CULEX QUINQUEFASCIATUS IN UTAH

The Culex pipiens complex is worldwide in distribution. It consists primarily of two subspecies Culex pipiens pipiens and Culex pipiens quinquefasciatus. They are taxonomically recognized as subspecies due to the fact that they normally occupy different distributions, pipiens occurring in the more northern and more southern temperate regions of the world and quinquefasciatus occurring primarily in tropical and warm temperate regions. When their distribution overlaps they readily interbreed to form viable hybrids. However, for convenience they are usually referred to without the subspecies designation as they will be here.

Barr (1957) reported that in the United States that only pipiens is usually found north of 39°N and only quinquefasciatus is generally present south of 36°N. When their ranges overlap in the intermediate areas between 36°N and 39°N they readily interbreed to form hybrids.

Taxonomically they are difficult to separate reliably as females or larvae. Males can be separated by structures in the male genitalia, the dorsal and ventral arms of the phallosome. (Fig. 1, Barr, 1957). In quinquefasciatus the dorsal arms extend in a parallel fashion terminating in a narrowed point. The ventral arms flare out and are distinctly separated from the terminal part of the dorsal arms at their intersection. This distance is known as the DV measurement. In pipiens the dorsal arms also flare out laterally to a blunt point and overlap the tips of the ventral arm so there is virtually no DV measurement. Hybrids show an intermediate condition (Fig. 1). Separation of the larvae is generally not reliable although the siphon of quinquefasciatus is usually more inflated basally than pipiens which tends to be more slender and less tapered, apically. In the female there are wing and abdominal characters that are useful. The wing character is the ratio of the length of cell R₂ divided by the length of its petiole. R₂₊₃ is usually longer and cell R₂ is shorter than in pipiens. Linam and Nielsen (1962, Fig. 2) studied this ratio. Examined were 95 pipiens females, 9 of which were from outside of the United States and 58 specimens of quinquefasciatus, 12 of which were from outside of the United States. Their results indicated that 81% of the quinquefasciatus specimens had a wing ratio between 2.6 and 3.6, with 90% below 3.6. In pipiens almost 95% had wing ratios in excess of 3.6. Linam and Nielsen also examined the abdominal bands on all of these specimens on abdominal tergites of segments III and IV. 91 of 95 pipiens specimens had unbroken basal bands extending to the lateral margin. In contrast, 56 of 58 specimens of quinquefasciatus had broken or narrowed bands laterally on tergites III and IV.

Culex quinquefasciatus in Utah

Rosay and Nielsen (1973) studied 233 females and 176 males from the Salt Lake area, 41°N. Wing and abdominal banding characters were examined for the females, and the male genitalia were studied using the DV/D ratio where D was the measured distance between the tips of the dorsal arms of the phallosome and DV was the average of the distances from the outside of the dorsal arms to the extreme tips of the ventral arms. A ratio of 0.2 or less was indicative of pipiens with 0.6 or more characteristics of quinquefasciatus. The ratio of all males ranged from 0.0 to 1.5. Using the DV/D ratio 3.4% were determined to be quinquefasciatus. 90.9% considered pipiens and 5.7% intermediate. A small series of the pipiens complex, 8 males and 5 females were reared from the St. George area in Washington County. The DV/D ratio in these specimens range from 0.6 to 1.4 with an average of 0.8. These all appeared to be quinquefasciatus. All of these females also had the indented or broken basal pale bands on abdominal tergites indicative of quinquefasciatus.

Because most of 233 females from the Salt Lake area were from light traps there was considerable damage to the abdominal scaling and this character was not considered reliable in these specimens. Wing measurements, however, were made. The ratios ranged from 2.8 to 10.0. Using the ratio indicated that 3.9% were quinquefasciatus, 55.4% were pipiens and 40.8% were intermediate.

Discussion and Conclusions

Based on the above studies, all of the mosquitoes from St. George were quinquefasciatus. Of the Salt Lake determinations, 3-4% of both males and females were quinquefasciatus. Barr's study in 1957 of the pipiens complex in North America included 41 Culex pipiens complex males from Salt Lake County. Only one of these was quinquefasciatus. At that time this represented the northern most record for quinquefasciatus at 41°N. I have recently re-examined the Utah collection of the pipiens complex and have found additional records. The first collection of quinquefasciatus males in St. George was
in October of 1938, the most recent, September 14, 1967. In the Salt Lake City area the first quinquefasciatus male was collected on August 26, 1940 with additional collections August 17 and September 21 of 1972. Males determined to be intermediate were collected on September 30, 1963 and August 7, 17, 20 and September 12, 1972.

Based on Barr's work it would appear that quinquefasciatus cannot normally survive winters north of 36°N although during the warm months it moves north to about the 39°N, interbreeding in the intermediate areas to form hybrids. As collections of quinquefasciatus and intermediates have only been collected in late summer in Utah it would appear that these forms which are absent in spring and early summer must move from the southern areas into northern areas each year. Unfortunately, in Utah no data on the distribution of the pipiens complex is available between St. George in Washington County and Utah County in the north. Thus we do not have data to support this hypothesis. Also, we do not know whether this apparent invasion of quinquefasciatus occurs on a yearly basis or occurs sporadically depending on climatic conditions. More data is obviously needed and further studies are underway.

References Cited

Barr, A. R. 1957. The distribution of Culex p. pipiens and C. p. quinquefasciatus in North America. Amer. J. Trop. Med. Hyg. 6:153-65.

Linam, J. H. and L. T. Nielsen. 1962. Notes on the taxonomic separation of adult females of Culex pipiens L. and Culex quinquefasciatus Say. Mosq. News 22:390-93.

Rosay, B. and L. T. Nielsen. 1973. Proc. 26^th Ann. Meeting of the Utah Mosq. Abate. Assoc. pp. 28-31.