Our latest publication in the Australian Journal of Entomology marks the end of a three year research project investigating the biology of a unique introduced mosquito species, Culex molestus, in Australia.
We generally think of nuisance-biting mosquito problems being confined to tropical regions, or at least warm summer conditions. Well, imagine you’re in London in late September 1940. You’re taking shelter in the underground during The Blitz. It is crowded and cold. You’re bitten by mosquitoes too. You’re being bitten by Culex molestus. It is often commonly referred to as the London Underground mosquito and has already been the subject of some fascinating research that has shown how the mosquito has adapted to life within the London underground.
Culex molestus was first described from Egypt in 1775. The mosquito is unique in that it is closely associated with subterranean habitats across the temperate regions of the world, from underground train networks to flooded basements to septic tanks. The species has adapted to these habitats by gaining the ability to mate without the need to swarm (a phenomenon known as stenogamy) and by dropping the requirement of a blood meal to develop the first batch of eggs (a phenomenon known as autogeny). You can read about our previously published work on this here.
The Culex pipiens subgroup of mosquitoes includes a number of globally important vectors of disease-causing pathogens but there are distinct genetic and biological differences between these species that influence their role in transmission cycles. There are four member of the Cx. pipiens subgroup in Australia, Culex australicus, Culex globocoxitus, Culex quinquefasciatus and Culex molestus.
The last of these species, Cx. molestus, had not been the focus of substantial research for over 50 years until a research project by the Department of Medical Entomology and University of Sydney commenced in 2010. The project was designed to address the gaps in our knowledge of these species with a view to assisting in the assessment and management of disease risk associated with this species.
This work was primarily undertaken by Nur Faeza Abu Kassim as part of her PhD candidature with generous support from Ministry of Higher Education Malaysia and Universiti Sains Malaysia.
How did the mosquito get to Australia?
The most cited theory to explain the introduction of Cx. molestus into Australia is that it was through military movements into Melbourne during World War II. This was based on an absence of this species in Victoria during the pre-WWII period. Our research supported this theory.
There were no reports of this species in Australia prior to the 1940s. A review of distribution records for this species confirmed the presence of the species at over 230 locations confirmed that the mosquito has spread throughout the southern parts of Australia and in coastal regions as far north as Tweed Heads (NSW) and Geraldton (WA). No specimens have been reported from Queensland or Northern Territory.
Molecular analysis of specimens collected from throughout Australia, with reference to specimens from Asia, North America and Europe, indicated that Australian Cx. molestus shared the strongest genetic similarity with specimens from Asia. Perhaps the mosquito hitched a ride from Japan into the Pacific and then, with US military, in Australia?
Buzzing (and biting) about all year long?
One of the interesting findings of our research was that the mosquito was active throughout the winter months around Sydney. Analysis of weekly trapping over a 13 month period indicated that the species does not display diapause. As well as generally being a cool-temperate climate mosquito species, perhaps the subterranean habitats provided a little “insulation” from the cold, keeping water temperatures just a little warmer than above ground pools and ponds?
Most of the other nuisance-biting pests disappear during the cooler months. There will occasionally be a few about, particularly during warmer winter days. However, for most local pest mosquitoes, it seems to be the minimum daily temperatures that drive mosquito activity more than maximum daily temperatures. In the case of Cx. molestus, they soldier on regardless.
What about the public health risks?
One of the last unanswered questions regarding the potential public health impacts of Cx. molestus is in relation to the ability of this mosquito to spread local and/or exotic viruses. While local viruses (e.g. Ross River virus) have been isolated from field collected specimens, there is yet to be a thorough investigation of the ability of this species to transmit endemic pathogens such as Murray Valley encephalitis virus or Kunjin virus.
I was involved in a research project assessing the risks posed in eastern Australia due to potential introduction of West Nile virus. Laboratory investigations and field collections provided some valuable information but, due to prevailing environmental conditions at the time, there were very few Cx. molestus collected during the study. We need to complete some of this work to gain a better understanding on how important a role Cx. molestus may play in local disease risk.
One of the key implications of our research is that it highlights the need for urban planners and engineers to consider the risks posed by above and below ground water storage for creating mosquito habitats. While much of my work previously has concentrated on the creation of wetlands and rehabilitation of other habitats in association with urban development, rainwater and storm water storage structures should be adequately designed to reduce mosquito risk.
The full reference for our most recent paper is below:
Kassim NFA, Webb CE and Russell RC (2013) Australian distribution, genetic status and seasonal abundance of the exotic mosquito Culex molestus Forskal (Diptera: Culicidae). Australian Journal of Entomology 52: 185-198 [online]
ABSTRACT. Culex molestus was probably introduced into Australia in the 1940s and represents a potentially important nuisance-biting pest and vector of disease-causing pathogens in urban areas. The aims of this study were to review the literature to determine the current and historical distribution of Cx. molestus in Australia, analyse the genetic similarity of specimens collected from various locations in Australia with reference to specimens from North America, Asia and Europe, and document the seasonal abundance of this mosquito in the Sydney region. Results showed that Cx. molestus is common in southern Australia, but there was no evidence that this mosquito is found north of latitude 28.17°S. Molecular analysis indicated that specimens from various locations throughout Australia shared strong genetic similarity and that it was most likely introduced from Asia, possibly through multiple introductions over the past 70 years. Analysis of the seasonal abundance of Cx. molestus indicated that the species does not display diapause during the cooler months. Consideration should be given to the unique biology and ecology of this species when assessing the public health risk and the surveillance methods required in the management of Cx. molestus within urban areas of Australia.
You can read a media release from the University of Sydney here. Our research was picked up by the local and international media in the past week or so too. You can read about our work in the Daily Telegraph, Newcastle Herald and Sydney Morning Herald.
Previous publications as part of this research project include:
Kassim NFA, Webb C.E. and Russell RC (2012) The importance of males: larval diet and adult sugar-feeding influence reproduction in the mosquito Culex molestus. Journal of the American Mosquito Control Association 28: 312–316
Kassim NFA, Webb C.E. and Russell RC (2012) Is the expression of autogeny by Culex molestus Forskal (Diptera: Culicidae) influenced by larval nutrition or by adult mating, sugar feeding or blood feeding? Journal of Vector Ecology 37: 162–171
Kassim NFA, Webb C.E. and Russell RC (2012) Culex molestus Forskal (Diptera: Culicidae) in Australia: colonisation, stenogamy, autogeny, oviposition and larval development. Australian Journal of Entomology 51: 67-77