A week or so into Spring and the east coast of Australia was already battling bushfires and drought. It is unlikely to get much better with predictions of a hot and dry summer ahead. What does this mean for mozzies?
Every month, the Bureau of Meteorology produces a three month forecast of temperature and rainfall. We often look to these forecasts to predict the likely outlook for nuisance-biting and public health risks. In turn, these predictions can assist our monitoring and control programs conducted with local authorities.
Can you predict future mosquito populations?
While it is relatively straight forward to predict potential peaks in mosquito populations, it can be difficult to predict the magnitude of those abundance peaks. It is even harder to predict outbreaks of mosquito-borne disease caused by Ross River virus and other endemic pathogens, particularly across different ecological zones. One of the issues is that the key mosquito species involved in outbreaks can change from year to year depending on local conditions. Rainfall and tides can both potentially drive outbreaks of disease. They may drive the abundance of different species. Outbreaks typically occur when mosquito populations are large but large populations of mosquitoes don’t necessarily result in outbreaks of disease (probably because disease outbreaks are driven by local wildlife, not just weather).
Is a hot and dry summer ahead?
We’ve had an unusually warm spring and as we move towards summer, Australia is going to remain hot (in fact, we’re heading towards the hottest calendar year on record). The chances of the November 2013 to January 2014 maximum and minimum temperatures exceeding our long-term median temperatures are greater than 60% over most of Australia. In some places 80%. From a mosquito perspective, it is often the minimum temperatures that drive activity. Even with unseasonably hot weather in late Autumn or early Spring around Sydney, we generally don’t see a response from mosquitoes so long as overnight temperatures remain cool. From the current outlook, it seems that our minimum daily temperatures will be relatively warm. This is good for most mosquitoes but may mean that many semi-permanent wetlands and other habitats may dry up.
Things don’t get much better with regard to rain. Throughout most of coastal Australia, there is a little less than a 50% chance of recording median rainfall. However, in some of the key areas of coastal mosquito activity, the north coast of NSW and SE QLD, there is less than a 40% chance of median rainfall.
What does this mean for the summer mozzies?
For freshwater mosquitoes that rely on inundation of flood pains, wetlands and coastal swamp forests, the dry conditions are unlikely to be favourable. However, you may be surprised to learn that one of our most important pest mosquitoes actually prefers dry conditions over wet, why is that?
Mosquitoes are a diverse group of insects. There are dangers in generalising across species when discussing their response to environmental conditions (this makes predicting changes resulting from a changing climate difficult). While the majority of Australia’s mosquitoes are associated with freshwater habitats, one of the key pest mosquitoes is found in close association with coastal estuarine wetlands. One of Australia’s worst nuisance-biting and pathogen transmitting mozzies is the saltmarsh mosquito, Aedes vigilax.
Estuarine wetlands, particularly saltmarshes, can be harsh environments. They’re highly ephemeral and highly saline. Not many animals call them home. However, they are still ecologically important environments and some of our most threatened. They’re under pressure from urbanisation, mangrove invasion and sea level rise. To learn more about Australian saltmarshes, check out this wonderful publication, Australian Saltmarsh Ecology.
As harsh an environment as it is, Aedes vigilax has adapted to it perfectly. The mosquito lays dessication resistant eggs at the base of saltmarsh vegetation, particularly Sarcocornia quinqueflora and Sporobolus virginicus.Those eggs stay on the marsh for months (perhaps even years) waiting for the right conditions of temperature and inundation resulting from tides and/or rainfall. During summer, the immature stages of mosquitoes hatch and can emerge from the wetlands in about a week. A large percentage of the mosquitoes will lay a batch of autogenous (not requiring a blood meal) eggs. These guarantee the next generation. After that job is done, the mozzies head off looking for a blood meal. In their 1000s. They disperse over 5km from the wetlands.
It may seem counter intuitive but “wetter” summers actually produce less abundant saltmarsh mosquito populations. We’ve seen this over the past few summers that, under an influence of La Nina conditions that produced substantial rainfall, populations of saltmarsh mosquitoes were generally down. Similar results have recently been reported from Florida (where an ecologically similar mosquitoes, Aedes taeniorhynchus, is present). Why?
Saltmarsh mosquitoes thrive under hot and dry conditions for a number of reasons. Firstly, if pools and ponds on the saltmarsh regularly dry up, there are fewer opportunities for fish (and other predatory aquatic invertebrates) to establish populations. Secondly, if the saltmarsh remains inundated, mosquitoes are unable to lay their eggs in their preferred locations and will be forced to lay eggs higher in the marsh where they are less likely to be inundated by rain or tides.
The perfect conditions for these mosquitoes are created when we get below average summer rainfall. The wetlands are only inundated by the, generally monthly, spring tides. With the post-tidal flooding pools drying up within two weeks, mosquitoes can still complete development and lay eggs in preferred locations. Low rain = high mozzies!
Are mosquitoes and mosquito-borne disease risks another environmental hazard of the Australian landscape?Just as debate surrounds how urban planning can address bushfire risks to new developments, do authorities need to consider “hazard reduction” or “mosquito-aware urban design” strategies for mozzies? Not just where new residential developments encroach on existing wetlands but also the creation of mosquito habitats through the inclusion of constructed wetlands and other water conservation strategies.
Managing future mosquito risks associated with saltmarsh mosquitoes can be difficult. Urbanisation of our coastal regions, both on the east coast and west coast of Australia, has brought many more people into close contact with mosquitoes. In addition, as we try to right the wrongs of past environmental damage, the rehabilitation of wetlands may inadvertently increase the productivity of some wetlands.
What impact future climate change may have on saltmarsh mosquitoes is difficult to predict. In regions where hot dry summers become more frequent, mosquito populations may increase. However, if “wetter” summers are experienced, mosquito populations may actually be kept at moderate levels. Sea level rise may mean that a greater proportion of saltmarsh is more regularly inundated but that may actually adversely impact mosquitoes as it will promote the landward movement of mangroves, increase the duration of saltmarsh inundation and with a greater volume of tidal water entering the marsh, a greater chance of hungry fish being introduced.
It looks like a long, hot and dry summer will mean a boost in coastal mosquito populations but while there is still plenty of gaps to be filled in our knowledge of saltmarsh mosquitoes and their interactions with climatic and environmental conditions, at least we know there is an upside to all this mosquito activity….the bats won’t go hungry.