The invasive mosquito species Aedes albopictus (Skuse, 1894) is rapidly spreading in Europe, posing an increasing threat because of its high vector competence for chikungunya and dengue virus. An integrative and eco-friendly control of these populations is required to prevent mosquito-borne disease outbreaks. Traditionally-used insecticides or other chemical control agents are often expensive, harmful to the environment, strictly controlled or completely banned in several countries. Additionally, insecticide resistance is a potential threat. One possibility for biological control agents is the use of native aquatic beetles as natural predators of mosquitoes to boost Bacillus thuringiensis israelensis (Bti) interventions. Thirty predatory aquatic beetle taxa were caught in Belgium and kept at the Institute of Tropical Medicine’s insectary to test predation rate and prey choice on Aedes albopictus and Culex pipiens Linnaeus, 1758. Predation rates suggest at least four efficient dytiscid predators that are known to inhabit small, temporary habitats in Europe. Further experiments on prey choice reveal a clear preference for Aedes albopictus over alternative larval prey (Culex pipiens, Daphnia sp., Chaoboridae). We found a strong ecological overlap of the feeding niche of A. albopictus and the hunting zone of dytiscid predators in the benthic layer of small waterbodies. Our findings on the efficacy are very encouraging to further assess the potential of native predacious diving beetles as a biological control agent against the invasive A. albopictus in Europe.

Arbovirus, biodiversity, Dytiscidae, invasion, predation

The Asian tiger mosquito Aedes albopictus (Skuse, 1894) is an invasive species that rapidly spreads throughout Europe (Sherpa et al. 2019). It poses an increasing threat because of its high vector competence for arboviruses such as chikungunya, dengue and zika (Martinet et al. 2019; Ryan et al. 2019). Since 2000, an increase in autochthonous infections of dengue and chikungunya virus was observed in Europe (Giunti et al. 2023), for example, 65 autochthonous cases of dengue were recorded in France in 2022 (Gossner et al. 2018; Cochet et al. 2022).

Aedes albopictus females search for blood meals during the day and prefer human hosts in urban areas (Bonizzoni et al. 2013), resulting in potential pathogen transmission and biting nuisance. The establishment of invasive mosquitoes in a yet unaffected area is mainly influenced by climate conditions (Schindler et al. 2015; Ryan et al. 2019), globalisation and urbanisation (Deblauwe et al. 2022b). In addition, these mosquito species are also adapting to climatic changes (Kramer et al. 2020, 2021). At the local scale, interspecific interactions and, foremost, the competition for ecological niches may shape the micro-distribution and abundance of a given A. albopictus population (Deblauwe et al. 2015; Müller et al. 2018).

To date, there are no effective vaccines or treatments widely available for dengue and chikungunya virus (Flandes et al. 2023; Thomas 2023; Al-Osaimi et al. 2024). Hence, the prevention of those arboviral infections is primarily based on the control of the mosquito vector (Vontas et al. 2012; Bonizzoni et al. 2013; Abdelnabi et al. 2017). The insecticides used for space spraying, indoor residual spraying or container treatment lead to insecticide resistance, which can rapidly develop in mosquitoes (Su et al. 2019; Pichler et al. 2022; Vereecken et al. 2022). In addition, insecticide-based control efforts over the past decades have not been successful in controlling Aedes populations (Achee et al. 2019). In Europe, a biological control method with Bacillus thuringiensis israelensis (Bti) is widely used to control mosquito populations (Giunti et al. 2023); however, its activity period is limited (Kroeger et al. 2013; Pauly et al. 2022). In addition, cryptic and domiciliary larval habitats may frequently go unnoticed (Achee et al. 2019). The enrichment of native aquatic biodiversity might be a promising nature-based solution to control discontinuously distributed A. albopictus populations. This form of biological control can synergise other control strategies, such as Bti, by including cryptic or domiciliary larval habitats, which often remain undetected or unidentified in other control strategies (Achee et al. 2019; Donald et al. 2020). Additionally, biological vector control generally receives more public acceptance if compared to chemical control (Reuss et al. 2020).

