Do aquatic weeds promote mosquito breeding?

wetlands_sydneyparkConstructed wetlands are increasingly common in newly develop urban areas across Australia. One of the major drivers of the increasing prevalence of these wetlands is the threats posed to many urban areas along the east coast of Australia where a changing climate poses a serious need to better manage our water resources. The objectives of these wetlands are for stormwater or wastewater treatment, water storage, passive recreation, community eduction or simply aesthetic appeal. Aquatic macrophytes play an important role in achieving many of these objectives.

Increasingly, these wetlands play an important role in wildlife conservation. Constructed wetlands may provide refuges for plants and animals under threat from urbanisation and climate change. Whether it is providing habitats for frogs, birds or bats or off setting habitat loss from large scale urban developments, constructed wetlands (both estuarine and freshwater) are becoming a more common sight around our suburbs.

wetlandsignThe mosquito-borne disease risks associated with constructed wetlands have been well documented. While these wetlands are generally small, given their proximity to the community, they may increase the relative risk of mosquito impacts and must be addressed in urban design strategies.

One of the problems faced by authorities designing constructed wetlands is balancing the design specifications to meet each of the objectives of the wetlands. There usually isn’t a “one design fits all” approach. In some cases, the design of a wetland may predispose it to the production of mosquitoes. For example, take these recommendations from a recent publication by Shulse et al. 2012:

Our results indicate that wetlands designed to act as functional reproductive habitat for amphibians should incorporate shallows, high amounts of planted or naturally established vegetation cover, and should be fish-free.

A wetland like this, as well as being an ideal habitat for frogs, will also be a very suitable habitat for mosquitoes! Studies have shown that the tadpoles are unlikely to help control mosquitoes either. While it should be noted that mosquitoes are a natural part of wetland ecosystems, the threats of nuisance-biting and public health risks mean that effective mosquito management strategies will need to balance both human and environmental health risks.

There aren’t many studies that link specific aquatic plant species to mosquito species. There are some mosquito species in Australia, such as Coquillettidia linealis, that have a close association with aquatic plants. The immature stages of these mosquitoes don’t breath at the water surface like most mosquitoes. Instead, they attach themselves to the roots and submerged parts of the plant. They can be an important pest species and may play a role in transmission of Ross River and Barmah Forest viruses.

Culex annulirostris

One of the major freshwater pest mosquitoes in Australia, Culex annulirostris (Photo: Stephen Doggett, Medical Entomology, Pathology West – ICPMR Westmead)

While there is little evidence of macrophyte-specific risk factors for mosquito production, there are studies that demonstrate the link between vegetation structure and mosquito production. The design of the wetland plays an important role but so too will the choice of vegetation type. Unfortunately, while native aquatic plants may be suitable for constructed wetlands they can often be overtaken by invasive exotic weed species.

There hasn’t been a lot of work done on exotic aquatic plants in Australia and their associations with mosquitoes. In 2011, I was fortunate to have a couple of fourth year students from the University of Sydney Faculty of Agriculture and Environment work with me on a couple of small research project. One of those projects was to investigate the egg-laying behaviour of mosquitoes in response to three exotic aquatic plant species. The results of that paper have just been published in General and Applied Entomology, the Journal of the Entomological Society of New South Wales.

Like many regional societies, the Ent. Soc. NSW has struggled in recent times to attract new members and as some serious discussions have been underway as to the restructuring of the society, the latest issue of General and Applied Entomology was substantially delayed. It is only now, in September 2013, that the 2012 issue of the journal has been released.

The abstract of our paper is below:

Constructed wetlands are becoming more common in New South Wales as they are a key element of Water Sensitive Urban Design within new residential and industrial developments. As well as providing waste-water management, wildlife conservation, or improved amenity, they may also inadvertently enhance local habitats for mosquitoes. The diversity and abundance of aquatic macrophytes has been identified as a predictor of mosquito abundance but there is a paucity of information on species-specific mosquito-plant associations. This study was to determine whether two pest mosquito species, Culex annulirostris and Culex quinquefasciatus exhibited an ovipositional preference when exposed to three aquatic plant species (Salvinia molesta, Eichhornia crassipes, and Cyperus haspens) in laboratory tests. Significantly more egg rafts were laid in association with S. molesta than either E. crassipes or C. haspens by Cx. annulirostris. This result suggests that control of S. molesta may reduce the suitability of habitats for mosquitoes. There was no significant difference in the mean number of egg rafts laid by Cx. quinquefasciatus in association with the three plant species. These results highlight the need for appropriate management of aquatic weeds in wetlands to ensure the environmental and human health risks are minimised.

We compared the egg-laying preferences of two mosquito species commonly associated with constructed wetlands. Culex annulirostris is one of the major freshwater pest mosquitoes in Australia and it has been demonstrated that this species plays an important role in the transmission of pathogens such as Ross River virus and Barmah Forest virus. The second species used in experiments was Culex quinquefasciatus, commonly known as the brown house mosquito, this species is widespread in urban areas of Australia and, as well as being found in wetlands, will also utilise storm water structures and other highly polluted man-made water sources.


Andrew Ironside releasing mosquitoes into the experimental cages (thanks Ikea) at the University of Sydney

Gravid mosquitoes were released into small cages in the presence of buckets containing water and one of three plant species. Three aquatic plant species, each exhibiting a different growth form, were Salvinia molesta (salvinia) is a small free-floating water plant, Eichhornia crassipes (water hyacinth) is a medium sized free-floating plant and Cyperus haspens (dwarf papyrus) is an emergent macrophyte. The number of egg rafts laid in associations with each of the plant species was recorded.

The most interesting result we found was that Culex annulirostris laid more eggs in association with the floating weed salvia. This exotic species is considered a “weed of national significance” and should be controlled for a range of reasons. Looks like reducing the suitability of a habitat for mosquitoes may be an additional reason!

Interestingly, the egg-laying preferences were not consistent between the two species. While Culex annulirostris had a strong preference for salvinia, Culex quinquefasciatus did not display a preference for any of the plant species. Previous studies have identified differences in the differing response of these two species to characteristics of habitats.

Webb_constructedwetlands_penrithThe implications of this research are that there may be close associations between the egg-laying preferences of mosquito species and local aquatic plants. There is still much work to be done to characterise these relationships between pest mosquitoes and Australian aquatic plants. The results of our short study here indicate that invasive aquatic weeds like salvina may increase the suitability of habitats for mosquitoes. The risk that these aquatic weeds may increase the pest and public health risks associated with constructed wetlands close to urban areas in Australia provided further justification for the management of these plants.

A salvinia weevil (Photo: CSIRO)

A salvinia weevil (Photo: CSIRO)

One of the interesting ways in which infestations of salvinia are controlled in Australian wetlands is through the release of the salvinia weevil, Cyrtobagous salviniae. Originally imported and released by CSIRO in QLD around 1980, this salvinia chomping weevil has been released at a number of sites to control the spread of this weed. While some studies have indicated that this species may not provide long-term control of salvinia unless environmental conditions are favourable, there is great potential for this biological approach to aquatic weed control in some regions.

It looks like our insect friends may be both friend and foe of wetland managers across Australia. Filling the gaps in our knowledge regarding the site-species relationships between insects and aquatic plants may help better manage our natural and constructed urban wetlands.

The full reference for our recent publication is:

Webb CE, Ironside A. and Mansfield S. (2012) A comparison of ovispoistion preference in the presence of three aquatic plants by the mosquitoes Culex annulirostris (Skuse) and Culex quinquefasciatus (Say) (Diptera: Culicidae) in laboratory tests. General and Applied Entomology 41: 21-26.

You can download the full paper here.