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West Nile Virus Monitoring


TRCA assists the Public Health Units of Peel, Durham, York and the City of Toronto by monitoring larval mosquito populations in selected natural wetlands and storm water management ponds on TRCA lands across its jurisdiction.

The TRCA West Nile Virus (WNV) Monitoring Program was established in 2003 to conduct vector larval surveillance for the presence of two key vector mosquito species, Culex pipiens and Culex restuans, on TRCA properties.  The work complements WNV vector source reduction activities carried out by TRCA's Regional Health partners in Durham, Peel, York and the City of Toronto.  In addition to monitoring, the WNV program includes public education and collaboration with Regional Health Units.  Concerns raised by citizens or staff are addressed through TRCA's Standing Water Complaint Procedure, and TRCA participates on Regional WNV advisory committees, and in information sharing/notification about vector hot spots.

Surveillance comprises an ongoing seasonal assessment of selected TRCA natural wetlands and Storm Water Management Ponds (SWMPs) to determine the presence of WNV vector larvae, the abundance of both vector and non-vector larvae, and to identify vector "hot spots". 

For more information on West Nile Virus see our Fact Sheetpdf icon


Our Approach

Selected monitoring sites are visited four times per season from May through September. Using a standard dipper, 10 dips are collected, the larvae counted and pooled into a replicate sample.  Four replicate samples are taken per visit and subsequently identified to species in the lab.

Risk ranking for each site is assessed for each vector species based on the average number of vector larvae found in 4 replications. A site is ranked as:

nil/no risk site if no vector larvae are present

low risk site if the average number of vector larvae collected is below 2 per sample

moderate risk site if the average number of vector larvae collected is between 2-30 per sample

high risk site if the average number of vector larvae 10 dips is greater than 31 per sample

Measurements of water temperature, pH, electrical conductivity, total dissolved solids and dissolved oxygen are also taken to quantify the relationships between mosquito species presence/abundance and water quality parameters.  Qualitative Information about water clarity, the type of predators present at the time of site visit, marginal and total vegetation is also recorded.

Results and Highlights

TRCA has monitored mosquito larvae in 36 wetlands and 9 storm water management ponds over the last 5 years.  Results have consistently shown differences between natural wetlands and storm water ponds in terms of the species and abundance of mosquito larvae found.



The majority of the mosquito larvae collected in natural wetlands over the past 5 years have been non-vector species.  This suggests that naturally functioning wetlands do not generally support high numbers of vector species. Wetlands were also found to have a highly diverse mosquito community, with the non-vector Culex territans being the most common species found.  Overall, natural wetlands had a higher total abundance of mosquito larvae and showed increases in years with higher precipitation.


Storm Water Management Ponds (SWMP)

Storm water management ponds were found to have an overall lower total abundance of mosquito larvae than wetlands, and a lower species diversity, but a higher proportion of vector species.  The vector Culex pipiens was the most common mosquito species found in storm water management ponds.


Wetlands have high species richness and abundance of mosquitoes, with most being non-vectors for WNV

Storm water management ponds have a lower species richness and abundance with a higher proportion and abundance of vectors of WNV

Since both wetlands and SWMPs contain occasional isolated pockets of high densities of larvae, continuous monitoring is warranted

Overall, the number of both wetlands and SWMPs ranked as high risk sites is low


All mosquitoes are carriers of WNV

Presence of large number of mosquitoes is always a serious concern for WNV

Wetlands pose significantly higher risks than SWMPs






Wetlands and West Nile Virus - Fact and Fiction.  2010 GTA Conservation Authorities Watershed Monitoring Forum


Ontario Ministry of Health and Long-term Care -

Peel Public Health -

Toronto Public Health -

York Region Public Health and Safety -



Climate Change Implications

The increase in frequency or magnitude of storm events caused by a changing climate may lead to the creation of more stagnant pools or prolong the period that water stands in poorly drained areas or storm water management ponds.  This in turn could provide additional habitat for West Nile Virus vector mosquito larvae. 

The WNV Surveillance program has a substantial baseline of data collected from natural wetlands and SWMPs through years of moderate and normal seasonal precipitation.  Data collected in the future will help identify significant changes in the presence of WNV vector mosquito larvae that may result from more severe weather.  TRCA will work with the various Regional Health Units to manage standing water and to treat areas with high numbers of WNV vector larvae.

West Nile Virus:

West Nile Virus (WNV) is a mosquito borne virus that normally cycles between birds and adult mosquitoes. On rare occasions, the virus is spread to humans through the bite of an infected mosquito. This is likely to happen when the virus amplifies in the bird population through a cycle of bird-mosquito-bird transmission. The cycle is driven predominantly by mosquitoes that feed exclusively on birds (enzootic vectors).  As greater numbers of birds become infected, there is an increased chance that the virus will spill over into the human population through mosquitoes that feed on both birds and mammals (bridging vectors).