Megastigmus stigmatizans (Fabricus, 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 : 52

Megastigmus stigmatizans (Fabricus, 1798)

Measurements: ex A. kollari (n=7), body length: 1.7–2.9 (2.3) mm; maximum width: 0.9–1.5 (1.3) mm; ex A. quercustozae (n=1; measured specimen without mean information), Body length: 5 mm; maximum width: 3.1 mm.

Similar to the larva of M. dorsalis in general appearance and most morphological characters, but differs in the following ways:

Larger size. Body slightly wider at the mid-region and tapers more abruptly posteriorly ( Fig. 1 D; Fig. 6 E). Truncate incised anterior margin of vertex prolonged into a relatively wide medial groove, which is as wide as the frontal pit ( Fig. 3 D). Frontal pit rounded ( Fig. 6 F). Antenna situated far to the anterior margin of medial frontal pit; antennal setae situated at a mid-distance between the medial frontal pit and the antennae, clearly above the antennae and below the median frontal pit. Antero-medial setae of the antennal area long (length is half of the distance between antennae) and the latero-clypeal seta are 0.5 × as long as width of labrum. Mandibles large, heavily sclerotized; left mandible has five teeth ( Fig. 5 G) while the right mandible with four teeth ( Fig. 5 H); first tooth acute, more than twice as long as second; apex of second and third teeth blunt; fourth tooth has a rounded apex; third and fourth teeth widely separated, with the separation being as wide as the width of fifth tooth.

Biology: Megastigmus stigmatizans is a polyphagous parasitoid of cynipid galls on oaks of the section Quercus . It is associated mainly with large lignified galls of agamic generations of Andricus species ( Askew 1966, Nieves-Aldrey 1984). We have observed that the larvae of M. stigmatizans attack the larva of A. kollari occupying the central cell of unilocular galls ( Fig. 6 G). This is in contrast to the closely related species M. dorsalis which attacks Synergus umbraculus occupying secondary cells in the gall tissue.

Materials examined. ex A. kollari , on Quercus faginea : Spain, Madrid: Dehesa de Arganda (20/iv/05) (n=4); on Q. pyrenaica: El Escorial (2/vii/03) (n=3); ex A. quercustozae , on Q. faginea : Spain, Ciudad Real: Puerto de Despeñaperros (15/x/04) (n=1). JLN-A 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 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 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.