The recent discovery of Aedes albopictus (Skuse) in the Torres Strait of northern Australia was quite alarming as this mosquito is known to be a significant pest species and arboviral vector in the regions it has already become established. With few studies conducted on Ae. albopictus populations from the Torres Strait a project was established to identify the potential range expansion of this species into Australia, as well as potential implications for local arbovirus transmission cycles.
To supplement theoretical projections addressing the range expansion of Ae. albopictus into Australia, laboratory based trials were conducted to assess the performance of a Torres Strait Ae. albopictus population under a range of Australian climates. First instar larvae were placed in individual microcosms and maintained on a natural food resource, under average climatic conditions representing different cities of Australia’s east coast. The population performance of Ae. albopictus under the different climates was assessed and compared using a population performance index. Results suggested that Ae. albopictus could survive in northern Australian and during the summer months in southern Australia, but were not able to survive winter as larvae in southern Australia. Therefore, the potential for Ae. albopictus eggs to remain viable when exposed to four different temperature (7°C, 17°C, 27°C and 33°C) and relative humidity (35%, 55%, 80%) combinations was assessed. Following embryogenesis and 3 months exposure to the different combinations, those eggs maintained at high temperatures and low humidity were not viable. However, those eggs maintained under cooler climates remained viable after 3 months.
Another ecological principle that has the potential to influence the colonisation and establishment of Ae. albopictus in Australia is cohabitation with native species that occupy the same habitats. To test this hypothesis cohabitation experiments were performed with Ae. albopictus and the endemic Aedes notoscriptus, which has been found occupying the same larval habitats as Ae. albopictus in the Torres Strait. To investigate the influence of environmental factors on cohabitation between the two species different climates, food resource levels, food resource types and species proportions were provided. Survivorship proportions and per capita rate of change (λ’) were compared. Results suggested that Ae. notoscriptus would not prevent the establishment of Ae. albopictus in Australia, probably due to the negative effect conspecific densities had on the endemic species.
Laboratory-based infection and transmission trials were conducted to determine the potential for Ae. albopictus from the Torres Strait to become involved in arbovirus transmission cycles in Australia. The four major Australian endemic arboviruses, Murray Valley encephalitis virus (MVEV), Kunjin virus (KUNV), Ross River virus (RRV) and Barmah Forest virus (BFV) were tested in this study as well as the exotic Japanese encephalitis virus (JEV), which range expanded into Australia during the 1990’s. Following exposure to viruses and 10-14 extrinsic incubation period, it was demonstrated that Ae. albopictus from Masig Island were moderately competent vectors of RRV and BFV, and less efficient vectors of MVEV, KUNV and JEV.
As Ae. albopictus is a competent vector of both chikungunya virus (CHIKV) and RRV experiments were conducted at different temperatures (22°C and 28°C) to assess potential differences in virus growth kinetics and extrinsic incubation periods required for transmission. It was evident for both viruses that lower temperatures resulted in lower infection and transmission rates although there were higher body titres. The minimum time between Ae. albopictus consuming an infected blood meal and transmitting CHIKV was 2 d at 28°C and 4 d at 22°C, and 4 d for RRV irrespective of temperature.
The outcomes of this thesis demonstrate the introduction of Ae. albopictus from the Torres Strait islands into Australia could facilitate a range expansion throughout the east coast of Australia, where it could become involved in arbovirus transmission cycles. When exposed to average tropical Australian climates Ae. albopictus is able to proliferate throughout the year. Under average temperate Australian climates Ae. albopictus will proliferate through the summer and overwinter as eggs. Although cohabitation with the endemic Ae. notoscriptus is detrimental to survivorship and population performance of Ae. albopictus, there is no evidence that it would prevent establishment. Aedes albopictus is capable of transmitting the major Australian endemic arboviruses, as well as the exotic JEV, DENV-3 and CHIKV. High transmission rates and low extrinsic incubation periods for RRV and CHIKV under average summer temperatures in temperate Australia suggests the establishment of Ae. albopictus on the east coast of Australia has the potential to increase enzootic and epizootic arbovirus transmission as far south as Melbourne