Urban wetlands in Sydney, Australia, are are actively managed to reduce the impact of an invasive fish, Gambusia holbrooki, known locally as the plague minnow, on threatened green and golden bell frogs, Litoria aurea. The plague minnow preys on bell frog eggs and tadpoles. To reduce the effect of predation by this invasive fish species on native frogs, the local authority routinely drains some wetlands to create a fish-free environment for the frogs. But research suggests that the fish-free environment also benefits a local pest mosquito species, Culex annulirostris, a nuisance biter and vector of arboviruses. (Photo credit: Jayne Hanford)

By Jayne Hanford and Cameron Webb, Ph.D.

As our lives become increasingly urbanized, green space is ever more valuable for people and the plants and animals we share our cities with. But sometimes our efforts to enhance local habitats can have unintended consequences. Our research indicates that some strategies to manage urban wetlands, critical resources in “green” cities, may increase pest and public health risks, even though they’re helping local wildlife such as threatened frog species.

Jayne Hanford

In Sydney, Australia, a series of small, seasonally interconnected wetlands just 14 kilometers (8.5 miles) from the central business district and in the geographical center of the city are actively managed by the local authority to reduce the impact of an invasive fish, Gambusia holbrooki, known locally as the plague minnow, on threatened green and golden bell frogs, Litoria aurea. The plague minnow preys on bell frog eggs and tadpoles and is considered a key threat to frog survival. To reduce the effect of predation by this invasive fish species on native frogs, the local authority routinely drains some wetlands to create a fish-free environment for the frogs.

The question had been asked, what does draining and refilling these ponds mean for mosquitoes? Mosquitoes are a major concern for local authorities in Sydney and, while key pest species are associated with estuarine wetlands, there are some important pests, such as Culex annulirostris, found in freshwater wetlands across the city.

Cameron Webb, Ph.D.

Culex annulirostris is the most important pest mosquito species associated with freshwater habitats in Australia, being a nuisance biter and vector of arboviruses including Ross River, Barmah Forest, Murray Valley encephalitis, and West Nile (Kunjin subtype) viruses.

Our research, published in late March in the Journal of Medical Entomology, showed that, when water from these drained wetlands (as well as some undrained wetlands) was returned to the laboratory, Culex annulirostris avoided laying eggs in the water from wetlands containing fish. So, while this wetland management program is creating opportunities for frogs, it may also create more opportunities for mosquitoes.

There has not been a resulting increase in pest mosquitoes locally. Perhaps this has been due to the relatively small number of fish-free habitats created as part of the draining program. However, the results do highlight a potentially problematic issue for wetland creation and management in our growing urban areas.

Constructed wetlands are popular tools for managing urban storm water. Where existing wetland habitat is limited, new constructed wetlands may increase opportunities for mosquitoes to thrive, potentially impacting human wellbeing. But, when wetlands are designed with only mosquito control in mind, we miss out on valuable opportunities to increase resources for plants and animals in our urban areas and improve ecosystem services, as well as to create attractive recreation spaces for people.

Culex annulirostris is the most important pest mosquito species associated with freshwater habitats in Australia, being a nuisance biter and vector of arboviruses including Ross River, Barmah Forest, Murray Valley encephalitis, and West Nile (Kunjin subtype) viruses. (Photo credit: Flickr/John Tann, CC BY 2.0)

Urban wetlands in Sydney, Australia, are are actively managed to reduce the impact of an invasive fish, Gambusia holbrooki, known locally as the plague minnow, on threatened green and golden bell frogs, Litoria aurea. The plague minnow preys on bell frog eggs and tadpoles. To reduce the effect of predation by this invasive fish species on native frogs, the local authority routinely drains some wetlands to create a fish-free environment for the frogs. But research suggests that the fish-free environment also benefits a local pest mosquito species, Culex annulirostris, a nuisance biter and vector of arboviruses. (Photo credit: Jayne Hanford)

Urban wetlands in Sydney, Australia, are are actively managed to reduce the impact of an invasive fish, Gambusia holbrooki, known locally as the plague minnow, on threatened green and golden bell frogs, Litoria aurea. The plague minnow preys on bell frog eggs and tadpoles. To reduce the effect of predation by this invasive fish species on native frogs, the local authority routinely drains some wetlands to create a fish-free environment for the frogs. But research suggests that the fish-free environment also benefits a local pest mosquito species, Culex annulirostris, a nuisance biter and vector of arboviruses. (Photo credit: Jayne Hanford)

So, what’s a wetland manager to do?

There is no doubt that balancing ecological and human health issues is a challenging task. Despite the many benefits urban wetlands can provide to people, if a wetland is also creating a mosquito problem for nearby residents, it is unlikely to gain the community support needed to maintain an attractive and ecologically functional space. Related research we’ve conducted suggests that considering the ecology of specific mosquito species that may be locally problematic and addressing their specific habitat requirements may deliver ecologically valuable wetlands that minimize mosquito risks.

The key to maintaining a happy and workable equilibrium between potential mosquito risks of urban wetlands and their benefits to biodiversity and human well-being lies in considering the specific risks and habitat requirements of locally abundant mosquito species, understanding the ecological interactions occurring between species brought together by these created environments, and understanding how identified risks might be mitigated through both wetland design and wetland management, rather than painting all mosquitoes with the same broad brush.

Jayne Hanford is a Ph.D. candidate in the Integrative Ecology Lab at The University of Sydney in Australia. Email: jayne.hanford@sydney.edu.au. Cameron Webb, Ph.D., Hanford’s advisor, is a clinical lecturer in the Department of Medical Entomology at The University of Sydney and NSW Health Pathology. Email: cameron.webb@sydney.edu.au.