﻿Dyscrituluseuropaeus sp. nov. (Hymenoptera, Braconidae, Aphidiinae): description of a new aphid parasitoid species with an identification key for species of the genus

﻿Abstract The braconid genus Dyscritulus Hincks is a small member of the subfamily Aphidiinae, distributed in Europe and Central Asia. All its species are highly specialized parasitoids of aphids of the genera Drepanosiphum Koch and, probably, Periphyllus van der Hoeven which are mostly associated with maple and sycamore trees (genus Acer). Upon examination of specimens from the Naturalis Biodiversity Center, Leiden, we unexpectedly noted unusual variability in morphological characters compared to other known Dyscritulus species. Further inspection of other material previously identified as Dyscritulusplaniceps Marshall, 1896 revealed additional specimens with the same morphological variability. Here we describe a new species of the genus, Dyscrituluseuropaeussp. nov., associated with Drepanosiphum aphids on Acer.

When Dyscritulus was first described, it bore a different generic name. Marshall (1896) described the genus Dyscritus Marshall, 1896, with the monotypic species Dyscritus planiceps Marshall, 1896(Viereck 1914. Morley (1933) described an additional species, Dyscritus suffolciensis Morley, 1933. However, almost half a century after the first mention of the genus, William Hincks (1943) changed the generic name to Dyscritulus Hincks, 1943; reason being that the name Dyscritus existed prior to the Marshall`s description (Dyscritus Scudder, 1868). Starý (1959) wrote a detailed revision of the genus and its species

Materials and methods
The specimens examined in this study were collected in Spain, France and Serbia. Two females from Spain (Málaga) and one from France (Mt Ventoux) are from the collection of Naturalis Biodiversity Center, labelled without a specified sampling method, but were collected by using a sweep net. The remaining specimens from Serbia were collected by rearing during 2006-2013 and are deposited in the collection of University of Belgrade, Serbia (Faculty of Biology, Institute of Zoology). Plant material with aphid colonies was kept in plastic containers covered by mesh for several weeks, under laboratory conditions, until the emergence of parasitoids. Live aphids were preserved in 96% ethanol for further identification. Parasitoids were either transferred to 96% ethanol or dry mounted. After examination under a ZEISS Discovery V8 stereomicroscope (Carl Zeiss MicroImaging GmbH, Göttingen, Germany), specimens were dissected and slide mounted in Berlese medium. Photographs of the dissected specimens were taken with a Leica DM LS phase contrast microscope (Leica Microsystems GmbH, Wetzlar, Germany). The obtained photographs were stacked in Helicon Focus software (v. 7.6.1; www.heliconsoft.com). ImageJ software (Schneider et al. 2012) was used to measure all important taxonomical characters. Morphological terminology follows Sharkey and Wharton (1997).  1B) (in one female specimen apical antennomera is half-divided, giving the impression of 22 antennomeres) (while D. planiceps has 23-24), coloration of first (F 1 ) and second (F 2 ) flagellomere ( Fig. 1C) (F 1 and first half of F 2 yellow, while in D. planiceps F 1 , F 2 and first half of F3 are yellow), shorter R1 vein ( Fig. 1H) (pterostigma/R1 length ratio is 2.0-2.2 vs 1.5-1.6 in D. planiceps), more elongated petiole (Fig. 1F) (ratio of petiole length and width at spiracles level is 1.7 vs 1.5 in D. planiceps) and narrower ovipositor sheath (Fig. 1G). From D. pygmaeus and D. dzhungaricus it is differentiated by number of antennomeres (21 vs 15 in both D. pygmaeus and D. dzhungaricus) and presence of areola on propodeum. It is most similar to D. trjapitzini, however it differs in following morphological characters: number of antennomeres (21 vs 22 in D. trjapitzini), more elongated petiole (ratio of petiole length and width at spiracles level is 1.7 vs 1.5 in D. trjapitzini), color of F 1 and F 2 (F 1 and half of F 2 yellow vs F 1 and F 2 entirely yellow with darker apices in D. trjapitzini) and shorter F 1 (4.5-4.6 length to width ratio vs 5 in D. trjapitzini).
