The yellow-legged hornet (Vespa velutina nigrithorax Lepeletier, 1836) (Hym.: Vespidae) and the European hornet (Vespa crabro L., 1758) (Hym: Vespidae) occupy different geographic ranges with different native fauna. As top insect predators, these two Vespa species have successfully established non-native populations far from their endemic ranges, adding to economic difficulties and environmental problems (Beggs et al., 2011; Barbet-Massin et al., 2020). Both are invasive species that permanently occupy new habitats with negative consequences for the natural environment and/or humans (Monceau et al., 2015; Otis et al., 2023). The natural range of V. crabro is temperate Eurasia, from the United Kingdom through the Iberian and Italian Peninsula to the Far East, Eastern Russia and Japan. It was accidentally introduced into eastern North America in the 1840s (Bequaert, 1932; Shaw and Weidhaas, 1956; Landolt et al., 2010; Bass et al., 2022). One of the most devastating enemies of the western honey bee (Apis mellifera L., 1758) (Hym.: Apidae), the yellow-legged hornet is currently spreading rapidly and become the most abundant Vespa species in the world (Park et al., 2024). V. v. nigrithorax is widely distributed and occupies different habitats in tropical and subtropical regions of Asia, from Pakistan throughout most of Indochina, Indonesia, and Taiwan, Southeastern Asia and the Malay Penninsula and to North America (Choi et al., 2012; Monceau et al., 2014; Lioy et al., 2023; Otis et al., 2023; Hoebeke et al., 2024). Its high invasion potential has been demonstrated by its rapid dispersal through Europe, Korea, and Japan (Robinet et al., 2017; Lioy et al., 2023; Otis et al., 2023).

The emergence and successful spread of this invasive hornet into new habitats poses a serious threat to the conservation of the native insect fauna, due to their predation and competition abilities (Snyder and Evans, 2006; Cini et al., 2018; Carisio et al., 2022). The competitive interactions can emerge among Vespa species when their habitats overlap within the same biotope. This phenomenon relates to several factors, such as overlapping nesting sites, access to resources, prey acquisition, and partially overlapping seasonal phenology (Beggs et al., 2011; Barbet-Massin et al., 2020). Each species efficiently exploits the organic resources available in its endemic range, thereby effectively occupying its ecological niche (Landolt et al., 2010; Lioy et al., 2023). However, this ecological success has also facilitated the colonization of new, previously uninhabited environments, driven by factors such as increased transportation, urbanization, and, importantly, climate change. V. crabro has historically occupied Eurasian regions (Bequaert, 1932; Shaw and Weidhaas, 1956; Snyder and Evans, 2006; Landolt et al., 2010). Whilst V. crabro is a generalist predator, it has not been reported to cause significant losses of honey bees, or environmental damages within endemic nor newly introduced ranges (Cini et al., 2018; Rome et al., 2021). The yellow-legged hornet by way of contrast has a semi-specialised diet which mainly targets honey bees and social wasps of the genus of Vespula and Polistes, and Diptera species (Laurino et al., 2019; Rome et al., 2021). Its presence in regions it has invaded raises various problems, due to its high predation of A. mellifera, causing a serious economic challenge for the beekeeping industry (Monceau et al., 2014; Rome et al., 2021; Diéguez-Antón et al., 2025).

The competition between predator and prey is an ongoing evolutionary contest, enacted via traits and strategies that evolve to maximize the chances of the predator to successfully catching food resources, and for local prey populations survive the attack and live on (Brodie and Brodie, 1999; Kwon and Choi, 2020). The problematic situation could arise when the ranges of two Vespa species begin to overlap. In such cases, the native Vespa species may experience competitive pressures, or even displacement following the arrival of a previously absent Vespa species (Snyder and Evans, 2006; Cini et al., 2018; Carisio et al., 2022; Lioy et al., 2023). However, in areas where the endemic species coexists with the ecologically similar and invasive V. v. nigrithorax, its dominance may be challenged. Previous studies have shown that V. v. nigrithorax can compete with other Vespa species within its invasive range in Asia (Choi et al., 2012; Park et al., 2024). Moreover, it has rapidly become the most common hornet species in Korea near Busan within six years (Choi et al., 2012; Lioy et al., 2023; Park et al., 2024).

