City of Toronto Larviciding Program
Reducing mosquito populations that can transmit West Nile Virus (WNV) is an important in component of the WNV program. Larviciding of City catch basins and stagnant water sites where immature mosquitoes (larvae) have been identified reduces the risk of WNV.
Larvicide Type and Application
Larvicide is applied by licensed applicators and/or trained technicians that are approved by the Ontario Ministry of the Environment and under the authority of Toronto’s Medical Officer of Health.
The City of Toronto applies larvicide to catch basins that hold water and breed mosquitoes from June to the end of
August, however the exact start date will vary, depending on larvae mosquito surveillance results. The City also monitors stagnant water sites or artificial bodies of water such as storm water ponds to determine if treatment is necessary.
Types of Larvicide
The City of Toronto will be using three different larvicides: methoprene, BS (Bacillus sphaericus) and Bti (Bacillus thuringiensis israelensis). Methorprene and BS will be used to treat catch basins. Bti will be used in other areas to reduce mosquito breeding.
Targeting the Culex Mosquito
Toronto’s mosquito control program aims at reducing the Culex mosquito population.
Culex mosquitoes are the most common carrier of West Nile Virus (WNV) in our region and although they prefer to bite birds, they will also bite humans. Surveillance activities in Toronto and elsewhere in North America have identified storm water catch basins as significant breeding sites for Culex mosquitoes, including some surface bodies of water.
2019 Larviciding Dates
Start Date: June 17, 2019.
End Date: To be determined
Health and Environmental Impacts
- The larvicides used by the City of Toronto break down quickly in the environment and are harmless to plants, mammals, fish, birds and insects other than mosquitoes and blackflies.
- Methropene, BS and Bti are not harmful to humans when used to control mosquitoes although direct contact may cause mild eye and skin irritation.
- BS and Bti are biological pesticides derived from naturally occurring soil bacteria. They work by killing mosquito larvae before they develop into adults by damaging the digestive system of mosquito larvae.