Several studies highlight the importance of the Mediterranean basin as one of the thirty-four biodiversity hotspots in the world (Mittermeier et al., 2004; Fonseca, 2009; Hewitt, 2011), probably because it was a climatic refugia during the Quaternary glacial cycles (Rand, 1948). Consequently, the fauna and flora of the Iberian Peninsula are quite well studied, but paradoxically, Fontaine et al. (2012) remark that Old World, and because of that, Iberian Peninsula, can be considered as an important reservoir of unknown species. Despite the importance of describing new species and improving knowledge on the spatial distribution of existing species, these aspects are often forgotten. The spatial distribution of several species in Old World is often only partially assessed, based on museum or amateurs’ records, underestimating the real distribution of the species.

Cecidomyiidae is the second most species rich family of thread-horns (Diptera: Nematocera) in the world with particular importance in the Iberia where 292 species (out of the total 6200 reported species) occur (Gagné & Jaschhof, 2014; Carles-Tolrá, 2002; Sánchez et al., 2012; Sánchez, 2016). This family is mainly characterized by the establishment of a highly specific trophic relationship between a fly and a plant species: the induction of a gall. This type of trophic specialism can be considered as an intermediate situation between parasitism and herbivory (Redfern & Shirley, 2011) because the gall is a reaction of the host-plant to the oviposition by the galler insect in their inner tissues. This reaction causes the growth of an abnormal plant structure that results from hypertrophy followed by hyperplasia of the vegetal cells, providing a nutritive tissue that feeds the galler larvae; simultaneously, the insect larvae induces a physiological cascade to promote the growth of the nutritional tissue by the host-plant (Nieves-Aldrey, 1998). In many cases, the specificity of the relationship is so high that the gall is considered as the extended phenotype of the inductor genes (Stern, 1995; Crespi et al., 1997), allowing possible to identify the galler insect only by the presence of the gall and by its morphology. Some species within this family are of concern for agriculture because they are considered pests of several crops (i.e. Contarinia nasturtii (Keiffer, 1888) on broccoli, Stenodiplosis sorghicola (Coquillett, 1899) on sorghum, Contarinia maculipennis Felt, 1933 on orchids, etc.). On the other hand, the high specificity between gallers and host-plants supports the use of gallers as biological control agents against invasive plant species, decreasing the probability of unwanted direct non-target effects on non-target species (Pearson & Callaway, 2003; Thomas et al., 2004; López-Núñez et al., 2017).

The genus Cystiphora Kieffer, 1892 belongs to Cecidomyiidae and includes six species in Europe, three of those recorded in Spain: C. sanguinea (Bremi, 1847), C. schmidti (Rübsaamen, 1914) and C. sonchi (Vallot, 1827). All members of this genus are gall inductors on the leaves of several species of Cichorieae (Asteraceae) and some of them were introduced into Nearctic and Australasian Regions as biological control agents for introduced Asteraceae weeds (Carles-Tolrá, 2002; Skuhravá & Skuhravý, 2009; Gagné & Jaschhof, 2014); for example: C. sonchi and C. taraxaci (Kieffer, 1888) were introduced into Canada to control Sonchus arvensis L. and Taraxacum officinale (L.) Weber ex F.H. Wigg respectively; and C. schmidti was introduced in Australia, California and Argentina against Chondrilla juncea L. (Gagné & Jaschhof, 2014).

The midge C. sonchi is a leaf-gall inductor in species of sow-thistle (Sonchus arvensis L., S. asper (L.) Hill, S. oleraceus L., S. maritimus L. and S. tenerrimus L.), and in sporadic situations in bulbous hawksbeard (Aetheorhiza bulbosa Cass.) (Peschken, 1982; Peschken et al., 1989). This is the single gall species induced by a midge described in sow-thistle; the gall is a dark red blotch with about 5 mm in diameter, located on the leaves. The larvae feed between the upper and lower epidermis of the leaf. In many cases, one leaf can harbor several blotch galls, distributed in rows throughout the leaf length. Depending on the season, the larvae can pupate in the leaf (in spring generations) or in the ground (in winter generations); the adults emerge and after mating females lay the eggs on the underside of mature leaves, avoiding the youngest and oldest leaves (Peschken, 1982). This species was released for the first time in Canada, from 1981 to 2008, as a biological control agent against S. arvensis, which was accidentally introduced with imported seeds of fodder, promoting losses in crops production (Peschken, 1982). The occurrence of this species is mostly scarce, with some records of locally abundant, but it has an Euro-Siberian distribution, being recorded in almost 30 European and in five Asian countries (Peschken, 1982; M. Skuhravá, comm. pers., 2017). The presence of C. sonchi in Iberian Peninsula is only confirmed in Spain (Skuhravá et al., 2006) on three localities in the Basque Country (in 1901), Catalonia (in 1902) and in Balearic Islands (in 1997; Fig. 1) (Trotter, 1902; Cogolludo, 1921; Skuhravá & Skuhravý, 2004; Blanes-Dalmau et al., 2017). From Portugal there is only one dubious cite in Tavares (1907) pag. 129: “[…] Cecidomyia, probably new […]” in S. oleraceus; but this material was not preserved and is not available to be confirmed.

We report for the first time C. sonchi for Portugal mainland, expanding its known distribution to the further west corner of Europe. Three populations were spotted in Coimbra (Portugal) (Figs. 1-2):

Assuming that the three records of C. sonchi in Spain and the new records now confirmed in Portugal were most probably not representative of the real distribution of the species in the Iberian Peninsula, we did an additional search to increase distribution records. A systematic search was performed through the Global Biodiversity Information Facility (GBIF) (www.gbif.org) and a set of different citizen science biodiversity databases: iNaturalist (www.inaturalist.org), Naturdata (naturdata.com), Biodiversity4all (www.biodiversity4all.org) and Biodiversidad Virtual (BV; www.biodiversidadvirtual.org). iNaturalist and GBIF are global biodiversity databases, while the others are developed and focused in Portugal (Naturdata and Biodiversity4all) and in Spain (BV). In these databases, any registered citizen can upload a photograph of a species and a community of specialists can either identify the species or validate the identification made by the citizen. Although such citizen science database can carry some identification uncertainty by lacking of specimens to examine (Gardiner et al., 2012), in this case the clear morphology of the gall and its host-plant specificity confer a high confidence to the identification based on photographs. Records were only found in GBIF and in BV, in total of eight more records for Spain, two of them being a novelty for Spain and the rest contributed to extend its distribution area (Table 1, Fig. 1A).

Concluding, the three new populations in Portugal together with the new records in Spain expand the distribution of C. sonchi in the Iberian Peninsula, from the north-west (A Coruña, Spain) to the west (Coimbra, Portugal) and further south (Seville, Spain).