Revision of the Palaearctic Gasteruption assectator aggregate, with special reference to Sweden (Hymenoptera, Gasteruptiidae)

Abstract The Palaearctic species of the Gasteruption assectator aggregate (Hymenoptera, Gasteruptiidae) are revised and three species are recognised. Two species are re-instated: Gasteruption boreale (Thomson, 1883), stat. n. and Gasteruption nigritarse (Thomson, 1883), stat. n., and both are excluded from the synonymy with Gasteruption assectator (Linnaeus, 1758). The general distribution of both species is given for Europe and in detail for Sweden. A key to the valid Palaearctic species of the Gasteruption assectator aggregate is given; key characters and primary types are illustrated. Four new synonyms are listed: Foenus fumipennis Thomson, 1883, Trichofoenus breviterebrae Watanabe, 1934, and Gasteruption margotae Madl, 1987, are synonymized with Gasteruption boreale (Thomson, 1883) and Gasteruption brevicauda Kieffer, 1904, with Gasteruption undulatum (Abeille de Perrin, 1879).


Introduction
The predator inquiline wasp Gasteruption assectator (Linnaeus, 1758) (Hymenoptera, Gasteruptiidae) has been considered a very common species with wide intraspecific variation concerning both morphology and colouration (e.g. van Achterberg and Talebi 2014). When working on an updated revision of the Nordic Gasteruption species the first author noticed that the specimens from a restricted geographical range could be clustered into three separate morphospecies. The discovery of a couple of hitherto unknown features of the females made it fairly easy to separate the morphospecies involved. Further studies of a larger number of specimens and conclusions drawn from practical field work showed that the three morphospecies have a significant difference in geographical distribution and habitat preference in Sweden. Studies of the type material of the supposed synonyms of G. assectator showed that Foenus borealis Thomson, 1883, and F. nigritarsis Thomson, 1883, are the oldest available names for these valid species. Nearctic synonyms listed by Smith (1996) are not accounted for here because their types need to be studied first. The synonymisation of Gasteruption brevicauda Kieffer, 1904, with G. assectator made by Madl (1987a) is here rejected and it is considered to be conspecific with G. undulatum (Abeille de Perrin, 1879), syn. n.

Gasteruption assectator (Linnaeus, 1758) sensu stricto
Type material. High resolution photos of the lectotype female of G. assectator in the Linnaean collection coll. no 2652-"49 assectator" (Figs 1-3) designated by van Achterberg and Talebi (2014) was studied. The specimen has an unusually short ovipositor and the pilosity of the sheath is longer than average, but within the variation of the species. The holotype female of G. brevicauda (Figs 4-7) from Algeria (Orléansville) was examined and the specimen, with its strongly sculptured mesoscutum, the strong antero-lateral teeth of the pronotum aswell as the orange hind tarsus clearly belongs to Gasteruption undulatum (Abeille de Perrin, 1879). The synonymisation with G. assectator made by Madl (1987a) is here rejected and G. brevicauda Kieffer, 1904 Diagnosis. Temples in dorsal view less parallel-sided and usually shorter than of G. boreale, head in dorsal view transverse, mostly distinctly wider than long. Occipital carina indistinct and not reflexed. Face mostly slightly narrower than that of G. boreale. Hypostomal bridge narrow, at most 0.5 times mandibular base (Fig. 25). Mesoscutum in most cases distinctly reticulate-coriaceous and without satin sheen (Fig. 26), medioposteriorly in front of scutellum distinctly rugose. Mesosoma and head silvery pilose. Mesosomal surface with a fatty gloss, quite distinct from the more opaque satin sheen in G. boreale. Antenna slightly longer than in G. boreale, with sixth segment about 1.8 times longer than wide and subapical segment about 1.5 times longer than wide. Hind coxa dorsally striate-rugose. Hind tibia and basitarsus with white ring which might be interrupted ventrally. Metasoma mainly black with lateral orange patches on tergites 2-4 often merged. Fore and middle tibiae with small, but quite distinct white or yellow patch basally. Ovipositor sheath black or brown, 1.0-1.3 times as long as hind tibia and without prominent bristles but with thinner adpressed pubescence, appearing nearly naked (Fig. 28). The pilosity of equal intensity all over the surface not becoming scarcer towards the tip. In some specimens, especially when the sheath parts  are twisted as in the lectotype female, the pilosity might be slightly raised. The species is closely related to G. boreale (Thomson, 1883) and G. nigritarse (Thomson, 1883), but the female can be distinguished by the slightly longer ovipositor without conspicuous bristles. The male is distinguishable by its slightly shorter head in dorsal view and the often more distinctly reticulate-rugose mesoscutum without satin sheen. Distribution. G. assectator is the most widespread and common species of the assectator aggregate in Europe. Towards its northern distribution limits in northern Scandinavia it seems to be confined to coastal areas with more favorable climate than inland areas.