Mesosoma. Mesoscutum with wide, deep notaulices, almost reaching prescutellar groove, dividing mesoscutum into three lobes (Fig. 1D). In lighter-colored specimens notaulices are paler and give an impression of reaching only half of mesoscutum. Two longitudinal rows of setae are present along the sides of notaulices. Lateral lobes of mesoscutum are covered with short dense setae  at the proximity of prescutellar groove, which is deep and smooth. Scutellum with somewhat longer setae along the outer margins. Propodeum areolated, with distinct central areola, in some specimens with slightly irregular lateral carinae (Fig. 1E). In smaller specimens, central areola is somewhat wider. Ex- ternal areolae with 4-5 short setae on each side, dentiparal areolae smooth or with one long seta. Fore wing with marginal setae that are longer than the surface setae (Fig. 1H). Pterostigma triangular, 3.1-3.5 times as long as wide. Vein R1 (=metacarpus) shorter than pterostigma (pterostigma length/R1 vein ratio 2.0-2.2). Vein r&RS distinctly colored in the proximal part, subequal to R1 vein length (1.0-1.15); the rest of the r&RS is colorless, reaching almost to the outer margin of the wing. Veins m-cu and 2M are colored throughout.
Metasoma. Petiole convex, 1.7 times as long as wide at spiracle level, with distinctly prominent transversal and longitudinal carinae (Fig. 1F). One central short longitudinal carina is feebly visible. On dorso-lateral sides two prominent longitudinal carinae are visible. Spiracular tubercles located closer to the anterior part of the petiole. Along the sides of the petiole, 4-5 long setae are present. Ovipositor sheaths elongated (2.2-2.4 length/width ratio), narrowed towards the apices, with 4-5 short setae across dorsal and ventral sides (Fig. 1G). Several campaniform sensillae situated at apical portion of ovipositor sheaths.
Colour. Upper part of head is brown, lower part, clypeus and mouthparts yellow (except for darker apices of mandibles). Scape, pedicel and annellus are yellow. F 1 is almost entirely yellow, except for narrow darker ring at apex; first half of F 2 yellow, second half is brown. Remainder of antennae dark brown. Mesoscutum and propodeum brown, petiole light brown. Legs yellow. Metasoma (=abdomen) and ovipositor sheaths brown. Fore wing venation brown.
Body length. 1.9 mm. Etymology. The name of the new species is derived from its current distribution. Distribution. Europe. Aphid host. Drepanosiphum platanoidis on Acer pseudoplatanus and Acer spp. Note. The morphology of the cocoon (Fig. 3) is typical for the genus, with external pupation, as described by Starý (1959). Number of antennomeres less than 20; propodeum without areola, with short diverging carinae at the posterior part (Fig. 4A) (Fig. 1F); first half of F 2 yellow and remainder dark brown; F 1 4.5-4.6 times as long as wide (Fig. 1C); number of antennomeres 21 (Fig. 1B)

Discussion
Throughout the last two centuries, museum and other institutional collections were considered important components of research, particularly in the field of taxonomy and systematics (Suarez and Tsutsui 2004). During the past two de-cades, museum collections have played an important role in identifying ecological responses to global change and habitat loss (Pyke and Ehrlich 2010;Kharouba et al. 2019). For the study of the subfamily Aphidiinae, the specimens preserved in museum collections are of crucial importance. Rare Aphidiinae species, holotype and paratype specimens used for research of taxonomy, systematics, morphology and phylogeny of the group are frequently acquired from museums and institutional collections (Petrović 2022). Recently, an increasing number of species have been described based on specimen loans (Davidian 2016;Tomanović et al. 2020;Petrović et al. 2021). Moreover, a member of Dyscritulus (D. dzhungaricus) was described from the collection of the Zoological Institute of the Russian Academy of Sciences (Davidian 2018). The first specimens of D. europaeus sp. nov. were also discovered in the museum collections of the Naturalis Biodiversity Center: only after re-examination of material previously identified as D. planiceps were additional individuals belonging to this species found.