Alternative strategies are mainly focused on adult control and involve Wolbachia bacteria (Caputo et al. 2023), sterile insect technique (Balatsos et al. 2024), release of insects carrying a dominant lethal gene (Dobson 2021), attractive toxic sugar baits (Chiu et al. 2024), mass-trapping (Jaffal et al. 2023), spatial repellents, insecticide treated materials (Senapati et al. 2019), antipathogen genetic modifications and lethal ovitraps (Achee et al. 2019; Jones et al. 2021; Ogunlade et al. 2023). For juvenile stages, the use of a new entomopathogenic fungi (Cafarchia et al. 2022) or autodissemination (such as pyriproxyfen) (Pleydell and Bouyer 2019) are proposed alternative strategies (Achee et al. 2019). Biological mosquito control is so far mainly based on predatory copepods, Toxorhynchites larvae and fish; however, none of these control agents is currently used in Europe (Baldacchino et al. 2015; Achee et al. 2019).

Biological vector control increases the necessity to identify the most locally effective natural predators of Culicidae, which is especially true for areas recently invaded by Aedes albopictus (Younes 2008; Bofill and Yee 2019). The use of native cyclopoid copepods already proved to be a successful method to control first larval instars of A. albopictus (Pauly et al. 2022) and is applied in the Americas, Asia and Oceania (Baldacchino et al. 2015). Toxorhynchites species show positive results in the control of Aedes species, but they are native to the Tropics with some species found in Asia and North America (Donald et al. 2020; Malla et al. 2023). Moreover, fish such as mosquitofish (Gambusia sp.) showed no preference towards mosquito larvae, were introduced far outside its natural distribution range and some species are now considered invasive species (Alcaraz et al. 2008; Jourdan et al. 2021; von der Leyen 2022).

In contrast, many Dytiscidae or predacious diving beetles show a preference to feed on mosquito larvae (Culler and Lamp 2009; Bofill and Yee 2019). The presence of Dytiscidae caused Culicidae populations to decrease significantly (Lundkvist et al. 2003; Chandra et al. 2008; Culler and Lamp 2009; Bofill and Yee 2019). Both larvae and adults of Dytiscidae are considered ubiquitous top predators in lentic systems (Yee 2014), particularly in fishless waters (Larson et al. 2000; Bofill and Yee 2019). Adults of most dytiscid species are capable of active dispersal due to their ability to fly (Bofill and Yee 2019) and many are pioneers occupying freshly-formed waters (Lundkvist et al. 2003; Reyne et al. 2020). They are also found in urban areas (Lundkvist et al. 2002; Liao et al. 2020), which is of paramount importance since urbanisation decreases species diversity and favours Aedes albopictus population growth (Perrin et al. 2022).

Dytiscidae are known to migrate, entering a large variety of aquatic habitats and may even have seasonal habitat-shifts or winged migrations (Nilsson and Holmen 1995). Larson et al. (2000) mention that adults of many Dytiscidae are known to disperse readily and are frequently encountered across a wide variety of waterbodies. This also includes artificial habitats as observed by Bameul (1990), Shaverdo et al. (2013) and Nilsson (2024). Fransiscolo (1979) states that isolated individuals can be found anywhere and refers to specimens that were found near and in the sea. Many species reproduce in ephemeral waterbodies, such as small ponds, bogs or ditches with aquatic vegetation. Some even choose small ponds, pits or ditches with little vegetation (Galewski 1971).

Since mosquito larvae are an important prey item for Dytiscidae (Galewski 1971), predacious diving beetles are known to naturally colonise habitats with mosquito larvae, which has been observed by Lundkvist et al. (2003). Especially Agabus species seem to prefer smaller waterbodies (Davy-Bowker 2002; Lundkvist et al. 2003).

