Megastigmus dorsalis (Fabricius, 1798)

Nieves-Aldrey, José Luis, Nieves, Maria Hernández & Gómez, José F., 2008, Larval morphology and biology of three European species of Megastigmus (Hymenoptera, Torymidae, Megastigminae) parasitoids of gall wasps, including a comparison with the larvae of two seed-infesting species, Zootaxa 1746, pp. 46-60 : 49-52

Megastigmus dorsalis (Fabricius, 1798)

Measurements: ex gall Andricus kollari (agamic generation) (n=1; measured specimen without mean information), body length: 3.3 mm; maximum width: 1.7 mm; ex gall A. grossulariae (agamic generation) (n=4), body length: 1.9–3.3 (2.3) mm; maximum width: 0.9–1.9 (1.3) mm.

Description: The body is composed of head and 13 segments (three thoracic, 9 abdominal and one anal segment) ( Fig. 1 A). Body 1.7–1.9 × as long as broad; cylindrical or slightly fusiform, a little broader in the mid-region, tapering slightly anteriorly and posteriorly; anal segment truncated, about two times broader than long. Colour is whitish and integument is smooth except for anterior 1/3 of T1–T3, which are rugose with irregular papillate sculpture, also visible on the genal area of head ( Fig. 3 A). Body sparsely setose; T1–T2 with 9–15 long setae almost as long as length of a thoracic segment; the third thoracic segment present 3–5 setae, each much shorter than a thoracic segment, while the abdominal segments appear almost bare; only a pair of inconspicuous short setae are visible on pleural region of A8; two pairs on A9 (one ventral, one pleural) and two pairs on the anal segment. The body is ventrally curved in the lateral view ( Fig. 2 A); the spiracles are visible on T2–A7. Anterodorsal protuberances visible on T2–A9 ( Fig. 2 A); ventral margin of body segments slightly convex.

Head (anterior view) ( Fig. 3 A) ovate, 1.2 times as wide as high; upper margin of vertex medially incised; concave medially, medial incision of vertex is prolonged as a groove ending in a deep oval pit; incised upper margin of vertex forms an ~90º angle with the median groove, and is generally narrower than the frontal pit. The head has a chaetotaxy pattern composed of 5–7 pairs of setae; antero-medial setae of vertex long, situated on the truncated margin of vertex; antero-medial setae are level or slightly above of the anterior margin of frontal pit, its length varying from one-half to almost as long as the distance between antennae. A pair of long genal and hypostomal setae are present with a pair of latero-clypeal setae with length varying from half to two thirds of width of labrum. The clypeal setae are very short, and the antenna is button-like, situated slightly below the margin of frontal pit. Ventral margin of clypeus indistinct or incomplete. Labrum sub-rectangular, its anterior margin fringed into small acute flaps or lobes.

Mouth parts ( Fig. 4 A). The maxillae are differentiated from labium, divided into two mostly differentiated parts. Two pairs of maxillary palps are visible, with a concave labium and two pairs of short labial setae present, the ventral one being longer.

Mandibles ( Figs 5 A, 5B). The mandibles are strongly sclerotized and exposed in part; the upper half of mandibles are visible under labrum. The right and left mandibles have four teeth, and a fifth very small tooth is visible in the left mandible. First tooth more than twice as long as second; apical part of first two teeth is slightly blunt, and the third and fourth teeth are triangular, acute and are not separated by a wide gap.

Remarks. Va ri at io n: Adult individuals reared from galls of the sexual generation of Andricus grossulariae Gir. , on Quercus suber L. in Spain ( Fig. 6 C), differed slightly in coloration (yellow colour more extensive) and few other features (relative lengths of A2 and A3, infumate area below stigma) from typical M. dorsalis reared from A. kollari galls, but they could not be identified as a separate species, according actual revisions of the group de Megastigmus species associated to oak gall-wasps in Europe. However, their larvae differ from the larvae of M. dorsalis from A. kollari described above in the following ways: the frontal pit was more rounded; the groove connecting it with vertex was slightly wider ( Fig. 3 B); the antenna and antennal setae were situated higher on the frons, at the level of the frontal pit; antennal setae and latero-clypeal setae were much longer, being nearly as long as the separation of antennae and width of labrum, respectively.

Biology: A detailed account of the biology of Megastigmus dorsalis was published by Askew (1966); see also data from Spain ( Nieves-Aldrey 1984; Pujade-Villar 1993). The species is a polyphagous parasitoid that attacks cynipid galls induced by Andricus , Cynips , Neuroterus , Biorhiza , and Callirhytis , on Quercus species. Megastigmus dorsalis have at least two generations in a year. Askew (1966) stated that in galls of A. kollari , M. dorsalis larvae were found in cells of cynipids inquilines of Synergus spp. We have now confirmed this association. In galls of A. kollari , the larvae of M. dorsalis were found in peripheral cells occupied by the inquiline Synergus umbraculus (Olivier) ( Fig. 6 B), whereas the central cell of the gall-inducer Andricus was usually attacked by a larva of the closely allied species Megastigmus stigmatizans ( Fig. 6A ).

Material examined. ex Andricus hispanicus (= A. kollari of authors), on Quercus faginea : Spain, Cádiz: La Suara-Jeréz de la Frontera (16/x/04 (n=1); on Q. pyrenaica , Spain, Madrid: El Escorial (ix/03 (n=2). Ex A. grossulariae (agamic generation), on Q. pyrenaica : Spain, Madrid: El Escorial (13/iii/06) (n=4). Ex Andricus grossulariae (sexual generation), on Quercus suber : Spain, Cáceres, Adeanueva del Camino (23/v/06) (n=2). All materials JLN-A leg.

