Hyposoter Förster, 1869

The genus Hyposoter was erected by F̂rster (1869) but, like many of his genera, with no type species designated. A type species, Limnerium (Hyposoter) parorgyiae Viereck, 1910, which is found in North America, was supplied by Viereck (1914). Until the 1960s most European species currently placed in Hyposoter were placed in Anilastus F̂rster (= Anilasta Thomson), now regarded as a synonym of Campoletis F̂rster, or in Limneria Holmgren (= Limnerium Ashmead), now regarded as a synonym of Olesicampe F̂rster. Following Townes’ (1970) definitions of the genera, Aubert and Horstmann, in various publications, were responsible for transferring the majority of the relevant species to Hyposoter.

Morphologically Hyposoter is very close to both Olesicampe and Phobocampe F̂rster. Species of the latter genus, which also attack the larvae of Lepidoptera, are generally separable from Hyposoter by wing venation and the characteristic short propodeum. Species of Olesicampe, which attack exposed Tenthredinoidea (Hymenoptera), are much harder to distinguish morphologically from Hyposoter, although they tend to have heavier mandibles and longer temples, and less pronounced flanges on the lower edge of the mandibles. Wahl (1991) suggested that Olesicampe might be derived from within Hyposoter, a thesis supported by Quicke et al. (2009).

The third author has obtained CO1 barcode sequences for a large number of Norwegian and German campoplegines, including 14 identified species of Hyposoter. This work is ongoing, and analysis of it remains at a very preliminary stage: it would be premature to form any firm conclusions at present. However, all phylogenetic trees so far produced using various alignments show Hyposoter as at present constituted as polyphyletic, and intermingled with a number of genera, including Olesicampe, some Diadegma, and some Tranosemella. Phobocampe, in contrast, appears to be well separated, and at some distance from Hyposoter. In addition, ongoing work by Dr Seraina Klopfstein on the genus Diadegma is already showing (S. Klopfstein pers. comm.) that the latter genus is not monophyletic.

It therefore seems that the generic classification will need to be extensively revised to conform to a more accurate phylogeny when a more detailed molecular analysis has been completed (Klopfstein et al. 2022). Meanwhile, because of the morphological similarity, it is sometimes not easy to be completely certain that a species is correctly attributed between Hyposoter and Olesicampe unless the host is known. Despite the usually much longer ovipositor found in Diadegma F̂rster, certain species now included in that genus might also be confused with Hyposoter, and vice versa.

Hyposoter species are parasitoids of lepidopteran larvae, and generally kill their hosts before full growth. The cocoons of some species are formed inside the larval skin of the hosts, usually in cases where the larvae are spiny or hairy, or occasionally aposematic, but in other species outside. The external cocoons mostly have a characteristic (though not diagnostic) tubular shape, and often incorporate dark coloured meconium into the otherwise paler-coloured structure, which gives them the appearance of a bird dropping. We take this opportunity to correct a misidentification in the British checklist (Broad, 2016), where figure 2a is labelled as a cocoon of Hyposoter sp.: it is in fact the cocoon of a Microplitis sp. (Braconidae, Microgastrinae). Typically Hyposoter cocoon formation is in exposed positions and emergence is generally quite rapid (though it can be a few weeks after pupation if cocoons are removed from the wild, where they usually experience insolation). Where the overwintering strategy is known, it is in the larval stage of the host, the host being killed in the spring (Shaw et al. 2016; Broad et al. 2018).