Does wetland rehabilitation need mosquito control?

Webb_estuarinewetlands_SydneyOlympicPark_2014Mosquitoes can be more than a nuisance. They pose health risks but could also erode the good will of the community for wetland rehabilitation projects. Wetland rehabilitation needs mosquito control.

It’s a warm February evening. A small and anxious group of residents have gathered in a local community hall to discuss the implications of a local wetland rehabilitation project. Some are angry. One of the first questions comes from an elderly gentleman. Close to tears, he explains how his grandchildren no longer visit due to the plagues of mosquitoes that engulf his property day and night. “What are you guys going to do about it?” he pleads.

I learnt a valuable lesson that night. Trying to explain the best mosquito repellent to use doesn’t go down too well when an audience is facing some significant nuisance-biting impacts around their homes. It doesn’t matter how much DEET is in the repellent, it may well work but is it something you need to wear all day just to get the day to day jobs done around the house? Something more substantial is required and, with hindsight, should have been in place before the first waves of mosquitoes left the local wetlands.

Coastal wetlands are under threat

Sea level rise and climate change  is putting pressure on saltmarshes and urbanisation is eating away land that would otherwise accommodate a landward shift in estuarine habitats. There is nowhere for saltmarshes to spread to so they’re destined to be swallowed up by mangroves. While the mangroves are valuable themselves, they don’t provide the same critical habitats required by many of the internationally protected migratory shorebirds that rely on saltmarshes. Saltmarsh habitats could well disappear from much of the east coast in coming decades if sea levels rise as expected and mangroves continue their march landward.

webb_landinglightswetlandsEstuarine wetlands and mosquitoes

Saltmarshes are home to one of our most important pest and vector mosquito species. While it is important to remember that Aedes vigilax is an Australian native animal and just as much a part of our wetland ecosystems as fish and birds, there is little doubt that it can have substantial impacts with regard to nuisance-biting and the transmission of Ross River virus.

Historically, many of the saltmarshes along the east coast were drained or filled to enable increased cattle grazing (although much of it was under the guise of protection from flooding). Tidal flows were cut off with the construction of levee banks and installation of flood gates. Notwithstanding the impacts of grazing, without tidal exchange, the habitats became brackish water to freshwater dominated systems with a dramatic change in vegetation. Saltmarsh and sedgeland vegetation was steadily replaced by reeds and rushes. Invasive plants such as Phragmites quickly took over many of these wetlands.

webb_floodgates_march2011Bringing back the tides

To combat the degradation of wetlands and impending loss due to climate change, there has been some ambitious wetland rehabilitation projects planned. One of the largest in the southern hemisphere is the Hexham Swamp Rehabilitation Project. Much can be learned from the experience in this wetland just west of Newcastle, NSW, and applied to rehabilitation projects, not only in Australia but overseas as well.

Rehabilitation of Hexham Swamp involved the staged opening of existing floodgates to reinstall tidal flows to an otherwise freshwater system. Many aspects of this project were considered and it is unsurprising that one major issue was the possible impact of mosquitoes. Mosquito populations were something of legend in this area, enough so that there is a “big mosquito” outside the local bowling club affectionately known as “Ossie the Mossie” (coincidently, “Ossie” celebrated her 20th birthday in March 2014).

One of the important considerations when assessing mosquito risk was that there had been a dramatic transformation of the areas surrounding the wetlands in the last 20 years. What was once agricultural land was now residential. This is the same situation right along the east coast of Australia, the rapid urbanisation and swelling residential populations along the coast have put people in the firing line of Aedes vigilax.

The prospect of mosquito control was raised in the early stages of the rehabilitation planning but there was great reluctance from the local authorities to head down that path. The problem is that broad scale mosquito control and ecological rehabilitation are often seen at opposite ends of the wetland management spectrum. I’ve experienced this many times first hand, from scepticism regarding the non-target impacts of biological larvicides to “Apocalypse Now” jokes as helicopters go about routine spraying of local wetlands.

Webb_MosquitoCollectionsIs there such a thing as “environmentally friendly” mosquito control?

The hangover from the actual and perceived impacts of pesticide use more than 50 years ago is still present in the minds of many responsible for managing local wetlands. I say perceived as the development of environmentally sustainable mosquito control programs over the past two decades may not be appreciated amongst those charged with managing wetlands and wildlife.

I feel we need to continue building the case for the range of mosquito control strategies available for our coastal wetlands. Not only do we need to convince local authorities that mosquito populations can be minimised without adversely impacting the local environment but that mosquito control should be a critical consideration in wetland rehabilitation projects. It also has the potential to reduce mosquito-borne disease.

We know that the direct non-target impacts of Bacillus thuringiensis israelensis and s-methoprene are minimal and there is growing evidence that the indirect impacts on local wildlife due to reduced mosquito populations isn’t a major concern. Well-designed projects can also minimise the frequency of treatments while reducing peaks in mosquito activity.

It seems our coastal bats populations are mostly eating moths, not mosquitoes so there is unlikely to be any significant impact on these bats resulting from reduced food. There is no reason why the judicious use of larvicides can’t knock the top off abundant mosquito populations, reduce the pest impacts on local community and not pose a risk to local wildlife. Perhaps it should be considered a critical component of wetland rehabilitation?

redkneeddoteral_kooragangisland_march2015Mosquito control and wetland rehabilitation

In speaking with wetland managers, I try to instil with them the importance of mosquito control. There is a risk that swarms of nuisance biting mosquitoes may erode the good will in the community for wetland rehabilitation. These systems, particularly in the early phases of rehabilitation don’t represent pristine environments and while there may not be a desire to establish ongoing mosquito control programs, some control may prove useful while the wetland comes back into balance with the changed environmental conditions.

Rehabilitation takes time and while there is substantial breakdown of freshwater vegetation it is not going to be ideal for fish and other mosquito predators. It is likely to provide ideal conditions for mosquitoes. Over time, however, fish are likely to increase in both their abundance and penetration into the wetlands and greater tidal flushing will make many of the wetland habitats generally unsuitable for mosquitoes.

Perhaps there is benefit in undertaking control as a show of good will to the local community? After all, engagement with the local community will be critical in the success of wetland rehabilitation projects.

The restoration of tidal flows to Hexham Swamp resulted in an initial increase in the abundance of Aedes vigilax. These increases resulted in substantial nuisance-biting impacts. However, in subsequent seasons, the populations of Aedes vigilax levelled out to comparable levels to those of the surrounding estuarine wetlands. The net result has generally been that the long-term moderate increases in Aedes vigilax populations have been balanced by reductions in Culex annulirostris and Coquilettidia linealis populations as the wetlands shifted from freshwater to saline. The health of the wetlands, as well as the local estuary, is improving.

Mosquito control is only a short-term fix and if the rehabilitation of estuarine wetlands is not carefully planned, there may well be ongoing mosquito impacts. However, reducing any initial impacts will go a long way to ensuring continued engagement of the community with the local wetlands. Cost and the operational considerations may be a barrier for organisations unfamiliar with broadscale mosquito control but these issues can be overcome with the expertise that exists in many parts of the country.

In summary, it is important that mosquito management be considered in the planning process of major wetland rehabilitation projects. There is little doubt that such projects will be required into the future as saltmarsh habitats and other estuarine wetlands are threatened but protection wetlands shouldn’t mean increasing mosquito populations. A balance is required between conservation of environmental health and protection of human health.

A longer version of this article originally appeared in “Mosquito Bites” (the Bulletin of the Mosquito Control Association of Australia).

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.