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Facts About...


What is larviciding?

Mosquitoes have a 3 stage lifecycle in the water: egg, larva and pupa. Larviciding involves the use of approved chemical or biological pesticides to control mosquitoes at the larval stage, before they can mature into adult mosquitoes and disperse.

Why is Durham Region larviciding?

Durham Region’s West Nile virus (WNv) Vector Control Plan includes the use of pesticides to help reduce and control WNv mosquito populations. The use of larvicides is considered when other measures to reduce and control mosquito breeding sites are inadequate.

Research shows that roadside catch basins are prime breeding sites for mosquitoes associated with WNv. As a result, monitoring and larviciding of regional and municipal catch basins have become necessary components of the Vector Control Plan. Surveillance and/or treatment of standing surface water on regional and municipal property will be based on a WNv risk assessment.

What about private backyard catch basins?

Catch basins in residential backyards may be larvicided under Durham Region’s Vector Control program on a request basis; subject to a local risk assessment.

When will larviciding take place?

Larviciding programs are best conducted through the late spring and summer months. Durham Region contracts a licensed pest control company to conduct its larviciding program. Generally, three catch basin treatments are planned; with the first occurring in mid-June, the second in mid-July and the third in mid August each year. The timing of larvicide treatments is based on larval mosquito surveillance results.

Who regulates the use of larvicides?

The management of pesticides is a federal and provincial responsibility. Health Canada’s Pest Management Regulatory Agency (PMRA) is responsible for assessing pesticides to determine their safety, merit and value. The Ontario Ministry of the Environment (MOE) regulates the sale, use, transportation, storage and disposal of federally registered pesticides in Ontario under the Pesticides Act and Ontario Regulation 63/09. The application of certain larvicides requires a permit from the MOE, and can only be applied by trained and licensed personnel.

What larviciding products will be used?

The Ontario Ministry of the Environment has currently approved only three (3) larvicides for use in Ontario, methoprene, Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus (Bsph).





What is it and how does it work?

Methoprene is a synthetic insect growth regulator that was first registered for use in 1977. This chemical pest control agent prevents mosquito larvae from maturing into adults.

Bacillus thuringiensis israelensis, commonly known as Bti, is a bacterium found naturally in soils. Once in the digestive system of the mosquito, it secretes an enzyme that destroys the insect’s stomach walls. The insect usually stops feeding within hours, and dies within days.

Bacillus sphaericus is a bacterium found naturally in soil and aquatic environments. Once in the digestive system of the mosquito, it secretes an enzyme that destroys the insect’s stomach walls. The insect usually stops feeding within hours, and dies within days.

How will it be used?

Altosid® pellets will be used in targeted catch basins, when mosquito larvae are identified. It is applied in pellet form, and gradually sinks to the bottom of the water. Over the next 21 days, it is slowly released to the surface in small amounts, and eventually eaten by the larvae. Methoprene breaks down rapidly in the environment. Methoprene is not sprayed into the air.

Vectobac® or Aquabac® granules will be applied directly to the surface of stagnant water (ponds, ditches) when mosquito larvae are identified. Bti is suspended in the surface of the water, and is ingested by the larvae. In some instances, Bti may have to be reapplied on a weekly basis since it biodegrades quickly in the environment with exposure to sunlight and microorganisms. Bti is not sprayed into the air.

Vectolex® granules or water soluble pouches may be used for larval mosquito control in catch basins. A single application of Bsph is effective in controlling mosquito larvae for an extended period of time. Bsph is not sprayed into the air.

Are there any risks associated with the product?

Studies indicate that methoprene is of low toxicity and poses little risk to people when used according to manufacturer’s instructions. Since methoprene will be applied directly to the catch basin, human or animal exposure is minimal. It is known to be slightly toxic to some aquatic life.

Bti only becomes toxic in the alkaline stomachs of mosquito and blackfly larvae. Following a review of human health risk assessments, Health Canada has determined that Bti does not pose any health risks to humans and other mammals. Studies have shown that even if Bti spores are ingested or inhaled, they are eliminated without any adverse health effects.

Health Canada has determined that Bsph is not harmful to human health when used according to manufacturer’s instructions. Bsph has minimal effect on non-target species of aquatic life.

Will I be kept informed?

Prior to the application of the larvicide, the Durham Region Health Department will provide advance notification of the treatment sites in accordance with MOE requirements (i.e. newspaper notice and/or posting along the perimeter of the treatment area). Up to date information will also be posted on our website at:

Are there any products available for household use?

Bti and Methoprene products are available for household use and can be purchased at some garden supply and hardware stores. These products can only be used to control mosquito larvae in self-contained bodies of water (private ponds, farm dugouts and ornamental ponds), where no outflow beyond the property exists. Bti and Methoprene are not approved for public use in catch basins. Remember to always follow manufacturer's instructions on the proper use of the product.

More information can be obtained from

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

April 22, 2013