Don't Kill the Buzz

FAQs : Mosquito Control Sprays

  • The active ingredient used in mosquito sprays is a class of insecticides called pyrethroids.
  • They are synthetic derivatives of pyrethrins which are six naturally occurring chemicals with insecticidal properties found in the flowers of the plant Tanacetum cinerariaefolium, formely named Chrysanthemum cinerariaefolium
  • They are more toxic to insects and last longer in the environment than pyrethrins.
  • Permethrin, deltamethrin, and cypermethrin are three commonly used pyrethroids.
  • Perythrins and perythroids are broad spectrum adulticides : they kill adult mosquitoes.
  • They disrupt the electrical signaling in the insect’s nervous system, resulting in repetitive nerve firing, eventually causing paralysis and death.
  • They achieve this by binding to the ɑ subunit of sodium channels in the membranes of neurons, preventing them from closing – this leads to permanent depolarisation.
  • In addition to their effects on sodium channels, these compounds are known to affect chloride channels, including GABA-dependent ones (inhibitory neurotransmitter).
  • Local mosquito control companies use ultra low volume (ULV) backpack sprayers, which dispense very fine aerosol droplets that can move through the air and kill mosquitoes on contact.
  • They are typically mixed with a synergist compound (not an insecticide) such as piperonyl butoxide (PBO), which enhances their effectiveness by inhibiting the insect’s natural detoxification mechanism – consequently, less may be used to achieve the same result.
  • They are applied to dark, damp areas, on dense plant materials (trees, shrubs, hedges, ground cover), under decks, stairways.
  • As broad-spectrum adulticides, they are highly toxic to a wide variety of insects, including native pollinators and pest predators (e.g. ladybugs, spiders, dragonflies).
  • Insects are also part of the base of the food web, without which other wildlife cannot survive, including many species of backyard birds.
  • Pyrethroids are extremely toxic to fish, crustaceans and amphibians and should never be sprayed directly onto water; unfortunately, they can enter lakes, ponds, rivers, and streams from rainfall or runoff from agricultural fields. 
  • Pyrethroids are more toxic to insects than to mammals and birds due to insects having a lower body temperature and more sensitive sodium channels in their nervous system; when applied properly, they do not pose an unreasonable risk to mammals or birds.
  • In air, all pyrethrins and many pyrethroids are broken down or degraded in 1 to 2 days by sunlight or other compounds found in the atmosphere

  • They can also be degraded by sunlight on the surfaces of water, soil, or plants. 

  • Pyrethroids are not all equally susceptible to sunlight, their degradation ranging from a few days to a few hundred days.

  • Pyrethroids bind strongly to soil where they can persist for a longer period of time; they are eventually degraded by microorganisms in soil and water.

  • They are not very mobile in soil; consequently, they are not easily taken up by the roots of plants and vegetation and do not usually leach into groundwater.

  • Runoff however can wash these chemicals from our yards into surface waters, where they can poison aquatic organisms such as fish and crustaceans.

Although mosquito control services aim to spray only backyard areas where mosquitoes rest and hide, insecticide sprays can drift 100 metres or more depending on the wind speed and direction.

  • Even if flowering plants are avoided during application, insecticides sprays can drift 100 metres or more. The potential contamination of the pollen and nectar can last  several days or weeks, and the residue on leaves can be toxic to caterpillars for weeks or months.
  • There is no way for companies to spray insecticides in your yard without also killing other insects they come in contact with, including bees, butterflies, caterpillars, ladybugs, dragonflies and other beneficial insects, along with the mosquitoes.
  • Pyrethroids are extremely toxic to fish, crustaceans and amphibians and should never be sprayed directly onto water.
  • To reduce risk to these aquatic species, a spray buffer zone is required by Health Canada’s Pest Management Regulatory Agency (PMRA).  A spray buffer zone is a no spray zone between the area being sprayed and a sensitive area.
  • Even if a mosquito control service follows all label directions requiring setbacks when spraying near bodies of water, the products can unfortunately enter lakes, rivers, streams, ponds and wetlands due to rainfall or runoff.
  • If applied properly, risk to humans is relatively low. 
  • However, individuals who are particularly sensitive to ingredients within the sprays may experience adverse effects such as eye, skin, nose and/or throat irritation, breathing problems, and nausea.
  • Children and pregnant women are also at higher risk of adverse health effects due to faster pyrethroid penetration into the body.
  • Exposure to large amounts can cause stinging skin, dizziness, headache, or nausea that might last for several hours.
  • Larger amounts might cause muscle twitching, reduced energy, and changes in awareness. 
  • Even larger amounts could cause convulsions and loss of consciousness that could last for several days.