Onyeka (1983) found eight species of Dytiscidae in 107 litre artificial containers, with 46.3% of 432 specimens testing positive for Culex pipiens/torrentium antiserum. Dytiscidae also inhabit tree holes, phytotelmata and water-filled leaves (Kitching and Orr 1996; Miller and Bergsten 2016). Kehl and Dettner (2007) observed them in wells, cattle troughs, swimming pools and rain barrels. Balfour-Browne (1940), Yanoviak (2001) and Nilsson (2024) noted dytiscids in open tub aquaria, paddling pools and plastic cups and pans. Bameul (1990) identified 16 species in an urban pool. James (1965) and Bay (1974) found dytiscids in rock pools preying on Aedes atropalpus, which breeds in the same habitats as A. albopictus (Kesavaraju et al. 2011; Farajollahi and Price 2013). Young (1954) reported Laccophilus sp. in puddles, rain barrels, tin cans and water-filled tyres. Bashir et al. (2017, 2018) observed them with A. albopictus larvae in temporary pools and latex collection cups, while Sulaiman and Jeffery (1986) noted Dytiscidae preying on A. albopictus larvae.

Until now, research on predation by Dytiscidae focused mainly on their habitat characteristics (Ohba and Ushio 2015), type of prey (Culler and Lamp 2009), preferred larval stage of the prey (Chandra et al. 2008; Younes 2008) and dytiscid stages (larvae and adult) predating on mosquito prey (Bofill and Yee 2019). For example, larval Acilius sulcatus collected in India consumed 34 specimens of late instar Culex quinquefasciatus in 24 h (Chandra et al. 2008). Though literature supports the statement that predacious diving beetles are effective predators of mosquito larvae, it is unknown which species are the most effective and suitable predators, particularly in areas newly invaded by Aedes albopictus (Ohba and Takagi 2010). Some studies in India (Kumar et al. 2014) and Malaysia (Sulaiman and Jeffery 1986) showed positive results using Dytiscidae as a natural predator for A. albopictus in laboratory and field conditions. Bashir et al. (2018) proposed to use a dytiscid species as an efficient biological control agent against A. albopictus larvae. In general, literature on the topic is mainly focused on Asia and the United States of America. Only very little research has been conducted on whether and which dytiscid species in Europe would be suitable for integrated biological control of mosquitoes, especially for A. albopictus. Are all dytiscid species equally suitable as biological control agent against mosquito larvae? Additionally, would aquatic beetles prefer mosquito larvae over other aquatic invertebrates? We here provide research on predatory aquatic beetles for Europe.

We hypothesise that dytiscid species are potentially good biological control agents given that they: 1) show high feeding rate with preference towards Culicidae; 2) are common and widespread throughout Europe and 3) occur in the same region and habitat as Aedes albopictus larvae.

In this study, we aimed to assess the feeding preference of 30 predacious diving beetle taxa comparing: 1) mosquito larvae with other aquatic invertebrates and 2) Aedes albopictus to Culex pipiens larvae. Based on the experimental data, we evaluated whether predacious diving beetles have the potential as a biological control agent against A. albopictus and compared the overlap in the field distribution of the most efficient predator species with the points of entry (PoEs) of A. albopictus in Belgium.

This study is a first step to understand the value of the use of native Dytiscidae and Noteridae species as a biological Aedes albopictus control tool. Firstly, a high feeding rate on A. albopictus has been observed in several dytiscid species. Based on Kehl and Dettner (2007), Nilsson and Hájek (2024) and expert judgement, these species resulted in a top ten of best predators for A. albopictus larvae. Two prey-choice experiments carried out with a selection of the top ten predators revealed a clear prey preference towards mosquito larvae and especially towards A. albopictus larvae. For Belgium, a significant distribution overlap was detected for three efficient predator species (Agabus bipustulatus, A. nebulosus and Laccophilus minutus) and ten points of entry where Aedes albopictus has been introduced in Belgium between 2007 and 2020 (Deblauwe et al. 2022b). The results are very encouraging to further explore the use of Dytiscidae as a potential biological control agent against Aedes albopictus.

