Adapted from Ontario Ministry of the Environment and Health Canada Fact Sheets.



(The use of trade names is for your information only and in no way endorses these products.)

Since 1999, the spread of West Nile virus across North America has brought with it a number of challenges. The first preventive measure, and the least challenging, is to vaccinate all horses with the standard 2-shot West Nile virus vaccine. This is followed with booster vaccinations as recommended by your veterinarian. The greatest challenge is how to control the mosquitoes responsible for spreading the virus. There are approximately 75 species of mosquitoes in Canada, all of which hatch their eggs in water and feed on blood. Some species feed primarily on birds, some on reptiles and amphibians, some on mammals (including humans), and some feed on both birds and animals. Mosquitoes that feed on both birds and mammals tend to predominate in late summer. It is the control of these species, the so-called "bridging vectors," that is critical in preventing the transfer of West Nile virus from infected birds to mammals such as horses and humans.

Life Cycle of the Mosquito

Mosquitoes have a four-stage life cycle: egg, larva, pupa and adult. Only female mosquitoes need a blood meal, which they require to develop their eggs. Both males and females feed on nectar for their energy source. During its life, a female mosquito may take two or three blood meals and develop several hundred eggs each time. Mosquitoes can live four to eight weeks. All female mosquitoes lay eggs in or around water. Some species leave their eggs in spots that will flood later, such as mud at the edge of a drying pond, while others lay them in tree holes that flood in rains. The eggs hatch into larvae. At the water surface, the larva changes to a pupa before emerging as an adult mosquito. The entire life cycle can be completed in less than 10 days if the temperature is favourable. Most mosquito species survive the winter as dormant fertilized eggs. However, the mosquito species of concern with respect to the spread of West Nile virus is Culex pipiens. Culex species are thought to be the primary bridging vector in the transfer of West Nile virus from infected birds to humans and horses. A fertilized Culex female can survive over winter in sheltered places such as animal burrows, cellars and sewers, emerging in the spring to take a blood meal prior to laying her eggs.

Reducing Breeding Areas for Mosquitoes - the First Line of Defence

Culex mosquitoes breed in stagnant water. Therefore, eliminating standing water and thereby reducing the number of mosquito breeding sites is the first line of defence against mosquito bites and West Nile virus. The following methods are used to eliminate standing water:

  • Identify areas where water accumulates on your property after a one-centimetre rainfall. These can include depressions and tire ruts in the soil. If the water remains for more than seven days, you have a potential mosquito-breeding site. Mark these areas on a site map for assessment after each significant rainfall and for employing prevention options. Fill in depressions and ruts with gravel or other material.
  • Eliminate structures that accumulate water wherever possible. These include cans, jars, discarded tires, clogged roof gutters, yard decorations and stock tanks. Empty bird baths every other day. Cover rain barrels with a tight-fitting fly screening.
  • Identify locations of catch basins and entrances to drains where water accumulates. These are areas that may require the application of larvicidal treatments. Unclog any ditches to allow water to flow. Use a sump pump to drain water from temporary pools of water that may accumulate on your property.
  • Drain or cover swimming pools, children's wading pools or similar conveniences that are not in use. Use appropriate pool chemicals to ensure water does not stagnate. Consider opening the pool in early April to prevent larva development that can occur if the pool is not opened until late May or June.
  • Drill large holes in old tires used to hold down tarps so that water drains.
  • Aerators or any method that creates water surface movement in ponds, e.g., windmill-driven aerators, will reduce or prevent mosquitoes from breeding.
  • Consult the Ministry of Natural Resources (MNR) before stocking any natural water body with fish. Artificial ponds can be stocked with fish (e.g., bass, rainbow trout) that eat mosquito larvae. Do not stock ponds that have an outflow to natural water sources with non-native fish.
  • Goldfish can be used in stock tanks that are not being emptied every three to four days. They will control both algae growth and mosquito larvae.
  • Wetlands must not be drained or altered in any way, unless there is an exceptional circumstance of significant human health risk from disease vector mosquitoes. Consultation with, and permission from, the MNR and appropriate Conservation Authority would be required.

Reducing Exposure - the Second Line of Defence

Taking precautions to protect against mosquito bites is your second line of defence.