N.B.  These exposures to larger amounts are not likely.

  • Cats (but not dogs) are very sensitive to pyrethroid exposure because they lack the liver enzymes that effectively break it down into harmless products.
  • Pyrethrin and pyrethroid toxicity in cats is often the result of using topical flea treatments labelled for dogs.
  • Pyrethroids are fat soluble – contact with the skin, digestive tract, and respiratory tract results in their penetration into the body.

  • Exposure can occur by eating contaminated foods, by breathing air that contains these compounds or by dermal contact.

  • Pyrethroids are quickly broken down into metabolites that leave the body mainly in the urine, but also in faeces and breath.

  • Their concentration in urine increases as the amount and duration of exposure goes up; if exposure levels are very high or if exposure occurs over a long time, they can build up in fatty tissue and remain in the body for a little longer.

  • All pesticides used, sold, or imported into Canada, are regulated by Health Canada’s Pest Management Regulatory Agency (PMRA).
  • Permethrin (a pyrethroid) was re-evaluated in 2019; the re-evaluation decision along with the risk mitigation measures for human health and the environment can be found here.
  • Eliminate breeding spots around your home : remove or drain sources of stagnant water such as clogged gutters, kids’ playsets, bird baths, empty pots, etc.
  • Cover up : Wear light-coloured pants and long sleeve shirts, socks, closed-toe shoes and hats.
  • Avoid being outdoors for too long at dawn or dusk when ,mosquitoes are prevalent
  • Create a landscape that attracts mosquito predators such as dragonflies, spiders, birds and bats.
  • Use natural repellents such as oil of lemon eucalyptus.
  • Mosquitoes serve as pollinators.
  • Mosquito larvae are part of the aquatic food chain : they serve as food to many aquatic predators including fish and birds.
  • Adult mosquitoes serve as food to birds, bats, frogs and other insects.

Agency for Toxic Substances and Disease Registry.  ‘Public Health Statement for Pyrethrins and Pyrethroids.’ (Page last reviewed:  March 25, 2014),or%20runoff%20from%20agricultural%20fields.

Bernard C.B., Philogène B. J.  ‘Insecticide synergists: role, importance, and perspectives’ J Toxicol Environ Health (1993 Feb).,otherwise%20break%20down%20insecticide%20molecules.

Chrustek A., Hołyńska-Iwan I., Dziembowska I., Bogusiewicz J., Wróblewski M., Cwynar A., Olszewska-Słonina D. (2018) ‘Current Research on the Safety of Pyrethroids Used as Insecticides’, Medicina (Published online 2018 Aug 28).

Ensley S. M. (2018) ‘Veterinary Toxicology, Third Edition, Chapter39 – Pyrethrins and Pyrethroids’, Science Direct (Available online 9 February 2018)

Hołyńska-Iwan I., Szewczyk-Golec K. (2020) ‘Pyrethroids: How They Affect Human and Animal Health’, Medicina (Published online 2020 Oct 30).

Laskowski D.A., (2002)  ‘Physical and chemical properties of pyrethroids’, Rev Environ Contam Toxicol.,5%20to%20430%20d%20anaerobically.

Mizejewski D., Weber D. (2022) ‘What You Need to Know Before Spraying for Mosquitoes’, NWF Blog.

Mosquito Buzz.

Mosquito Hero.

Peach D. A. H. (2019) ‘The bizarre and ecologically important hidden lives of mosquitoes’, Earth Touch News Network.

Ranatunga M., Kellar C., Pettigrove V. (2023) ‘Toxicological impacts of synthetic pyrethroids on non-target aquatic organisms: A review’, Environmental Advances (Available online 25 May 2023).

Smitley D., Brown D., Finneran R., Elsner E., Landis J., Shrewsbury P., Herms D., Palmer C. L., (2019) ‘Potential impact of mosquito and nuisance insect sprays on pollinators’ Michigan State University, MSU Extension, Pollinators and Pollination.

Wikipedia ‘Pyrethroid’ (last edited on 29 November 2023).