic pressures, including organic and nutrient pollution and agrochemical contamination, particularly in regions where freshwater systems are heavily modified. This dual status—as both functional predator and vulnerable bioindicator—underscores the species’ potential value in freshwater monitoring programs [13]. The increasing involvement of citizen science initiatives has also contributed substantially to detecting new occurrences and should be incorporated into future monitoring efforts to better understand distributional dynamics [16].