In Europe, competition has also been hypothesized between V. v. nigrithorax and the native V. crabro, although direct field-based confirmation remains limited (Cini et al., 2018; Carisio et al., 2022). There are clearly visible morphological differences between the workers of the Vespa species studied for, example in terms of colour and body size (Kovac and Stabentheiner, 2012; Smith-Pardo et al., 2020; Do et al., 2022; Mattila et al., 2022) These differences are the result of adaptive processes and predation strategy linked to the biotope of the original habitat (Araújo et al., 2004; Perrard et al., 2013, 2016). Among hornet species, the flight capacity is a major driver of success, most exemplified by bee-hawking activity, a specialised predation stategy found in the case of V. v. nigrithorax (Tan et al., 2007; Sauvard et al., 2018; Cappa et al., 2021).

The morphology associated with flight affects various aspects of performance, such as lifting capacity, speed, endurance, and manoeuvrability (which refers to the ability to change the velocity and the direction of movement) (Marden, 1987; Sauvard et al., 2018). Several morphological parameters and the ratios calculated from them can be used to infer flight performance and locomotion ability (Marden, 1987; Byrne et al., 1988; Danforth, 1989; Sauvard et al., 2018; Kwon and Choi, 2020). There is currently a scarcity of data on insect morphological characteristics derived from three-dimensional micro-CT imaging (Jonsson, 2023; Toulkeridou et al., 2023). Overall, relatively few studies in entomology have employed non-invasive imaging techniques such as micro-tomographyto investigate insect anatomy. The principal advantage of this technique lies in its ability to capture detailed, comparable internal structures and morphological features without resorting to dissection or other destructive methods, thus preserving specimens in their natural state (Jonsson, 2023; Moraes et al., 2023; Toulkeridou et al., 2023).

Based on these morphological features, our present research aimed to investigate the fundamental differences in the body structure of the two Vespa species, V. crabro and the invasive V. velutina nigrithorax, to understand and describe the external morphology in more detail, and to compare the two species flying abilities by calculating several flight coefficient factors. Our work also aimed to confirm the differences in morphology related to hunting strategies of the two species using modern 3-D imaging techniques and digital microscopy. Using these data, it is possible to compare the predation success and ecological competitiveness of two generalist predators within a specific habitat by examining their morphological traits. By introducing a generalist predator into a newly occupied habitat, the study can effectively highlight the strengths and weaknesses of this predator when competing with endemic species, providing valuable insights into ecological adaptations and competitive interactions.

The paradox of the Vespa genus lies in the fact that its various species are highly successful organisms within their respective native habitats, whereas in our case, both examined species have successfully colonized various regions (Monceau et al., 2015; Otis et al., 2023; Hoebeke et al., 2024). The different endemic ranges of these two species may be reflected in the very distinct colouration patterns shown, from both the frontal and dorsal views (Kovac and Stabentheiner, 2012; Do et al., 2022; Mattila et al., 2022). Our study confirmed that the dorsal colouration of V. v. nigrithorax dorsal colouration is much darker (9.5% bright; 34.3% reddish-brown; 56.2% black) than that of V. crabro, suggesting that darker colourn may relate to thermal regulation, crypsis, or mimicry (Joron, 2009; Perrard et al., 2014). Furthermore, the colouration of V.crabro displays a more vivid and variegated dorsal tone (30.9% yellow; 41% redish brown; 28.1% black) with more bright yellowish colours, which may well serve as aposematic signalling against predators (Joron, 2009; Perrard et al., 2014). The different colouration in different areas may certainly play a role in terms fo the ecological specialization and hence success of the two species. In the eastern Palaearctic, within its native range, V. v. nigrithorax does not appear to have any natural predators that exert significant population-regulating pressure. In contrast, within the western Palaearctic, the European bee-eater (Merops apiaster; L. 1758) (Aves: Meropidae), which feeds on hymenopterans, including bees and wasps, has been identified as a potential natural enemy of V. v. nigrithorax (Onofre et al., 2023). Moreover, the cryptic black colouration of V. v. nigrithorax may, in some cases, effectively conceal it from visual predators, thereby reducing its likelihood of being preyed upon (Endler, 1988; Keszthelyi et al., 2024).

