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revised Thursday July 13 2017

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West Nile



What is larviciding?
Where in Peel will larvicides be applied?
When will larviciding take place?
What larvicides are being used?
Will people be exposed to larvicides?
How are Peel residents being informed about larviciding in Peel?

What is larviciding?

Larviciding is one measure to reduce mosquito breeding. It involves the use of products (larvicides) to control mosquito larvae, which are an immature form of mosquitoes that live in water. There is strict regulation by the Ministry of the Environment and Climate Change on the use of larvicides. Larvicides can only be applied by trained and licensed applicators, who must obtain a permit from the Ministry of the Environment and Climate Change.

In Peel, larvicides are being applied to standing water as granules, pellets or briquets.

Not all mosquito breeding sites will be treated with larvicides. Some sites breed types of mosquitoes that are not important in WNV transmission. Other sites contain mosquito types that are poorly controlled by larvicides.

Where in Peel will larvicides be applied?

The Region of Peel will apply larvicides in approximately 85,000 catch basins (roadside storm drains) in Mississauga, Brampton and in urban areas of Caledon. Catch basins are an artificial environment designed to hold stagnant water for long periods of time. The only option to prevent mosquitoes from breeding there is through the use of larvicides.

Larger artificial bodies of water, such as storm water ponds, are usually not significant mosquito breeding sites. These and other areas will be monitored for larvae and may be treated with larvicides, if necessary. Natural areas of Peel close to population centers, such as marshes and ponds, will also be monitored for larvae. Larvicides may be applied in those areas if there are large numbers of mosquitoes that can carry WNV.

When will larviciding take place?

Larviciding is carried out in the city of Brampton, the city of Mississauga and the town of Caledon each year. Larviciding of surface water sites starts in early June and continues until the end of September. A paint dot on the catch basin grate is used to identify that the catch basin has been treated. The following paint dots on the catch basins will indicate treatment:

First round
Second round
Third round

What larvicides are being used?

The larvicides are methoprene, Bacillus thuringiensis var. israelensis (Bti) and Bacillus sphaericus (Bs).

Methoprene interferes with the development of mosquito larvae into adult mosquitoes. Methoprene is highly effective, has low environmental impact and poses no risk to the health of mammals, including humans. Methoprene is being used in roadside catch basins.

Research has shown that methoprene, when used to reduce mosquito breeding according to directions, does not pose a risk to human health. Direct contact may cause mild eye and skin irritation. Methoprene breaks down quickly in water and soil. It will not remain in the ground and will not leach into ground water.

Bti and Bacillus sphaericus are biological pesticides which kill mosquito larvae before they develop into adults. The active ingredients in Bacillus sphaericus and Bti are naturally occurring soil bacteria. Both products have a similar mode of action - larvae ingest the larvicide, consisting of Bti or Bs spores. The bacteria damage the gut of the mosquito larvae, causing the larvae to starve to death. Bti is being used in surface water breeding sites while Bacillus sphaericus is being used to treat catch basins that drain directly into sensitive sites.

Will people be exposed to larvicides?

No. Larvicides will be applied directly to the water in the mosquito breeding sites. They will not be sprayed into the air. Larvicides will be applied in areas to which people have no access (e.g. catch basins). Other areas where the larvicides may be applied, such as roadside ditches, will have signs posted notifying the public about the use of the larvicides there.

How are Peel residents being informed about larviciding in Peel?

To conform with provincial regulations, the Region of Peel has been informing Peel residents in advance about the timing and location of larvicide applications through a larvacide notice (PDF) in local newspapers.

In addition, signs notifying the public about larvicide applications will be posted at some mosquito breeding sites where larvicide may be used, such as roadside ditches.

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Revised: Thursday July 13 2017


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