Preserve and protect? Exploring mosquito communities in urban mangroves

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This is a special guest post from Dr Suzi Claflin. Suzi found herself in Sydney, Australia, (via Cornell University, USA) in 2015 to undertake a research project investigating the role of urban landscapes in determining mosquito communities associated with urban mangroves. She was kind enough to put this post together to celebrate the publication of our research in Wetlands Ecology and Management!

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Sometimes you’ve got to make hard choices for the greater good. These situations can arise anywhere, but here – as usual – we are concerned with mosquitoes. There’s a balancing act carried out by public health officials and wetland managers trying to both preserve endangered habitat and protect human health. In this guest post, I’ll explain the science behind research I recently published in collaboration with Dr Cameron Webb, and suggest one way forward for addressing human and environmental health concerns in urban wetlands.

During my PhD, I studied how the landscape surrounding small-scale farms affects the spread of a crop virus and the community of insect pests that carry it. When I came to Australia to work with Cameron, I was surprised to find myself applying the same type of landscape ecology to mosquitoes and mangroves in urban Sydney.

The misfortune of mangroves

Mangroves are real team players. They provide a range of services to the surrounding ecosystem and to the humans lucky enough to live near them. Mangroves are extremely effective at protecting the shoreline (but this can sometimes be a problem). They prevent erosion by gripping the soil in their complex root systems and buffer the beach by serving as a wave break. By filtering sediment out of the water that flows over them, mangroves also prevent their neighbouring ecosystems, such as coral reefs and seagrass forests, from being smothered.

Despite all their good work, mangroves have an almost fatal flaw; they prefer waterfront property. Unfortunately for them, so do humans. Urban and agricultural development has eaten away at mangroves, leaving them highly endangered.

The mosquito menace

Mozzies are a public health menace, because they spread human diseases like Ross River virus (RRV). Because of this, public health officials rightly spend time considering how to supress mosquito populations in order to reduce the risk of disease transmission.

Here’s where things get tricky: mangroves are great for mosquitoes.

That leaves public health officials and wetland managers in a difficult position. On the one hand, mangroves are delicate, at-risk ecosystems that need to be preserved. On the other, mangroves and surrounding habitats potentially harbor both the animal carriers of the RRV (e.g. wallabies) and a load of mosquitoes, which means that people nearby may need to be protected.

How can we do both?

 

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Dr Suzi Claflin trapping mosquitoes in the mangroves along the Parramatta River, Sydney, Australia.

 

The potential power of prediction

This is a hard question to answer. One approach is prediction: using measurements of the environment, like rainfall and tide level, to estimate what the mosquito community will look like in a given region. The mosquito community determines what management actions, like spraying an insecticide, need to be taken, based on the threat it poses to public health.

We set out to explore how the way we use land (e.g. for residential areas or industrial areas) near urban mangroves affects the mosquito communities that live in those mangroves. The project involved dropping over retaining walls, slipping down banks, and tromping through muddy mangroves along the Parramatta River in Sydney. We set mosquito traps (billy cans of dry ice with a container on the bottom) and left them overnight to capture the mozzies when they are most active. We did this at two points in the summer, to see if there was any change over time.

We found that yes, the way we use land around a mangrove makes a difference. Mangroves with greater amounts of bushland and residential land in the surrounding area had fewer mosquitos, and fewer species of mosquitos. On the other hand, mangroves with greater amounts of industrial land surrounding them had a greater number of mosquito species, and those surrounded by greater amounts of mangrove had more mosquitos.

And, just to muddy the waters a bit more (pun intended), several of these relationships changed over time. These results show that although prediction based on the surrounding environment is a powerful technique for mangrove management, it is more complicated than we thought.

Another way forward: site-specific assessments

Our work suggests another way forward: site-specific assessments, measuring the mosquito community at a particular site in order to determine what management approaches need to be used. This is a daunting task; it requires a fair number of man-hours, and mangroves are not exactly an easy place to work. But it would be time well spent.

By assessing a site individually, managers can be confident that they are taking the best possible action for both the mangroves and the people nearby. It turns out that the best tool we have for striking a balance between environmental and public health concerns, the best tool we have for preserving and protecting, is information. In mangrove management—as in everything—knowledge is power.

Check out the abstract for our paper, Surrounding land use significantly influences adult mosquito abundance and species richness in urban mangroves, and follow the link to download from the journal, Wetlands Ecology and Management:

Mangroves harbor mosquitoes capable of transmitting human pathogens; consequently, urban mangrove management must strike a balance between conservation and minimizing public health risks. Land use may play a key role in shaping the mosquito community within urban mangroves through either species spillover or altering the abundance of mosquitoes associated with the mangrove. In this study, we explore the impact of land use within 500 m of urban mangroves on the abundance and diversity of adult mosquito populations. Carbon dioxide baited traps were used to sample host-seeking female mosquitoes around nine mangrove forest sites along the Parramatta River, Sydney, Australia. Specimens were identified to species and for each site, mosquito species abundance, species richness and diversity were calculated and were analyzed in linear mixed effects models. We found that the percentage of residential land and bushland in the surrounding area had a negative effect on mosquito abundance and species richness. Conversely, the amount of mangrove had a significant positive effect on mosquito abundance, and the amount of industrial land had a significant positive effect on species richness. These results demonstrate the need for site-specific investigations of mosquito communities associated with specific habitat types and the importance of considering surrounding land use in moderating local mosquito communities. A greater understanding of local land use and its influence on mosquito habitats could add substantially to the predictive power of disease risk models and assist local authorities develop policies for urban development and wetland rehabilitation.

