Controlling Mosquitoes
on Horse Farms and Rural Properties
Adapted from Ontario Ministry of the Environment and Health
Canada Fact Sheets.
Table of Contents
- Introduction
- 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
- Larvicides
- Bti (Bacillus thuringiensis israeliensis)
- Methoprene
- Adulticides
- Alternatives to Pesticides
- Summary
- Protect Yourself and Your Horse
- Reference
- More Information
Introduction
(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
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
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
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.
Summary
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.
Reference
- 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.
More Information:
Ontario Ministry of
Agriculture, Food and Rural Affairs
Pesticides and Mosquito Control
Ontario
Ministry of the Environment
Health Canada
Human Health
Ontario
Ministry of Health and Long-Term Care - Toll-free hot line - 1-877-234-4343
Health Canada
