Megastigmus pictus (Förster)

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 : 173-177

Megastigmus pictus (Förster)

(figures 2, 3, 19, 37, 56, 75, 95, 113, 132, 151)

Torymus pictus Förster, 1840: 31 . Lectotype X (Boucĕk, 1970b: 268), Germany, coll. Mayr (NMW [examined]); 5 X paralectotypes (NMW [examined]).

Megastigmus seitneri Hoffmeyer, 1929: 327 . Holotype X (mounted on slide), 5 X paratypes, Lellinge Skov , Denmark (ZMUC [examined]); Allotype W (Hussey, 1962: 39–40), Great Britain (Hussey collection [not examined]). Synonymy by Boucĕk, 1970b: 268.

Female

Body length (without ovipositor) 3.0 mm. Body colour black with yellow-brown patterns. Head yellowish except a brown patch extending from scrobes to ocellar area and occipital area. Pilosity pale on clypeal and supraclypeal area, black on remainder of head. Antenna brownish. Pronotum mostly orange-yellow, with a large dark brown spot on the middle of anterior margin which continues in a dark median band narrowing towards the posterior margin; mesoscutum with mid-lobe and notuli brownish black to black, and lateral lobe yellow with indefinite light brown markings; axilla yellowish with a large brown spot on the outer part; scutellum brownish black with two lateral yellow spots on anterior margin; lateral panel of metanotum black but dorsellum yellow; mesopleuron mostly light brown except upper mesepimeron and pleural suture dark brown; metapleuron dark brown. Pilosity black on thoracic dorsum. Legs mostly yellowish except hind coxa brown at basis. Wings subhyaline but extremely setose. Propodeum black except callus yellowish. Dorsum of gaster black, extending laterally in large bands in the proximal part and in narrow bands in the distal part; remainder of gaster orange-yellow. Ovipositor sheaths black.

Head about 1.5× as broad as long in dorsal view, with distinctly protruding eyes (figures 2, 3). Scape elongate, slightly longer (1.1×) than pedicel, anellus and first funicular segment combined; following funicular segments about twice as long as broad (figure 19). Pronotum and mesoscutum strongly cross-striated. Scutellum elongate, nearly 1.4× as long as broad, striate-reticulate in the anterior part, the frenal area smooth (figure 132). Forewing stigma elongate-oval, 1.6× as long as wide; upper part of stigmal vein comparatively elongate, slightly longer (1.1×) than stigma width; uncus short, 0.3× as long as upper part of stigmal vein (figure 56). Propodeum with a median carina (figure 132). Ovipositor sheaths much shorter (0.7×) than body, nearly (0.9×) as long as thorax and gaster combined. Distal part of dorsal valve of ovipositor with the third median tooth enlarged (figure 95).

Male

Body length 2.4 mm. Head yellow except a T-shaped black area on vertex around ocelli. Pilosity pale on lower face below antenna, black on remainder of head. Antenna brown, with scape and pedicel yellowish beneath. Pronotom yellowish with a triangular, light brown spot on middle of anterior margin; mid-lobe of mesoscutum dark brown with two large lateral yellow lunules along notuli, lateral lobes yellow; scutellum mostly yellow with a brown patch resembling a four-branched star, which extends longitudinally from above the frenal line to the posterior margin and laterally along the frenal line; axilla yellow with a light brown spot along anterior suture; mesopleuron and metapleuron yellow. Pilosity black on thoracic dorsum, with four pairs of hairs on scutellum. Legs yellow, including coxae. Propodeum black with two small, lateral yellow spots on the posterior part. Gaster mostly yellow with a black dorsum, extending laterally in three distinct large, triangular stripes in the mid-part; last segment entirely yellow.

Head rounded, nearly 1.3× as broad as long in dorsal view. Scape elongate, slightly longer (1.1×) than pedicel, anellus and first funicular segment combined; following funicular segments about twice as long as broad (figure 19). Thoracic sculpture similar to that of female with cross-striae on pronotum and mesoscutum, the anterior part of scutellum striate-reticulate, and the frenal area smooth (figure 151). Forewing stigma elliptical, 1.2× as long as wide; upper part of stigmal vein comparatively short, about 0.4× as long as stigma length; uncus 0.6× as long as upper part of stigmal vein (figure 75); basal cell of forewing with 30 hairs including hairs on basal and cubital setal lines. Propodeum with a distinct median carina (figure 151). Aedeagus short, rounded, digitus with three teeth (figure 113).

