Culex (Culex) pipiens Linnaeus

subspecies pallens Coquillett, 1898 —original combination: Culex pallens (subspecific status by Tanaka 2004). Distribution: China, Japan, Mexico, South Korea, United States (continental) (Wilkerson et al. 2021).

subspecies pipiens Linnaeus, 1758 —original combination: Culex pipiens. Distribution: Temperate regions of Africa, Asia, Australia, Europe and North and South America (for specific country records, see Wilkerson et al. 2021).

Without question, the taxonomic status of Cx. pipiens and its allied forms has received more attention than any other culicine taxon. As pointed out in a review of the taxonomic history of the species (Harbach 2012), the essence of decades of work conducted on Cx. pipiens was to determine whether it is a single polytypic species or a species complex. As a starting point, Edwards (1932a) listed Cx. pipiens and its sister species Cx. quinquefasciatus Say, 1823 (as Cx. fatigans Wiedemann, 1828) as separate species, and Cx. pallens as a variety of the former. In a review of the systematics of these nominal species, and taking into consideration evidence of hybridization in areas where the distributions of Cx. pipiens and Cx. quinquefasciatus overlap, Mattingly et al. (1951) and Mattingly (1967) concluded that they should be considered members of a single polymorphic species comprised of subspecies pipiens and quinquefasciatus and several varieties, with pallens being one of them. Beginning with Belkin (1962), the recognition of Cx. quinquefasciatus as a separate species gradually gained acceptance, resulting in the current recognition of Cx. pipiens and Cx. quinquefasciatus as closely related sister species and pallens as a subspecies of the former species (Smith & Fonseca 2004; Harbach 2012; Fonseca et al. 2009; Aardema et al. 2020; Wilkerson et al. 2021).

Culex pipiens pallens has been regarded as a subspecies since the studies of Japanese mosquitoes by Tanaka et al. (1979) and Tanaka (2004). Since then, genetic and molecular studies have shed light on the status of pallens in eastern Asia. A multilocus genotype analysis conducted by Fonseca et al. (2009) revealed the occurrence of hybridization between Cx. p. pallens and Cx. quinquefasciatus in China, South Korea and southern Japan, but not in northern Japan. However, Ohashi et al. (2014) found that Cx. p. pallens and Cx. pipiens hybridize in northern Japan, but hybrids have lower fitness than the offspring of either parental taxon. Finally, recent detailed analyses of DNA sequence data conducted by Aardema et al. (2020), which included sequence for specimens of pallens from China, cogently support the hypothesis that pallens is a genetically distinct entity derived from ancestral hybridization between Cx. pipiens and Cx. quinquefasciatus. The authors note, however, that the “hypothesis warrants further examination.” The presence of pallens in Mexico and the United States must also be assessed. As indicated by Mattingly et al. (1951), some authors have conjectured that the pallens form was introduced into California, presumably from Japan. Despite the concerns and implications of hybridization, we feel it is prudent at this time to accept the evidence for the genetic distinction of pallens provided by Aardema et al. (2020) and hereby re-establish its original species status pending further investigation: Culex (Culex) pallens Coquillett, 1898.

It is interesting to note that Cx. pipiens and Cx. quinquefasciatus are listed as species in the Encyclopedia of Life, but Cx. pallens is not included; thus, it needs to be included with the Culex species listed therein. Oddly, the nomen dubium Culex molestus Kollar, 1832 (in Pohl & Kollar 1832) is listed as a species, probably mistakenly included instead of Cx. molestus Forskål, 1775, a recognized synonym and physiological variant (molestus form) of Cx. pipiens. The list of mosquitoes in the Encyclopedia of Life was not compiled by mosquito taxonomic experts and does not, in many cases, reflect the current taxonomy of various taxa.

Thirty-six nominal taxa (disregarding the name melanorhinus Giles, 1900, which was proposed as a replacement name for Cx. pallipes Macquart, 1838) are currently recognized as synonyms of Cx. pipiens. We consider all synonymous taxa named from localities in Europe (England, France, Germany and Portugal), northern Africa (Algeria, Egypt and Tunisia), Mexico and the United States to be synonyms of Cx. pipiens. These include all of the synonyms listed by Harbach (2018) and Wilkerson et al. (2021), as well as Cx. comitatus Dyar & Knab, 1909a (California) and Cx. quinquefasciatus race dipseticus Dyar & Knab, 1909a (western Mexico and California), which were listed as synonyms of Cx. pipiens pallens Coquillett, 1898. Only a single nominal species, Cx. osakaensis Theobald, 1907 (type locality: Osaka, Honshu Island, Japan), is retained as a synonym of Cx. pallens.