Megastigmus aculeatus (Swederus)

Roques, A. & Skrzypczyńska, M., 2003, Seed-infesting chalcids of the genus Megastigmus Dalman, 1820 (Hymenoptera: Torymidae) native and introduced to the West Palearctic region: taxonomy, host specificity and distribution, Journal of Natural History 37 (2), pp. 127-238 : 145-148

Megastigmus aculeatus (Swederus)

(figures 11, 32, 48, 69, 87, 107, 124, 145)

Pteromalus aculeatus Swederus, 1795: 221 –222. Lectotype X, Sweden (?NHMS [not examined]).

Megastigmus transversus Walker, 1833: 117 . Syntype X, England (BMNH [not examined]). Synonymy with collaris by Mayr, 1874: 137.

Torymus (Megastigmus) collaris Boheman, 1834: 332 . Syntypes X and W, ‘ Westrogothia , Smolandia and Scandia’, Sweden (NHMS [6 X, 3 W examined]). Synonymy with aculeatus by Milliron, 1949: 289.

Torymus punctum Förster, 1840: 31 . Syntype X, Germany (NMW [not examined]). Synonymy with collaris by Mayr, 1874: 137.

Megastigmus vexillum Ratzeburg, 1848: 182 . Syntypes X and W, Germany (destroyed). Synonymy with collaris by Mayr 1874: 137.

Megastigmus flavus Förster, 1859: 109–110 . Syntypes X and W, Tyrol, Austria (female) and Aachen , Germany (male) (NMW [1 X examined]). Synonymy with collaris by Mayr, 1874: 137.

Megastigmus collaris flavus Förster : Mayr, 1874: 138 [subspecies]

Megastigmus cynorrhodi Perris, 1876: 222 . Syntypes, France (MNHN [lost, not examined]). Synonymy with aculeatus by Milliron, 1949: 289.

Female

Body length (without ovipositor) 3.4 mm. Colour predominantly brownish yellow with brown to black markings. Head light brownish yellow except ocelli and occipital carina partly surrounded with black. Pilosity pale on lower face, dark on dorsum of head. Antenna dark brown. Thorax mostly brown except anterior part of pronotum light brown, and posterior part of mid-lobe of mesoscutum, scutellum and dorsellum orange-yellow. Pilosity on thorax black. Legs tawny to brownish yellow, except front tarsus and distal segments of other tarsi brown. Wings subhyaline; forewing stigma light brown without any infuscation. Propodeum dark brown to black except latero-posterior margin brownish yellow. Gaster dark amber on the dorsum and brownish yellow laterally. Ovipositor sheaths black.

Head about 1.4× as broad as long in dorsal view. Antennal scape much longer (about 1.8×) than pedicel, anellus and first funicular segment combined. Funicular segments 1–6 about 1.5× as long as wide (figure 11). Pronotum and mid-lobe of mesoscutum with coarse cross-striation. Scutellum 1.1–1.2× as long as broad, with coarse, arcuate cross-striae on the anterior part, the frenal area nearly smooth with a few weak longitudinal furrows on the sides (figure 124). Forewing stigma roughly oval, less than 1.5× as long as broad; upper part of stigmal vein comparatively short, smaller than one-third of stigma length (figure 48). Propodeum with a double median carina and distinct sublateral carinae. Ovipositor sheaths about 1.2× as long as body. Distal end of the dorsal valve of ovipositor bearing small cutting teeth, the first median tooth lanceolate (figure 87).

Male

Body length 2.9 mm. Body light brownish to brownish yellow, with some brown to black markings. Head brownish yellow except ocelli surrounded with black. Pilosity pale on lower face, black on remainder of head. Antenna orange-yellow with scape and pedicel lighter. Thoracic dorsum mostly light brownish with some dark brown patterns: a narrow band along anterior margin of pronotum, mid-lobe, anterior part of lateral lobes and transscutal articulations of mesoscutum, a patch on outer part and lower internal part of axilla, and a patch on lateral panel of metanotum. Lateral parts of thorax light brownish except mesepisternum, mesepimeron and metapleuron dark brown. Pilosity on thoracic dorsum black. Legs light brownish except hind coxa at base and claws dark brown. Wings subhyaline; forewing stigma light brown without any infuscation. Middle part of propodeum black, callus yellowish. Gaster with the terga black on dorsum and the sides orange-yellow except the two last terga entirely orange-yellow.

