Megastigmus milleri Milliron, 1949

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 : 166-170

Megastigmus milleri Milliron

(figures 17, 36, 54, 73, 93, 111, 130, 149)

Megastigmus pinus Parfitt of Crosby, 1913: 162, 168 (in part according to Milliron, 1949) [misidentification].

Megastigmus milleri Milliron, 1949: 323–326 . Holotype X, ex. Abies grandis, Crescent City, CA , USA (USNM [examined]); 39 X, 22 W paratypes, same data as holotype (USNM [3 X, 2 W examined]).

Female

Body length (without ovipositor) 4.2 mm. Body colour black and golden yellow. Head colour as follows according to Milliron (1949; no more head at present on holotype): face mostly yellow except the sides of clypeus with a more or less distinct black circular spot and the antennal scrobes black; frons yellow with a narrow black line extending from the scrobe extremity to the eye; vertex and occiput black but temple and gena mostly yellow. Pilosity pale on lower face, black on remainder of head. Antenna brownish yellow except scape, pedicel and anellus yellow beneath. Thoracic dorsum predominantly yellow with black patterns as follows: two oblique lateral stripes on middle part of pronotum, a transverse band covering anterior part of mid-lobe of mesoscutum, a longitudinal median band on lateral lobe of mesoscutum which extends to axilla (leaving yellow only on outer surface and posterior inner angle of mid-lobe of mesosctum and on inner angle of axilla), and lateral panel of metanotum. Scutellum with a small, lighter (dark brown) spot on middle of anterior margin. Mesopleuron and metapleuron entirely black. Pilosity on thoracic dorsum black. Fore coxa yellow, mid-coxa and hind coxa black with yellow distally, remainder of legs yellowish except hind femur infuscated dorsally. Propodeum black except a longitudinal yellow spot on callus. Gaster with first apparent tergum black; the following terga with transverse, rectangular black bands alternating with similar transverse yellow bands which are broadening progressively; last tergum black; gaster sides and sterna mostly yellowish. Ovipositor sheaths black.

Head about 1.5× as broad as long in dorsal view. Scape slightly shorter (0.97×) than pedicel, anellus and first funicular segment combined (figure 17); pedicel 0.9× as long as first funicular segment. Pronotum and mesoscutum with strong crossstriae. Posterior margin of mid-lobe of mesoscutum at least twice as wide as anterior margin of scutellum (figure 130). Scutellum 1.2× as long as broad, the anterior part striate-reticulate, the frenal area with longitudinal, irregular wrinkles (figure 130). Forewing stigma oval-elongate, 1.5× as long as wide; upper part of stigmal vein about half as long as stigma length; uncus half as long as upper part of stigmal vein (figure 54). Propodeum with a sharp median carina. Ovipositor sheaths 1.2× longer than gaster and thorax combined, 0.9× as long as body. Distal part of dorsal valve of ovipositor with blunt teeth, the second median tooth larger than the others (figure 93).

Male

Body length 3.3 mm. Colour black and yellow. Head black except face, lower part of temple, gena and a small spot above eye, yellow. Pilosity pale on lower face, black on the remainder of head. Antenna brownish yellow. Pronotum predominantly black with the sides, the lateral panel and a transverse bilobed band on posterior margin yellow. Remainder of thorax mostly black except outer surface of lateral lobe of mesoscutum, tegula and dorsellum yellow, and prepectus brownish. Pilosity on thoracic dorsum black. Fore coxa yellow, mid-coxa brown, hind coxa mostly black except at distal extremity; remainder of legs yellow. Propodeum black. Gaster with first apparent tergum black, the following terga black on dorsum with the sides orange-yellow except the last tergum entirely orange-yellow.

Head about 1.4× as broad as long in dorsal view. Antennal scape short and medially expanded, only 0.9× as long as pedicel, anellus and first funicular segment combined; pedicel 0.7× as long as first funicular segment (figure 36). Anterior margin of scutellum much shorter than posterior margin of mid-lobe of mesoscutum; scutellum 1.2× as long as broad, the anterior part transversely striate-reticulate, the frenal area with longitudinal, irregular wrinkles (figure 149). Forewing stigma broadly oval, nearly 1.4× as long as wide; upper part of stigmal vein comparatively, 0.3× as long as stigma length; uncus shorter (0.7×) than upper part of stigmal vein (figure 73). Propodeum with a sharp median carina. Aedeagus narrow and elongate, digitus with four teeth (figure 111).

Variation

The above description is based on the type material from North America. In Europe , body length varied from 2.7 to 4.3 mm in females, from 3.0 to 3.5 mm in males, the smallest examined individuals originating from Denmark (ex. Abies grandis ). The relative length of the exserted part of female ovipositor varied from 0.9 to 1× the body length. Most of the female specimens examined from Europe fitted the type description. A few variations in colour were observed as follows: oblique black stripes on pronotum reduced to small spots, mesopleuron and metapleuron extensively yellow, anterior part of scutellum black, yellow patch on central part of propodeum. In males, most French specimens differed from the type in having two rounded yellow spots on posterior margin of pronotum and the scutellum with two longitudinal yellow bands. In both sexes, the median carina on propodeum was sometimes bifid, especially in the posterior part .

Sex ratio Apparently balanced in the areas of introduction in Europe (AR).