Gasteruption boreale (Thomson, 1883), stat. rev.
Type material. In Thomsons collection in MZLU four males and one female are placed at the label Foenus borealis. Hedqvist (1973) states that the type series by Thomson only consisted of three males and one female but this is probably a simple miscalculation. The female (from Norway) was originally selected as lectotype by Hedqvist (1973) but the selection was declared as invalid (van Achterberg and Talebi 2014) because the listed original locality of the type series (= Lappland) excludes the selection of a lectotype from Norway. One male (Figs 8-11) was designated lectotype by van Achterberg and Talebi (2014) and the species was synonymized with Gasteruption minutum (Tournier, 1877). All males and the female in the type series belong to one distinct species (see key below). The wider malar space exhibited by the lectotype is clearly shorter than the mandibular base and fits within the range of this species and is not as long as in G. minutum. The type series of Gasteruption fumipenne consists of the lectotype from Gotland. The size, habitus, antennae and smooth sculpture on the mesoscutum of the lectotype (Figs 12-13) indicates, despite the lacking metasoma, that it concerns a female of G. boreale. This is the only specimen known from the Baltic island Gotland, but the type locality ("Olle hau"= Ulla hau, Fårö, Gotland, Sweden) was at the time of the collection an active sand dune field with old pines. It was quite a different ecosystem than at the mainland of Gotland where G. assectator is the most common (and now only?) of the three species. The other specimen under this label is a male of G. assectator from Scania (Skåne). The synonymisation with G. assectator made by Schletterer (1889) is rejected and F. fumipennis is to be regarded as a new synonym of G. boreale (Thomson).
The type series of G. margotae consists of the male holotype (Figs 14-15) from Finland (Madl 1987c); the holotype is a typical male of G. boreale (Thomson). The study of the holotype shows that the synonymisation with G. assectator made by Madl Figures 8-11. Lectotype of Gasteruption boreale (Thomson). 8 habitus lateral 9 head dorsal 10 labels 11 head anterior. 8 9 10 (1990) is unjustified after resolving the G. assectator aggregate and that G. margotae is clearly a new synonym of G. boreale (Thomson).