Variation

The above description is based on the type material of M. pictus . In the other examined specimens, body length varied from 1.9 to 3.3 mm in females, from 2.4 to 2.9 mm in males. Female colour was highly variable. The dark median band on pronotum was absent in the lightest specimens (e.g. from Poland, MS). Scutellum colour varied from black with two anterior lateral yellow spots to yellowish with a median longitudinal brown band crossed by a transverse brown band on the posterior part. Both forms were observed in the Alps (AR). Darker specimens (e.g. emerged from Larix gmelinii, Les Barres arboretum, France, AR) showed a thoracic dorsum quite entirely black (mid-lobe of mesoscutum, scutellum except two small yellow spots, metanotum except dorsellum yellow, and propodeum). The median carina on propodeum was often double posteriorly.

Male colour was not very variable but the dark parts often turned from brown to black. The specimens observed by Hussey (1962) in Great Britain presented a pronotum entirely yellow and a mid-lobe of mesoscutum with black areas limited to spots on anterior margin. In a few individuals from Southern Poland, the longitudinal brown band on scutellum extended to the anterior margin (MS). Hussey (1962) did not notice a distinct carina on propodeum but observed four teeth on digitus.

Sex ratio

The species usually reproduces by thelytokous parthenogenesis, males being very scarce (16 W: 2123 X, i.e. 0.74%, in Poland; Skrzypczyńska, 1981a).

Hosts

Specific to seeds of Eurasian species of larch ( Larix spp. , Pinaceae ). Observed to damage seeds in the natural range of L. decidua (= L. europaea ; among others, Skrzypczyńska, 1973, 1974, 1977, 1984a; Roques, 1983; Krístek et al., 1992; Da Ros and Battisti, in press), L. decidua var. polonica (Kapuściński, 1966; Karpiński, 1967; Skrzypczyńska, 1974, 1977, 1984a), L. gmelinii (= L. dahurica ; Stadnickii et al., 1978; Zhang and Zhou, 1990; Xu and He, 1995; AR), L. gmelinii olgensis (Xu and He, 1995) , L. gmelinii principis-ruprechtii (Xu and He, 1995) , L. sibirica (Stadnickii and Grebenshchikova, 1977; Stadnickii et al., 1978), L. sukaczewii (Stadnickii et al., 1978) . The species has extensively colonized seeds of the same larch species planted outside their natural range, additionally attacking L. × czekanowski (Ostermeyer, 1990), the Japanese larch L. leptolepis (Hussey, 1962; Ostermeyer, 1990; Grijpma and van de Weerd, 1991; Xu and He, 1995) and L. × eurolepis (Hussey, 1952, 1962). However, no attack was noticed on larch species introduced from North America to France (Ostermeyer, 1990). Records on spruce (e.g. Stadnickii, 1971) probably referred to M. strobilobius while earlier records on Rosa spp. (e.g. Zacher, 1932; Cĕrmak, 1952; Sorauer, 1953) corresponded to M. rosae (Boucĕk, 1971) .

Distribution

Palearctic, probably originating from the native range of Eurasian larch species. Has largely colonized larch plantations and forests from Great Britain to far-eastern Asia (northeastern China; Zhang and Zhou, 1990; Roques et al., 1995; Xu and He, 1995; AR). In Europe, recorded from Austria (Vikberg and Valkeila, 1977); former Czechoslovakia (Cĕrmak, 1952; Boucĕk, 1954; Hrubík, 1973; Krístek et al., 1976, 1992); Denmark (Hoffmeyer, 1929, 1931b; Jespersen and Lomholdt, 1983; Ochsner, 1998; Jensen and Ochsner, 1999); Estonia (Luik and Voolma, 1988); Finland (Hellén, 1933; Vikberg and Valkeila, 1977); France (Roques, 1983; Ostermeyer, 1990); Germany (Zacher, 1932; Escherich, 1938; Boucĕk, 1970b; Lessmann, 1974b; Schwenke, 1982); Great Britain (Laidlaw, 1931; Hanson, 1952; Hussey, 1952, 1954a, 1962; Boucĕk, 1970b); Ireland (Boucĕk, 1970b); Italy (Da Ros, 1997; Da Ros and Battisti, in press); The Netherlands (Sorauer, 1953; Grijpma and van de Weerd, 1991); Poland (Adamczyk et al., 1969; Banaszak and Szmidt, 1987; Kapuściński, 1966; Karpiński, 1967; Skrzypczyńska, 1973, 1974, 1977, 1981a, 1984a, 1996; Szmidt, 1986); probably in Romania (Olenici, 1990); Russia from the European part to Siberia (Irkutsk), at least (Okunev, 1958; Golutvina et al., 1972; Bekreneva and Tropin, 1975; Smetanin, 1977; Nikol’skaya and Zerova, 1978; Stadnickii et al., 1978; Gusev, 1984; Zerova and Seryogina, 1994); Sweden (Hansson, 1991); Ukraine (Padii, 1952; Jurchenko, 1973); former Yugoslavia (Boucĕk, 1977). In the Alpine range, the insect seems limited to low altitude stands (Roques, 1983; Da Ros and Battisti, in press).

