Wild species 2010: chapter 18

Insects: Mosquitoes

Culicidae - A family of slender, long-legged flies, belonging to the insect order Diptera (the “true flies”). Their bodies are covered in scales, the colour and position of which are used to distinguish species. They are notorious for the females’ blood-feeding habit which is required to mature their eggs. The immature stages (eggs, larvae and pupae) are generally found in standing water.

Photo of a Permanent Marsh Mosquito
Photo: Permanent Marsh Mosquito, Anopheles walkeri © Tom Murray

Quick facts

  • There are about 3000 species of mosquitoes in the world, of which 80 live in Canada.
  • When excluding species ranked as Extinct, Extirpated, Undetermined, Not Assessed, Exotic or Accidental, the great majority (95%) of mosquitoes in Canada have Canada General Status Ranks (Canada ranks) of Secure, while 5% have Canada ranks of Sensitive.
  • A total of three species of mosquitoes are Exotic in Canada.
  • Mosquitoes first appear in the fossil record during the Mid-Cretaceous (about 89-99 million years ago) and remains are often found in more recent amber deposits. A blood-fed mosquito trapped in amber figured in the movie Jurassic Park, but that individual would predate the first known fossil!
  • Mosquitoes transmit a number of diseases to humans including malaria, arboviral encephalitis, yellow fever, and dengue, and they are also vectors of disease to livestock and wild animals.
  • Although only females blood-feed, both males and females require sugar meals, which they generally get from floral nectar. In the process, mosquitoes can serve as pollinators – in the Arctic, they are important pollinators of orchids.
  • Mosquito larvae are an important source of food for other aquatic organisms including fish, larval dragonflies, and other invertebrates. Adult mosquitoes are a source of food for insectivorous birds, bats, and other animals.
  • Not all species require a blood meal – some are able to carry over enough nutrients from their larval stage to develop a batch of eggs without having to take a blood meal. This is common in northern species.

Background

Mosquitoes, like all true flies, undergo complete metamorphosis: egg, larva, pupa, and adult. The first three stages are aquatic, whereas the adults are aerial or terrestrial.

Mosquitoes are often categorized according to the type of larval habitat in which they develop: containers (rain buckets, tires, tree holes), transient standing water (flooded areas, snowmelt pools, ditches), permanent standing water (ponds, swamps, marshes) and even slow moving streams. Each larval habitat is exploited by a particular set of mosquito species. For example, species capable of breeding in transient flooded areas generally have drought-tolerant eggs and may even require a drying out period before eggs will hatch.

Blood-feeding behaviours can also differ among species. Some species are able to use energy reserves gained during the larval stage to develop their first batch of eggs, and a few species are able to do this throughout their adult lives (e.g., Wyeomyia smithii and Toxorhynchites rutilis). However, in most species, the females must take a blood-meal in order to develop eggs. Some species blood-feed preferentially on amphibians and reptiles, whereas others specialize on birds or mammals. There are no species in Canada that are specialist human-feeders, but several of the more generalist feeders will include humans as a blood-meal source if humans are available.

Mosquitoes have been the target of many control programs over the years due to their role in disease transmission. Many Anopheles species are vectors of human malaria, whereas Culex species tend to carry enzootic arboviruses such as West Nile virus (WNV), St. Louis encephalitis, and Eastern Equine encephalitis (EEE). Historically, Anopheles quadrimaculatus was a vector of human malaria in southern Ontario (Fisk, 1931), but with improved sanitation and the use of window screens, endemic malaria transmission in Canada has disappeared.

Status of knowledge

Despite the fact that mosquitoes have attracted some attention, the life history, distribution, and habitats requirements of many Canadian mosquito species are not well known. While many people are familiar with the nuisance species or those that transmit disease, the majority of species do not feed on humans at all. Many of these species are rarely collected and their life history and larval breeding habitats are poorly understood, making assessment of their status in Canada difficult.