The examined type series of G. breviterebrae (Fig. 16) consists of the holotype female and a paratype male from Sakhalin (Far East Russia). The holotype shows the typical features of G. boreale (Thomson), viz., the less sculptured mesoscutum and the bristly ovipositor sheath. The slightly aberrant red marks on the metasoma fall within the geographical variation of G. boreale. The synonymisation with G. assectator made Diagnosis. Head in dorsal view almost parallel-sided behind eyes, elongate, about as wide as long (Figs 9, 18). Occipital carina indistinct and not reflexed. Frons with satin sheen. Mesoscutum smooth, weakly rugose/shagreened with satin sheen, medioposteriorly in front of scutellum rugose-reticulate 27). Mesosoma and head silvery pilose. Mesosoma with a satin sheen, quite distinct from the rather matt gloss occurring in G. assectator. Whitish pubescence of eye of female mostly distinctly longer and denser than of G. assectator. Antenna slightly shorter than in G. assectator with sixth segment about 1.5 times longer than wide and subapical segment about 1.2 times longer than wide. Only apical half of hind coxa weakly striate dorsally. Hind tibia and basitarsus with white ring which might be interrupted ventrally. Metasoma mainly black with orange lateral patches on tergites 2-4 which might be partially reduced, especially in northern specimens. Inner sides of tibiae often red brown to orange with white or yellow basal patch indistinct on fore and middle tibiae. Ovipositor sheath black or brown, 0.7-1.0 times as long as hind tibia, its apical half entirely with stout, rather scarce black bristles angled backwards at about 45° (Fig. 29). The species is closely related to G. assectator (Linnaeus) but the female can be distinguished by the shorter ovipositor sheath, the less sculptured mesoscutum and the more scarce promi- 14 15 nent bristles on the apical half of the ovipositor sheath. The male is hard to distinguish from G. assectator and identification is not always possible with certainty. In most cases the male of G. boreale can however be separated from G. assectator by its slightly more elongated, parallel-sided head in dorsal view, the more or less enlarged malar space and its less sculptured mesoscutum.
Description. Female. Length of body 6-11 mm (fore wing 3.5-5.5 mm) Head. Temples parallel-sided behind eyes in dorsal view. Occipital carina not raised. Frons and vertex with satin sheen. Malar space short, at most about 0.5 times mandibular base. Hypostomal bridge narrow, at most 0.5 times mandibular base. Eyes with dense white pubescence. Antenna short; sixth segment about 1.5 times longer than wide and subapical segment about 1.2 times longer than wide.
Mesosoma. Surface largely smooth with satin sheen, mesoscutal sculpture of almost equal intensity as on vertex. Antesternal carina narrow, non-lamelliform. Pronotal sides with very small pointed teeth antero-ventrally, but these are sometimes entirely absent. Upper half of mesopleuron mostly considerably weaker sculptured than its more rugose lower part.
Legs. Hind tibia stout as in G. assectator. Hind coxa often with weaker rugae apically than on basal half, dissolving amidst rugose background. Hind tibial spurs often brighter than hind tibia. Metasoma. Ovipositor sheath entirely black or brown, 0.7-1.0 times as long as hind tibia, its apical half entirely with stout, black bristles angled backwards at about 45° (Fig. 29).
Colour. Black. Mandible apically, hind tibial spurs and patches laterally on tergites 2-4 reddish brown. Patches rarely intercepted. Fore and middle tibiae mostly with indistinct yellow or ivory basal patch. Inner side of tibiae often orange. Basal ivory ring of hind tibia usually indistinct.
Distribution. As Thomson's name implies this species is most common in boreoalpine areas. In northern Europe it is quite widespread and common at higher latitudes and high altitude sites in the southern part, but becoming scarcer towards the southern lowlands in Sweden. Specimens are examined from Austria, Bulgaria, Finland, Germany, Netherlands (den Dolder, de Bilt, Tilburg, Wageningen, Groesbeek, Rhenen, Velp, Maastricht, Drimmelen, Rotterdam, Voerendaal), Norway, Russia, Serbia and Sweden.
Biology. The species occur in (boreal) landscapes dominated by coniferous forests where it can be locally common. Many of the sites in Scandinavia are at high altitude. G. boreale is lacking from more open localities as well as in regions dominated by deciduous forests. It has been observed searching high stumps of Pinus trees and it also attends old wooden walls and artificial bee nests in gardens. Probably it is a kleptoparasitoid of Hylaeus spp. (Thomson, 1883), stat. rev.
