Description of the Puparium of Eumerus vestitus Bezzi, 1912 (Diptera: Syrphidae) Reared from Supermarket Plums in Israel

Abstract

With over 300 species, the genus Eumerus Meigen, 1822 is one of the largest in the Syrphidae family. Regarding its immature stages, information is scarce, since they are described for only 22 species. The known larvae of Eumerus can be phytophagous on the underground parts of live plants or saprophagous in decaying plant materials of many sorts. Eumerus vestitus Bezzi, 1912 is a widely distributed species with records in the Afrotropical, Oriental, and Palearctic Regions. In the Palaearctic, E. vestitus is present in the Arabian Peninsula, Cyprus, Greece, Israel/Palestinian Authority, North Africa, and Syria. It is listed as an endangered species in the IUCN European Red List of Hoverflies due to the destruction of its habitat. Several puparia of E. vestitus were obtained from larvae collected in supermarket plums in Israel. In this work, the E. vestitus puparium is described in detail using a scanning electron microscope and stereomicroscope and compared with those of other members of the Eumerus obliquus (Fabricius, 1805) group, which E. vestitus belongs, and with members of other species groups. Our work also confirms a long-standing and broad occurrence of E. vestitus in Israel and records plum fruit as a new host plant for the species.

1. Introduction

With over 300 species, the syrphid genus Eumerus Meigen, 1822 (Diptera: Syrphidae) has a worldwide distribution through introduction events of Eumerus aurifrons (Wiedemann, 1824), Eumerus figurans Walker, 1859, Eumerus funeralis Meigen, 1822, Eumerus narcissi Smith, 1928, Eumerus obliquus (Fabricius, 1805), and Eumerus strigatus (Fallén, 1817) in the Nearctic and Neotropical Regions due to international trade of bulbs or other underground storage organs [1,2,3,4]. Eumerus belongs to the tribe Merodontini and can be distinguished from phylogenetically related genera [5] by a combination of features, including a usually round or roundish basoflagellomere, a scutellum narrower than the head, a hind femur without a triangular expansion, and the tergum I pilose laterally, among other characters [6]. At the moment, Eumerus is divided into 12 species groups based on DNA and morphological data, which will be henceforth referred to as follows: barbarus group, basalis group, clavatus group, minotaurus group, olivaceus group, obliquus group, ornatus group, pulchellus group, strigatus group, torsicus group, triangularis group, and tricolor group [7,8]. Adults are the best known stage of Eumerus life cycles, but many taxonomic problems still await resolution, and a complete revision of the genus is to be performed [9].

In comparison with adults, the information about the immature stages (i.e., egg, larva, and puparium) is very limited. Immatures are known for 36 Eumerus species, while descriptions are available for only 22 species [10,11,12,13]. Krivosheina & Krivosheina [14] presented a description of an Eumerus larva but were unable to identify it to species level. According to Rotheray & Gilbert [15], the Eumerus larva feeding regime is between phytophagy and saprophagy, since they feed on semi-liquid decay in dead or live plants. However, the larva of Eumerus larvatus Aracil, Grković & Pérez-Bañón in Aracil et al. [16] is completely phytophagous, while the larva of E. strigatus is completely saprophagous [11]. In some cases, it has been found that the larva can be saprophytophagous, such as in Eumerus alpinus Rondani, 1857, since clusters of scraped and decomposed plant material have been found in its head skeleton [11]. Nevertheless, we do not know if E. alpinus produced its own decaying material or whether the decomposing plant matter was already there when the gravid female reached the plant. Rotheray [17] diagnosed the Eumerus larva and puparium as having a posterior respiratory process (PRP) at the tip of a barely extended anal segment, mouth hooks with fleshy mandibular lobes, and a dorsal lip with a setal fringe, among other characters. Later on, Ricarte et al. [10] considered the number of spiracular openings in the spiracular plate to distinguish Eumerus larvae from other genera, but this feature has to be considered together with characters mentioned by Rotheray [17] and not alone.