Because of the recent introduction of West Nile virus to Canada in 2001, some provinces have initiated mosquito surveillance programs, which have contributed greatly to our knowledge of species distribution and abundance in those areas. The last major mosquito survey occurred during the late 1970s after an outbreak of St. Louis encephalitis virus and resulted in the publication of The Mosquitoes of Canada by Wood et al. (1979), which remains an invaluable resource for information on mosquitoes in Canada to this day.

Little is known about certain Aedes species, such as Aedes implicatus and Aedes intrudens, apparently because they are the first to emerge in the spring and the adults are short-lived. Thus, a lack of early spring sampling may be causing these species to appear rarer than they actually are. How biologists sample mosquitoes also has had a significant impact on the knowledge of mosquito species distribution and abundance. For example, many surveys use only light traps to collect adults. However, not all species are attracted to light and therefore go undetected in these surveys. Sampling of larval habitat is essential in these cases and yet, for some species the larval habitat remains unknown.

The status of knowledge about mosquitoes differs significantly among provinces and territories. Newfoundland and Labrador recently coordinated a project through Memorial University that looked at changes in mosquito ecology resulting from land use changes and the potential for West Nile virus in Newfoundland (Hustins, 2006). This study resulted in one of the best data sets for mosquito distribution and abundance of any province or territory and will provide an excellent foundation for future studies. Other recent surveys in the Maritime Provinces have added several species of mosquitoes to each provincial list, and greatly increased our knowledge in general. On the opposite end of the spectrum is Nunavut, a vast land with an abundance of mosquitoes. Very little is known about the mosquito fauna in the territory – only 11 species have been confirmed to occur there, compared to 33 in the Northwest Territories and 31 in the Yukon.

Richness and diversity in Canada

Within Canada, mosquito species richness is highest in Ontario and Quebec, with 64 and 50 species, respectively (figure 18). There are many species (Aedes triseriatus, Aedes hendersoni, Anopheles barberi, Toxorhynchites rutilus, Orthopodomyia alba, and Orthopodomyia signifera) whose larvae occur in large tree holes found in older hardwood forests in southern Ontario and Quebec, particularly in the Carolinian zone. All of these species are rare (except Aedes triseriatus, whose larvae also occur in artificial containers), and the conservation of this habitat is likely a requirement for their continued survival in Canada. British Columbia is also rich in mosquitoes, with 46 species. Some Canadian species (Aedes togoi, Aedes aboriginis, Aedes nevadensis, Aedes aloponotum, Aedes sierrensis, and Aedes melanimon) are known only from British Columbia.

Despite the tremendous hordes of mosquitoes that are common in the northern territories, species diversity is lowest in these regions. Some Aedes species are common only in the north (Aedes impiger, Aedes nigripes, Aedes churchillensis, and Aedes rempeli) and their eggs require long periods of extremely cold temperatures in order to hatch the following year.

Species spotlight - Puget Sound Mosquito

The Puget Sound Mosquito (Aedes aloponotum) is only known in Canada from the southwestern corner of British Columbia. It is a large, light brown mosquito with sharp white bands on the legs and abdomen. The larvae have been found in the woodlands of the lower Fraser Valley and southern Vancouver Island. In the Fraser Valley, larvae were found in woodland pools and open pools with grassy bottoms from March until May. Adult females bite humans readily, particularly in the evening. But very little is known about this species – we are ignorant about mating behaviour, host preference and its ability to transmit disease.

In the Fraser Valley, woodland habitat is being developed rapidly, putting this mosquito at risk of extirpation in both British Columbia and Canada. Although it was once found at Burnaby Lake, it has not been collected there in the last 10 years (Belton, 1978; Belton, personal communication). The geographic distribution of Aedes aloponotum does extend south into the USA, but is limited there as well, restricted to western Washington and Oregon. Recent abundance data are not available and the status of this species in this rapidly developing part of the USA is currently unknown. In Canada, this species has a general status rank of Sensitive.