The obtained results showed evidence for efficacy of predacious diving beetles to predate on Aedes albopictus larvae. Following our observations, of the topmost predatory dysticid species are Agabus undulatus, A. bipustulatus, A. nebulosus, Rhantus exsoletus, Hyphydrus ovatus, Laccophilus minutus, Hygrotus impressopunctatus, H. parallellogrammus, Hydroporus figuratus and Ilybius quadriguttatus. The tested beetle larvae proved to be good predators, which is in line with Chandra et al. (2008), Culler and Lamp (2009) and Bofill and Yee (2019). However, dytiscid larvae were not included in the topmost predator list, since the active dispersal of the predators is necessary for biological control. Adults are found in a larger variety of waterbodies, while dytiscid larvae are generally ecologically more specialised than adults (Galewski 1971). It is important to note that Dytiscidae show different types of feeding behaviour in their life stages. Larvae can inject digestive protease enzymes and feed on the prey’s body contents (Yee 2014). In the majority of dytiscids, the first larval stage has shorter mandibles and feeds mostly on planktonic Crustacea, but older larval stages have longer mandibles and hunt insect larvae such as Culicidae (Galewski 1973). Especially larvae from the genera Agabus, Colymbetes, Hydroporus and Rhantus are known to feed mainly on mosquitoes (Galewski 1971). Adults have chewing mouthparts and are thus gape-limited (Yee 2014); however, they will attack any prey they can overtake (Galewski 1971).

The efficient predation on mosquito larvae by Agabus species are in line with Ohba and Takagi (2010), where they found a predation rate on fourth instar Culex tritaeniorhynchus of 100% and 55% for Agabus conspicuous and A. japonicus, respectively. The feeding rate for Hyphydrus ovatus is comparable to the observation in Ohba and Takagi (2010), with a 44% predation rate for H. japonicus on Culex tritaeniorhynchus. A similar predation rate was reported for Laccophilus minutus on C. pipiens and L. difficilis on C. tritaeniorhynchus of 43% and 39%, respectively. Evidence for predation on Aedes atropalpus was found for Laccophilus sp. (James 1965). The predation rate for Hydroglyphus geminus is in line with a 16% feeding rate in H. japonicus. In contrast to current observations, Rhantus suturalis had a feeding rate of 99% on C. tritaeniorhynchus (Ohba and Takagi 2010).

The prey choice experiments showed a clear feeding preference towards Culicidae, specifically to Aedes albopictus. When A. albopictus was depleted, a switchover to Culex pipiens was observed several times, which is in line with Culler and Lamp (2009) and Ohba and Ushio (2015). Aedes albopictus was presumably preferred due to its foraging behaviour, since its larvae tend to feed in the benthic zone in a flexing behaviour. In contrast, C. pipiens filter-feeds near the water surface and has a motionless behaviour to avoid predation (Yee et al. 2004; Ohba and Ushio 2015). Three videos demonstrating this behaviour are added as supplementary material (Suppl. materials 2–4). Predacious diving beetles tend to hunt their prey in the benthic zone (Lundkvist et al. 2003); hence, they encounter more A. albopictus larvae. Chances are high for introduced A. albopictus to encounter a suitable predator, since the distribution of these predacious diving beetles largely overlap with at least two species present in the known PoE of A. albopictus in Belgium. Some dytiscids occupy similar habitats as A. albopictus (Table 3) and are active all year round, except when freezing (Foster and Friday 2011).

In addition to Aedes albopictus, there are also two other invasive mosquitoes in Europe, i.e. A. japonicus (Theobald, 1901) and A. koreicus (Edwards, 1917) (Deblauwe et al. 2022b). Both species share a similar larval foraging behaviour as A. albopictus and, therefore, similar results in predation by dytiscids are expected. These species and especially A. japonicus, often occur in more natural habitats in their non-native range (Smitz et al. 2021; Deblauwe et al. 2022b) and, therefore, have an even larger overlap in habitat preference with native Dytiscidae. Most probably the foraging behaviour of these mosquito larvae dictates in large part the suitability of aquatic beetles as native predators.