Head about 1.5× as broad as long in dorsal view. Antennal scape slightly longer (about 1.1×) than pedicel, anellus and first funicular segment combined (figure 32); funicular segments more elongate than in female, about 1.8× as long as broad. Pronotum and mid-lobe of mesoscutum with dense cross-striae. Scutellum nearly 1.3× as long as wide, the anterior part densely striate-reticulate, and the frenal area nearly smooth with sparse areolae (figure 145). Forewing stigma broadly oval, about 1.5× as long as broad; upper part of stigmal vein comparatively short, about 4× as small as length of stigma; uncus comparatively long, 1.1–1.2× as long as upper part of stigmal vein (figure 69). Aedeagus short and conical, digitus with three teeth (figure 107).

Variation

The above description is based on the type material of M. collaris . In the other specimens we examined, body length varied from 2.6 to 4.0 mm in females, from 2.5 to 3.2 mm in males. Large variations in female colour were observed. The type material of M. flavus as well as female specimens from western and central Europe presented lighter colour patterns, with a thorax mostly brownish yellow to yellow and dark brown patterns limited to outer surface of axilla and anterior margin of mesoscutum. By contrast, females originating from south-central ( Italy, Switzerland) and southeastern Europe ( Bulgaria, Croatia, Greece) were usually darker with the following parts coloured in black: a spot on the anterior margin of pronotum, a more or less extended band covering the anterior half of the mid-lobe of mesoscutum, the outer surface of axilla, and the frenal line.

Males showed only little variations in colour. The dark patterns on thorax varied from brown to black. Some specimens from the French Alps were lighter with no dark brown band on anterior margin of pronotum, the dark brown colour on midlobe of the mesoscutum reduced to two small spots on anterior part, and lateral parts of thorax and legs (excpet claws) entirely light brownish.

Sex ratio

Strongly skewed in favour of females because of thelytokous parthenogenesis. Males usually constituted less than 7% of the populations sampled in western and central Europe: 0% (Lessmann, 1974b) in Germany; 1.1% (Eichhorn, 1967) and 2.3% (Kurir, 1975) in Austria; 0–7% in France (AR).

Hosts

Develops exclusively in seeds of Rosa spp. (Rosaceae) , earlier records from other plant species having not been confirmed (Milliron, 1949). Seed damage has been observed in most of the native and naturalized species of Rosa growing in Europe and Eurasia: R. alpina (Milliron, 1949; Boucĕk, 1977; USNM), R. andreae (USNM) , R. arvensis (AR) , R. beggeriana (Zerova and Seryogina, 1994) , R. blanda (USNM) , R. canina (= R. monticola ; Kapuściński, 1948; Eichhorn, 1967; Lessmann, 1974b; Kurir, 1975; Krístek et al., 1992; Xu and He, 1995; Ochsner, 1998; Jensen and Ochsner, 1999; AR; USNM), R. cinnamomea (Nikol’skaya, 1934; Kapuściński, 1948; Zerova and Seryogina, 1994; Skrzypczyńska, 2000), R. collina (Eichhorn, 1967; AR), R. davurica (= R. dahurica ; Milliron, 1949; Zerova and Seryogina, 1994; USNM), R. ferruginea (= R. glauca , = R. mayeri , = R. rubrifolia ; Kapuściński, 1948; AR; USNM), R. gallica L. (Milliron, 1949), R. glutinosa (Kapuściński, 1948; AR), R. jundzilli (Kapuściński, 1948; Milliron, 1949), R. mollissima (= R. mollis ; Milliron, 1949; USNM), R. montana (AR) , R. multiflora (Kamijo, 1962; USNM), R. pendulina (Kurir, 1975; AR), R. pouzini (AR) , R. rubiginosa (= R. eglanteria ; Balduf, 1957; Eichhorn, 1967; Syrett, 1990; Xu and He, 1995; AR), R. rugosa (= R. kamtschatika ; Kapuściński, 1948; Milliron, 1949, Kamijo, 1962; Valentine in Boucĕk, 1988; Xu and He, 1995; USNM), R. spinosissima (= R. pimpinellifolia ; Kapuściński, 1948; Krístek et al., 1992; AR), R. turkestanica (Zerova and Seryogina, 1994) and R. villosa (Eichhorn, 1967) . Additional Rosa species are likely to be attacked, records often having noted only Rosa sp. as host (Seitner, 1916; Hoffmeyer, 1931b; Zacher, 1932; Escherich, 1938, 1942; Kangas, 1940; Prisiazhniuk, 1949; Pavlovskii and Bei- Bienko, 1950; Cĕrmak, 1952; Sorauer, 1953; Boucĕk, 1954, 1970a, 1977; Novitzky, 1954; Györfi, 1962; Lozovoi, 1965; Vikberg, 1966; Nikol’skaya and Zerova, 1978; Schwenke, 1982; Jespersen and Lomholdt, 1983).