Species spotlight - Permanent Marsh Mosquito

The Permanent Marsh Mosquito (Anopheles walkeri) is known in Canada from Saskatchewan to Nova Scotia and the larvae occur primarily in large marshes and wetlands with stable, permanent water levels. Once considered the most abundant Anopheles species in southwestern Ontario (Wood et al., 1979), it is now one of the least common, except in large conserved marshes, such as those in Point Pelee National Park, and Rondeau and Long Point Provincial Parks (Thielman and Hunter, unpublished data). Once common in Wainfleet, Ontario, Anopheles walkeri is now rarely collected in the area, and the large marsh (known as the Wainfleet Marsh) was drained for agricultural purposes sometime after the 1970s. A few specimens have been recently collected from the nearby Mud Lake Conservation Area, but the dominant Anopheles mosquitoes there, and in the nearby Wainfleet Wetlands Conservation area are Anopheles punctipennis and Anopheles quadrimaculatus.

Anopheles walkeri is the only Anopheles species to overwinter in the egg. Overwintering eggs must be subjected to a lengthy period of cold conditioning before they will hatch. According to weather station data for the Niagara region, the average temperatures in January and February are 5 to10 degrees warmer now than they were 40 years ago. This means that the eggs of Anopheles walkeri may not be getting cold enough temperatures for as long as is necessary for the eggs to hatch the following spring, resulting in population declines. Recent studies have also shown that this species may require large, pristine marshes and wetlands for their survival (Thielman and Hunter, unpublished data). While Anopheles punctipennis, Anopheles quadrimaculatus and Anopheles walkeri often occur together in this type of habitat, Anopheles punctipennis and Anopheles quadrimaculatus also occur in roadside ditches, forest pools, and artificial containers. Anopheles walkeri is only collected from large bodies of water with stable water levels and emergent vegetation such as cattails.

Species spotlight - Pitcher Plant Mosquito

The Pitcher Plant Mosquito (Wyeomyia smithii) is known in Canada from Saskatchewan to Newfoundland. It is an interesting species whose larvae occur in the rainwater that collects in the pitcher-shaped leaves of the Northern Pitcher Plant (Sarracenia purpurea). This plant is found in bogs throughout the boreal regions of Canada and Wyeomyia smithii occur almost everywhere the Northern Pitcher Plant is found, except British Columbia, Alberta and the Northwest Territories. This species overwinters as third instar larvae, frozen inside the water in the leaves of the Northern Pitcher Plant. No blood feeding has ever been recorded for this species, even though their proboscis appears to be fully functional and capable of taking blood.

The Northern Pitcher Plant is usually recognized as a carnivorous plant. Pitcher Plant Mosquitoes do not seem to have a special adaptation to prevent them from being digested by the plant. Instead, the plant does not seem to do a lot of active digestion by itself, but relies mainly on the aquatic community of bacteria, protists, and midge and mosquito larvae to do the digestion for it, using the minerals released during predation and digestion.

Results of general status assessment

The report Wild Species 2010 marks the first assessment for mosquitoes. The rankings were completed in March 2009. They were based on a comparison of historical data using published literature and current data obtained from recent surveys, primarily for West Nile virus surveillance. The Northwest Territories, Newfoundland and Labrador, Prince Edward Island, New Brunswick and Ontario had the most rigorous datasets, having recently conducted thorough mosquito surveys that involved both larval and adult collections. Other provinces had fewer data available, because of either a lack of sampling, limited duration or type of collections, or the lack of publishing results of mosquito surveillance projects. The amount of abundance data, old and new, varied among provinces and territories – in general, there was very little available. This resulted in a large number of species being assigned a rank of Undetermined.

The majority of Canada’s 80 species of mosquitoes have Canada ranks of Secure (63 species, 78%, figure 18 and table 25). Three species (4%) have a Canada rank of Sensitive. There are 11 species (14%) with a rank of Undetermined.

