Mosquito management for 2020/21 was the most intense it's ever been, a City of Mandurah report has revealed.
Relentless tidal inundation to the wetland breeding grounds proved havoc for the management effort with 25 aerial larviciding treatments completed over a total of 6808.1 hectares.
This surpassed the previous record of 5446 hectares in season 2011/12.
The task to keep down the pesky insects is held by a City of Mandurah team in conjunction with the Peel Mosquito Management Group (PMMG) and Department of Health.
At a City council meeting on November 23, councillors were presented with the findings of the 2020/21 Mosquito Management Annual Report.
By the end of September 2020, the Bureau of Meteorology confirmed that a La Nina event was underway and was likely to persist into early 2021.
La Nina occurs when equatorial trade winds become stronger, changing ocean surface currents and drawing cooler deep water up from below. This results in a cooling of the central and eastern tropical Pacific Ocean.
Those significant rises in tidal inundation events and intensity lead to greater hatching events, which means more intervention by the management program is necessary.
The La Nina event in Spring 2020 was felt immediately by the program with significant tidal inundations over a six week period from late August into early October resulting in 1,059 hectares being treated.
"When we get tides of about 0.7 metres or above that results in flooding to wetlands," Mosquito operations officer Scott Severn said.
"The increase in tides up to one metre high were significant because it meant all breeding sites were flooded and there was mass hatching across the region."
The size of aerial treatments increased from late August and this remained the case until late May.
January 2021 brought the most intense and challenging workloads for the program with four large scale treatments being required and resulted in an area of more than 1400 hectares being treated in four weeks. The only other time this intensity of aerial larviciding has been required was in January 2012.
The increase in mosquito hatching was felt across the Peel region with a total of 119 mosquito complaints/enquiries recorded for the 2020/21 season compared to 17 the previous years.
According to the report, the majority of complaints in relation to mosquito nuisance were received early in the year and were "no doubt a direct result of the spikes in abundance of the vicious biting Aedes vigilax species during these months".
Ross River Virus (RRV) cases increased in Mandurah over the season with 47 recorded compared to 29 in 2019/20.
Similarly, season 2020/21 recorded a state wide increase of 156 per cent with 816 reported cases compared 318 in the previous season.
A range of factors including the environmental conditions, virus replication cycles and an increase in the abundance of mosquitos statewide can be assumed to have contributed to the elevated incidence of RRV across the state.
Cases of Barmah Forest Virus for Mandurah did not increase and there was one case reported for 2020/21, as was the case in 2019/20.
Residents can find out more about mosquitoes in Mandurah by visiting, https://www.mandurah.wa.gov.au/live/residents/health-and-safety/pest-control