(Figs 100, 101) (https://doi.org/10.6084/m9.figshare.11405247)
Pamphilius masao Shinohara, 2005: 55; Shinohara & Hara, 2005: 275; Shinohara & Lelej, 2007: 937, 942; Taeger et al., 2010: 88; Sundukov & Lelej, 2012: 109; Sundukov, 2015: 250; Sundukov, 2017: 104; Shinohara, 2019: 10; Shinohara, 2020: 23, 253.
Material examined. About 3240 specimens, including the type series. Five specimens are from the Russian Far East (Shinohara 2005; present work). New collection data: RUSSIA: Sakhalin: 1♀, Matolozovo, 2–3. VII. 1991 (NSMT).
Distribution. Russia (Sakhalin), Japan (Hokkaido, Etorofu Is., Kunashiri Is., Shikotan Is.) (Shinohara 2005; Sundukov 2015).
Host plant. Betulaceae: Betula ermanii Cham., Alnus maximowiczii Call. (Shinohara 2005; Shinohara & Hara 2005).
Remarks. This species is a member of the P. vafer complex (Shinohara 2005) or the P. vafer subgroup of the P. vafer group (Shinohara 2002b), which is a complex of birch- or alder-feeding species having much in common morphologically and genetically (see also comments under P. alnivorus). The larva of P. masao is a solitary leaf-roller. The egg is laid on the upperside and the larval leaf-roll is also on the upperside of the leaf (Type III of Shinohara 2005). This character is known otherwise only for P. confusus and P. flavipectus from Japan (Shinohara & Kojima 2006).
Pamphilius masao is fairly common in montane birch forests in Hokkaido, Japan (2769 Japanese specimens recorded by National Museum of Nature and Science 2021), but only a few specimens were recorded from Sakhalin, Russia (Shinohara 2005).