The obliquus group consists of nine species, as follows: Eumerus effossus Gilasian & van Steenis in Gilasian et al. [18], Eumerus incilis Smit in Smit et al. [19], Eumerus lyneborgi Ricarte & Hauser in Ricarte et al. [20], E. obliquus, Eumerus punctifrons Loew, 1857, Eumerus rufotibialis Randeković & Vujić in Aracil et al. [12], Eumerus similis Gilasian & van Steenis in Gilasian et al. [18], Eumerus unicolor Loew, 1858, and Eumerus vestitus Bezzi, 1912 [12,19,20]. The adults of the obliquus group are distinct in having the following combination of characters: very deep and clear punctuation on the body (i.e., thorax and abdomen), a hind margin of the scutellum pollinose, swollen hind femur; eyes meeting for a certain linear distance in males, and a frons that is extensively and densely pollinose in females, among other characters [20]. Currently, the third-stage larva (L3) and puparium of E. obliquus and E. rufotibialis [12,21,22] and the second-stage larva (L2) of E. lyneborgi [13] are the only species within the obliquus group with available descriptions of immature stages.

Eumerus vestitus is one of the 82 species present in Europe, with a distribution range concentrated in Greece and Cyprus [23]. In addition, it is known to occur in the Arabian Peninsula, Guinea-Bissau, India, Pakistan, Sudan, and Syria [23,24,25,26,27]. Shaumar & Kamal [28] listed E. vestitus from Egypt; however, El-Hawagry & Gilbert [29] mentioned that it was wrongly identified and was actually Eumerus efflatouni Curran, 1938, following Dirickx [30], who considered all ‘vestitus’ from Egypt to belong to a different species from the Afrotropical vestitus. At the moment, there is uncertainty regarding whether E. efflatouni is a synonym of E. vestitus, mentioned as tentative possible synonym by Smit et al. [19], which is the only work that considered it as a synonym, whereas the IUCN Red List of Threatened Species does not [25]. Eumerus vestitus was assessed under the IUCN categories and criteria and included in the European Red List of Hoverflies as Endangered due to degradation of habitats caused by tourism and recreation activities [25]. At the moment, information regarding the immature stages of E. vestitus is scarce, as despite being known from several host plants [23], no morphological description of the larva is available.

The finding and rearing of larvae of E. vestitus in supermarket plums in Israel prompted us to describe the remaining puparia upon adult emergence. Thus, the aim of this work is to expand the knowledge of the immature stages of Eumerus by describing the puparium of E. vestitus in detail and highlighting the importance of pest control management in airports and commercial ports.

2. Materials and Methods

Examined material and adult identification. Numerous larvae of Eumerus were observed in fresh plums bought at the Victory supermarket in Rehovot (Israel; 31.91° N 34.80° E) by Wolf Kuslitzky on 26 July 2023. Once the larvae pupariated, the puparia were reared in a container that was checked regularly until adult emergence on August 20, 2023. Nine adults (7♀ and 2♂) (Figure 1) were procured from these puparia. The adults were identified by Mike Mostovski and confirmed by Pablo Aguado-Aranda and Antonio Ricarte. The exact country of origin of the plums is unknown, although there are only a few possible options. Locally grown plums are harvested in Israel from May to September, whereas plum imports were negligible in 2023 [31]. Additionally, another seven adults (5♀ and 2♂) identified as E. vestitus had been reared from puparia collected on rotten potato tubers on 2 August, 1973 by M. Kaplan on the premises of the Agricultural School Mikve Israel (Holon, Israel; 32.03° N 34.78° E). All material is deposited at the entomological collection of the Steinhardt Museum of Natural History, Tel Aviv University (Tel Aviv, Israel), except for three puparia that are deposited at the Entomological Museum of the Mississippi State University (MS, USA).

Sample preparation and study. The preparation procedure of the puparia (n = 10) for study follows Orengo-Green et al. [13]. Each puparium was cleaned with a brush to remove any dirt from the specimen. Measurements of puparium and posterior respiratory process (PRP) follow Orengo-Green et al. [32]. Photos were produced as stacks of individuals images made with a Canon 7D camera (Canon Inc., Ōta, Tokyo) from the Biology Department at the University of Puerto Rico—Mayagüez Campus (Mayagüez, Puerto Rico). Stacks were made using Zerene stacker software T2024-11-18-1210 (Zerene Systems LLC, Richland, WA, USA). The drawing of the head skeleton was made from stacks of individuals images taken from a Leica DMC 5400 camera (Leica Camera AG, Wetzlar, Germany) attached to a Leica M205 C binocular stereomicroscope (Leica Camera AG, Wetzlar, Germany) from the Entomological Museum of the Mississippi State University (MS, USA). Stacks were made with Leica Application Suite LAS^®, v.4.12.0. (Leica Microsystems, Wetzlar, Germany). The shading of the head skeleton drawing indicates the level of sclerotization (lighter = weak, darker = heavy). A Zeiss Gemini 1 Sigma 560 scanning electron microscope (Zeiss, Baden-Württemberg, Germany) operating in variable pressure mode from the Biology Department at the University of Puerto Rico—Mayagüez Campus was used for a more detailed description of the anterior respiratory process (ARP), PRP, and pupal spiracles. Three puparia (one dorsal, one lateral, and one in ventral view) were prepared on aluminum stubs with double adhesive carbon tape, and they were not gold-coated.