Coinciding with a higher feeding rate in November, lipid concentration was much lower in November compared to April for Agabus bipustulatus, which may indicate a shortage of lipids before winter (Arrese and Soulages 2010). Agabus bipustulatus is known to be active during winter (Classen and Dettner 1983) and to hunt underneath ice (pers. observ. K. Scheers). For A. undulatus, no differences in weight, lipid, protein and glycogen content were found, which coincides with their disinterest to feed in November. This storage in energy and reduced feeding behaviour could indicate an overwintering strategy in the soil, which is also observed for A. paludosus (Classen and Dettner 1983). Average glycogen and protein concentration was doubled in November compared to April for both Laccophilus minutus and Hyphydrus ovatus. However, the difference was not significant. This observation is in line with their active behaviour in winter, since glycogen is synthesised to sugar alcohols as an adaptation to cold (Arrese and Soulages 2010). Further studies on the feeding behaviour of Dytiscidae should include seasonal effect and age of the used specimens to avoid seasonal bias.

From a European biological control perspective, Agabus bipustulatus seems to be the most suitable predator to reduce mosquito larvae, especially Aedes albopictus larvae. The species is known to occur in artificial containers (Onyeka 1983; Reyne et al. 2020) and temporary pools (Eyre et al. 1986), has a high dispersal rate (Kehl and Dettner 2007), has a broad habitat spectrum and is very common throughout Europe (Nilsson and Holmen 1995). In addition, Aedes cantans and Culex pipiens/torrentium antiserum was found in gut smears of field-caught adult and larval Agabus bipustulatus (Service 1977; Onyeka 1983), confirming its suitability as a biological predator against mosquito larvae. Further laboratory and field research on predation of mosquito larvae by A. nebulosus, Hygrotus impressopunctatus and Laccophilus minutus is required to assess whether these species are suitable biological control agents, since these species are largely understudied.

Currently, there is no literature available on the release of diving beetles in Europe. In terms of ecosystem impact, the introduction of additional native diving beetles as proposed in the present study could potentially compensate for loss of biodiversity, especially in biodiversity-poor areas commonly associated with Aedes albopictus infestations (Perrin et al. 2022; Giunti et al. 2023). Competition between Dytiscidae will be minimal, since adults are capable of active dispersal (Kehl and Dettner 2007); however, there is potential for competition and cannibalism amongst diving beetle larvae (Inoda and Kamimura 2004). Evaluating these interactions will be essential for predicting the success and sustainability of using Dytiscidae as biocontrol agents.

When considering the introduction of diving beetles as biological control agents against A. albopictus, it is crucial to account for several important non-target effects. Although the present study includes various prey species, such as Chaoboridae and Daphnia sp., further extensive field studies are necessary to include all naturally occurring prey and predators. This broader assessment will ensure a comprehensive understanding of the ecological impacts. Historical evidence indicates that generalist and specifically non-native predators often proved to become problematic. For instance, the introduction of the cane toad in Australia (Shine et al. 2020) and the mosquitofish in various regions (Jourdan et al. 2021) led to significant ecological disruptions due to their generalist feeding habits and competition with native species. In contrast, the use of native species, such as copepods (Giunti et al. 2023) and Toxorhynchites mosquitoes (Malla et al. 2023), could reduce negative ecological impacts.

This form of biological control may synergise with another biological control method that is already widely used in Europe: the use of Bti. This form of integrated vector control may work well with predacious beetles, since they are not affected by Bti (Shaalan and Canyon 2009) and studies showed that predator cues even increased mortality on Culex pipiens and C. quinquefasciatus when combined with Bti (Op De Beeck et al. 2016; Delnat et al. 2020. In addition, the dytiscids can invade cryptic or domiciliary breeding sites, which often remain undetected or unidentified during Bti application (Achee et al. 2019; Donald et al. 2020).

