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ARCHIVED - Using Pesticides to Control Mosquitoes

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Who decides whether to use pesticides to control the spread of West Nile virus?

Provincial and local health authorities are responsible for deciding whether to use pesticides to control the spread of West Nile virus. Some of the factors they may consider include:

  • the number of mosquitoes in the area
  • the extent of West Nile virus activity in local mosquito populations
  • the degree of risk the mosquitoes pose to people in the area
  • the size of the area to be treated
  • the number of people who live nearby
  • whether it is early or late in the mosquito season
  • whether other options for mosquito control would be effective in reducing risks to people
  • how the community feels about pesticide programs
  • the extent of local West Nile virus activity in birds, horses, other animals and people

If provincial and local authorities decide to spray, what products would they use?

It depends on the time of year and the current stage of development for local mosquito populations. There are four stages in a mosquito's life cycle: egg, larva, pupa and adult.

Egg: Mosquitoes lay eggs in clusters, in standing water. The eggs float on the water's surface, sometimes in rafts of a hundred or more. Most eggs hatch into larvae within 48 hours.

Larva: Larvae live in the water and come to the surface to breathe. They shed their skin four times and become larger each time. After the fourth molt, a larva becomes a pupa.

Pupa: This is a resting, nonfeeding stage that lasts for about two days. When the mosquito is fully developed, the pupal skin splits and an adult mosquito emerges.

Adult: The adult mosquito rests on the water's surface while all of its parts harden and its wings dry. After this, it is ready to fly.

Mosquitoes can only be controlled when they are larvae or adults. Products that control larvae are called larvicides. Products that control adult mosquitoes are called adulticides.

How do larvicides work?

Larvicides are chemical or biological agents that are applied directly to standing water where mosquito larvae live. They control mosquito populations by preventing the larvae from developing into adults. Larvicides are most effective when used early in the mosquito season, from May to July.

How do adulticides work?

Adulticides are chemical agents. They are applied to vegetation where mosquitoes rest, or are sprayed into the air as fine droplets. They control mosquito populations by killing the majority of adult mosquitoes. Adulticides are applied in very low concentrations. They are typically used at night or early in the morning, when mosquitoes are most active and most people are indoors.

Which type of pesticide product do authorities prefer to use?

In general, the use of larvicides is preferred to the use of adulticides, for the following reasons:

  • Larvicides are more targetted because they are applied directly to mosquito breeding areas.
  • Larvicides control mosquito populations before they become blood-feeding adults.
  • Larvicides may reduce the need to use adulticides. This may result in an overall reduction in pesticide use.

Larvicides and adulticides are part of an integrated approach to mosquito control that also includes surveillance and reducing sources of standing water where the larvae live. It may be necessary to use adulticides where there are large numbers of adult mosquitoes, or where there is an imminent risk of human infection.

What steps do provincial/local health authorities take to ensure public safety when pesticides are used?

All workers who carry out mosquito control programs are licensed by provincial authorities and are trained in the safe use of pesticides. If local authorities decide that the risk of West Nile virus infection warrants the use of pesticides in a certain area, they will notify the public in advance. This will enable people to take precautions and minimize their exposure to pesticide products.

Are mosquito control products safe?

Pesticides are carefully regulated and must be registered by Health Canada's Pest Management Regulatory Agency (PMRA) before they can be used in Canada. As part of the registration process, the products undergo a rigorous scientific assessment to determine whether they can be used safely when used according to label directions. The assessment looks into many areas, including risks to human health and the environment and whether the product is effective. The PMRA refuses to register pesticide products that are ineffective or pose unacceptable risks.

What can people do to help control mosquito populations?

Mosquitoes need standing pools of water to breed, so it is a good idea to get rid of standing water from such items as pool covers, saucers under flower pots, pet bowls, wading pools and old tires. Empty bird baths twice a week. Cover rain barrels with screens, and clean eaves troughs regularly to prevent clogs that can trap water. For more information on steps you can take to reduce mosquito breeding grounds on your property, see Health Canada's factsheet: West Nile Virus: How to Protect Yourself and Your Family. For more detailed information see the Fact Sheet on Effective Control of Mosquitos Around Your Home at Link.

For more more information about larvicides and adulticides, the regulation of pesticides, and reducing mosquito populations, visit the Mosquito Control page of the Pest Management Regulatory Agency Web Site at Link, or call 1-800-267-6315 (toll-free in Canada).