Mosquitoes (Diptera, Culicidae) are amongst the most socioeconomically important animals in the world, with many species being vectors of disease-causing pathogens, including bacteria, helminths, protozoa and viruses. While not subjected to the same mosquito-borne disease burden as the tropics, Finland nonetheless has three known mosquito-borne viruses which cause disease in humans. Sindbis virus (Togaviridae: Alphavirus), which causes Pogosta disease, as well as Inkoo Virus and Möhkö strains of Chatanga virus (Peribunyaviridae: Orthobunyavirus). Three insect-specific flaviviruses have also been isolated from mosquitoes in Finland, Hanko, Ilomantsi and Lammi viruses (Flaviviridae: Flavivirus) as well as one Negevirus, Mekrijärvi negevirus.
Knowledge of the mosquitoes, however, had become outdated, with no significant records of mosquitoes published since 1979 when distribution maps for each of the 38 recorded species were created using historical and contemporary collections. Additionally, the viruses which have been isolated from mosquitoes in Finland have been found in pools of unidentified specimens, which means that no vector or invertebrate host species have been confirmed for five of the seven aforementioned viruses.
The aims were therefore to increase the knowledge of the composition and distribution of the Finnish mosquito fauna and the viruses associated with them. Adult and immature mosquito collections were made around the country in all 19 regions (corresponding to the European NUTS-3 divisions) between 2012–2018. The main collection efforts were made in 2014–2017 along transects in Lapland, southern Finland and the mainland of the Åland Archipelago (Ahvennanmaa). All specimens were morphologically identified, where possible, and stored in one of a variety of ways suitable for either virus cell culture experiments, virus PCR experiments, DNA studies or morphological studies.
Study I was the first to focus on the mosquitoes of the Åland Archipelago, which is situated between Finland and Sweden in the Baltic Sea. From the collections made on mainland (Fasta) Åland in late 2015 and across 2016, 12 new species records were observed. This included the first country record for Aedes geminus Peus and reconfirmation of Anopheles maculipennis s.s. Meigen and Ochlerotatus sticticus (Meigen) following their recent removal from the fauna of Finland. It also recorded what later became the only confirmed record of Dahliana geniculata (Olivier) in Finland. The Finnish mosquito fauna increased from 38 to 41 species.
Study II used DNA sequences extracted from specimens of the Anopheles maculipennis complex to identify which species are present in Finland. This resulted in the discovery of a new country record for Anopheles daciae Linton, Nicolescu & Harbach, a sibling species of An. messeae Falleroni. The Finnish mosquito fauna increased from 41 to 42 species.
Study III utilised records for all 52,466 specimens from 1,031 collections to create distribution maps for 40 of the 43 recorded species that were collected for this thesis. Specimens of Aedes rossicus Dolbeškin, Gorickaja & Mitrofanova, Culiseta subochrea (Edwards) and Ochlerotatus cyprius (Ludlow) were not collected, nor mapped, but have been recorded previously. These new collection data significantly extended the recorded distributions for several species, while other common species were still found across the country. Questions were raised about the presence of Ochlerotatus riparius, since only adult specimens were identified, which are easily confused with closely related species. This study provides a solid foundation for future studies to build upon.
Study IV explored the RNA viromes of nine man-biting Ochlerotatus species collected in Finland using next generation sequencing. In total, 514 viral polymerases were sequenced, which grouped into 159 species belonging to 25 families or equivalent taxonomic groups as follows: Aliusviridae (1), Aspiviridae (1), Botybirnavirus (8), Chrysoviridae (5), Chuviridae (14), Endornaviridae (2), Flaviviridae (9), Iflaviridae (17), Negevirus (41), Partitiviridae (55), Permutotetraviridae (6), Phasmaviridae (13), Phenuiviridae (58), Picornaviridae (5), Qinviridae (7), Quenyavirus (2), Rhabdoviridae (21), Sedoreoviridae (10), Solemoviridae (15), Spinareoviridae (1), Togaviridae (1), Totiviridae (205), Virgaviridae (7), Xinmoviridae (9) and Yueviridae (1). Twelve of these species have previously been described, while 147 were novel viruses. The host-vector associations of these viruses are yet to be established.
Overall, studies I–IV contribute a wealth of contemporary knowledge about the mosquitoes of Finland. More research is required to complete our understanding of mosquito distributions in the country and mosquito-virus associations and interactions, but this thesis provides a solid foundation upon which future research can now be built.