Diagnosis. Adults are larger, have darker forewing and more distinctly purple wing apex than those of S. magdalenae. The male genitalia of S. nylandriella, S. oxyacanthella and S. pyri closely resemble each other and are somewhat variable. Although there is some overlap, nylandriella can usually be separated from oxyacanthella by the longer distal process of valva and the slightly longer aedeagus if compared with the length of genital capsule. S. pyri has larger genitalia, slightly longer and thinner distal process on valva, usually more pointed inner lobe and more prominent protuberances on uncus. The female of oxyacanthella has a smaller accessory sac than nylandriella and pyri a more strongly folded and less arcuate accessory sac. However, in most cases, these three species can easily be separated by external characters.
Male. Wingspan: 4.6-4.9 mm. Head: frontal tuft ochreous yellow to yellowish brown; collar and scape white to yellowish white; antenna approximately half as long as forewing. Forewing: glossy olive grey or bronzy brown with faint purple tinge; darkest and with more distinct purple tinge in apical third; terminal cilia concolorous but paler at tips. Hindwing: greyish brown; without androconial scales. Abdomen fuscous.
Female. Wingspan: 4.6-5.0 mm. Head: antenna slightly shorter than half length of forewing. Otherwise similar to male. Male genitalia. Vinculum with small, widely separated lateral corners. Uncus with small, widely separated, latero-posterior protuberances. Gnathos variable, usually with long transverse bar, short to very short horns and short, pointed anterior process. Valva with moderately long distal process and rounded inner lobe; distal process extending distinctly farther posteriorly than uncus. Aedeagus 1.1-1.3 times length of genital capsule; cornuti varying from half-circular, obtuse to sharply pointed. Genital capsule wide.
Female genitalia. Bursa copulatrix large, occupying at least two-thirds length of abdomen. Accessory sac large, arcuate, and sometimes convoluted distally; reticulate field distinct. Ductus spermathecae longer than bursa copulatrix.
Host plant: usually on Sorbus aucuparia but also found on the closely related S. domestica in botanic gardens in Hamburg (Buhr, 1937), and in Finland occasionally on Malus. Egg: on leaf underside, away from rib. Larva: green. Mine : a narrow, much contorted gallery, often following leaf margin, or a contorted gallery following the ribs, occasionally the midrib; early mine with thin linear frass, which usually soon becomes thicker; thereafter often with broad dispersed or coiled frass, leaving narrow clear margins; infrequently frass remains linear throughout. Mines with linear frass can be distinguished from S. magdalenae by longer and wider gallery, usually wider than 1.5 mm. It seems impossible to separate vacated mines with coiled frass from those of S. oxyacanthella, which occasionally feeds on the same foodplant.
Widely distributed in Denmark and Sweden up to Upl. Norway: østfold. From almost all districts in Finland. - In west and central Europe (Schoorl et al., 1985).
Voltinism: univoltine in northern part of its range. Larvae feeding from June until August, commonest in first half of July. These larvae usually give rise to adults in May and June of the next year, but in England and The Netherlands there appears to be a small second generation with larvae in September and October.
Description based on Johansson and Nielsen (1990)