Megastigmus dumicola Bou č ek, 1982

The larva of M. dumicola is described here for the first time.

Measurements: ex Plagiotrochus sp. (n=1; measured specimen without mean information), body length: 2.2 mm; maximum width: 0.7 mm; ex P. kiefferianus (n=29), body length: 2.2–2.5 (2.3) mm; maximum width: 1.2–1.3 (1.3) mm.

Larvae of these species are similar to the larva of M. dorsalis in general appearance ( Fig. 1 C) and in most morphological characters. The following diagnostic characters can readily separate the larvae of the two species:

The blister-like sculpture on the genae is almost absent. The truncate and incised upper margin of vertex forms an obtuse angle with the median groove, while the median groove is relatively long and narrow (narrower than the frontal pit). The frontal pit is shield-shaped ( Fig. 3 C), and the antero-medial setae of the vertex are shorter (shorter than larger diameter of frontal pit) and are situated lower. The insertion of antennae and antennal setae is clearly below the ventral margin of the frontal pit. The antennal setae are long, almost as long as the separation of antennae, as are the latero-clypeal setae (0.7 times as long as the width of the labrum). The anterior margin of the clypeus is distinct and well marked ( Fig. 4 C); both mandibles have four teeth ( Fig. 5 D, 5E).

Remarks: M. dumicola is an uncommon species distributed only in the Western Mediterranean. It was described by Bouček (1982) from insects collected in “maquis” vegetation in Southern France. The adult of M. dumicola can be easily distinguished from the other entomophagous Megastigmus species by its distinctly petiolate metasoma and its reddish brown coloration ( Bouček 1982, Roques & Skrzypczynska 2003).

Biology: The biology of this species was unknown when it was described. Later biological data were cited by Askew and Nieves-Aldrey (1988) from materials reared from galls in Spain. The species is a monophagous parasitoid, associated only with woody galls of Plagiotrochus spp. on Quercus coccifera , and more rarely on Q. ilex ( Fig. 6 D).

Material examined. ex Plagiotrochus sp. on Quercus ilex : Spain, Madrid: Monte el Robledal (20-x-02) (n=1); Ex P. kiefferianus , on Quercus coccifera : Spain, Madrid: Puente de San Antonio (21- iv/05) (n=29). J.LN-E leg.

Figures

FIGURE 1. Ventral view of final-instar larvae of Megastigmus. A. M. dorsalis ex Andricus kollari (asexual). B. M. dorsalis ex Andricus grossulariae (sexual). C. M. dumicola ex Plagiotrochus kiefferianus (asexual). D. M. stigmatizans ex Andricus kollari (asexual). E. M. pistaciae ex seeds of Pistacia lentiscus F. M. aculeatus ex seeds of Rosa sp.

FIGURE 2. Lateral view of final-instar larvae of Megastigmus. A. M. dorsalis ex Andricus kollari (asexual). B. M. dumicola ex Plagiotrochus kiefferianus (asexual). C. M. stigmatizans ex Andricus kollari (asexual). D. M. aculeatus ex seeds of Rosa sp.

FIGURE 3. Anterior view of the head of the final-instar larvae of Megastigmus. A. M. dorsalis ex Andricus kollari (asexual). B. M. dorsalis ex Andricus grossulariae (sexual). C. M. dumicola ex Plagiotrochus kiefferianus (asexual). D. M. stigmatizans ex Andricus kollari (asexual). E. M. pistaciae ex seeds of Pistacia lentiscus F. M. aculeatus ex seeds of Rosa sp. Arrows point at antennal setae and anterior margin of clypeus.

FIGURE 4. Anterior view of mouthparts of final-instar larvae of Megastigmus. A. M. dorsalis ex Andricus kollari (asexual). B. M. dorsalis ex Andricus grossulariae (sexual). C. M. dumicola ex Plagiotrochus kiefferianus (asexual). D. M. stigmatizans ex Andricus kollari (asexual). E. M. pistaciae ex seeds of Pistacia lentiscus F. M. aculeatus ex seeds of Rosa sp.

FIGURE 5. Mandibles of the final-instar larvae of Megastigmus. A. M. dorsalis, left mandible anterior view. B. M. dorsalis left mandible, posterior view. C. M. pistaciae, right mandible, posterior view D. M. dumicola, left mandible, anterior view E. M. dumicola, right mandible, anterior view. F. M. aculeatus, right mandible, posterior view G. M. stigmatizans, left mandible, anterior view. H. M. stigmatizans, right mandible, anterior view.

FIGURE 6. A. Cross section of a gall of Andricus kollari showing a larva of M. dorsalis in a secondary cell of the inquiline Synergus umbraculus, and the central cell occupied by the larva of M. stigmatizans. B. Larva of M. dorsalis in a peripheral cell of a gall of A. kollari. C. Larva of M. dorsalis in a gall of Andricus grossulariae (sexual gen.). D. Crosssection of a gall of Plagiotrochus kiefferianus with larvae of M. dumicola E. Larva of M. stigmatizans. F. Detail of head of the larva of M. stigmatizans. G. Cross section of a gall of A. kollari showing the central cell with a larva of M. stigmatizans. H. A seed inside a hip of Rosa sp., attacked by a larva of M. aculeatus.