Three species (4%) have Canada ranks of Exotic since they were recently introduced from Asia. Ochlerotatus japonicus was first discovered in 2001 in the Niagara Region, has since spread to Quebec and Nova Scotia and is now well established. Aedes togoi, another resident of Japan, was first discovered along the southern British Columbia coast in the 1950s. Aedes albopictus is known from only two specimens collected in 2001 in the Niagara Region and thus may not have become established in Canada.

Figure 18. Results of the general status assessments for mosquito species in Canada in the Wild Species 2010 report.
bar graph (see long description below)
Long description for Figure 18

Figure 18 shows results of the general status assessments for mosquito species in Canada in the Wild Species 2010 report. The bar graph shows the number of mosquito species ranked as Extinct, Extirpated, At Risk, May Be At Risk, Sensitive, Secure, Undetermined, Not assessed, Exotic, and Accidental in Canada, each province and territory and the 4 oceanic regions. Of the 80 species occurring in Canada, 3 were ranked as Sensitive, 63 as Secure, 11 as Undetermined and 3 as Exotic. Of the 31 species occurring in the Yukon, 13 were ranked as Secure and 18 as Undetermined. Of the 33 species occurring in the Northwest Territories, 1 was ranked as Sensitive, 14 as Secure and 18 as Undetermined. Of the 11 species occurring in Nunavut, 4 were ranked as Secure and 7 as Undetermined. Of the 46 species occurring in British Columbia, 1 was ranked as Sensitive, 35 as Secure, 9 as Undetermined and 1 as Exotic. Of the 44 species occurring in Alberta, 2 were ranked as Sensitive, 31 as Secure and 11 as Undetermined. Of the 42 species assessed in Saskatchewan, 5 were ranked as Sensitive, 25 as Secure and 12 as Undetermined. Of the 46 species occurring in Manitoba, 27 were ranked as Secure and 19 as Undetermined. Of the 64 species occurring in Ontario, 2 were ranked as Sensitive, 43 as Secure, 17 as Undetermined and 2 as Exotic. Of the 50 species occurring in Quebec, 29 were ranked as Secure, 20 as Undetermined and 1 as Exotic. Of the 38 species occurring in New Brunswick, 1 was ranked as Sensitive, 24 as Secure and 13 as Undetermined. Of the 34 species occurring in Nova Scotia, 6 were ranked as Sensitive, 18 as Secure, 9 as Undetermined and 1 as Exotic. Of the 31 species occurring in Prince Edward Island, 16 were ranked as Secure and 15 as Undetermined. Of the 33 species occurring in Newfoundland and Labrador, 1 was ranked as Sensitive, 15 as Secure and 17 as Undetermined. There were no species listed as occurring in the oceanic regions.

 

Table 25. Canada ranks of mosquito species as determined by the National General Status Working Group.
Canada rank Number and percentage
of species in each rank category
0.2 Extinct 0 (0%)
0.1 Extirpated 0 (0%)
1 At Risk 0 (0%)
2 May Be At Risk 0 (0%)
3 Sensitive 3 (4%)
4 Secure 63 (78%)
5 Undetermined 11 (14%)
6 Not Assessed 0 (0%)
7 Exotic 3 (4%)
8 Accidental 0 (0%)
Total 80 (100%)

Threats to Canadian mosquitoes

Ask most Canadians if they would be sad if mosquitoes became extirpated or extinct and the reply will be “Why no, that would be great!” This view is based on the misconception that all mosquitoes are bad, either because they are annoying nuisances or because they may transmit diseases to humans. The fact is that, of 80 species in Canada, the majority would prefer to feed on animals other than humans. Currently, the major vectors of human disease in Canada are Culex pipiens and Culex restuans (in the east) and Culex tarsalis (in the west), all of which are competent vectors of West Nile Virus. Occasionally, Culiseta melanura transmits Eastern Equine Encephalitis. There are other non-human mosquito-borne diseases such as dog heartworm, transmitted by Aedes mosquitoes. However, an increased use of pesticides against disease-carrying mosquitoes may potentially upset the balance by killing harmless mosquitoes unnecessarily, while at the same time artificially selecting for pesticide-resistance among vector mosquitoes.

