Photo by Seamus Doherty
Constructed wetlands are generally large, semi-permanent water bodies designed and constructed to capture and filter storm water and other surface water flows. In South Australia, as elsewhere, the design objectives of constructed wetlands may also include the provision of local amenity and habitat for wildlife.
As with any large, natural body of water, a constructed wetland in an urban setting has the potential to produce populations of mosquitoes; these in turn have the potential to pose a risk to the surrounding community through the transmission of disease.
Explainer Mosquito Management and Urban Wetlands
Although constructed wetlands are designed and operated to manage the risk associated with the generation of populations of nuisance mosquito species, due to the understandable concern by the public, monitoring of mosquito populations and water quality has been undertaken around urban Adelaide by Healthy Environments, Healthy People Research Group at the University of South Australia with the support of the Department for Environment and Water, Natural Resources Adelaide and Mount Lofty Ranges.
Trapping for this season’s Urban Wetland monitoring program commenced in October 2018 and ran until March 2019. Once again, trapping was conducted monthly throughout the season with 36 CO2– baited EVS (encephalitis vector survey) traps set across the twelve wetland sites. All potential larvae sites were continuously monitored throughout the season.
Trapping conducted in March 2019 indicated most urban wetland sites contained few mosquitoes; the one exception was Ridge Park. At Ridge Park, the number of mosquitoes trapped was close to the historical mean. While overall mosquito numbers were low it is interesting to note that the long term average adult female mosquito collections were exceeded at three locations: St Clair, Felixstow and Brown Hill Creek wetland with all these locations having higher than average adult female mosquitoes (Figure 1). Domestic environments are the most likely sources for larvae of mosquitos such as Culex quinquefasciatus and Aedes notoscriptus at Felixstow and Brown Hill Creek Wetland while populations of Coquillettidia linealis are responsible for the increased collections at St Clair.
This season the most abundant species collected over March was the nuisance Aedes notoscriptus (below) representing half of all individuals collected. Over half of all individuals (34) were collected at a single location, at Ridge Park. This species is considered to be a peridomestic in and around human habitations) mosquito preferring to utilise containers or similar habitats to rear larvae. This species is usually associated with domestic environments and is unlikely to use the wetland to rear mosquito larvae.
The second most abundant species collected across urban Adelaide wetlands was Cq. linealis representing 20% of all individuals collected. Unlike other species collected the larvae of Cq. linealis attach to aquatic plants and are a species that requires management in urban wetlands. Regular maintenance of wetlands such as ensuring emergent vegetation is managed so to prevent it overgrowing the wetland is sufficient to prevent Cq. linealis becoming a pest to residents.
Like in the previous season Culex quinquefasciatus, continues to be common across Adelaide’s urban wetlands. This was the third most common species collected in March with 14% of all individuals collected belonging to this species (Figure 2). Like Cx. quinquefasciatus it is unlikely that Ae. notoscriptus originated in the wetlands as it prefers containers, most likely in the gardens in the areas surrounding the wetlands or alternatively containers or tree holes in the park surrounding the wetland.