The phylogenetic position of Dyscritulus within the tribe Praini is uncertain. The results of parsimony analysis, which considered both morphological and life history characters of members of Praini, grouped Dyscritulus together with Areopraon and Pseudopraon as monophyletic, and positioned Dyscritulus as a sister group to these two genera (Tomanović et al. 2006). Several studies have included D. planiceps in molecular phylogenetic analysis at the family (Dowton et al. 2002) or subfamily (Kambhampati et al. 2000;Sanchis et al. 2000) level. However, the tribe Praini was, besides Dyscritulus, represented only by genus Praon (Sanchis et al. 2000;Dowton et al. 2002) or genera Praon and Pseudopraon (Kambhampati et al. 2000).
Except for D. planiceps and D. europaeus sp. nov., the three remaining species of Dyscritulus have unknown aphid hosts. Dyscritulus trjapitzini was reared from mummies found on Quercus L. and Fagus orientalis Lipsky. It might be that D. trjapitzini parasitizes some other aphid genera, or that the aphid host belonging to Drepanosiphum or Periphyllus was mummified on Quercus sp. and F. orientalis by accident. Dyscritulus europaeus sp. nov. originates from Drepanosiphum aphids (D. platanoidis) or aphid mummies collected on Acer.
While D. planiceps is distributed throughout Europe, D. pygmaeus has been recorded only twice since its description, in Finland and Hungary (Hellen 1971;Polgar 1983). Dyscritulus trjapitzini is known only from its type locality Georgia (Abkhazia) (Davidian 2018) and D. dzhungaricus is described from Kazakhstan (Davidian 2019) and also reported from Russian Siberia and Mongolia (Davidian 2020), In this article, the new species has a southern European distribution, having been recorded in Spain (Málaga), Serbia and France (Mt. Ventoux, southern France). However, the possibility that its distribution range spans across entire Europe is not excluded. According to the currently available data, D. planiceps and D. europaeus sp. nov. are sympatric in southern Europe. Nevertheless, the revision of the available material should clarify their true distribution. As in our case, where important morphological differences were overlooked and samples were initially identified as D. planiceps, it is possible that material preserved in other collections also contains both species.
Although heavy infestations of Drepanosiphum aphids do not kill sycamore and maple trees, they significantly affect their health and appearance. With heavy infestations, trees produce smaller leaves at maturity and the growth of the stem wood is reduced (Dixon 1971). It has been shown that in the absence of aphids, sycamore could achieve nearly twice their normal annual growth. In addition, severely attacked leaves succumb to chlorosis, and the heavily produced honeydew covers the leaves and provides a suitable surface for the development of sooty moulds (Speight 1980). In urban areas, chemical control using insecticides is generally avoided. Among the natural enemies, parasitoids play an important role in regulating size of an aphid population. Aphidius setiger Mackauer, 1961, Trioxys cirsii Curtis, 1831, T. falcatus Mackauer, 1959, T. acericola Starý & Mackauer, 1971 and Falciconus pseudoplatani Marshall, 1896 are commonly found parasitizing aphids on Acer trees (Kavallieratos et al. 2016). Compared to them Dyscritulus specimens are collected rather rarely. The probable reason is their highly specialized life cycle and specific seasonal activity. The new species is an additional member of a highly specialized group of Drepanosiphum parasitoids that may contribute to the natural control of aphid outbreaks. The holotype and paratype specimens of D. europaeus sp. nov. were collected in both urban and protected areas (national parks), i.e., on cultivated and forest trees. Even after its revision (Starý 1959) and additional species descriptions (Davidian 2018(Davidian , 2019, the biology of the genus still remains poorly known. It is likely that these five currently known species represent only a fraction of its actual diversity. Accordingly, further efforts should be made towards revealing the distribution, trophic associations and life cycle of the genus. Likewise, an integrative approach of morphological and molecular methods with a sufficient sample size should reveal the phylogenetic relationships among congeners.