Rhinusa tetra (Fabricius, 1792)

Gosik, Rafal, Caldara, Roberto, Tosevski, Ivo & Skuhrovec, Jiri, 2024, Description of immature stages of Rhinusa species (Coleoptera, Curculionidae, Mecinini) with a focus on diagnostic morphological characters at the species and genus levels, ZooKeys 1195, pp. 1-94 : 1

2) Rhinusa tetra (Fabricius, 1792)

Material examined.

21 mature larvae; 5 ♂ and 2 ♀ pupae. Italy, Lombardia, Linarolo (Pavia), ex Verbascum thapsus , 25.08.2015; 19 mature larvae; 7 premature; 3 ♂ and 4 ♀ pupae, Italy, Alto Adige, Castelrotto (Bolzano), ex Verbascum blattaria L., 10.08. 2017, all leg., det. R. Caldara.

Description of mature larva

(Figs 6A, B , 7A-E , 8A-C ). Measurements (in mm). Body length: 3.25-6.50 (avg. 5.90). The widest place in the body (meso- and metathorax) measures up to 2.25. Head width: 0.85-0.90 (avg. 0.87).

General. Body elongate, slender, curved, rounded in cross section (Fig. 6A ). Prothorax smaller than mesothorax. Metathorax as wide as mesothorax; each divided dorsally into two folds (prodorsal fold much smaller than postdorsal fold). Pedal folds of thoracic segments prominent, conical, well isolated. Abdominal segments I-V of similar size, next segments tapering towards posterior body end. Abdominal segments I-VI each divided dorsally into two folds almost identical in size. Segments VII-IX dorsally undivided. Epipleural folds of segments I-VIII conical, well developed. Laterosternal and eusternal folds of segments I-VIII conical, well isolated. Abdominal segment X divided into four folds of equal size. Anus situated ventrally, almost completely hidden in segment IX.

Thoracic and abdominal spiracles unicameral; thoracic spiracles (Fig. 6A ) placed laterally close to mesothorax; abdominal spiracles (Fig. 6A ) placed medially on segments I-VIII.

Colouration. Light yellow to brownish head (Fig. 6B ). All thoracic and abdominal segments whitish (Fig. 6A ). Cuticle densely covered with fine asperities.

Vestiture. Setae on body thin, hair-like, different in length (minute to medium).

Head capsule (Figs 6B , 7A ). Head wide, endocarinal line present, reaching to 1/2 of the length of frons. Frontal sutures on head wide, indistinct. A single pair of stemmata in the form of small black spots (st) placed laterally, close to the end of the frontal suture. Des1 very short, located in middle part of epicranium; very long des2 located anteriorly; very long des3 placed almost on the border of the frontal suture; very short des4, located laterally; and long des5 placed anterolaterally above stemma (Fig. 7A ). Fs1 and fs2 minute, located posterolaterally; fs3 absent; fs4 medium, located anteriorly; and long fs5 located anterolaterally, close to antenna (Fig. 7A ). Les1 and les2 medium; single short ves. Epicranial area with four pes.

Antennae placed distally of the frontal suture, on the inside; membranous and distinctly convex basal article bearing one conical, moderately elongate sensorium, plus two sensilla basiconica (Fig. 7B ).

Clypeus (Fig. 7C ) trapezoidal, ~ 2.5 × as wide as long with two relatively long cls, localised posterolaterally, with one sensillum between them; anterior border almost straight.