Dr Suzi Claflin completed her PhD at Cornell University exploring environmental factors driving the spread of an aphid-borne potato virus on small-scale farms. She is now a postdoctoral research fellow at the Menzies Institute for Medical Research in Hobart, TAS. In her spare time she runs her own blog, Direct Transmission, focusing on disease and other public health issues (check it out here). To learn more about her doctoral research, follow this link!

Why are mosquitoes so bad this summer?

Swarms of mosquitoes are descending on Sydney. It’s been one of the worst starts to summer for pesky mozzies and it’s only going to get worse.

Mosquitoes need water and there’s been plenty of it. Water filling up our wetlands or backyard buckets provides habitat for mosquitoes. Warm humid weather also helps keep mosquitoes alive longer. That’s right, there are more mosquitoes and they’re living longer!

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The saltmarsh mosqutio, Aedes vigilax, is closely associated with tidally influenced estuarine wetlands along most of Australia’s coast (image: Stephen Doggett)

Predicting boosts in mosquito numbers isn’t always easy. The reason is that mosquitoes respond to the pattern of rainfall, not just the quantity.

Rain, Rain, Rain

We’ve had over 160mm of rain in Sydney in just the first 10 days of December. The long-term average is 70mm. It is when this rain is falling that makes the difference. If we’d had it all over a couple of days, there would have been one hatch of mosquitoes. Just one generation of adults buzzing about. Problem is we’ve had steady rain. It’s been spread out and so have been the hatches of mosquitoes. It keeps the wetlands flooded too so any wrigglers that have hatched from eggs won’t be stranded in a drying puddle.

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Total daily rainfall recorded at Sydney Olympic Park (Data source: Bureau of Meteorology)

The rain is one thing but tides play a role too. While backyard mozzies rely on rainfall, our biggest nuisance biting pest, the saltmarsh mosquito (Aedes vigilax), relies on a combination of rainfall and tidal flooding of local wetlands. Funnily enough, they generally love a hot dry summer.

Swimming with the tide

In a cruel twist of fate, we’ve had tidal flooding of local wetlands at the same time as all the rain. The tides were nothing compared to the “king tides” we experienced last summer but, in combination with the rain, it resulted in complete flooding of our local estuarine wetlands. Maximum occupancy for mosquitoes.

Current monitoring along the east coast, particularly in areas close to estuarine wetlands, are recording above average numbers of mosquitoes. The collections are currently dominated by saltmarsh mosquitoes but there are plenty of “backyard” mozzies too. The current generation of mosquitoes was triggered by environmental factors in mid to late November.

Problem is, we’re going to see more as the next generation is currently wriggling about in our wetlands and backyard habitats. They’ll be emerging over the next few days….

While the current rain and tides are playing their part, something happened way back in October that set the wheels in motion for the marauding mozzies.

Springing into Summer

The hottest spring on record set the scene. Then, around the middle of October, there was about 100mm of rain that followed a few days after a series of higher than expected tides. Historically, regardless of rain, tides or temperature, there is rarely a substantial increase in mosquitoes. This year was different. it was a perfect storm of climatic conditions that “woke up” local mosquito populations a month or so early.

The early start to the season took everyone by surprise. The state-wide mosquito monitoring program generally only kicks off in coastal areas in December and there has never been any mosquito control in local wetlands in October. While I was filming a piece for Chanel 9 News along the banks of the Parramatta River, we were eaten alive by mozzies. I’d never seen so many that early in the season.

henandchickenbay_stormfrontMy experience in the past is that once mosquito populations gain some momentum, only an extended hot and dry period will slow them down. Each mosquito can lay up to (or beyond) 100 eggs. The more mosquitoes, the more eggs. The more eggs, the more mosquitoes. The cycle continues. The short-term forecasts are for  warm and humid weather along with continuing storms. Unfortunately, there may not be a break from mosquitoes for a while yet.

What can you do the beat the bite?

Here are five tips that will help beat the mozzie bites this summer.

1. Tip out, drain or cover any water-holding containers in the backyard. They may be buckets, discarded tyres or a taupaulin covering an old trailer or boat. Anything that collects water can be used by mosquitoes. Make sure your rainwater tank is correctly screened and your roof gutters are clean and free-flowing.

2. Screen your windows. This may not help when you’re outside but it will at least stop them coming inside. If you live near wetlands, give some serious thought to creating a screened outdoor area. There are lots of flexible screening options available. This is common place in some parts of North America and Europe, I don’t know why we don’t do it more of ten here in Australia.

3. Cover up or wear repellent. If you’re outdoors, particularly at dawn or dusk, cover up with long sleeved shirts and long pants. Pale colours tend not to attract so many mosquitoes. Apply repellent to any exposed areas of skin. There are lots of tips on using repellents here and here and here.

4. Sprays and coils and sticks and zappers. There are plenty of products available that contain insecticides. Aerosol “fly sprays” will help get rid of a mozzie buzzing about indoors but the better option are the “plugin” zappers that heat a small reservoir of insecticide. These are very effective and safer to use indoors than burning a mosquito coil. Mozzie coils and sticks should only be burnt outdoors or in sheltered areas. Pick a product that contains an insecticide. The botanical (e.g. citronella) based products will hep reduce the total number of bites but not prevent them all.

5. Listen out for warnings. Stay tuned to local radio, or keep your eye on the website of local health authorities. Mosquito bites can be pretty annoying but there is also concern regarding mosquito-borne diseases, particularly those such as Ross River virus. There is a number of state-based surveillance programs in place and warnings are often issued when elevated levels of pathogen activity is detected in local mosquitoes or there is an increase in human disease reported.

Join the conversation on Twitter. Is this the worst summer for mosquitoes in your part of Australia?