  • Wear light-coloured long-sleeved shirts, long pants and socks when outdoors. Place cooler sheets and "fly-masks" on horses to reduce the total body area that is exposed to mosquitoes. "Fly-masks" are made from see-through netting and do not obstruct the horse's vision.
  • Use an insect repellent on yourself and your horses and apply according to label directions. Apply initially to horses in small areas in case a horse is sensitive to the product.
    • Several products containing pyrethrin, resmethrin and permethrin are available for use on horses. They can be found in various concentrations, formulated as sprays, wipe-ons and ointments. These products are more effective as fly repellents than mosquito repellents.
    • DEET-based (N,N-diethyl-m-toluamide) repellents are the most effective. There is only one DEET-based insect repellent registered for use on both horses and their riders: Vet Tek Mustang PCP No. 22000. It is currently being reformulated from a 35% to a 30% DEET-based product. In a study comparing the efficacy of insect repellents against mosquito bites, a 23.8% DEET provided a mean complete protection time of 301.5 + 37.6 minutes (4.4 - 5.7 hours) protection to humans. Complete protection refers to the interval from application to the first bite.(1) Riders should be warned that DEET-based products may cause damage to rayons, acetates, dynel, plastics or painted surfaces.
  • Avoid placing horses outdoors between dusk and dawn when mosquitoes are most active. Since face flies bother horses from late morning to late afternoon, it will be difficult for horse owners to find an insect-free time when their horses can graze on pasture.
  • Avoid riding horses or placing horses in areas that are favorable mosquito habitats, e.g., low wet pastures or bush areas. Pastures that are open to the breeze are preferred.
  • Ensure that your barn has tight-fitting screens over the windows and doors. Large fly screens, which pull across or down to cover the entrances to alleyways, are commercially available.
  • Use yellow incandescent lights or fluorescent lights in the barn. These are less attractive to mosquitoes.

Cautions When Using Insecticides on Horses

Many of the products used by horse owners will be supplied in ready-to-use hand sprayers containing permethrin and/or pyrethrin in various concentrations (e.g., 0.5%). Horse owners should always read the label carefully and use according to manufacturer's directions. The following are examples of recommendations taken from these products:

  • Avoid contact with eyes, nose and muzzle. Do not saturate the hair or soak skin. Repeat treatment daily or as directed by the product label and when necessary. Do not use on newborn or nursing foals. Do not cover horses with a blanket immediately after treating. Do not apply to the back of horses prior to saddling. If a horse develops skin irritation after use, cease using this product and consult a veterinarian. Do not use on horses intended for food. Do not contaminate feed or water troughs. Avoid breathing the mist. These products are toxic to fish.

Pesticides for Mosquito Control - the Third Line of Defence

Rural dwellers and farmers can hire a licensed pest management company to properly assess their needs and safely apply pesticides to control mosquitoes on their property. These companies are listed under Pest Control in the Yellow Pages of the telephone book. In Ontario, a commercial pest management company must have an Operator's licence issued by the Ministry of the Environment to employ licensed applicators. An applicator, in turn, must hold a Mosquito/Biting Flies licence to apply larvicides (pesticides used to control mosquito larvae) or adulticides (pesticides used to control adult flying mosquitoes). The Ministry of the Environment strongly recommends that rural landowners and farmers focus efforts on personal protection and removing mosquito-breeding sites to reduce the chances of being bitten by mosquitoes before considering the use of pesticides.


Larvicides are used in circumstances where reducing or removing standing water is not feasible. They could be used for West Nile virus control programs to reduce the immature stages of mosquitoes before they develop into adult mosquitoes and disperse. Larvicides are applied as a liquid or as granules/pellets and are consumed by the mosquito larvae. Mosquito larvae reduce their feeding prior to each moult and, therefore, precise timing of larvicide applications is needed with some products for effective control of mosquito larvae. The Ministry of the Environment (MOE) allows only two larvicides - Bacillus thuringiensis israeliensis (Bti) and methoprene - to be used under permit in Ontario.