Mouth parts. Labrum (Fig. 7C ) ~ 2.2 × as wide as long, with three piliform lrs, various long; lrs1 elongated, located posteromedially, on small protuberance, lrs2 medium, located lateromedially, and lrs3 short, located laterally; anterior border almost straight. Epipharynx (Fig. 7C ) with two elongated finger-like als identical in length, three piliform ams variable in length, and two short, finger-like mes; labral rods (lr) distinct, kidney-shaped. Mandibles (Fig. 7D ) bifid, cutting edge straight; two medium piliform and short mds, both located close to lateral border. Maxillolabial complex: maxilla brownish sclerotised (Fig. 7E ) stipes with one stps, two pfs and one short mbs, stps and both pfs1-2 elongated; mala with six finger-like dms variable in length; four medium piliform vms. Maxillary palpi two-segmented; basal palpomere distinctly wider than distal one; both palpomeres equal in length; basal palpomere with short mpxs and two sensilla, distal palpomere with a group of six apical sensilla in terminal receptive area. Prementum (Fig. 7E ) close to oval-shaped, with a single elongated prms; ligula with rounded margin and two, short ligs; premental sclerite broad, sclerotised, cup-shaped, posterior extension short with thick apex. Labial palpi one-segmented; palpi with a single pore, and four apical sensilla in terminal receptive area; surface of labium smooth. Postmentum (Fig. 7E ) with three pms, medium pms1 located posteromedially, long pms2 located mediolaterally, and elongated pms3 located anterolaterally; membranous area partially covered with knobby asperities.

Thorax. Prothorax (Fig. 8A ) with eight elongated and single medium prns; two elongated ps; and single short eus. Mesothorax (Fig. 8A ) with a single short prs; two medium and one short pds (ordered: medium, medium, short); one short as; two medium and one short ss; one medium eps; one medium ps; and single minute eus. Chaetotaxy of metathorax (Fig. 8A ) almost identical to that of mesothorax. Each pedal area of thoracic segments with five elongated and one short pda.

Abdomen. Segments I-VIII (Fig. 8B, C ) with one very short prs (segment VIII without prs), three pds of various length; one medium and one long ss; single elongated eps; one medium ps; one short lsts; and two minute eus. Abdominal segment IX (Fig. 8C ) with one short and two minute ds; two minute ps; and two minute sts.

Description of pupa

(Figs 9A-C , 10A-C ). Measurements (in mm). Body length: 3.75-5.25; body width: 2.40-2.75; thorax width: 1.05-1.75; rostrum length: up to 0.70 ♂ and ♀.

Body. Integument white, with some parts dark sclerotised; moderately elongated. Elongated head protuberances (h-pr) present on head above eyes. Rostrum moderately elongated, on both sexes almost 2.8 × as long as wide and protruding mesocoxae. Pronotum trapezoidal 2 × as wide as long. Pronotal protuberances (p-pr) flattened, sclerotised, separated at bases. Mesonotum slightly narrower than metanotum. Abdominal segments I-VI almost identical in size, VII semicircular, segment VIII narrow, segment IX reduced. Abdominal segment VIII dorsally with rounded, well developed abdominal protuberance (a-pr). Urogomphi (ur) very short, ending with sclerotised, sharp apexes (Fig. 9A-C ).

Chaetotaxy. Well developed, setae medium to short. Head with one medium sos and one medium os. Rostrum with a single rs (Fig. 10A ). Pronotum with two as, two ls, single ds, and four pls, all equal in length. Dorsal parts of meso- and metathorax with three setae of various length, placed medially. Apex of femora with two long fes (Fig. 10A-C ). Abdominal segments I-VII dorsally with five setae dorsally, variable in length: first, second and fourth minute, third and fifth medium; first seta placed posteromedially, second to fourth placed close to posterior margin of the segment, fifth placed below stigma. Segment VII with four elongated setae dorsally, segment VIII with three elongated setae dorsally. Each lateral part of abdominal segments I-VIII with a single short seta. Ventral parts of abdominal segments I-VIII with four setae (first distinctly longer than other setae). Abdominal segment IX with three short setae ventrally (Fig. 10A-C ).

Remarks and comparative notes.

