West Nile Virus

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West Nile Virus and Mosquito Habitat
- Natural Environments - Wetlands & Flood Plains
- Man-make Environments - Storm Water Management Ponds, Catch Basins, Irrigation and Drainage Ditches, Small Private Ponds

West Nile Virus and Mosquito Habitat

One misconception is that any type of standing water, such as wetlands and waterways, may produce large numbers of virus-infected mosquitoes and that all potential sites should be drained, filled, sprayed or managed to eliminate the possibility of WNV transmission. However, not only is it impossible to eradicate all mosquitoes, but not all water bodies are home to the mosquito species that propagate WNV. Instead, mosquito-producing habitats vary depending on the mosquito species. Since not all mosquito species that feed on humans transmit WNV, control measures for West Nile Virus will vary depending on the habitat.

In general, activities that encourage the presence of natural mosquito predators and reduce the amount of artificial or disturbed habitat will assist in reducing the mosquito species that carry West Nile Virus.

Natural Environments

Wetlands

Mosquitoes are common in natural wetlands. However, the mosquitoes that are responsible for transmitting West Nile Virus, such as Culex species, are not common in natural wetlands (Chipps et al. 2002). Results from monitoring studies conducted in southwestern Ontario (Perth County) in the summer of 2002 show the same trend.

Healthy wetlands have features that reduce the number of mosquitoes. Mosquitoes are an important part of the food chain and healthy wetlands are home to hundreds of mosquito-eating aquatic insects (beetles, back swimmers, water striders, dragonflies, etc.), birds, amphibians and bats. This balanced predator-prey relationship provides natural mosquito control. In addition, the water in a wetland is constantly stirred by the wind and water levels fluctuate, all of which helps reduce the number of mosquitoes.

Recognizing the valuable role of wetlands, their preservation is extremely important. If a wetland is disturbed by humans or if other life forms are eliminated through the incorrect use of pesticides, it is possible that the number of mosquitoes in a wetland may actually increase. Therefore, it is important to preserve the natural balance in a wetland. Considering the number of wetlands that have been lost, each remaining wetland is that much more important to the health of the ecosystem.

Flood Plains

Culex mosquito species are not common in natural flood plains but Aedes species are. Flood plains are also home to a variety of other species that prey on mosquitoes, and altering these areas to eliminate mosquitoes will also affect these species as well as increase sedimentation, erosion, etc., downstream. Altering fish habitat is strictly regulated under the Federal Fisheries Act. Since Culex and Aedes are relatively weak fliers, the best preventative measure is to avoid these areas.

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Man-Made Environment

Storm Water Management Ponds

Storm water management ponds are designed to hold and treat runoff (i.e. rain or melting snow) from nearby land. Runoff may be contaminated with soil, salt, oil and other residues. In the pond, pollutants settle out and are trapped by the soil as the water slowly percolates down to the water table. These ponds are often constructed in subdivisions and help to keep sediments and contaminants out of rivers and streams as well as reducing erosion and flooding. Thus, they provide a great number of benefits to the community as a whole.

Storm water management ponds are generally not breeding areas for the virus-transmitting mosquito species. In fact, the chance of finding a significant number of Culex species in a storm water pond is much, much smaller than of finding them in artificial containers in your backyard. There are several reasons for this:

Wet Ponds are constructed to hold large volumes of water to improve water quality and will contain water most of the time. They tend to be deep (Culex species primarily breed in shallow areas), have a large surface area (the wind will destroy egg rafts of Culex species so they do not breed in these environments) and have steep banks along the sides (which discourages Culex from breeding there). These ponds usually have a concrete bottom so that sediments and debris can be regularly cleaned out.
Dry Ponds are designed to help reduce flooding during wet periods but otherwise will be dry. These areas usually have channels that water flows through during a storm (Culex will not breed in moving water), and will then slowly dry up. These types of ponds should not hold water for more than 48 hours as the potential for mosquitoes using them will increase. Some of these areas are more naturalized with vegetation that may attract mosquitoes (as well as their predators).

Storm water management ponds that are perceived to be mosquito breeding areas should be monitored for the mosquito species responsible for transmitting West Nile Virus. If the local Medical Officer of Health decides that a storm water management pond has raised the risk of West Nile Virus spreading, the pond may be treated to control mosquito larvae.

Catch Basins

Catch basins are one of the main breeding sites for Culex species. The depth of the catch basin does not appear to affect mosquito population numbers, but organic material is very important (i.e. putting grass clippings, leaves and other plant material into sewers will increase the number of mosquitoes). Catch basins in newer subdivisions have fewer Culex species because of the lack of vegetative material, while older neighbourhoods have higher numbers of these mosquitoes.

Catch basins in older neighbourhoods with dense population and/ or retirement homes nearby should be monitored for the mosquito species responsible for transmitting West Nile Virus. If the local Medical Officer of Health decides that the catch basins in a neighbourhood are at a high risk of West Nile Virus, the catch basins may be treated to control mosquito larvae.

Irrigation and Drainage Ditches, Small Private Ponds

Drainage ditches provide prime mosquito producing habitat because these areas typically contain warm, standing water that is loaded with nutrients. Sites can be made unsuitable for mosquitoes to lay eggs in by:

designing steep slopes and gravelled shore lines,
manipulating water levels,
controlling emergent vegetation,
improving drainage,
infilling small wet pockets,
running a narrower and deeper ditch through centres of broader ditches.

If the property owner has shown that virus-transmitting mosquitoes are present in these areas, that these water bodies are located entirely within the boundaries of their property (with no direct or indirect outflow beyond their property boundary), and that the property is not accessible to the general public, then an over-the-counter biological larvicide (such as Bti or AquaBac) may be applied without a permit.

Golf Courses and Gravel Pits

In semi-natural environments (such as golf courses and gravel pits), there may be a mixture of natural and artificial water features. Each area should be examined separately for its potential to breed mosquitoes. Mosquito populations can be reduced or they can be discouraged from laying eggs by:

stocking ponds with fish,
adding a fountain or aeration unit to create surface (wave) movement,
redesigning ponds to discourage Culex species (i.e. steep banks, deep water, mow vegetation to the edge),
regularly skimming the surface along the edges (where water tends to be stagnant) to remove mosquito larvae,
using larvicide in ponds where there is no drainage off the site.

Encouraging personal protection measures (i.e. long sleeved shirts, DEET on clothing, avoiding dawn and dusk) for people using the area will further reduce the risk of exposure to West Nile Virus.

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