The morphological terminology of puparia follows Rotheray [33]. For the head skeleton, it follows Courtney et al. [34], Rotheray [33], and Campoy et al. [35].

3. Results

3.1. Eumerus vestitus Puparium (Figure 2)

Description: Length: 13.1–13.5 mm; width: 5.6–6 mm; height: 5.3–5.7 mm (n = 10). Oval-shaped, flat ventrally. Surface covered with dense pubescence. One pair of observable lappets below the PRP. Ventrally with one pair of prolegs with crochets in the mesothorax and from the first to seventh abdominal segments. The prolegs of the seventh abdominal segment are united medially and just in front of the anus (Figure 3). Prolegs with three to four rows of highly developed and sclerotized crochets pointing backwards; crochets of larger size in the anterior row and smaller size crochets in the posterior row. The last pair of prolegs with crochets facing forward; crochets of smaller size in the anterior row and larger in the posterior row. A pair of small, sclerotized ARP located in the prothorax. A pair of pupal spiracles located on the dorsal side of the first abdominal segment. Color: light brown. ARP (Figure 4): height: 57–60 μm; width: 43–45 μm. Light brown color, cylindrical, and with smooth surface. Pupal spiracles (Figure 5A): dark brown color, cylindrical, tapering with a round tip; surface covered with yellow tubercles with 5–8 opening holes (Figure 5B). Smooth surface. PRP (Figure 6A): shiny reddish-brown, with a conspicuous transverse ridge. Inclined upward from the transverse ridge. Length above the transverse ride: 0.43–0.45 mm; length below the transverse ridge: 0.49–0.51 mm; width at the transverse ridge: 0.49–0.50 mm. Surface above the transverse ridge with small puncture; surface below the transverse ridge with smooth lines (Figure 6D). Spiracular plate (Figure 7) with three pairs of irregular spiracular openings, four pairs of long feathery interspiracular setae, one pair of perispiracular glands between the first and second spiracular openings, and one pair of ecdysial scars in the center of the spiracular plate. Head skeleton (Figure 8): mouth hooks sclerotized, with some parts that are highly sclerotized; mandibular lobes fleshy and fused with slightly sclerotized mandibles; sclerotized mortar; optical depression highly sclerotized; tentorial arm and tentorial bar highly sclerotized; dorsal and ventral cornu not sclerotized; dorsal cornu almost half the length of the ventral cornu; shark-fin shaped dorsal cornu; ventral cornu with a slightly sclerotized pestle; slightly sclerotized pharyngeal ridges located at the ventral cornu. Soft food pebble found at the mandibular lobes and food particles in the pharyngeal ridges.

Figure 2. Puparium of E. vestitus. (A) Lateral view; (B) dorsal view.

Figure 2. Puparium of E. vestitus. (A) Lateral view; (B) dorsal view.

3.2. Eumerus vestitus Development Sites

In

Table 1. Host plants of the early stages of Eumerus vestitus. New records reported in this work are in bold.

Host Plant	Locality of Origin
Banana	India [23]
Grape	Mex (Alexandria, Egypt) [24]
Plum	Unknown (probably Israel)
Potato tuber	Cyprus, Greece, Israel/Palestinian Authority [24], and Israel
Paw-paw (rotten stems)	Tel-el-Kebir (Egypt) [24]
Tomato	India [23]
Watermelon	Tul-Karam [Tulkarm] (Israel/Palestinian Authority) [24]

, a list of the host plants of E. vestitus and the countries of their origin is provided.

3.3. Update of the L3/Puparia Taxonomic Key of Eumerus Species

With the description of E. vestitus, the taxonomic key of the L3/puparia of Eumerus species of Ricarte et al. [10] will be modified as follows. Eumerus lyneborgi was not added since only the L2 is described.