Distribution

Widely distributed in the Holartic and Australasian region. Recorded from the eastern part of North America (Milliron, 1949; Balduf, 1957; Peck, 1963; Grissell, 1979), Ethiopia (Milliron, 1949; USNM), South Africa (AR), Iran (Milliron, 1949; USNM), Iraqi (USNM), China (Milliron, 1949; Xu and He, 1995; USNM), Japan (Milliron, 1949; Kamijo, 1962) and Australasia including New Zealand (Boucĕk, 1988; Syrett, 1990). In Europe, known from Austria (Eichhorn, 1967; Kurir, 1975); Bulgaria (AR); Czech Republic (Cĕrmak, 1952; Boucĕk, 1954; Krístek et al., 1992); Denmark (Hoffmeyer, 1931b; Jespersen and Lomholt, 1983; Ochsner, 1998; Jensen and Ochsner, 1999); France (Vayssières, 1931; AR; USNM); Finland (Hellén, 1933; Vikberg, 1966); Germany (Milliron, 1949; Eichhorn, 1967; Lessmann, 1974b); Great Britain (Laidlaw, 1931; Boucĕk, 1970b); Greece (AR); Hungary (Györfi, 1962; AR); Italy (AR); The Netherlands (AR); Poland (Kapuściński, 1948); Russia: from the European part to the Caucasus, Ural, Siberia and Far East (Kniazheckii, 1949; Milliron, 1949; Prisiazhniuk,1949; Pavlovskii and Bei-Bienko, 1950; Nikol’skaya, 1952; Nikol’skaya and Zerova, 1978; USNM); Spain (AR); Sweden (Swederus, 1795; Hansson, 1991; AR, USNM); Switzerland (Milliron, 1949; MS; USNM); former Yugoslavia: Croatia, Serbia; Morocco (Boucĕk, 1977; AR).

Comments

Two other species of Megastigmus , M. nigrovariegatus and M. rosae , develop in Rosa seeds in the West Palearctic region. Females of M. aculeatus can be easily distinguished by the relative size of the ovipositor sheaths, which are 1.2× as long as body length whilst they are only 0.7–0.8× as long as body length in the two other species, and by the absence of infuscation around the forewing stigma (wide infuscation in M. nigrovariegatus and slight infuscation in some specimens of M. rosae ). Males of M. aculeatus differ from those of M. nigrovariegatus by the absence of infuscation around the forewing stigma, and from those of M. rosae by the lighter colour of thorax (mostly brownish yellow versus mostly brownish black).

A third species, M. aculeatus nigroflavus Hoffmeyer , was recognized in Japan (Kamijo, 1962) and Russia (Milliron, 1949). It was also observed in North America (Milliron, 1949; Balduf, 1957; Peck, 1963; Grissell, 1979; Nalepa and Grissell, 1993) but suspected to be introduced with Rosa seeds imported from Eurasia and Japan (Milliron, 1949). It was first considered as a variety (Hoffmeyer, 1929; Peck, 1963) but recent genetic studies confirmed that the North American populations of M. aculeatus nigroflavus correspond to a valid separate species (Roques and Roux, unpublished). Although the species has not been recorded from Europe yet, one specimen was found emerging from Rosa seeds imported from Germany to the USA (Hoboken Poe, 16 May 1966, USNM). We examined the type specimens that emerged from Rosa multiflora (1 X, 1 W, Japan, 1928, Hoffmeyer [ZMUC]; 4 X paratypes, same data as holotype [USNM]). The female is smaller in size than that of M. aculeatus , with brown and amber pattern of gaster more intensified dorsally, and ovipositor sheaths are only 0.8–0.9× as long as body length (as long as body length according to Nalepa and Grissell, 1993). Two specimens which emerged from hips of R. rubiginosa in the southern French Alps (Saint Crépin, June 1990) fitted that description but the record needs to be confirmed by a larger series. Male is lighter than that of aculeatus and more sharply sculptured, with a stigma surrounded by a narrow, inconspiscuous infuscation as noted by Milliron (1949). Sex ratio seems different from that usually observed in M. aculeatus , with a larger proportion of