This species is one of the most common and widespread of the genus Rhinusa . It has been reported in all of Europe, Siberia, North Africa, the Middle East, central Asia, and northern India. It was accidentally introduced in North America, where it is currently distributed in several states of the USA and Canada ( O’Brien and Wibmer 1982; DiGirolomo et al. 2019). Although it was proposed as a potential candidate for the biological control of invasive common mullein, Verbascum thapsus , it was not used in North America in this regard. In the southern part of its area of distribution, it can be confused with other species of the group, such as R. verbasci (Rosenschoeld, 1838), R. moroderi (Reitter, 1906), and R. weilli Caldara, 2014, from which it can be separated only by the shape of the female rostrum, most easily if observed in lateral view ( Caldara 2014). It is well known that adults of R. tetra are highly variable in size (2.0-4.5 mm), even among specimens collected from the same plant. Additionally, the rostrum length of the female distinctly varies among populations living on different plants, probably related to oviposition inside seed capsules of different sizes and thicknesses, as suggested to occur in R. dieckmanni (Behne, 1988) ( Toševski et al. 2023).

Biological notes.

This species was reported to feed on several species of Verbascum . Caldara et al. (2012) verified the following plant associations: Verbascum blattaria , V. boerhavii L., V. creticum (L.) Cav., V. lychnitis L., V. nigrum , V. phlomoides , V. phoeniceum L., V. pulverulentum , V. speciosum Schrader, V. thapsiforme Schrader, and V. thapsus . Adults were also collected on Scrophularia auriculata L., S. canina L., and S. laevigata Vahl.

Marquess (2000) provides very detailed information on the biology of this species. Adults feed on the dorsal surfaces of leaves and on the seed capsules of the host plant. Once a plant flowers, mating occurs on the entire inflorescence. The female usually oviposits 1-3 eggs per seed capsule. Egg hatching occurs 7-11 days after deposition. Larvae feed on seeds within capsules and consume the majority of them. Pupation occurs within the seed capsule, and the emergence of the adults occurs ~ 25 days later. Adults exit the seed capsule by chewing through the hardened pericarp.

Rhinusa antirrhini group

Adult diagnosis. Rostrum in dorsal view from base to antennal insertion with a trapezoidal outline, with dorsal part narrower than ventral part, in male with distinctly visible scrobe; rostrum in lateral view with dorsal margin abruptly narrowed in apical part; first elytral interstria apically covered with dense vestiture.

Figures

Figure 6. Rhinusa tetra (Fabricius, 1792) mature larva A habitus B head, frontal view.

Figure 7. Rhinusa tetra (Fabricius, 1792) mature larva, head and mouth parts A head B antenna C clypeus and labrum (left side), epipharynx (right side) D left mandible E maxillolabial complex (schemes). Abbreviations: at-antenna, lr-labral rods, sb-sensillum basiconicum, Se-sensorium, st-stemmata, setae: als - anterolateral, ams - anteromedial, cls - clypeal, des - dorsal epicranial, dms - dorsal malar, fs - frontal epicranial, les - lateral epicranial, ligs - ligular, lrs - labral, mbs - malar basiventral, mds - mandibular dorsal, mes - medial, mpxs - maxillary palp, pes - postepicranial, pfs - palpiferal, pms - postmental, prms - premental, stps - stipital, ves - ventral, vms - ventral malar.

Figure 8. Rhinusa tetra (Fabricius, 1792) mature larva, habitus A lateral view of thoracic segments B lateral view of abdominal segment I C lateral view of abdominal segments VII-X (schemes). Abbreviations: Th. 1 - 3 - number of thoracic segments, Abd. 1 - 10 - number of abdominal seg, setae: as - alar, ds - dorsal, eps - epipleural, eus - eusternal, lsts - laterosternal, pda - pedal, pds - postdorsal, prns - pronotal, prs - prodorsal, ss - spiracular, ps - pleural, sts - sternal.

Figure 9. Rhinusa tetra (Fabricius, 1792) pupa habitus A ventral view B lateral view C dorsal view.

Figure 10. Rhinusa tetra (Fabricius, 1792) pupa habitus A ventral view B dorsal view C lateral view (schemes). Abbreviations: a-pr-abdominal protuberances, h-pr-head protuberances, p-pr-pronotal protuberances, ur-urogomphi, setae: as - apical, d - dorsal, ds - discal, fes - femoral, l, ls - lateral, os - orbital, pls - posterolateral, sos - supraorbital, rs - rostral, v - ventral.