Darwin wasps are severely understudied across most of the world, and the subfamily Campopleginae, to which Hyposoter belongs, is particularly poorly resolved. These are koinobiont endoparasitoids, meaning the female wasp oviposits into an early larval stage of its host and the host continues to develop while the parasitoid larva grows inside it, eventually killing the caterpillar before it pupates. All Hyposoter species with reliable host records attack lepidopterans from a wide range of moth and butterfly families. Until recently, identifying them was nearly impossible: the only available European identification key was more than a century old, incomplete, and riddled with synonyms and mistakes. A monumental 2023 taxonomic revision finally made reliable species-level identification feasible, and this new study is among the first to exploit that breakthrough at national scale.

The scale of the discovery is striking for a country in the middle of Europe. The previous Swiss checklist of Darwin wasps included only six Hyposoter species, largely because no reliably identified material was available at the time. The researchers found that most of the newly recorded species had been sitting unidentified in museum collections for decades, many collected between 1880 and 1970 by two prolific Swiss collectors, Theodor Steck and Jacques de Beaumont. De Beaumont, a former curator and director of the cantonal museum of Zoology in Lausanne, accumulated new species at a notably steeper rate than Steck, not simply because he collected more intensively, but because he visited more heterogeneous localities across a broader elevational range.

Using two non-parametric species richness estimators, Chao-I and the abundance-based coverage estimator, the team calculated that Switzerland probably harbours between 54 and 61 Hyposoter species. These estimators work by accounting for rare species, using the number of singletons and doubletons, species represented by only one or two specimens, to infer how many undetected species remain. The confidence intervals are large, allowing for more than eight species so far unrecorded, and the species accumulation curve has not quite levelled off. For comparison, Germany has 42 recorded species and the United Kingdom 35, meaning Switzerland might host as much as 76 to 87 percent of the Western Palaearctic fauna, a proportion comparable to other well-studied ichneumonid groups.

The study also reveals a clear altitudinal pattern. Most Hyposoter species and specimens were collected below 1,000 metres, with 39 species having a median recorded elevation below that threshold. Only three species appear to be high-altitude specialists, and many show remarkably broad elevational ranges, with 10 species collected both below 500 metres and above 1,500 metres. Hyposoter caedator was recorded at the lowest altitude, 270 metres in Münchenstein, while Hyposoter culminator was found at the highest, 2,145 metres in Alp Flix in the Grisons. This diversity peak at low elevation contrasts with distribution patterns of other Darwin wasps in the Western Palaearctic, where higher-elevation regions are often surprisingly species-rich.

One of the most surprising findings concerns Hyposoter albosignatus, a species originally described from Northern Wales at less than 100 metres elevation. In Switzerland, its median altitude exceeds 1,750 metres, suggesting it may be a montane specialist whose lowland populations elsewhere remain undetected or unrecorded. The researchers found no distinct morphological patterns along the altitudinal gradient, such as darker colouration due to thermal melanism or size variation, leaving the ecological drivers of these distributions open to future investigation.

Phenological data show that the first Hyposoter species appear in April, with several clearly bivoltine or plurivoltine, appearing in two separate clusters per year. Some species, such as Hyposoter didymator and Hyposoter notatus, span from early April until early October, while others with at least 10 individually collected specimens appear only within one or two months, suggesting univoltine life cycles. More specimens were generally collected in the second half of the year, although with a notable gap between 1,000 and 1,500 metres after August.

The study also contributes novel host records for five species, reared from Swiss caterpillars. Hyposoter barrettii was reared from the plume moth Oxyptilus tristis, confirming its apparent specialization on Pterophoridae. Hyposoter didymator emerged from the noctuid Lacanobia aliena, Hyposoter dolosus from the erebid Spiris striata, and Hyposoter placidus from the copper butterfly Lycaena hippothoe, matching previous records from Andorra and Spain. A fifth, uncertain record links Hyposoter validus to the pine lappet moth Dendrolimus pini. Each record was evaluated using a five-digit quality code assessing host identity, ecological consistency, rearing audit, identification reliability and specimen accessibility.

Beyond the faunistic inventory, the paper resolves several taxonomic tangles. The researchers formally synonymized three names, including Hyposoter caedator corsicator, a Corsican subspecies whose diagnostic characters proved too variable to justify separation. In another case, Hyposoter monensis was found to be a junior synonym of Phobocampe nigra, but because the resulting name would clash with an older Hyposoter niger, the junior synonym becomes the valid name under the International Code of Zoological Nomenclature. A third synonym placed Hyposoter castaneus within the genus Lemophagus, illustrating how misidentifications can even cross generic boundaries.

The new species, Hyposoter beaumonti, was described from four females collected in Western Switzerland between 1955 and 1962, all by Jacques de Beaumont, after whom it is named. It can be distinguished by a combination of characters including dark metasomal tergites with narrow yellow rims, a dark first tarsomere of the hind leg with a narrow yellow basal ring, and a very short ovipositor. The male remains unknown, and its biology is undocumented, a reminder of how much ecological information still lies dormant in museum collections awaiting careful study.

The researchers emphasize that the discovery of so many unrecorded species was principally due to extensive identification efforts and the recent availability of a robust identification key, underscoring the critical role of up-to-date taxonomic revision. They also highlight the value of by-catch material from Malaise trap projects, which were often focused on hoverflies or non-ichneumonid parasitoids but yielded significant Hyposoter specimens because all non-target insects had been sorted to at least family level. The complete digitized specimen data are now openly available, a step the authors argue is crucial for making the ecological and biological information stored in natural history collections accessible for future research.

Subject of Research: A review of Hyposoter parasitoid wasps ( Hymenoptera , Ichneumonidae ) in Switzerland

Article Title: A review of Hyposoter parasitoid wasps ( Hymenoptera , Ichneumonidae ) in Switzerland

Article References: A review of Hyposoter parasitoid wasps ( Hymenoptera , Ichneumonidae ) in Switzerland. (n.d.). https://doi.org/10.3897/alpento.10.175647

Image Credits: AI Generated

DOI: 10.3897/alpento.10.175647

Keywords: review, Hyposoter, parasitoid, wasps, Hymenoptera, Ichneumonidae, Switzerland, scientific research