Diagnosis. Head dorsally more parallel-sided than in G. assectator, elongate and about as wide as long. Occipital carina indistinct and not reflexed. Mesoscutum superficially reticulate and (especially laterally) rugose, medio-posteriorly in front of scutellum more rugose-reticulate. Mesosoma laterally and face with long, thick golden pubescence. Hind tibia and basitarsus darker, often with the basal ring lacking or interrupted. Fore and middle tibiae often with large distinct ivory patch covering about one third of tibia. Metasoma mainly black with well-defined orange lateral patches on tergites 2-5 which might be partially reduced, especially in northern specimens. Ovipositor sheath entirely black or brown, 0.7-1.0 times as long as hind tibia and its apical half dorsally with stout, black bristles angled backwards at about 45°. The bristles are all conspicuously widened and bent apically, reminiscent of "velcro" (Fig. 30). The species is closely related to G. assectator (Linnaeus) and G. boreale (Thomson), but the female can be distinguished by the stout velcro-like bristles dorsally on the apical half of the ovipositor sheath, its denser pubescence of head and mesosoma and its broader hypostomal bridge. The male is distinguishable by its broader hypostomal bridge as well as the thick golden facial pubescence.
Description. Female. Length of body 8-11 mm (fore wing 4.0-5.5 mm) Head. Temples parallel-sided behind eyes in dorsal view. Occipital carina not raised. Frons and vertex with satin sheen. Malar space short, at most about 0.5 times mandibular base. Hypostomal bridge at least 0.7 times width of mandibular base, medio-laterally often with distinct transverse striae. Face covered with dense golden pubescence.
Mesosoma. Surface vaguely reticulate and strongly shagreened. Antesternal carina narrow and non-lamelliform. Pronotal sides with very small pointed teeth antero-ventrally, but these are sometimes entirely absent. Legs. Hind tibia rather stout as in G. assectator. Hind tibial spurs and hind tibia mostly similarly coloured.
Metasoma. Ovipositor sheath entirely black or brown, 0.7-1.0 times as long as hind tibia, its apical half dorsally with stout, black bristles angled backwards at about 45° and conspicuously widened and bent backwards apically.
Colour. Black. Mandible apically orange. Sometimes small patch on hind tibia baso-ventrally white or ivory. Northern specimens of both sexes often with entirely black hind tibia. Fore and middle tibiae often with large distinct ivory patch covering about one third of tibia. Fore, middle and hind tarsus black. Patches laterally on tergites 2-5 reddish brown. Last sternite apically often extensively orange. Colour of hind tibial spurs variable, mostly black or dark brown.
Biology. The species primarily occur in small-scale agricultural landscapes where it is to be found especially on walls of log barns (Fig. 23). An association with the bees Hylaeus difformis and/or Hylaeus pictipes is highly probable, at least in Scandinavia and is based on observed behaviour of the wasps. Gasteruption nigritarse seems to have diminished dramatically in Scandinavia during the last century, probably due to the loss of habitat and is only known from a couple of localities. Mesoscutum and coxae more opaque, less rugose, smoother and with satin sheen (Figs 17-18, 27). Sculpture of mesoscutum and head similar, shagreened (Fig. 27). Ovipositor sheath 0.7-1.0 times as long as hind tibia, in dorsal view its apical half with more scarse distinct straight bristles, angled backwards at about 45° (Fig. 29).....G. boreale (Thomson, 1883), stat. rev. -Mesoscutum and coxae more shiny and rugose, with a "fatty" gloss ( Fig. 26). Sculpture of mesoscutum distinctly rougher than that of head, reticulate-coriaceous (Fig. 26). Ovipositor sheath 1.0-1.3 times as long as hind tibia, in dorsal view normally only with dense bright adpressed pubescence, thus appearing naked in lower magnifications (Fig. 28)  Mesoscutum and upper half of mesopleuron rather smooth with small puncture-like grooves and with satin sheen almost of the same intensity as vertex ( 27). Head, mesosoma and coxae more opaque and with satin sheen. Sculpture of mesoscutum similar to that of head (Fig. 27). Head in dorsal view more elongated and its temples more parallel-sided behind eyes (Fig. 10)