Rearing of Dytiscidae remains a major challenge due to their high rate of food consumption and their intrinsic cannibalistic behaviour (Inoda and Kamimura 2004; Inoda and Kitano 2013). Dytiscid larvae feed on dissolved body fluids, resulting in food refusal and irreversibly spoiling the water (Inoda and Kamimura 2004). A first solution could be deploying the larval stages in close proximity with Aedes albopictus populations to avoid the laboratory rearing to adulthood. Another possibility would be using the rearing technique developed for mass rearing of juvenile lobsters (Homarus gammarus), which could resolve the rearing problem by keeping juveniles separately in clean water with a high quality food source (Schmalenbach et al. 2009). Moreover, the terrestrial pupation stage of water beetles in soil is an additional element that is absent in many other reared aquatic animals and which makes rearing on larger scale more difficult. Once resolved, mass-reared Dytiscidae could be released in A. albopictus infested areas to aid population control. In addition, species such as Agabus bipustulatus have been found to live over two years, meaning a frequent release of these species would be unnecessary (Davy-Bowker 2002).

The results also underline the suitability and possibly important role of native predators in the ongoing battle against invasive species, such as the vectors of mosquito-borne diseases. Good habitat quality and high native predatory insect densities can prevent the establishment of invasive mosquitos (Juliano and Lounibos 2005). Moreover, high species richness of these native predators can assure the presence of suitable species which co-occur in the same habitat and have a compatible foraging behaviour. Restoration of degraded aquatic habitats is an important issue, however, often limited to larger bodies of water and lotic environments. In the light of the management of invasive mosquito species, also small fishless temporary waterbodies should be taken into account in nature management and restoration projects (Liao et al. 2020). Though the habitat overlap between Aedes albopictus and Dytiscidae is not 100% secure, we do observe an influence of separation of entomological disciplines: Culicidae specialists aim to find A. albopictus for monitoring purposes and do not record other species found in these habitats, while Dytiscidae specialists focus on natural habitats and rarely search specimens in tyres or artificial containers in gardens. From this perspective, it would be beneficial to underline the importance of recording these findings.

During this study, a potential bias was created since the beetles were fed solely under laboratory conditions and, therefore, forced to feed on selected prey, which might differ from their natural preference. They were fed with Culex pipiens and A. albopictus larvae, both accounting for the diverse feeding strategies of mosquitoes. In addition, Chaoborus sp. larvae were included since they resemble mosquito larvae and are very common in lentic waters. Daphnia sp., generally found in ephemeral ponds and small waterbodies, were added to include a completely different type of prey. Chironomidae were not included in this study since they prefer waters with sediment, which was beyond the scope of this study. Therefore, it is assumed that prey choices most likely available were added to the study and, hence, reducing the influence of bias.

We provide some evidence on the efficacy of Dytiscidae to predate on Aedes albopictus larvae. In total, the feeding rate of thirty aquatic beetle taxa on A. albopictus larvae were tested, accounting for almost 25% of the total Dytiscidae diversity in Belgium and one out of two Noteridae species present (Scheers et al., in press). When giving Dytiscidae the choice to feed on taxa other than mosquitoes, such as Chaoboridae and Cladocera, the tested species still preferred Culicidae larvae, with a clear preference towards A. albopictus over Culex pipiens. The distribution of best-predating species of Dytiscidae in our tests shows a clear overlap with the PoE where A. albopictus has been found in Belgium in recent years. Several species which are widespread in Europe, such as Agabus bipustulatus, A. nebulosus, Hygrotus impressopunctatus and Laccophilus minutus also show a high predation rate and clear preference for Aedes albopictus. Agabus bipustulatus seems to be the most suitable predator for Aedes albopictus, since this species is known to occur in artificial containers and temporary pools, has a high dispersal rate and is very common throughout Europe. Given these promising results for a nature-based solution, the use of Dytiscidae as a biological control agent against A. albopictus should be further investigated.

We would like to thank Marre van de Ven and Chaymae Kadi for their contribution to the research. We are grateful to the laboratory and technical staff at the Institute of Tropical Medicine Antwerp, Belgium, especially Luka Wouters, Leen Denis and Karen Jennes for their support of lab experiments. We would like to thank Tom Vermeire for his additional data on Dytiscidae in artificial habitats in Belgium. We also thank Dr. Juliane Hartke for her help during the experiments and Prof. Konrad Dettner for his insights into the seasonal effects. We thank Prof. Norbert Becker for providing us with the Aedes albopictus colony.