The seventh mosquito surveillance period for the 2018-19 season along the River Murray commenced on the 17th of March and included the seven district councils of the Murray River, South Australia, with 35 traps set. Overall mosquito numbers exceeded collections made in March 2018 in all councils with the exception of Alexandrina Council. Collections also approximated or exceed historic mean collections in all councils except Renmark Paringa Council.
While mosquito numbers have climbed in the upper river and exceeded collections in the previous season overall collections remain low at the majority of locations in the upper sections of the River Murray. Collections have reduced in the more southern regions of the Murray River (Figure 1). The Bureau of Meteorology three-month outlook indicates rain-induced mosquito issues continue to remain unlikely. This does not exclude issues such as river management such as wetland inundation or other river manipulations. The high numbers collected in the District Council of Loxton Waikerie and Berri Barmera Council are of concern as these collections do not correlate to any rain event or increase in river flow. This increase in mosquito numbers also includes high numbers of Culex annulirostris a species considered to be a major vector of arbovirus in inland Australia. The nearby Yatco Lagoon has recently undergone a drying event and been refilled. Inundations such as this have a risk of increasing the potential larvae sites for a variety of mosquito species1
There has been little significant rainfall along the length of the river so far in 2019. As a result, there has been little larvae activity, many of the areas holding water already colonized by aquatic invertebrates such dragonfly larvae. Therefor addition treatment was only conducted in Alexandrina Council conjunction with the surveillance trip in March 2019. The daily maximum temperatures have been high so far this year with the daytime temperature reaching 43.3°C at Murray Bridge and 41.1°C in Renmark on the 3rd of March. The temperatures have reached over 40°C on a number of other occasions this year producing poor conditions for the recruitment of many mosquito species.
Overall average mosquitoes collections along the River Murray exceeded the previous years collections in March in all councils with the exception of Coorong Council. Collections approximated the long term means within most councils however, they only around two thirds the long term mean in Mid-Murray council and a fraction of long term averages in Renmark Paringa Council. Collections made in District Council of Loxton Waikerie were over four times the average historic collections (Figure 1). Overall, mosquito collections from the ‘upper’ and ‘middle’ river councils were the most diverse of all regions with ten and eleven species identified from the 13 and 9 EVS traps collected respectively with the most common species Culex annulirostris only making up 72% of all specimens in the upper river councils while Culex quinquefasciatus was the most common species in the middle river comprising 27% of collections (Figure 2).
The reason for the presence of these species is hard to determine, however the source is probably environmental watering of the nearby Yatco Lagoon. This lagoon has been emptied but the water level has recently raised. Increases in the species Culex annulirostris is a concern and is one of the risks to the public that should be considered when conducting inundations, particularly in late summer until March.
Larvae sites would probably be on the far side between the river and Yatco lagoon, probably isolated pools. The best option for these locations is to be forewarned of watering or refilling events and conduct an inspection shortly afterwards to identify the sites prior to numbers building up.
Around 20% of the remainder of collections was made up of Anopheles annulipes along with a diverse array of species. Expected dry hot conditions, it is unlikely that this species mosquito will increase over spring in this region without further environmental watering or irrigation sources.
The middle river was the most diverse region, with eleven mosquito species collected, however, Culex quinquefasciatus was the most numerous comprising 27% of individuals collected (Figure 3). The majority of mosquitoes were collected from trap sites in the Rural City of Murray Bridge. Overall, the number of mosquitoes collected in the middle River Murray was much higher than the previous season and approximated the historical average collections within Rural City of Murray Bridge and was around two thirds the Mid Murray council (Figure 1). The section of the Morgan Conservation park being watered has subsided and no mosquitoes attempted to land when the site was investigated. Unfortunately, the nearby trap failed overnight however, there remains an abundance of dragonflies and damselfly nearby. This will provide an abundance of predation pressure on any mosquito larvae produced.
The middle river was the most diverse region, with eleven mosquito species collected, however, Culex quinquefasciatus was the most numerous comprising 27% of all individuals collected (Figure 3). As with the previous month, the majority of mosquitoes were collected from trap sites in the Rural City of Murray Bridge. Overall, the number of mosquitoes collected in the middle River Murray was much higher than the previous season and approximated the historical average collections within Rural City of Murray Bridge and was around two thirds the Mid Murray council (Figure 1). The section of the Morgan Conservation park being watered has subsided and no mosquitoes attempted to land when the site was investigated. Unfortunately, the nearby trap failed overnight however, there remains an abundance of dragonflies and damselfly nearby. Larvae of these insects are aquatic predators and heavily impact on mosquito larvae.
Various insects at Morgan Conservation park.
The diversity of adult female mosquitoes collected in the lower river was low with only six species identified. Aedes camptorhynchus no longer dominated collections, representing only 14% of individuals collected (Figure 4). Populations of Coquillettidia linealis also declined to represent only 9% of all female mosquitoes collected. The bulk of mosquitoes collected were Culex species such as Cx. molestus and Cx. globocoxitus.
Collections were broadly similar to the previous season in both Coorong District council and Alexandrina council. (Figure 1) In the Coorong council collections remained highest at Poltalloch Bluff. While Clayton Bay produced the highest collections in Alexandrina Council primarily Ae. camptorhynchus, and Cq. linealis.
The variation in the level of the lower lakes appeared to have little influence on the mosquito populations sampled in March 2019. Variation in lake level can from time to time result in the inundation of the coastal saltmarsh that fringes portions of the lower lakes, this can produce large populations Aedes camptorhynchus, particularly in spring.
Looking forward the Bureau of Meteorology season outlook suggests conditions are expected to continue to be drier than average over the next three months over the eastern portions of the state. There is no strong indication of either dryer or wetter conditions over the remainder of the state. Daily Maximum and Minimum temperatures are predicted to be much warmer than long term trends over the continent.
El Niño-Southern Oscillation (ENSO)
El Niño-Southern Oscillation (ENSO)
El Niño conditions remain neutral but near El Niño thresholds, with models indicating there remains a chance of an El Niño event developing in autumn.
The Indian Ocean Dipole (IOD)
The IOD is currently neutral with the weekly index weakly negative. However, the influence of this feature is limited over December through to April 2019.
ROSS RIVER VIRUS RISK FORECASTS
Upper River Murray risk for April to June: The predicted incidence rate for late summer and autumn in the upper Valley in SA is 0 per 100,000 population. The predicted number of notifications for the region for the next three months is 0, (95% confidence 0-9 ). This does not constitute above-average risk.
Mid River Murray risk for April to June: The predicted incidence rate for late summer and autumn in the middle Valley in SA is 10 per 100,000 population. The predicted number of notifications for the region for the next three months is 3, (95% confidence 0-57 ). This does not indicate above-average risk.
Lower River Murray risk for April to June: The predicted incidence rate for late summer and autumn in the lower Valley in SA is 0 per 100,000 population. The predicted number of notifications for the region for the next three months is 0, (95% confidence 0-7). This does not indicate above-average risk
14 Honey baited FTA cards have been dispatched from along the Murray river in arch. All cards have screened negative for arboviruses in the 2018-19 season. 14 Cards have been sent for screening from collections in March 2019.