In South Korea, the rising number of hornet nests reported to emergency services and the growing public fear of these insects reflect an increasing aversion to them in modern society. Yet despite this attention, the basic taxonomy of the country’s hornets has remained surprisingly incomplete, particularly for the immature stages. Traditionally, taxonomic studies on hornets have relied primarily on adult morphological characteristics, with molecular analyses added more recently to improve accuracy. Non-adult developmental stages, including eggs, larvae, pupae, and nests, have been excluded from integrative interpretations of the genus because of a lack of taxonomic knowledge. This gap matters, because phylogenetic trees built from larval traits could provide valuable insights when compared with the existing adult-based trees.

A new study published in the Journal of Hymenoptera Research set out to close this gap for South Korea, where eight hornet species and two subspecies have been documented, including the invasive V. velutina nigrithorax. For some of these, such as V. simillima xanthoptera, V. simillima, and V. crabro flavofasciata, only pictorial identification keys were previously available, and the literature contained only a brief description of V. dybowskii. The study aimed to develop identification keys and provide detailed morphological descriptions of the mature larval stage, specifically the fifth instar, for eight hornet species, including five indigenous to South Korea that had not been extensively studied. To date, morphological studies have covered the larvae of only 11 of the 22 recognized hornet species worldwide, and detailed species-level descriptions of variation between and within species remain scarce.

The researchers focused on fifth-instar larvae because most morphological analyses of Vespa larvae have concentrated on this stage, in which the head capsules are the most sclerotized and their traits are the most distinct and observable. Between 2021 and 2024, larvae were collected from nests of the eight Vespa species across South Korea. For each species, more than 80 individual larvae were observed and sampled, with one exception: V. binghami, which is rare and was sampled from a single nest containing 10 individuals. Mature larvae were identified by several key features, including body size, particularly head width, a strongly sclerotized and yellow-pigmented head, and a heavily sclerotized apical portion of the mandible. Larvae collected in 2021 and 2022 were stored in a freezer at minus 20 degrees Celsius until processing, while those collected in 2023 and 2024 were processed immediately.

The laboratory work combined several imaging techniques. Heads and body integuments were cleaned with a 10 percent potassium hydroxide solution. For scanning electron microscopy, three specimens per species were fixed in formaldehyde solution with a phosphate buffer, dehydrated through a graded ethanol series, treated with hexamethyldisilazane, dried, and gold-coated before imaging with a Hitachi SU8220 scanning electron microscope. Additional specimens were examined under a stereomicroscope, and for each species, 10 individuals were dissected to isolate mandibles, labrums, and various cranial parts, which were mounted on slides for examination with an optical microscope. Because specimens stored in a freezer may exhibit significant morphological changes due to dehydration, the researchers excluded distinct differences between freshly treated and frozen specimens, such as variation in cranial reticulation, from their observations.

The study recorded detailed measurements and structural traits for each species. These included body length, cranial width, the ratio of cranial width to mouthpart width, the angles formed between adjacent mandibular teeth, and the clypeal index, a measure of the relative height of the main disc of the clypeus. The resulting descriptions are strikingly specific. Vespa mandarinia, for example, showed a cranial width to mouthpart width ratio of 1.29 to 1.42, with a mean of 1.39, and a body length of 26 to 35 millimeters. Its mandible bears three heavily sclerotized teeth with blunt apices, and the outer surface of the mandible is entirely covered with numerous pits. Vespa ducalis, by contrast, can be identified by the distinctive orangish coloration of its cranium, a trait the authors describe as unique to this species, along with a large clypeal main disc more than three times the height of the ventral region and a relatively small third mandibular tooth.

Several other species showed equally characteristic features. Vespa crabro flavofasciata exhibits numerous dark spots along the dorsal and lateral margins of the clypeus, whose margins form a broad, rounded, almost semi-circular arc, a shape observed only in this species; the number of spots varied among individuals, possibly due to environmental or genetic factors. Vespa dybowskii is distinguished by an entirely weakly pigmented temporal band and by conical papillae and spicules confined to a narrow ventral zone on the palate, and its mandible lacks pits on the outer surface. Vespa binghami has distinctly thicker collar processes on the spiracles than other species, and its temporal band, antennal rim, and mandible are more heavily pigmented than in its relatives. Vespa analis parallela resembles V. velutina nigrithorax and V. simillima simillima in having a relatively narrow clypeus, but unlike them it has abundant pits on the outer surface of the mandible.