Comments

No other chalcid species has yet been observed to attack larch seeds in the West Palearctic. However, an additional species, M. inamurae Yano , exists in the Far East (Yano and Koyama, 1918). Because some specimens have been observed in the 1950s in seeds of Japanese larch imported from Japan to Great Britain (Hussey, 1954a), the species may have been introduced to Europe. Based on Yano’s original description, Hussey (1962) stated that females of M. inamurae differ by a more limited black margin on the anterior part of pronotum. However, this black margin is sometimes absent in the European specimens of M. pictus . We examined females of larch seed chalcids from far-eastern Asia, both Japanese specimens identified as M. inamurae (1 X, Shikotan, Kuriles Islands, Russia, 12 June 1944, K. Kamijo; 1 X, ex. L. leptolepis, Usuda , Nagano Pref., Honshu, J, April 1964, K. Kobayashi) and Chinese specimens identified as M. pictus (Heilongjiang, northeastern China; AR). Both exhibited highly variable colour patterns for thorax, ranging from the typical pattern of pictus to forms with an extended brownish coloration on mesoscutum, scutellum and axilla, yellow stripes being limited to a transverse band on scutellum and an oblique one at inner margin of axilla. No reliable difference thus appeared between females of M. pictus and M. inamurae . Hussey (1962) separated the males of the two species by the colour of pronotum, considered as entirely yellow in M. pictus whilst black anteriorly in M. inamurae . Similarly as for female, the variation in thorax colour observed in males of M. pictus did not allow Hussey’s key for separating the two species to be used.

Material examined

China: 2 X, ex. Larix gmelinii, Jagedaqi (Heilongjiang) , 28 July 1987, Zhang Xu-dong ( AR) ; 12 X, ex. L. gmelinii , Daï-ling (Heilongjiang), June 1994 ( AR) ; 22 X, ex. L. gmelinii , Xin-lin (Heilongjiang), June 1994 ( AR) . Denmark: 1 X holotype of M. seitneri and 5 X paratypes, Lellinge Wald , 1928 ( ZMUC) . France: 3 X, ex. L. decidua, Briançon (05), elev. 1200 m, July 1983 ( AR) ; 4 X, ex. L. decidua, St Sernin / Rance (12), June 1991 ( AR) ; 6 X, ex. L. decidua, Les Barres , May 1991, R. Ostermeyer ( AR) ; 5 X, Les Barres, ex. L. sibirica , June 1980 ( AR) ; 4 X, ex. L. x czekanowskii, Les Barres, June 1991, R. Ostermeyer ( AR) ; 5 X, ex. L. gmelinii, Les Barres (45), May 1993 ( AR) ; 3 X, ex. L. leptolepis, Les Barres , June 1991, R. Ostermeyer ( AR) ; 3 X, ex. L. leptolepis, Argonne Forest (51), June 1993 ( AR) .

Germany: 1 X lectotype and 5 X paralectotypes of M. pictus, Mayr coll. ( NMW) .

Italy: 2 X, ex. L. decidua, Cavedine (TN) , elev. 650 m, June 1993, N. Da Ros ( AR) . The Netherlands: 1 W, ex. L. leptolepis, Vaals seed orchard (Limburg), 14 July 1990, P. Grijpma ( AR) . Poland: 1 W, ex. L. decidua var. polonica, Rudy Raciborskie, Kotlarnia Forest , 24 April 1970 ( MS) ; 1 W, ex. L. decidua var. polonica, Skarżysko, Dalejów Forest , 30 March 1976 ( MS) ; 1 W, ex. L. decidua var. polonica, Skarżysko, Jastrzębie Forest , 6 April 1976 ( MS) ; 1 W, ex. L. decidua var. polonica, Odrowążek Forest (Swiętokrzyskie Mts. ), 21 April 1976 ( MS) ; 25 X, ex. L. decidua var. polonica, Forest Wojkowa (Beskid Sądecki) , 27 May 1990 ( MS) .