Controlling Mosquitoes on Horse Farms and Rural Properties
Adapted from Ontario Ministry of the Environment and Health Canada Fact Sheets.
Table of Contents
(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.
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.
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:
Taking precautions to protect against mosquito bites is your second line of defence.
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:
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.
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.
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.
Pesticides and Mosquito Control
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