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West Nile Virus
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Pesticide Use and West Nile VirusExperience has shown that the use of pesticides should be the last tool in an integrated pest management plan and that efforts should be focussed instead on personal preventative measures and reducing the source of affected mosquitoes. Pesticides kill or harm beneficial insects (e.g. bees and dragonflies), spiders and other wildlife. They may have serious long-term impacts on the environment and on human health. Pesticides can also create "pesticide resistance" which kills off the weaker insects, leaving behind stronger, more pesticide-resistant individuals. Mosquitoes are an important part of the food chain. Therefore, any control measure using pesticides must be thoroughly evaluated before an application is performed. The decision to use pesticides to reduce mosquito populations is made by local Medical Officers of Health and the local Health Unit. The Ministry of the Environment regulates the application of pesticides. If health officials decide the use of pesticides is necessary, chemical or biological pesticides may be used to reduce mosquito larvae in standing water. Mosquito control programs are not intended to completely eliminate mosquitoes, but rather to reduce their numbers below the associated risk of disease transmission. The goal is to stop the disease from spreading by reducing the number of disease-transmitting mosquitoes. Larval Mosquito Control Measures for West Nile Virus SpeciesLarvicides are pesticides that target the mosquitoes’ larval or immature life stage. The larval stage is the most effective life stage to control, since it accounts for 90% of the mosquito’s life (the adult stage only accounts for 10% of the mosquito’s life). The two types of larvicides that the Health Unit may use to control mosquito larvae are Methoprene and Bti (VectoBac). Methoprene (Altosid) is a chemical pesticide that stops the growth of the larvae, making it impossible for insects to mature to adults. Since the larvae remain in the food chain, the impact on aquatic mosquito predators is reduced. Methoprene is active only in wet environments and, if it dries, will lie dormant until the next rain. The wetting process releases the oil-based compound which then lies on the water’s surface and is slowly released. It has a short half life and, once exposed to sunlight, it is broken down quickly. Since methoprene provides residual control for 30 days, it is useful in catch basins where mosquitoes can re-populate the habitat within one or two weeks, and in ditches where wetting and re-wetting may occur. Once the methoprene is released, it degrades within 40 hours thus reducing the chance of it moving away from the application site and affecting other areas. Although Methoprene degrades rapidly in aquatic environments, it is moderately toxic to warm freshwater fish and slightly toxic to cold freshwater fish and birds. It is toxic to amphibians (frogs, toads and salamanders), crayfish, shrimp, crabs and other aquatic invertebrates. It should not be applied directly to habitats with these organisms. Bti (Bacillus thuringiensis subsp. israelensis) or VectoBac is a naturally occurring microorganism that kills mosquito larvae, leaving less food available in the food chain. The product is short-lived on foliage because the sun’s ultraviolet light rapidly destroys it and rain washes it onto the soil. It also has a limited residual life and must be reapplied frequently. The product is excellent for depressions or small ponds where mosquitoes would develop only once or twice over the course of the year. It can also be used in temporary or permanent pools in pastures and woodlands, roadside or irrigation ditches and golf courses, as well as waters near fish-bearing water, catch basins and sewage lagoons. Bti is toxic to the larvae of mosquitoes and black flies but does not affect other non-target aquatic invertebrate predators or parasites. Therefore, longer periods of suppression may result since these organisms can be conserved to aid in mosquito population management. However, there is some indication that certain uses of Bti can impact endangered and / or threatened species of moths and butterflies and use of this product in habitats where these species are found should be restricted. Adult Mosquito Control Measures for West Nile Virus SpeciesAdulticides, which kill adult mosquitoes, have the most potential for affecting the environment and are the least acceptable method of mosquito control. Adulticides can have harmful side effects on human health, provide only temporary relief of mosquito numbers (a couple of days), are not species specific (will harm several non-target organisms) and are not effective at controlling population numbers (must actually have direct contact with the mosquito to control it). However, if a comprehensive larvicide program is not undertaken or if human cases start to rise in the fall, adulticiding may be considered by the Health Unit to assist in reducing the spread of the disease. For more information, contact your local Public Health Unit:
Middlesex-London
Health Unit
Oxford County Board of Health Perth District Health Unit
Huron
County Health Unit Other websites of interest: |
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This page was last
revised on
April 20, 2012.
Copyright © 2012 Upper Thames River Conservation Authority 1424 Clarke Road, London, Ontario, Canada N5V 5B9 tel: 519-451-2800 fax: 519-451-1188 All rights reserved. |
