on Horse Farms and Rural Properties
Adapted from Ontario Ministry of the Environment and
Health Canada Fact Sheets.
Table of Contents
- Life Cycle of the Mosquito
- Reducing Breeding Areas for Mosquitoes -
the First Line of Defence
- Reducing Exposure - the Second Line of Defence
- Cautions When Using Insecticides on Horses
- Pesticides for Mosquito Control - the Third
Line of Defence
- Bti (Bacillus thuringiensis israeliensis)
- Alternatives to Pesticides
- Protect Yourself and Your Horse
- More Information
(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
- 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
- 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
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
- 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
- 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
- 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,
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
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.
- 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.
of Agriculture, Food and Rural Affairs
Pesticides and Mosquito Control
Ministry of the Environment
Ministry of Health and Long-Term Care - Toll-free hot line
For more information:
Toll Free: 1-877-424-1300