• 1a. Mesothoracic prolegs present………………………………………………………………..2
• 1b. Mesothoracic prolegs absent…………………………………………………………………5
• 2a. Head skeleton: mandibular hook serrated apically……………………………………….3
• 2b. Head skeleton: mandibular hook not serrated…………………………………………….4
• 3a. Head skeleton: ventral cornu without pestle………..Eumerus etnensis van der Goot, 1964
• 3b. Head skeleton: ventral cornu with a slightly sclerotized pestle………………E. rufotibialis
• 4a. Prolegs of the seventh abdominal segment united in a medial position of the segment. Head skeleton: ventral cornu with a slightly sclerotized pestle…………..……………………………………………………………….……………..E. vestitus
• 4b. Prolegs of the seventh abdominal segment separated. Head skeleton: ventral cornu without pestle…………………………………………………………………………..E. obliquus

4. Discussion

In the Syrphidae phylogeny, the sister group of Eumerus is Merodon Meigen, 1822 based on larval morphology [15] and also on DNA [36,37]. The larvae/puparia of Eumerus can be easily distinguished from those of Merodon by the presence of three pairs of spiracular openings in the spiracular plate, as opposed to four pairs in Merodon [10]. The present study confirms the validity of this character, since E. vestitus also has three pairs of spiracular openings. Four pairs of lappets on the anal segment were long considered typical of Alipumilio Shannon, 1927, Eumerus, and Merodon [15,38]. However, the number of lappets in Eumerus has lately been found to be variable; for instance, two pairs in E. rufotibialis [12], three pairs in E. alpinus [11], and just one pair of observable lappets in the puparium of E. vestitus (present study). Therefore, this character can no longer be taken into consideration to separate Eumerus from other genera.

The information about the immature stages of Eumerus is scarce; they have been described only for 23 species, including E. vestitus. This species can be distinguished from all congeners by the ornamentation of the surface and tip of the pupal spiracles, the location of the prolegs, the presence and number of crochets on the prolegs, the number of lappets, and the shape and surface ornamentation of the PRP, the last character being the most diagnostic. The pupal spiracles in E. vestitus are completely smooth and rounded at the tip, which is similar to E. etnensis [38], E. obliquus [22], and E. rufotibialis [12], but different from E. alpinus, Eumerus pulchellus Loew, 1848, and Eumerus superbus Shannon, 1927, which have granulated pupal spiracle tip [11,38], and from Eumerus nudus Loew, 1848, that has completely nodulated pupal spiracle tip [10].

Within the obliquus group, only four head skeletons (including E. vestitus) have been described [12,13,38]. It must be noted that the head skeleton of E. lyneborgi has been recorded for the second-stage larva, and it is known that there can be morphological changes between larval stages. The head skeleton of E. vestitus is similar to E. lyneborgi, E. obliquus, and E. rufotibialis, since it has a mandibular lobe fused to sclerotized mouth hooks, a small mandible, and pharyngeal ridges (not found in E. rufotibialis) [13,38]. However, the presence of the pestle in the ventral cornu has only been found in E. rufotibialis [12] and E. vestitus. The presence of pharyngeal ridges is indicative of the saprophagous diet of the larvae, since the function of these ridges is to filter and concentrate material in the decaying plant matter [15,39]. The presence of pharyngeal ridges is not exclusive to the obliquus group, as they can be found in other Eumerus species [11]. The presence of mouth hooks points to the phytophagous regime of the larva, since the mouth hooks are used for consuming living plant tissues. The combination of pharyngeal ridges and sclerotized mouth hooks suggests that the larva develops on a plant that is beginning to decay or in overripe fruit. This is consistent with our observation during the removal of the head skeleton from the E. vestitus puparium, as a soft pebble of plant material was found in the mandibular lobe and food particles were found in the pharyngeal ridges.

Regarding the prolegs, E. vestitus is similar to E. lyneborgi and E. obliquus in having one pair on the mesothorax and on each of the abdominal segments 1–7 [13,38]. However, E. vestitus differs from those two species of the obliquus group in that the prolegs of the seventh abdominal segment are united, giving the appearance of having a single medial proleg on the segment. The united prolegs on the seventh abdominal segment are not exclusive to E. vestitus. There are other species possessing this feature, such as Eumerus astropilops Hull, 1964, Eumerus compactus van Doesburg, 1966, E. etnensis, and E. rufotibialis [12,39]. The mesothoracic prolegs and the prolegs on the seventh abdominal segment are not always present in Eumerus larvae. For instance, E. figurans and E. pulchellus do not have mesothoracic prolegs [11,38], and E. strigatus and E. superbus have prolegs only on abdominal segments 1–6 [10,11].