Hosts

Specific to fir seeds ( Abies spp. , Pinaceae ). Recorded in native American areas from A. grandis , A. procera (= A. nobilis ) and A. magnifica shastensis Lemmon (Milliron, 1949; Peck, 1963; Grissell, 1979; Hedlin et al., 1980). In Europe, found in seeds of firs introduced from North America, Abies balsamea (Ochsner and Jensen, 1998) , A. concolor (Ochsner and Jensen, 1998) , A. grandis (Hussey, 1952, 1954a; Ochsner and Jensen, 1998; AR) and A. magnifica (Hussey, 1954b) but not on A. procera in Denmark (Ochsner, 1998; Jensen and Ochsner, 1999). It also shifted on to seeds of the native A. alba (Ochsner and Jensen, 1998; AR) and of some other Eurasian firs introduced to Denmark ( A. bornmülleriana , A. homolepis , A. nordmanniana and A. pinsapo ; Ochsner and Jensen, 1998).

Distribution

Originates from the western North America, the native range extending from California to British Columbia (Milliron, 1949; Peck, 1963; Grissell, 1979; Hedlin et al., 1980). Introduced to Europe, but apparently limited to the western part: Denmark (Ochsner and Jensen, 1998; Jensen and Ochsner, 1999); France (Da Ros et al., 1993; AR); Great Britain (Hussey, 1952, 1954a, 1954b; Hussey and Klingler, 1954); The Netherlands (AR). The important seed damage observed during 1999–2000 in several native stands of eastern France (Auger-Rozenberg, unpublished observations) suggests that the species is more widespread than present data indicate, in connection with continuous importations of A. grandis seeds which may be chalcid-infested (Hussey, 1954a).

Comments

Until today, six other species of seed chalcids were regularly observed in seeds of Abies spp. in the West Palearctic region: M. pinus , M. pinsapinis , M. rafni , M. schimitscheki , M. specularis and M. suspectus (an additional species, M. borriesi , has just been recorded from Denmark —see the chapter concerning M. suspectus for diagnosis). Females of M. pinus easily differentiate from these of the other species by the pale pilosity on thoracic dorsum. The thoracic dorsum is entirely or quite entirely black in females of M. pinsapinis , M. schimitscheki and M. suspectus whereas more or less extended light parts are observed on that of M. milleri , M. rafni and M. specularis . The females of M. rafni differ by the relative size of the exserted part of ovipositor, which is much more important than that of the two other species (1.2 versus 0.9–1.0 body length). The females of M. milleri can be differentiated from these of M. specularis by the colour of pronotum. Pronotum is mostly black in M. specularis except two yellow spots on posterior margin of pronotum whereas that of M. milleri is predominantly yellow, even in dark forms, except two isolated stripes or spots on middle. They also differ by the shape of stigma (1.5–1.6× as long as broad in M. milleri versus less than 1.5× in M. specularis ). The females of M. schimitscheki can be easily recognized from these of M. pinsapinis and M. suspectus by the thorax and the face entirely black except oral area, and by the stigma shape (1.6 versus 1.9–2.2× as long as wide). M. pinsapinis differs from M. suspectus by the colour of face (entirely yellow versus yellow with more or less extended black markings), temple (with a limited black patch not reaching gena versus a blackbrown patch extending to gena), prepectus, tegula, acropleuron (yellow to light brown versus black) and coxae (fore and mid-coxa yellow versus only distal part yellow; hind coxa brownish to black with yellow markings versus entirely black).

Males of fir seed chalcids are less easy to separate, except these of M. pinus which present a pale pilosity on thoracic dorsum whereas this vestiture is mostly dark in other species. Males of M. pinsapinis , M. schimitscheki and M. suspectus can be separated from others by the colour of the thoracic dorsum, which is entirely black whereas it presents more or less extended light parts in the other species. In males of M. schimitscheki , the three pairs of coxa are entirely black whereas they are mostly yellow in M. pinsapinis and M. suspectus . M. pinsapinis differs from M. suspectus by the colour of hind coxa (yellow with black markings versus black), the sculpture of the frenal area (rounded wrinkles versus coarse longitudinal carinae) and the shape of aedeagus. Male of M. specularis can be separated from that of M. milleri and M. rafni by the scutellum entirely black whereas it always presents some light patterns in the two other species, even in dark forms. M. milleri differs from M. rafni by the colour of pronotum (black with yellow sides and a yellow transverse bilobed band on posterior margin versus brownish yellow with a triangular black spot on anterior part which can extend to posterior margin in dark forms).

Material examined

Canada: 9 X, 8 W, ex. Abies grandis, Vancouver, B.C., 1993 , G. R. Hopping ( GM).

Denmark: 10 X, ex. A. grandis, Skramsø , East Jutland , May 1994, T . Jensen ( AR) .

France: 3 X, 2 W, ex. A. alba, Les Barres (45), May 1981 ( AR) ; 50 X, 50 W, ex. A. alba, Forest of Lafage (15), June 2000, M. Auger-Rozenberg ( AR) . The Netherlands: 10 X, 10 W, ex. A. grandis, Sleenerzand , April–May 1990, P. Grijpma ( AR) . USA: 1 X holotype, 3 X, 2 W, paratypes, ex. A. grandis, Crescent City, CA , 26 April 1915, J. M. Miller ( USNM) .