The fighting ability related to the success of a predator is largely influenced by body size (Kwon and Choi, 2020). Some earlier studies support the differences here found between the examined two Vespa species, including in terms of morphological and behavioural aspects. Thus, our results reinforce the study by Kwon and Choi (2020), completed using deeper morphological analyses. In our study V. crabro was here found to be larger than V. v. nigrithorax in every aspect, mosty the body volume and dry body weight, reflecting the clearly visible different anatomical structure. The correlation analysis revealed more structural differences in the body anatomy, while V. crabro displayed more uniformly strong correlations among body and thoracic parameters, suggesting a more robust body structure. In contrast, V. v. nigrithorax exhibited stronger correlations among wing traits and leg traits suggesting the key to the success of this species is related to its flight performance. The different body structure can be rooted in terms of direct ecological pressure, thereby supporting the results of Kwon and Choi (2020) in suggesting that V. v. nigrithorax tends to avoid direct competition (fighting, nesting sites) with native hornet species including V. crabro in its Asian range.

The predatory success of these two social wasp species is fundamentally based not only in their overall body size but in their flight performance capabilities (Tan et al., 2007; Sauvard et al., 2018; Cappa et al., 2021). To gain a deeper understanding of the structural differences between V. crabro and V. v. nigrithorax, and thereby their potential predatory success, we calculated multiple flight-related morphological ratios, analying them through the linear model developed specifically to assess key differences of flight and predation abilities.

Based on the analysed coefficients, V. v. nigrithorax has relatively longer legs in proportion to its body size, which likely supports enhanced aerial balance and prey-capturing ability (Dudley, 2002; Burnett and Combes, 2023). In contrast, V. crabro showed higher values in buoyancy-ability but not significant different to that of V. v. nigrithorax in terms of the relative thorax size and volume. However, in terms of wing loading and wing surface-to-body ratio, V. v. nigrithorax appears more adapted for hovering flight and rapid manoeuvrability. The higher wing surface, density, and shape related quotients emphasize the better flight performances of V. v. nigrithorax that appear unequivocally reflected in different hunting activity, the so-called bee-hawking behaviour. These findings are further supported by our linear model analysis, which compared distinct flight abilities between the species ( Figure 4 ; Table 4 ). According to this model, V. v. nigrithorax outperformed V. crabro in nearly all evaluated flight and predation-related traits as would be expected from empirical studies (Tan et al., 2007; Sauvard et al., 2018; Cappa et al., 2021; Herrera et al., 2023).

The relative success of these two distinct taxa is likely influenced by key morphological differences as revealed by our findings from multiple perspectives. Our research, has been driven by the aim to better understand how morphological traits alongside behavioural differences previously noted, can help explain their ecological performance. Specifically, we tried to examine how the generalist predator V. crabro and the semi-specialized, bee-adapted predator V. v. nigrithorax have morphologically adapted to specific predation strategies, such as bee-hawking.

Due to its ethological and ecological traits, V. v. nigrithorax appears to have gained a competitive edge over the native V. crabro in some special areas, i.e. bee-hawking. However, more empirical studies need to be preformed to ascertain whether or not V.v. nigrithorax can really affect native Vespidae species at the population-level. The invasive V. v. nigrithorax behavioural, morphological, and visual adaptations – particularly its efficiency in terms of honeybee predation – have positioned it as a major emerging threat to honeybee populations in western Palaearctic landscapes. These characteristics allow V. v. nigrithorax to become the dominant arthropod predator of honey bees in this region, regardless of the continued presence of its congener V. crabro.