Bti (Bacillus thuringiensis israeliensis)

In Ontario, rural landowners (and their full-time employees) who have a pond or dugout that is wholly contained on their property, with no outflow or connection in any way to surface waters, can apply specific products containing the larvicide Bti. This naturally occurring bacterium provides the most effective control at certain stages of a mosquito's life cycle, so repeated applications of this larvicide are necessary to achieve effective mosquito control. In the spore-forming stage of its life cycle, the Bti bacterium produces a protein crystal which, when ingested by mosquito and blackfly larvae, becomes toxic to the larvae. The insecticidal toxin biodegrades quickly in the environment through exposure to sunlight and microorganisms. Bti is available in a granular formulation (500-gram shaker cans or 5-kilogram bags) from local feed and hardware outlets, garden centres and pest control companies in Ontario. These vendors must hold a Pesticide Vendor's Licence. No licence or permit is required by a farmer or rural dweller to purchase or use a Schedule 3 product containing Bti. Products currently registered, available in Canada and classified for use in Ontario as Schedule 3 products include: AquaBac 200G Commercial PCP No. 26862, VectoBac 220G Commercial PCP No. 19466, and AquaBac 200G Domestic PCP No. 27374. Products sold in the United States, such as slow release dunks or pucks, are not registered in Canada.


Methoprene is an insect growth regulator. It comes in granular or pellet form and is applied directly to water. When mosquito larvae are exposed to methoprene, their life cycle is disrupted and they are prevented from reaching maturity and reproducing. Methoprene can impact some freshwater invertebrates but does not seem to have long-term adverse effects on their population. It is slightly toxic to some fish species. It degrades rapidly in water, being susceptible to transformation by sunlight and microorganisms. In Ontario, only a licensed applicator can apply this product and a permit would be required. MOE is only approving permits for use in catch basins or in sewage/sludge lagoons. Currently, the only registered product available in Canada is Altosid PCP No. 21809.


Adulticides are used to reduce the numbers of adult mosquitoes. Adulticides could be considered for use to control adult mosquito populations known to carry West Nile virus when these populations reach critical levels placing human health at risk. Adulticides must be applied when the target mosquito species is most active and applications are more effective when used under ideal weather conditions (e.g., a clear night, air temperature about 15°C or higher and wind velocity 5 - 8 km/h). Currently registered adulticide products contain malathion, propoxur, pyrethroids, and pyrethrins.

Rural landowners and farmers may use adulticides labeled for domestic use from a ready-to-use aerosol, fogger or tank sprayer on their own property without a licence. Most horse owners will restrict the use of adulticide use to the inside of barns using ready-to use hand sprayers (premise sprays) or mechanically-timed mist release units. Several adulticide products are available in garden centres and hardware outlets for domestic use.

Farmers who are certified agriculturists may purchase pesticides for land exterminations on their farm, including adulticides for mosquito control around livestock and in farm buildings. Users are cautioned to apply these products according to label directions, to ensure the application is to target areas such as trees and bushes, and to ensure that neighbours are not affected. Users should be aware that adulticides may be effective only for a short period of time, due to the ability of mosquitoes to fly short distances from other properties.

Alternatives to Pesticides

Scientific studies do not support claims that natural predators - such as bats, swallows, purple martins, dragonflies or other flying predators - are effective in appreciably reducing mosquito populations. According to Health Canada, citrosa plants do not produce enough citronella oil to be considered effective and citronella-based products appear to be potential dermal sensitizers which, therefore, may cause allergic reactions in some individuals. Ultraviolet or black lights, sonic devices, various mosquito traps and zappers have not been proven effective. There is inconclusive evidence on the effectiveness of mosquito traps using carbon dioxide and octinol. Scientific studies have not been published to support the feeding of garlic and apple cider vinegar to horses as a mosquito repellent.


There are no guarantees that will prevent you or your horse from being infected with the West Nile virus. Therefore, the best you can do is to manage your farms/properties in ways that will decrease the risk of coming in contact with an infected mosquito, and to provide your horse with protection by using the West Nile vaccine.

Protect Yourself and Your Horse

  • Ensure your horse is properly protected by using the West Nile virus vaccine.
  • Do, or have a professional do, a site evaluation and eliminate all mosquito breeding grounds, where possible. Evaluate the breeding areas that cannot be eliminated and choose the most appropriate method to prevent mosquitoes from breeding.
  • If you choose to use a pesticide, read the label carefully and observe all label precautions in order to protect people, pets and livestock.


    1. Fradin MS, Day JF. Comparative efficacy of insect repellents against mosquito bites. The New England Journal of Medicine 2002; 347 (1, July 4): 13-18.

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