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West Nile Virus


open grate in roadMosquitoes complete all 4 stages of their development, from egg to larva, pupa and then adult, in water. 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.

Research has shown that roadside catch basins are prime breeding sites for mosquitoes associated with WNV. Therefore, monitoring and larviciding of regional and municipal catch basins are necessary components of the Durham Region West Nile Virus Vector Control Plan (PDF).

Larviciding Techniques Applied in Durham Region

  • Catch Basin Larviciding Program
    Regionally and municipally-owned roadside catch basins throughout the Region are monitored for larval mosquito activity annually beginning mid-May. Three catch basin treatments are conducted during each WNV season. The first catch basin treatment normally occurs in mid-June, with subsequent treatments occurring mid-July and mid-August. In order to maximize effectiveness, the larvicide treatment periods are separated by a maximum of twenty-one (21) days.
  • Standing Water Larviciding Program
    Beginning mid-May each year, areas of standing/stagnant water will be assessed as potential larval mosquito development sites. Where remediation of identified larval mosquito development sites is not possible, a biological larvicide is applied after a local risk assessment.

For more information, please refer to Facts About… Larviciding, Facts About… Stagnant Water & Mosquito Prevention, Fact Sheet on the Use of Methoprene in Mosquito Control Programs (PDF), Fact Sheet on the Bacillus thuringiensis israelensis (Bti) (PDF).