Mosquito-borne diseases cause mortality and morbidity worldwide. Diseases, such as malaria, dengue and yellow fever, have emerged or re-emerged in recent decades and are often induced by anthropogenic land use changes. Deforestation, road construction and conversions of land use (e.g. from natural environment to urbanisation or agriculture) are examples of human modified environments. These modifications alter original habitats and species compositions and, in relation to mosquito-borne diseases result in novel juxtapositions of vectors, hosts and pathogens. Hot spots for the emergence or re-emergence of mosquito-borne diseases are tropical regions due to high biodiversity, vast land clearances and human encroachment into these areas. Furthermore, remote, tropical regions are especially vulnerable to the emergence of mosquito-borne diseases as surveillance can be logistically demanding and expensive and thus prevent early disease detection.
My thesis examined if mosquito communities respond to land use changes in tropical Australia and additionally, if a newly developed technique to capture adult mosquitoes can be applied in remote localities. I explored mosquito communities across anthropogenic disturbance gradients in several ways: (1) capturing adult mosquito from three habitats (man-made grassland, forest edge and rainforest interior) during the wet and dry season in a peri-urban environment near Cairns; (2) sampling immature mosquitoes from small, artificial containers from the same habitats; and (3) capturing adult mosquitoes from urban and sylvan habitats from four remote islands (Saibai, Boigu, Badu and Moa) in the Torres Strait where I used a novel trap design.
To evaluate if mosquito communities are influenced by anthropogenic land use, I carried out adult mosquito sampling in three habitats (man-made grassland, rainforest edge and rainforest interior) in the outskirts of Cairns and in two habitats (sylvan and urban) in the Torres Strait. I found that adult mosquito communities varied in response to anthropogenic modified habitats in both locations. The Cairns sampling revealed that the mosquito community from rainforest interior was distinctly different to the grassland community and that forest edge acted as an ecotone with shared communities from both forest interior and grasslands. I also found that important vector species (Aedes vigilax, Culex annulirostris) were able to persist all year round and occurred mainly in grasslands (Chapter 2). A similar pattern was evident from the Torres Strait sampling where urban and sylvan habitats supported distinctly different mosquito communities with disease-competent species, such as Aedes albopictus, Aedes aegypti and Culex quinquefasciatus occurring more in urban areas than in sylvan habitats (Chapter 5).
I sampled immature mosquitoes from small, artificial ovitraps across the three habitats in Cairns and from two trap locations (traps were either placed on the ground or above ground) to evaluate female oviposition preferences. I found that most species chose to lay their eggs in grassland traps and that none of the species preferred to oviposit in forests traps. I also found that traps located on the ground had four times more emergents than traps located above ground. Aedes notoscriptus, an important disease vector, was mostly reared from grassland traps. Additionally, I observed that water temperature (ranging between 13.7°C and 43.5°C) had no influence on the number of emergents and that mosquito eggs were able to hatch in instalments (Chapter 3).
Mosquito sampling in remote areas poses unique challenges for disease surveillance and detection. Sampling female mosquitoes is heavily dependent on attractants to lure them into traps. Carbon-dioxide (CO₂), in the form of dry ice or from gas cylinders, is commonly used. However, dry ice is unavailable in remote areas and gas cylinders are difficult or even prohibited to transport. I therefore aimed to assess the usefulness of CO₂ derived from sugar and yeast as an attractant and trialled different CO₂ concentrations in temperature controlled experiments. The concentrations which produced the most CO₂ were then compared to dry ice in field situations (in three tropical forest habitats). I found that traps baited with dry ice captured more mosquitoes than yeast-baited traps; but more importantly that there were no differences in the mosquito community composition (Chapter 4). An additional challenge is that most mosquito traps require a source of electricity which is rarely obtainable in remote field locations. I developed a novel sampling technique to capture mosquitoes by coupling a non-powered trap with CO₂ derived from sugar/yeast fermentation (Chapter 5).
Conventional disease detection methods involve sentinel animals, such as chickens or pigs, or large pools of dead mosquitoes which can be very expensive and labourintense in remote areas. A suitable alternative are honey-soaked Flinders Technology Associates cards (FTA® cards) which preserve viruses but at the same time deactivate them. Mosquitoes taking a honey-meal from the FTA® cards expel saliva which can be used for disease detection by eluting viral RNA. I used four FTA® cards in each of the non-powered traps in the Torres Strait and even though weak infections were initially detected, they were not significant (Chapter 5).
In summary, my thesis demonstrates that mosquito communities in peri-urban environments and on remote islands in tropical Australia are strongly influenced by land use. This could have potential impacts for disease transmission to humans, domestic animals and wildlife, especially where immense anthropogenic pressures continue to change natural environments irrevocably. Strong projected growth in human population and the subsequent demand for space will further impact already fragile environments.
My thesis may contribute to achieve a more cost-effective and logistically less demanding method to monitor mosquitoes in remote localities and thus allowing permanent and continuous disease surveillance
