Lyonetia ledi, Wocke, 1859

Huemer, Peter & Schmid, Juerg, 2021, Relict populations of Lyonetia ledi Wocke, 1859 (Lepidoptera, Lyonetiidae) from the Alps indicate postglacial host-plant shift to the famous Alpenrose (Rhododendron ferrugineum L.), Alpine Entomology 5, pp. 101-106 : 101

Alpine population of Lyonetia ledi

Material examined. 1♂: Switzerland, Graubünden, Ardez, SE Sur En , 1760 m, 46°45'38"N, 10°11'11.7"E, 6.8.2021 ex larva ( Rhododendron ferrugineum ), leg. Huemer ; 3♂: same data, but DNA Barcode TLMF 30911 , DNA Barcode TLMF 30912 , DNA Barcode TLMF 30913 ; 4♂: same data, but 21.8.2021 ex pupae, all leg Huemer coll. TLMF. 11♂, 6♀ same locality, 8.2021 e.l., e.p. leg. et coll. JS .

Adult (Figs 2 , 3 ). Head, tuft, and eye-cap as well as thorax glossy white; labial palpus white with some fuscous on outer surface; antenna about length of forewing, pale grey. Wingspan c. 7-9 mm; forewings glossy white; short oblique dark brown streak from tornus at half-length of wing, longitudinal patch at about 2/3, two dorsal streaks from tornus converging in disc at 3/4; large ochre-brown patch in distal fifth; costal ciliae with four blackish strigulae, large black apical dot followed by black line and apical scale-pencil. Hindwings grey. The variation of forewing markings is considerable, i.e. a short sub-basal streak below fold at about 1/5 or a complete medial streak as figured by Bengtsson and Johansson (2011) could not be observed, whereas the medial and postmedial markings are completely reduced in one specimen.

For exhaustive description of the adults including genitalia of both sexes see Bengtsson and Johansson (2011).

Biology. In Europe, Lyonetia ledi is a widespread leaf-miner of Rhododendron tomentosum ( Ericaceae), but in the northern part of the continent it is also regularly recorded from the unrelated Myrica gale ( Myricaceae), a species absent from large parts of Central Europe. Larvae of the newly discovered population from the Engadine mine the leaves of R. ferrugineum . The egg is laid on the upper side of a leaf. The tunnel-mine initially extends towards the base of the leaf, then turns and continues alongside the leaf rim towards the leaf tip, where a spacious blotch mine is formed. Only current year leaves, recognizable by their green underside are infested, while older leaves with the plant's name-giving rusty underside may contain mines from previous years only (Figs 4 , 5 ). The mine is hardly visible on the underside. Frass is firstly deposited in continuous line which later fills the complete tunnel-mine, whereas it is deposited in irregular flawy patches in the botch-mine. According to Kuroko (1964) frass may also be ejected through semicircular lateral slits along the border of the blotch mine on the lower side. The same author also reports the larva moving to a new leaf and starting to produce a new mine, an observation unconfirmed in alpine habitats. The final instar larva is light yellow with a light brown head and a brownish mottled thoracic shield and ca. 4.8 mm long (Fig. 6 ). On the underside of a nearby leaf, it constructs a X-shaped silken scaffold in the center of which it then pupates like in a hammock (Figs 7 , 8 ). The ca. 4.4 mm long pupa can be easily found when examining the underside of mined or adjacent leaves. First larvae were detected on the 29th of July 2021, two weeks later on the 12th of August 2021 only pupae were found. In captivity the adults emerged after about a week to 10 days between 6th and 21st of August. These data suggest hibernation of the adults as it is also reported from Sweden, with a flight period lasting from mid-August to October and again from April to May ( Bengtsson and Johansson 2011).

The larvae seem to be regularly infested by parasitic wasps of Diadegma cf. semiclausum , ( Ichmeumonidae ) (barcoded) and an unidentified species of Ichneumonidae as we found a number of their cocoons that were already empty.

Habitat (Fig. 9 ). Lyonetia ledi is considered as a tyrphobiontic species ( Spitzer et al. 1996) because it is restricted to peat bogs. The Swiss habitat is completely different and can be characterized as a northern exposed subalpine Larici-Piceetum plant-association dominated by Larix decidua Mill. and Picea abies (L.) H. Karst and interspersed Pinus cembra L., Betula pendula Roth and Alnus alnobetula (Ehrh.) K. Koch are also present in the adjacent area. This biotope is located in a north-facing, steep avalanche gully at the bottom of which remaining snow masses may persist into early summer and provide unique microclimatic conditions. Furthermore, the most infested Rhododendron shrubs are often rather puny specimens, growing in very shady places and thus unable to produce flowers.

Figures

Figure 6. Final instar larva of Lyonetia ledi on Rhododendron ferrugineum (Switzerland, Graubuenden, Ardez).

Figure 9. Habitat of Lyonetia ledi in Engadine / Switzerland with Rhododendron ferrugineum.

Figures 2, 3. Lyonetia ledi adults resting on leaves of Rhododendron ferrugineum (Switzerland, Graubuenden, Ardez).

Figures 4, 5. Leaf-mines of Lyonetia ledi on Rhododendron ferrugineum (Switzerland, Graubuenden, Ardez).

Figures 7, 8. Characteristic cocoon with final instar larva and pupa of Lyonetia ledi on Rhododendron ferrugineum (Switzerland, Graubuenden, Ardez).