A report on the mosquitoes of mainland Åland, southwestern Finland and revised list of Finnish mosquitoes.

To successfully implement surveillance or control strategies for mosquitoes, up-to-date knowledge of regional species composition is vital. The last report regarding mosquitoes (Diptera: Culicidae) in the Aland archipelago, southwestern Finland listed 19 species (Utrio, 1979). To determine the current species diversity, one collection trip was made to mainland Aland in 2015 and three in 2016. Mosquitoes (n = 3286) were collected as both adult and immature life stages from 88 collections within 29 1-km(2) areas. Fifteen of the 19 previously reported species were obtained, leaving the current status of four species uncertain. At least 11 species previously not reported from Aland, but confirmed on the Finnish mainland, were collected. Aedes geminus Peus was identified based on examination of the gonostylus, and represents a new species distribution for Finland. Anopheles maculipennis s.s. Meigen was confirmed from cytochrome c oxidase subunit I (COI) and internal transcribed spacer 2 (ITS2) sequence data and is reinstated on the list of Finnish species, along with Ochlerotatus sticticus (Meigen). Dahliana geniculata (Olivier) was found in two locations, in 2 months, indicating that there is an established population in Aland. The present data confirm that at least 27 species inhabit mainland Aland, rising to 31 when historical data are included. The Finnish mosquito fauna is increased from 38 to 41 species.