West Nile Virus
Mosquito Larvicides
The following document briefly outlines the human and environmental effects of larvicides used within the
Region of Waterloo by mosquito control contractors to minimize human risk of West Nile Virus infection. Pesticides, when applied by a licensed exterminator and with a permit, may be used for the promotion of health and
safety (i.e. control of West Nile Virus) under Ontario Regulation 63/09 made under the Pesticides Act.
Larvicides
Normal mosquito development includes four (4) stages of
growth: egg, larvae, pupa and adult. Larvicides are used to
control mosquito populations in the larval stage—to prevent
them from becoming biting adults.
Methoprene (“Altosid”)
Methoprene is annually applied to catch basins in Waterloo
Region. Methoprene is an insect growth regulator first
registered for use in Canada in 1977. Available in either a
granular or palletized form, methoprene is applied directly to
watery areas where mosquito larvae are found and where
water containment is assured.
Methoprene is considered a growth regulator because it
interferes with the larval stage of an insects’ life cycle—thus
preventing the emergence of the adult insect.
Methoprene is considered to be of low toxicity by Pest
Management Regulatory Agency (PMRA) and poses little
toxicological risk to human health when used appropriately.
It has very low acute oral and inhalation toxicity potential
and is not considered to be a skin or eye irritant and is not a
skin sensitizer. Methoprene is also considered to have very
low toxicity towards Mallard ducks and is only slightly toxic
to fish. Although methoprene is considered to be highly toxic
to freshwater invertebrates, field studies have demonstrated
that no lasting adverse effects on invertebrate populations
and other non-target aquatic organisms have been observed.
Methoprene degrades rapidly in water and is susceptible
to transformation by both sunlight and microorganisms.
Methoprene is not mutagenic nor is it considered a
carcinogenic compound. Methoprene does not have a high
potential for bioaccumulation.
Bacillus thuringiensis israelensis
(Bti) (“Aquabac/Vectobac”)
Bacillus thuringiensis israelensis, commonly referred to as
Bti, is used to control mosquito larvae in environmentally
sensitive areas or in situations where the flow of water is
not contained. It is annually applied to natural sites across
Waterloo Region. It is a bacterium found naturally in soils.
Since 1982, Bti has been used successfully as a biological
pest control agent against mosquito and blackfly larvae.
Bti works against mosquito larvae by producing protein
crystals which are toxic to the mosquitoes’ alkaline digestive
system. During feeding, the wall of the mosquitoes’ stomach
is essentially destroyed by ingesting the microscopic crystals
produced by the bacterium. Within hours the larvae stops
feeding and usually dies within a few days. Because Bti
is only effective during the immature, or larval, stage of
development, effective monitoring/surveillance is necessary in
order to determine the most appropriate time for application.
Bti is applied directly to the watery areas where mosquito
larvae are found. Bti poses little or no threat to human or
environmental health. The acidic digestive environment of
humans and animals will not activate Bti toxins. There have
been no documented cases of toxicity or endocrine disruption
in humans or other mammals in Canada or around the world.
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Several studies have shown that accidental ingestion or
inhalation of Bti spores would be eliminated without any
adverse health effects. Bti is only toxic in the alkaline
digestive environment of mosquitoes and blackflies and will
not affect other insects, honeybees, fish or birds. The U.S.
Environmental Protection Agency (EPA) has designated the
risk of Bti to non-target organisms as minimal to non-existent.
Bti biodegrades quickly in the presence of sunlight and
microorganisms and is considered “immobile” as it does not
move or leach with groundwater.
Bacillus sphaericus (“Vectolex”)
Bacillus sphaericus (BS) may be applied to catch basins and
natural sites in Waterloo Region. It is a naturally occurring
bacterium commonly found in soil. BS is effective against
Culex mosquito larvae found in polluted water or water rich
in organic material. As with Bti, BS is only effective against
feeding larvae. Upon ingestion, the granules of BS destroy the
larvae’s digestive tract, killing them within days.
References
Health Canada. “Fact Sheet on the Use of Methoprene in Mosquito Control
Programs.” p.1
2
Cornell University. “Extoxnet.” http://pmep.cce.cornell.edu/profiles/extoxnet/
haloxyfop-methylparathion/methoprene-ext.html
3
Health Canada. “Fact Sheet on the Use of Methoprene.” p.1
4
Ibid.
5
Cornell University. “Extoxnet.” p.4
6
“Extoxnet.” Cornell University. http://pmep.cce.cornell.edu/profiles/
extoxnet/24d-captan/bt-ext.html
7
Health Canada. “Fact Sheet on the Bacillus thuringiensis subspecies israelensis
Bti.” (April 2001) p.1
8
Ibid. p.2
9
Extoxnet, Cornell University. p.4. NOTE: Health Canada has determined that
products containing Bti do not pose any health risks to humans and other
mammals. Only products scientifically reviewed and found to be effective
and safe for use with minimal risk to human health and the environment are
registered by the Pest Management Regulatory Agency (PMRA).
10
Washington State Health Department. Larvicide: Bacillus sphaericus. See also:
http://pmep.cce.cornell.edu/profiles/insect-mite/abamectin-bufencarb/bacillussphaericus/b
1
The advantage of BS over Bti is the residual effect of the
larvicide which remains active for approximately 21 days. BS
is generally used in storm water management ponds, catch
basins and sludge or waste lagoons. A permit is required in
order to use BS.
BS has no measurable mammalian health effects and has
proven safe for use in the environment. Birds, fish and nontarget invertebrates have not been adversely affected by
exposure to BS in contained waters.
Questions or concerns?
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Talk to a Public Health Inspector at 519-575-4400.
Region of Waterloo Public Health
Health Protection and Investigation
www.regionofwaterloo.ca/ph
519-575-4400
TTY 519-575-4608
Fax 519-883-2241