Megastigmus nigrovariegatus Ashmead, 1890

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 : 170-173

Megastigmus nigrovariegatus Ashmead

(figures 18, 55, 74, 94, 112, 131, 150) Megastigmus nigrovariegatus Ashmead, 1890: 26 . Lectotype X (Milliron, 1949: 295), Greeley,

CO, USA (USNM [examined]); Neoallotype W (Milliron, 1949: 296), Minneapolis, USA

(University of Minnesota [not examined]).

Female

Body length (without ovipositor) 3.2 mm. Body brownish yellow with black, brown and yellow markings. Head mostly brownish yellow except supraclypeal area, ocellar area, temple, gena and occipital area dark brown. Pilosity pale on lower face, black on remainder of head. Antenna dark brown except scape and pedicel yellowish beneath. Pronotum yellow, contrasting with the much darker head. Thoracic dorsum brownish yellow except anterior part of mid-lobe of mesoscutum, outer surface of axilla and lateral panel of metanotum black; mesopleuron and metapleuron dark brown. Pilosity on thoracic dorsum mostly dark. Fore and midcoxa light brownish, hind coxa dark brown; remainder of legs yellow. Wings subhyaline; forewing stigma dark brown, distinctly surrounded by a clouded area. Propodeum black. Dorsum of the first two apparent gastral terga mostly brown, then becoming yellowish brown. Ovipositor sheaths black.

Head about 1.3× as wide as long in dorsal view. Antennal scape elongate, 1.1× longer than pedicel, anellus and two first funicular segment combined; pedicel elongate, about twice as long as broad, funicular segments 1–3 slender, 2.4× as long as broad (figure 18). Pronotum and mesoscutum with cross-striae. Scutellum about 1.2× as long as wide, the anterior part reticulate and the frenal area with longitudinal carinae (figure 131). Forewing stigma oblong, 1.3× as long as broad,; upper part of stigmal vein comparatively short, 0.3× as long as stigma length; uncus as long as upper part of stigmal vein (figure 55). Propodeum without median carina. Ovipositor sheaths much shorter (0.8×) than body, nearly as long as gaster and thorax combined. Distal part of dorsal valve of ovipositor with blunt teeth, the second median tooth a bit larger than others (figure 94).

Male (according to Milliron, 1949)

Body length 2.9 mm. Body brownish yellow with black and yellow markings. Head mostly dark brown with brownish yellow on lateral area of lower face and on parascrobal area, then extending along dorsal eye margin to gena. Pilosity pale on lower face, black on remainder of head. Antenna dark brown. Thorax mostly brownish yellow with a wedge-shaped brown spot extending on dorsum of pronotum from middle of anterior margin towards posterior margin; metapleuron dark brown with distal part lighter. Coxae dark brown except fore coxa posteriorly and anterior half of mid-coxa light brown; remainder of legs light brown. Wings subhyaline; forewing stigma darker than in female, surrounded by a broad clouded area. Propodeum dark brown except posterior margin lighter. Gaster with first apparent tergum blackish brown on dorsum and brownish yellow at side; following terga brownish black except some yellowish infuscation at side, and last two terga brownish yellow.

Head about 1.3× as wide as long in dorsal view. Funicular segment of antenna more elongate than in female, about twice as long as wide. Pronotum and mesoscutum with cross-striae; scutellum about 1.3× as long as broad, the anterior part reticulate and the frenal area with curved longitudinal carinae (figure 150). Stigma large, rounded, 1.1× as long as broad; upper part of stigmal vein comparatively very short, about 6.5× as small as stigma length; uncus much longer (1.7×) than upper part of stigmal vein (figure 74). Propodeum without median carina. Aedeagus very elongate, much more than in other Megastigmus spp. , digitus with six teeth (figure 112).