Probably one of the greatest threats to overall mosquito biodiversity is the destruction and degradation of wetlands. Water pollution and acidification will likely have a negative impact on many sensitive mosquito species. On the other hand, increased urban sprawl may be encouraging an increase in a number of species that do well in and near human dwellings. One such example is the rapid expansion of the range of the invasive species Ochlerotatus japonicus (Thielman and Hunter, 2006). Ochlerotatus japonicus and Aedes albopictus (currently a rare invasive in Canada) may outcompete native species (for a review, see Armistead et al., 2008).

Conclusion

In closing, it should be noted that the majority of work currently being done on mosquitoes in Canada is in relation to arboviral disease transmission. The problem with this focus is that often only species known to transmit diseases are identified and the rest are ignored, making accurate assessments of the status of many of our species difficult to establish.

Further information

Hinterland Who’s Who. 2008. Insect fact sheets: mosquito. (Accessed February 26, 2010).

Public Health Agency of Canada. 2009. West Nile Virus. (Accessed February 26, 2010).

Thielman, A. and Hunter, F. F. 2007. A photographic key to the adult female mosquitoes species of Canada (Diptera: Culicidae). Canadian Journal of Arthropod Identification. (Accessed February 26, 2010).

References

Armistead, J. S., Arias, J. R., Nishimura, N. and Lounibos, L. P. 2008. Interspecific larval competition between Aedes albopictus and Aedes japonicus (Diptera: Culicidae) in Northern Virginia. Journal of Medical Entomology 45: 629-637.

Belton, P. 1978. The mosquitoes of Burnaby Lake. Journal of the Entomological Society of British Columbia 75: 20-22.

Belton, P. 1983. The mosquitoes of British Columbia. British Columbia Provincial Museum Handbook No. 41. British Columbia Provincial Museum Publishing. Victoria: 185 pp.

Bourassa, J.-P. 2000. Le Moustique : par solidarité écologique. Les Éditions du Boréal, Montréal: 237 pp.

Darsie, R. F. Jr. and Ward, R. A. 2005. Identification and geographical distribution of the mosquitoes of North America, North of Mexico. American Mosquito Control Association, University Press of Florida: 383 pp.

Fisk, G. H. 1931. Malaria and the Anopheles mosquito in Canada. Canadian Medical Association Journal 25: 679-683.

Giberson, D. J., Dau-Schmidt, K. and Dobrin, M. 2007. Mosquito species composition, phenology, and distribution (Diptera: Culicidae) on Prince Edward Island. Journal of the Acadian Entomological Society 3: 7-27.

Gordh, G. and Headrick, D. H. 2001. A dictionary of entomology. CABI Publishing. Oxon, United Kingdom: 1032 pp.

Hustins, S. 2006. Mosquito ecology in relation to land use changes and potential West Nile virus in Newfoundland. M.Sc. Thesis, Memorial University of Newfoundland.

Sames, W. J., Duffy, A., Maloney, F. A., Townzen, J. S., Brauner, J. M., McHugh, C. P. and Lilja, J. 2007. Distribution of mosquitoes in Washington State. Journal of the American Mosquito Control Association 23: 442-448.

Thielman, A. and Hunter, F. F. 2006. The establishment of Ochlerotatus japonicus (Diptera: Culicidae) in Ontario, Canada. Journal of Medical Entomology 43: 138-142.

Webster, R. P., Giguere, M.-A., Maltais, P., Roy, J., Gallie, L. and Edsall, J. 2004. Survey of the mosquitoes of New Brunswick. (Accessed December 19, 2008).

Wood, D. M., Dang, P. T. and Ellis, R. A. 1979. The mosquitoes of Canada (Diptera: Culicidae). The insects and arachnids of Canada. Part VI. Agriculture Canada, Ottawa: 390 pp.

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