Aedes (Ochlerotatus) spencerii (Theobald)

Harbach, Ralph E. & Wilkerson, Richard C., 2023, The insupportable validity of mosquito subspecies (Diptera: Culicidae) and their exclusion from culicid classification, Zootaxa 5303 (1), pp. 1-184 : 42-43

Aedes (Ochlerotatus) spencerii (Theobald)

subspecies idahoensis ( Theobald, 1903a) —original combination: Grabhamia spencerii var. idahoensis (subspecific status by Nielsen & Rees 1959). Distribution: Canada, United States (continental) ( Wilkerson et al. 2021).

subspecies spencerii ( Theobald, 1901c) —original combination: Culex spencerii . Distribution: Canada, Mexico, United States (continental) ( Wilkerson et al. 2021).

Aedes spencerii was described, originally as a species of Culex , from four adult females collected in Manitoba Province of Canada, two from Stony Mountain, the type locality based on lectotype selection by Belkin (1968), and two from St. Boniface ( Theobald 1901c). Theobald provided a color illustration of the adult female in dorsal aspect ( Theobald 1901a [plates]): fig. 104, pl. XXVI), which shows the pattern of dark tergal scaling typical of subspecies spencerii : “Abdomen covered with creamy and white scales, with large, black-scaled patches on each side of the middle line” ( Theobald 1901c).

Subspecies idahoensis was originally described as a variety of spencerii based on an unspecified number of females collected at Market Lake, Idaho, United States ( Theobald 1903a); however, Townson (1990) reported the presence of two syntype females in the collection of the Natural History Museum, London. Theobald indicated that idahoensis only differed from the typical form in “abdominal ornamentation”, which he described as “Abdominal basal bands almost white, broad apical bands yellowish-white to white, very narrow, almost obliterated on some segments; the abdomen not pale at the sides, so that only broad basal and narrow apical pale areas are shown, the two apical segments are mostly pale scaled, and there are scattered pale scales on the others.” In contrast, idahoensis has traditionally been characterized as having dark-scaled abdominal terga with white transverse basal bands (see references listed below).

Seven COI sequences for Ae. spencerii are registered in GenBank, three from Saskatchewan Province, Canada (KR444402, 596 bp; KR443924, 551 bp; and KR440309, 596 bp); two from Manitoba Province, Canada (KF535006, 658 bp and KF535007, 658 bp); one from Utah State, USA (JX260472, 213 bp); and one from Colorado, USA (JX259672, 658 bp). BLAST comparison of sequences revealed that two of the sequences from Saskatchewan (KR444402 and KR440309) and the sequence from Colorado (JX259672) are identical, and the 551 bp of the third sequence from Saskatchewan (KR443924) is 99.82% similar to the corresponding segments of those three sequences. Additionally, a 589-bp sequence from Saskatchewan identified as “ Aedes sp. BOLD-2016” is 98.13% similar. The sequence from Utah (JX260472, 213 bp) was too short to obtain a meaningful comparison, and the two sequences from Manitoba were apparently obtained from misidentified specimens: KF535006 is identical with 11 COI sequences of Ae. dorsalis and KF535007 is 99.39% similar to a COI sequence from Alberta (KP047935) identified as “ Culicidae sp. BOLD:AAD7982”. No DNA sequences are available for specimens identified as subspecies idahoensis .

Ward (1984, 1992) and Wilkerson et al. (2021) were apparently not aware that Wood et al. (1979) had synonymized idahoensis with spencerii , with the following explanation: “The status of idahoensis has not been satisfactorily resolved. A few females in the Canadian National Collection have the characteristic abdominal pattern of idahoensis , but there are no males or larvae in the collection. According to Carpenter and LaCasse (1955), larvae of idahoensis have comb scales (13–29 instead of 7–13 in spencerii ), and each scale has a less pronounced apical spine. Males of the two species were not separated. Nielsen and Rees (1959) found ‘almost every degree of variation’ between the typical female abdominal pattern of idahoensis and that of spencerii in Utah, Montana, and Wyoming. Furthermore, they cited three other authors who described similar variations in spencerii . They concluded that idahoensis was a more southerly subspecies of spencerii and that a zone of integration occurred in the three states previously mentioned. Until detailed biological or genetical studies resolve the status of idahoensis , we have no choice but to consider it a synonym of spencerii .”

Darsie & Ward (1981, 2005) continued to recognize the two nominal forms as subspecies, stating in 1981: “Following Nielsen and Rees (338) [number of the article in the list of references cited] we recognize two subspecies under Ae. spencerii (Theobald) , the typical subspecies which inhabits the central plains of North America, and subspecies idahoensis (Theobald) , a more westerly and northwesterly form, also reported from southern British Columbia (135).” Based on published records, Darsie & Ward showed that the distributions of the two subspecies overlap in Montana, Nebraska, North Dakota, South Dakota and Wyoming. They indicated that only idahoensis was known to occur in Colorado; thus, the COI sequence in GenBank (JX259672) of the specimen from Colorado identified as spencerii sensu stricto suggests that either both subspecies occur in the state or idahoensis is conspecific with the typical form that occurs in Canada. More specimens and study are needed to resolve this question.

It is interesting to note that idahoensis , originally described as a variety of spencerii , has been regarded as a distinct species ( Dyar & Knab 1908; Dyar 1917b; Dyar 1928; Edwards 1932a; Gjullin 1937; Rees 1943; Yamaguti & LaCasse 1951 d; Carpenter & LaCasse 1955), a synonym ( Pratt 1956; Wood et al. 1979) and a subspecies ( Nielsen & Rees 1959; Smith 1966; Myers 1967; Darsie & Ward 1981, 2005). In as much as idahoensis is apparently morphologically distinct in the adult and larval stages ( Carpenter & LaCasse 1955; Darsie & Ward 1981, 2005) in areas southward and westward of an area of overlap or a zone of introgression with spencerii in Montana, Nebraska, North Dakota, South Dakota and Wyoming, we believe it is prudent to recognize it as a separate species, pending a comprehensive morphological and genetic analysis of specimens from throughout the distributions of the two forms: Aedes (Ochlerotatus) idahoensis ( Theobald, 1903a) . Aedes idahoensis is currently listed as a species in the Encyclopedia of Life.