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).

Australian Mammal Society Conference

IMG_5671On Tuesday 9 July I’m presenting some work at the Australian Mammal Society conference at the University of NSW. The title of my presentation is “The role of macropods in mosquito-borne disease: Implications for urban development and wetland rehabilitation” (my coauthors are Stephen Doggett (Medical Entomology, Westmead Hospital) and Mark Ferson (University of NSW/NSW Health).

Ross River virus causes around 5,000 cases of reported human disease every year but, as the symptoms can sometime be mild, the official data is probably an underestimate. The role of Australian wildlife in mosquito-borne transmission cycles is often overlooked. The emphasis is generally placed on mosquito populations and their relationship to environmental drivers of population abundance. We have some very good data on the types of mosquitoes responsible for transmitting RRV thanks to detection of the virus in wild caught mosquito populations and “vector competence” experiments in the laboratory.

There have only been a few studies looking at the role of wildlife. These studies have included serological surveys, isolation of pathogens from wildlife and laboratory studies investigating the titre and duration of viremia in infected animals. These studies have helped identify macropods as some of the key reservoir hosts of RRV in coastal Australia. However, we still don’t know much about how the local wildlife, mosquitoes and pathogens interact under the influence of local environmental and climatic conditions. In particular, how does the ecology of local macropods influence local mosquito-borne disease risk? To be even more specific, how may the conservation strategies of local wildlife at the urban fringe influence public health risks?

If you’d like to read more about RRV, there are two very good review papers here and here. For more on mosquito risk associated with urban development, see my piece here.

IMG_7562My presentation will concentrate on mosquito abundance and diversity, as well as the activity of mosquito-borne pathogens, from two estuarine river systems in Sydney. The Parramatta River and Georges River systems contain comparable mosquito habitat dominated by estuarine wetlands (i.e. saltmarsh and mangroves) but support very different populations of macropods. There aren’t any macropods along the Parramatta River. What we’ve found by studying these two systems can be used to assist in urban development plans where wildlife conservation may require increased awareness of mosquito population management.

Of course, I’m not suggesting that the conservation of macropods isn’t important. The point here is that local authorities must be aware that in regions where there are opportunities for interactions between mosquitoes and wildlife (particularly kangaroos and wallabies), public health risks will be higher. Increased mosquito populations in association with newly constructed or rehabilitated wetlands, particularly in urban areas, may risk only increase nuisance-biting impacts. However, at the fringes of our cities, the risks of disease caused by pathogens such as RRV must be considered. In these circumstances, mosquito management strategies should be more carefully considered.

The full abstract of my presentation is below:

Mosquito-borne disease risk in coastal regions of Australia is a concern for local authorities. Many gaps exist in our understanding of the drivers of mosquito-borne disease risk, particularly with regard to the role of interactions between mosquitoes and wildlife. Macropods have been identified as important reservoir hosts of mosquito-borne pathogens and the presence of kangaroos and/or wallabies is a critical factor in driving outbreaks of disease. What are the implications for urban development and wetland rehabilitation projects? To investigate the role of macropods in urban mosquito-borne disease outbreaks, mosquitoes and activity of Ross River virus (RRV) was investigated in two estuarine wetland systems in Sydney. The abundance and diversity of mosquitoes produced by the estuarine wetlands along the Parramatta River and Georges River are similar with the dominant mosquito Aedes vigilax. There are no macropods are present along the Parramatta River. Few isolations of RRV have been detected along the Parramatta River but significantly higher rates of RRV (as well as other mosquito-borne pathogens) have been detected from mosquitoes collected along the Georges River. In addition, public health investigation confirmed local acquisition of RRV disease in residents living along the Georges River. No locally acquired RRV disease has been confirmed from the Parramatta River region. The use of natural bushland wildlife corridors along the George’s River by macropods is increasing local disease risk in that region. The results have implications for urban planning where wetland creation and rehabilitation, as well as wildlife corridors, may increase local public health risks.

The full program for the Australian Mammal Society conference is available here.

UPDATE: There was a nice article by Nicky Phillips in the Sydney Morning Herald on 13 July covering this story too “Urban kangaroos, wallabies harbour Ross River virus