The most consequential finding, however, concerns the two species that could not be reliably separated. The study found no significant morphological differences between the larvae of V. velutina nigrithorax and V. simillima simillima. The antenna in V. simillima simillima is typically nearly hyaline, as in V. velutina, but may be slightly sclerotized, and this condition can vary even within a single colony. Mature larvae of V. velutina nigrithorax exhibit minimal variation within a colony but consistently retain traits comparable to those of V. simillima simillima, and the authors state that distinguishing them from some individuals of V. simillima simillima based solely on morphology is challenging. The identification key itself reflects this difficulty: the final couplet separates the two species only by the degree of sclerotization of the antennal rim and the extent of dotted patches on the frons, both of which vary within colonies. The close relationship between these two species is supported by adult and larval morphology as well as molecular data.

The study also introduced morphological features not previously noted by other authors, including paired suprafrontal marks that appear to branch from the median cranial sulcus just above the frons. This structure is observed in species other than V. analis parallela, V. simillima simillima, and V. velutina nigrithorax, though its function and phylogenetic significance remain unknown. Another previously overlooked characteristic is the relative position of the temporal band and the antenna, which the authors used in their species key. Some earlier traits proved less reliable than assumed: the median patch on the palate was previously considered a key characteristic for differentiating certain species, but this study also observed it in V. mandarinia and V. ducalis, with intraspecific variation in both, raising questions about its reliability for species identification.

Why are hornet larvae so hard to tell apart? The authors point to the protected environment in which these insects develop. Hornet larvae depend on nutritional inputs provided either by the queen during the solitary phase or by foraging workers in later stages, and eusocial species invest significant resources in caring for eggs and developing larvae, including regulation of nest temperature and humidity, structural fortification, and defense against intruders. In many social Hymenoptera, nests are protected from external conditions by being built in concealed sites or enclosed within a thick covering. In vespine species in particular, genetic changes affecting larval phenotypes may have been minimal throughout their evolutionary history given the stable conditions within the nest, exerting little selective pressure for morphological divergence among larvae. Compared to adults, which often differ markedly in body coloration and behavior, vespine larvae exhibit limited morphological differences across species. Polistine larvae, by contrast, show more distinct variation in features such as the size and distribution of hairs and bristles, which facilitates chaetotaxy-based analyses. Although vespine larvae may exhibit some species-specific tendencies in chaetotaxic patterns, the study found these generally insufficient for identifying similar species such as V. simillima simillima and V. velutina nigrithorax. The authors note that establishing species groups based solely on larval characteristics remains a challenging task, but that these characteristics could have significant implications for understanding hornet phylogeny. For now, the new descriptions and keys give South Korea its first comprehensive larval reference for eight hornet species, a practical tool for researchers and a reminder that even well-studied insects can keep their secrets hidden inside the nest.

Subject of Research: Morphological descriptions and identification keys for mature larvae of eight Vespa hornet species in South Korea

Article Title: Comparative morphological analysis of mature larvae of eight Vespa Linnaeus, 1758 species ( Hymenoptera , Vespidae ) in South Korea

Article References: Kim, J., Kwon, O.-S., Yamane, S., & Choi, M. B. (2025). Comparative morphological analysis of mature larvae of eight Vespa Linnaeus, 1758 species (Hymenoptera, Vespidae) in South Korea. Journal of Hymenoptera Research, 98, 435-465. https://doi.org/10.3897/jhr.98.141572

Image Credits: AI Generated

DOI: 10.3897/jhr.98.141572

Keywords: hornets, Vespa, larval morphology, taxonomy, South Korea, scanning electron microscopy, Vespa velutina nigrithorax, Vespa mandarinia, identification key, entomology, invasive species, Vespidae