Lasioptera Meigen, 1818

Lasioptera Meigen View in CoL , are polyphagous with a host range across several plant families ( Gagné & Jaschhof, 2017). In contrast, many cecidomyiid genera have a tight association with a single or a few plant genera (e.g. Yukawa et al., 2005; Gagné & Jaschhof, 2017). For example, 56 species of the genus Caryomyia Felt induce different sorts of galls only on the plant genus Carya Nutt. ( Juglandaceae ), 23 species of Celticecis Gagné on Celtis L. ( Cannabaceae ), ≥ 30 species of Daphnephila Kieffer on Machilus Nees ( Lauraceae ), 98 species of Haplodiplosis Rübsaamen on Salsola L. and allied genera ( Amaranthaceae ), 21 species of Hartigiola Rübsaamen on Fagus L. ( Fagaceae ), 17 species of Procontarinia Kieffer on Mangifera L. ( Anacardiaceae ), 80 species of Rabdophaga Westwood on Salix L. ( Salicaceae ), 253 species of Rhopalomyia Rübsaamen on Artemisia L. and some related genera ( Asteraceae ) and 26 species of Psectrosema Kieffer on Tamarix L. ( Tamaricaceae ) (e.g. Skuhravá, 1986; Yukawa et al., 2005; Gagné, 2008; Tokuda et al., 2008; Gagné & Moser, 2013; Gagné & Jaschhof, 2017). Such examples imply possible sequential evolution after their ancestor arrived at a specific host plant genus, after which it radiated.

The association of species of the genus Rabdophaga ( Diptera : Cecidomyiidae ) with Salix ( Salicaceae ) can been seen widely across the Holarctic Region ( Stelter, 1970, 1980; Nijveldt &Yukawa, 1982; Gagné & Jaschhof, 2017; Bowser et al., 2018). Rabdophaga rosaria (H. Loew, 1850) is one such species and has been known to induce rosette galls on terminal buds ( Fig. 1 View Figure 1 ) of ≥ 26 Salix species ( Table 1; Stelter, 1970; Nijveldt & Yukawa, 1982; Skuhravá et al., 2014; Bowser et al., 2018). Distribution records of these Salix species can be obtained from Newsholme (1992) and Argus (2004).

In recent years, we have been gathering specimens from rosette galls on Salix collected from various localities in the Holarctic Region, such as China, Georgia, Japan, Korea, The Netherlands, Poland, Russia, the UK and the USA (Alaska). Like many other gall-inducing cecidomyiids, R. rosaria has been identified mainly on the profile of its unique rosette-shaped galls on Salix species, because the appearance and structure of these galls are frequently specific to mono- or oligophagous gall inducers (e.g. Fukatsu et al., 1994; Stern, 1995; Stone & Cook, 1998; Nyman et al., 2011).

However, there are a few cases where different cecidomyiid species induce the same type of gall on the same host plant organ and species. For example, Orseolia oryzae (Wood-Mason, 1889) and Orseolia oryzivora Harris & Gagné, 1982 induce tube-like ‘silver shoot’ or ‘onion leaf ’ galls on rice, Oryza spp. ( Poaceae ), in tropical Asia and Africa, respectively ( Harris & Gagné, 1982). Asphondylia capsicicola Uechi, Yukawa & Tokuda, 2016 and Asphondylia gennadii (Marchal, 1904) induce pod galls on chili, Capsicum spp. ( Solanaceae ), in Southeast Asia and the Mediterranean, respectively ( Uechi et al., 2017). In these cases, we cannot identify gall inducers based solely on the shape of the gall and host plant information, particularly when species have a wide distributional range. We have been suspecting that R. rosaria is an example of the aforementioned case, because it is widely distributed across the Holarctic Region and induces galls on ≥ 26 Salix species, which belong to different subgenera. It is also worth examining whether the oligophagous host range, consisting of different plant subgenera, is related to the diversification of sibling species.

The purposes of this paper are as follows: (1) to ask whether all cecidomyiids that induce rosette galls on Salix species in the Holarctic Region are identical to R. rosaria , on the basis of DNA sequence data and morphological features; (2) to investigate whether the degree of DNA sequencing variation among insect populations is congruent with different host plants and localities; and (3) to construct a phylogenetic tree to evaluate whether clades of different populations are related to localities or sections of Salix . In the Discussion, we refer to possible diversification scenarios of R. rosaria along with host range expansions within a single plant genus.