The number of crochet rows on the prolegs of Eumerus larvae is variable: for instance, one row in E. alpinus [11], two rows in Eumerus pusillus Loew, 1848 [22], three rows (e.g., E. etnensis) [38], or four rows in E. vestitus. To the best of our knowledge, E. vestitus is the only Eumerus species with four rows of crochets on all prolegs (including mesothorax), since E. compactus has multiple crochet rows only on the mesothoracic prolegs and only two rows on the rest of prolegs [12]. Thus, the crochets arranged in four rows on all proleg-bearing body segments can be used as diagnostic for E. vestitus. Unlike the mesothoracic crochets and the crochets of abdominal segments 1–6, the crochets of the seventh abdominal segment in E. vestitus are forwardly orientated, a feature that is also observed in some other Eumerus species (e.g., E. etnensis and E. lyneborgi) [13,39]. Curiously, E. obliquus has the crochets backwardly orientated, despite belonging to the same species group as E. vestitus [22].

The presence of crochets on the prolegs is an adaptation that allows larvae to grip to the substrate in a liquid environment so they can move easily or prevent being carried away by the movement of water in aquatic species [15]. In E. vestitus, all the prolegs bear well-developed crochets, suggesting that the larva lives in liquid or liquified vegetal materials (e.g., decaying fruit). Some species, including E. obliquus itself, also have prolegs with crochets [22], whereas others, such as Eumerus ammophilus Paramonov, 1927, do not [14].

The most diagnostic morphological characteristic to distinguish E. vestitus from other immatures of Eumerus is the PRP shape and ornamentation. The PRP of E. vestitus in dorsal view is conical (wider basally and narrower apically) and, in lateral view, the PRP section above the conspicuous transverse ridge is somewhat bent upwards (Figure 6C). In addition, the ornamentation above and below the transverse ridge is well differentiated, with smooth lines below and fine punctures above. In E. lyneborgi, the PRP is square-shaped, devoid of a transverse ridge, and the entire PRP surface is completely smooth [13]. However, for E. lyneborgi, only the second-stage larva is described, and the larva may show different character states when developing into the third-stage larva, as shown in various studies [13,40]. In E. obliquus, the PRP is more like an elongated rectangle and it is not upwardly bent above the transverse ridge [22]. Larvae of other Eumerus species may show some of the individual PRP characters observed in E. vestitus; for instance, E. nudus has a cone-shaped PRP [10], Eumerus hungaricus Szilády, 1940 has the PRP bent above the transverse ridge [10], and several species such as E. pulchellus and E. pulsillus have fine lines below and fine punctures above the transverse ridge [22].

Even with the national and international prevention legislation and procedures in place, both the unintentional and intentional introduction of species outside of their native ranges have increased in recent years [41]. Introduced species can cause significant damage to agriculture, as Ceratitis capitata (Wiedemann, 1824) (Diptera: Tephritidae) [42]; human health, as Aedes albopictus (Skuse, 1895) (Diptera: Culicidae) [43]; and to the ecosystems, as Python molurus bivittatus Kuhl, 1820 (Squamata: Pythonidae) [44]. Anooj et al. [23] mentioned that E. vestitus entered India with commercial consignments of fruits, plants parts, and vegetables, but further studies must be carried out. This route is not surprising, since several species of Eumerus are known to have been introduced to the Nearctic and Neotropical Regions (e.g., E. obliquus [1]). Israel is considered as a top country in production of several fruits [45], including over 16,500 tons of plums and sloes in 2023 [46]. Thus, the presence of E. vestitus in Israel may affect the production of plums by modifying their quality and quantity at harvest. Nonetheless, further observations and studies are required to determine whether E. vestitus larvae cause damage to fresh produce or just colonize already decaying fruit (i.e., whether E. vestitus larvae are predominantly phytophagous or saprophagous). In addition, countries that import plums have to look into developing pest control management programs to avoid the spread of E. vestitus to other regions.