A review of the early stages and host plants of the genera Eumerus and Merodon (Diptera: Syrphidae), with new data on four species

Abstract

The genera Eumerus and Merodon (Diptera: Syrphidae) have a high taxonomic diversity (300+ species altogether), but life histories of most species are unknown. In addition, these hoverfly genera are recognised to be pests (ornamental plants and vegetable crops). In this paper, early stages of four hoverfly species are described, Eumerus hungaricus Szilády, 1940, Eumerus nudus Loew, 1848 and Merodon geniculatus Strobl, 1909, from Spain, and Eumerus strigatus Walker, 1859, from California, USA. Larvae of E. nudus were obtained from swollen roots of Asphodelus cerasiferus J. Gay. Larvae of E. hungaricus were found in bulbs of Narcissus confusus Pugsley. The host plant of the examined specimen of Eumerus strigatus is unknown. Larvae of M. geniculatus were reared from bulbs of different species of Narcissus L. Scanning electron microscope imaging was used to study and illustrate the anterior respiratory processes, pupal spiracles and posterior respiratory processes of the new early stages. A compilation of all available information on the early stages and host plants of Eumerus (21 spp.) and Merodon (15 spp) is provided, as well as an identification key to all known larvae/puparia of these genera. Eumerus elavarensis Séguy, 1961 is proposed as a new synonym of E. hungaricus and first data of this species are reported from Austria, Bulgaria, Spain and Turkey. In Eumerus, larvae are alleged to rely on the previous presence of decay organisms, but in the larvae of E. nudus the sclerotisation and size of the mandibular hooks suggest that this larva can generate decay from intact plant tissue.

Fig 1. Head skeletons of Eumerus and Merodon larvae, lateral view.

(A) Eumerus hungaricus. (B) Eumerus nudus. (C) Eumerus strigatus. (D) Merodon geniculatus. Abbreviations: d, dorsal cornu; l, mandibular lobe; m, mandibular hook; t, accessory tooth; v, ventral cornu. Scale lines: A, B and D = 0.5mm; C = 0.2mm.

Fig 2. Anterior respiratory processes of Eumerus and Merodon larvae.

(A) Eumerus hungaricus. (B) Eumerus nudus. (C) Eumerus strigatus. (D) Merodon geniculatus. Abbreviations: o, spiracular openings. Views: apical (B and C), apico-lateral (A and D). Scale lines: A and B = 25μm; B = 20μm; D = 50μm.

Fig 3. Pupal spiracles of Eumerus and Merodon puparia.

(A) Eumerus hungaricus. (B) Eumerus nudus. (C) Eumerus strigatus. (D) Merodon geniculatus. Scale lines: A, B and C = 0.1mm; D = 0.2mm.

Fig 4. Detail of the tubercles bearing spiracular openings in the pupal spiracles of Eumerus and Merodon puparia.

(A) Eumerus hungaricus. (B) Eumerus nudus. (C) Eumerus strigatus. (D) Merodon geniculatus. Abbreviations: o, spiracular openings. Scale lines: A = 10μm; B and D = 25μm; C = 20μm.

Fig 5. Posterior respiratory processes (PRP) of Eumerus and Merodon puparia, dorsal view.

(A) Eumerus hungaricus. (B) Eumerus nudus. (C) Eumerus strigatus. (D) Merodon geniculatus. Abbreviations: a, distance between the transverse ridge and the centre of the spiracular plate; b, width at the transverse ridge level; r, transverse ridge. Scale lines: A and C = 0.2mm; B = 0.25mm; D = 0.5mm.

Fig 6. Posterior respiratory processes (PRP) of Eumerus and Merodon puparia showing the spiracular plate.

(A) Eumerus hungaricus (only the basis of the spiracular setae are left in this specimen). (B) Eumerus nudus. (C) Eumerus strigatus. (D) Merodon geniculatus. Abbreviations: c, spiracular scar; o, spiracular openings; s, spiracular seta. Views: apical (A, B and C), apico-ventral (D). Scale lines: A = 0.1mm; B and C = 0.2mm; D = 0.25mm.