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Facts About...


Why should mosquito populations be reduced?

West Nile virus (WNv) is a mosquito-borne illness that can be transmitted to humans by the bite of an infected mosquito. Reducing mosquito populations helps to reduce the spread of WNv and other infections spread by mosquitoes, thus reducing the risk of disease in humans.

Why should we be concerned about stagnant water?

Mosquitoes lay their eggs in water that is stagnant, shallow and high in organic matter. Mosquitoes complete three stages of their life cycle in the water (egg, larva and pupa) in about 4 to 14 days. As a result, standing water must remain stagnant for a minimum of 4 days in order to support the mosquito’s life cycle. Elimination of potential mosquito breeding sites is the primary control measure used to reduce mosquito populations.

Not all bodies of water are ideal breeding sites for mosquitoes. For example, water accumulations on roof tops are generally not an issue due to wind action and a lack of organic matter.

How can I reduce or eliminate stagnant water…

Some sources of stagnant water can be drained or removed so mosquitoes cannot breed (e.g. change water in birdbaths weekly). Others sources, like ponds, require good design and construction plus regular maintenance to make them unsuitable breeding sites for mosquitoes. This includes ensuring a good location, drainage, wind and adjustment of water levels.

Some bodies of water such as ditches, storm water management ponds and natural wetlands cannot be removed as they play important roles in storm water drainage and maintenance of the environment.

…on my property?

The most common mosquito species associated with WNv is the Culex species. This species is usually an urban-dwelling, container-breeding mosquito. Culex mosquitoes generally do not fly very far. As a result, adult mosquitoes are usually found close to their breeding sites. The following simple steps can be taken to eliminate potential mosquito breeding sites in and around your home:

…in an ornamental pond?

What about ditches?

Ditches can be potential mosquito breeding sites if they are not properly designed and maintained. Regional and municipal public works departments monitor and maintain ditches on their property to ensure proper drainage and water flow. For more information, contact your local public works department.

Ditches on private property should be routinely maintained (e.g., mowed, sediment and debris removed) to prevent mosquitoes from breeding in the area. Alternatively, depressions and unnecessary ditches can be filled with clean soil to reduce the ponding of water. Check municipal “fill” by-laws prior to undertaking such action.

What about storm water management ponds?

Storm water management ponds (SWMPs) are not generally considered to be good mosquito breeding sites. Changes in water level and exposure to wind are some of the factors that help deter mosquitoes from laying their eggs along the surface of the water. However, where rock formations, vegetation and/or debris exist, there may also exist pockets of standing water where mosquitoes might be encouraged to breed.

In 2003, the Durham Region Health Department conducted a study of storm water management ponds to determine whether these conditions give rise to potential breeding sites. For a summary of this study please visit our website at - West Nile virus.

What about natural wetlands such as lakes, swamps or streams?

Research suggests that stagnant water pockets in urban areas (e.g. roadside catch basins, old tires) will tend to contain the highest populations of the species of mosquitoes commonly associated with WNv. While natural wetlands can also be potential mosquito breeding sites, they do not tend to produce the same mosquito species. Natural wetlands are balanced ecosystems that tend to contain predatory fish, birds, frogs and insects that will help control mosquito populations. These natural water bodies require special consideration, as they are an essential part of the water cycle and they play an important role in ensuring water quality and quantity. Since 2003, the Toronto and Region Conservation Authority (TRCA) has been conducting yearly monitoring of the larval mosquito populations in a number of natural areas (e.g. marshes, ponds, woodland pools, etc.) throughout Durham Region. The results of this monitoring program are available on the TRCA website. (see the TRCA contact information below).

It is recommended that you consult with the following agencies prior to implementing any stagnant water management activities in a natural wetland area:

Ontario Ministry of Natural Resources (Aurora Office)

50 Bloomington Road West, R.R. #2
Aurora, Ontario L4G 3G8
Phone: (905) 713-7387

Central Lake Ontario Conservation Authority

100 Whiting Avenue
Oshawa, Ontario L1H 3T3
Phone: 905-579-0411
Fax: 905-579-0994

Toronto Region Conservation Authority

5 Shoreham Drive
Downsview, Ontario M3N 1S4
Phone: 416-661-6600
Fax: 416-661-6898

For more information on West Nile Virus, visit our website at

More information can be obtained from

Durham Region Health Department, Environmental Help Line, 905-723-3818 or 1-888-777-9613.

April 22, 2013