Variation

The above description is based on the type material from North America. In other specimens from America, body length varied from 3 to 3.4 mm in females, from 2.9 to 3.3 mm in males. Body colour, especially of thorax, was highly variable in the native areas. Female colour ranged from dark forms where black extended to the anterior part of pronotum, anterior part of lateral lobes of mesoscutum and axilla to light forms almost entirely flame red, orange or light brownish yellow, black being limited to a small band on the anterior margin of mid-lobe of mesoscutum (Milliron, 1949; Balduf, 1957). The few female specimens yet found in Europe corresponded to dark forms.

Extreme colour variations occurred at a higher degree in males (see Balduf, 1957 for figures). The lightest specimens observed in North America were almost wholly flame red or yellow, black being confined to the limits of the ocellar triangle on head. In darker specimens, black was invading most of the dorsum including propodeum and gaster, yellow being confined to a small patch on the vertex adjacent to the inner edge of eye, two squared patches on the posterior half of pronotum, an elongate spot along notula on lateral lobe of mesoscutum, a small spot at inner angle of axilla, two rounded spots on anterior half of scutellum, tegula and dorsellum. The few male specimens found in Europe were of dark type, with the following yellow patterns: a triangular patch on vertex near eye, a large band covering the posterior half of pronotum, a triangular patch on posterior margin of mid-lobe of mesoscutum and inner angle of axilla, two large bands on the sides of scutellum extending to the frenal line, and dorsellum. The scutellum sculpture also appeared highly variable in both sexes (Milliron, 1949).

Sex ratio

Balanced in the USA (0.69 to 1.65; Balduf, 1945, 1957), the ratio of females to males is in favour of females in Nova Scotia (1.43 to 2.57; Gillan and Richardson, 1997). Sex ratio presently unknown in the countries of introduction.

Hosts

Develops specifically in seeds of Rosa spp. (Rosaceae) . Recorded in the native areas from numerous wild and cultivated species: R. acicularis Lindl. , R. arkansana Port. and Coult. , R. blanda Ait. , R. californica Cham. and Schlecht. , R. canina , R. carolina L. (= R. palustris Marsh. , = R. virginiana Mill. ), R. engelmanni Wats. , R. lunellii Greene , R. macounii Greene , R. spinosissima , R. pyrifera Rybd. , R. rubiginosa , R. rugosa , R. setigera Michx. , R. suffulta Greene , R. ultramontana Hell. , R. woodsii A. Goory , R. xanthina Lindl. (Balduf, 1945, 1957; Milliron, 1949; Peck, 1963; Grissell, 1979; Mays and Kok, 1988; Gillan and Richardson, 1997). For the moment, only observed on R. pendulina in France.

Comments

Two other species, M. aculeatus and M. rosae , co-exist with M. nigrovariegatus in Rosa seeds in the West Palearctic. Diagnostic characters for separating the three species are given in the chapter concerning M. aculeatus .

Distribution

Originates from North America. Widely distributed in the USA and Canada, from California to Alaska and from Nova Scotia to Vancouver Island (Balduf, 1945, 1957; Milliron, 1949; Peck, 1963; Grissell, 1979; Mays and Kok, 1988; Gillan and Richardson, 1997). A few specimens recently recorded from south-central and southeastern France (AR).

Material examined

France: 1 X, ex. Rosa sp. , Briançon (05), elev. 1200 m, 6 June 1987 ( AR); 1 X, 1 W, ex. Rosa pendulina, Briançon (05), elev. 1200 m, 23 June 1990 ( AR); 1 X, flying, Sousceyrac (46), June 1993, G. Bour ( AR); 1 X, beating of Rosa sp. , Brouis (06), 19 July 1990, J. Y. Rasplus ( AR). USA: Lectotype X, Greeley, CO ( USNM); 1 X, ex. Rosa sp. , Washington, DC, 4 March 1940, E. L. King ( USNM); 2 X, ex. Rosa sp. , Jenkinson Lake, CA, 15 June 1994, N. Rappaport ( AR); 1 W, ex. R. rugosa, Kingston, G. A. Thomson (USNM) .