Effects of mosquito sprays on humans, pets, and wildlife

Mosquito Shield sign in yardThis summer I’ve had several people ask me whether I had opinions about Mosquito Authority, Mosquito Joe, Mosquito Platoon, Mosquito Shield, and Mosquito Squad, companies that spray insecticides around residential properties. Neighbors of some of the treated properties were curious whether the chemicals are safe or had any other effects. I thought I’d put some thoughts online in case useful to others.

After a little digging, I think I’ve uncovered most of the active ingredients they use in their adulticide formulations (sprays that kill adult mosquitoes):

Mosquito Authority: bifenthrin, deltramethrin, permethrin
Mosquito Joe: lambda‑cyhalothrin
Mosquito Platoon: permethrin
Mosquito Shield: D-allethrinbifenthrin
Mosquito Squadbifenthrin, cyfluthrin

I still haven’t figured out whether companies permit their franchise owners to pick and choose which insecticides to use, so when multiple chemicals are listed above that doesn’t necessarily mean the sprays will contain both; I’ve found that different franchise locations use different chemicals. Please email me if you think I’ve made an error or if you know that a formulation has changed (which happens).

Are these chemicals safe for humans?

Chemical structure of permethinAll of the above chemicals are pyrethroids, synthetic neurotoxins that cause almost instant paralysis and death to mosquitoes. Permethrin, a common ingredient of lice shampoo, is shown at right.

Pyrethroids are relatively non-toxic for humans but they should not be viewed as either harmless or safe. If you spill enough on your skin you might experience itchiness, numbness, nausea, and respiratory problems (the full list is pages long). At very high doses pyrethroids can kill you, a fact known because some people have injected it (suicide) and in one case because somebody ate food that was cooked in pyrethroid concentrate (it resembles cooking oil). There are also scattered reports that some pyrethroids are carcinogenic and estrogenic but I don’t think such effects are shockingly large, and to date they seem to be restricted to mouse studies, not humans. Each pyrethroid listed above has a slightly different toxicity, so to get more information Google the name or read the linked material by clicking on their name.

Are pyrethroids safe for pets?

Cat in the grassDogs and chickens seem to be fine. Cats, however, lack sufficient levels of a liver enzyme that helps detoxify pyrethroids, and can thus develop what is called pyrethroid toxicosis. Among a long list of alarming symptoms is depression, and cats are already sullen enough. The real concern is that a cat could easily wander into a yard that has just been sprayed, and if the cat eats grass enough pyrethroids might be ingested to cause death. A good indicator of cat sensitivity to pyrethroids is the standard warning of keeping them away from pyrethroid-treated dogs. I.e., if you dose your dog with a large amount of pyrethroids (to kill fleas, e.g.), friendly cats that cuddle with the dog are at risk.

Do pyrethroids kill other animals? 

Yes, and this fact is seldom acknowledged on sprayers’ websites. In fact, I suspect if you confronted a franchise owner with the information below, he or she would get defensive and claim my information is incorrect.

I’ll start with highlighting that pyrethroids kill dragonflies, damselflies, fireflies, butterflies, hoverflieshoney bees, bumble bees, solitary bees, and ladybird beetles. In fact, pyrethroids can kill all types of insects in a yard except perhaps those that are deep under the soil or encased within galls. But these bolded examples are insects that a typical American might regret killing.

Monarch butterfly on swamp milkweed

For example, everyone loves monarch butterflies (I think), and the spray kills adults and caterpillars, even weeks later later due to the presence of insecticide dried onto milkweed leaves (Oberhuaser et al. 2006). And everyone loves honey bees. Even if honey bees are inside their hives when the pyrethroids are spayed, workers bring small amounts back to the hive the following day if they land on treated plants or if they find small puddles of water to drink. Sublethal amounts of pyrethroids can change honey bee behavior and make workers smaller. And then there are kids’ favorite, fireflies, which become most active in a yard precisely when mosquito-spraying franchises like to fog (evening).

Two-spotted long-honred bee (Melissodes bimaculatus) clamped to leafMy favorite group of unnoticed insects that are killed by evening pyrethroid applications are solitary bees, of which there are approximately 4,000 species in the United States. These are bees that collect pollen and nectar during the day but spend their evenings and nights in holes (e.g., mason bees) or clamped to low vegetation. At right is one from my front yard this summer (it’s a two-spotted long-horned bee, Melissodes bimaculatus). Everyone has native bees in their yards but few people realize it. But they are amazing pollinators and almost all adorable. So when pesticide applicators claim their pyrethroid sprays “don’t harm bees” or are “bee friendly”, that is entirely untrue (it’s a marketing slogan they were taught when they bought the franchise). FYI, one can apparently buy a bumper sticker about bee-killing yard spays.

Anyone using a spraying service is, therefore, killing all of the above and more. Insects are small and easy to ignore, but if you were to go out after a spraying and look very carefully, you’d find thousands of dead insects on the ground. And only a small percentage would be mosquitoes. If you go out immediately after a spraying you can even see the twitching that precedes death (movie). It’s not a pleasant way to go.

Pyrethroids kill all arthropods, in fact, not just insects. So if a yard is sprayed, likely 100% of spiders, mites, centipedes, and millipedes would be at risk. In total that could mean tens of thousands of individuals left twitching in the yard. Or perhaps more. In one estimate, an acre of land in Pennsylvania contains 425 million animals … and a good portion of them would be killed by pyrethroids.

Wren feeding babies in a gourdAnother concern with gassing the tens of thousands of arthropods in a back yard is that those bird species that eat arthropods will have a lot less to eat (or nothing at all). Population levels of swallows and flycatchers, for example, have dropped in last several decades and one explanation is that there are fewer insects to eat. One might argue that they can just go into an unsprayed, neighboring yard to find insects but that’s a lot to ask of birds that often have territories.

And then there are fish, which are acutely sensitive to pyrethroids. Franchise owners will generally avoid spraying near people’s fish ponds and bodies of water. Indeed, by law pyrethroids can’t be used near water, though there are many reports of franchises ignoring that regulation. Even if a property doesn’t have a pond or stream, pyrethroids are rather stable in the soil and tend to get washed into nearby streams after rains. It is increasingly accepted that runoff of pyrethroids into creeks might kill fish downstream.

Finally, pyrethroids are toxic to certain earthworms. This is probably especially true for those species that come out onto lawn surfaces in the evening, when pyrethroids are often applied. To be honest, many people don’t really care about earthworms (some hate them) but for people who love aerated lawns it should be pointed out that fogging with pyrethroids might result in dramatically less aeration and dramatically more odor from rotting earthworm carcasses. And, perhaps, result in robins that wonder where all the worms have gone.

Pyrethoids are engineered to last for weeks

Asian tiger mosquito on milkweed flower

As mentioned above in reference to monarchs, pyrethroids adhere to plant surfaces and stay active for weeks, so the risk to all the organisms mentioned above lasts for weeks if not longer. Here’s a nice description of pyrethroid persistence that I found on a Mosquito Squad FAQ:

“How can the barrier spray continue to kill mosquitoes for 21 days? Mosquitoes will feed on plant juices. When they attempt to feed on sprayed leaves, the residual from the spray will kill them.”

Mosquitoes don’t eat leaves, of course, but the quote is correct about the fate of insects that walk on treated leaves. Pesticide franchises like to claim that once their product is dry it is no longer toxic, but that claim makes no logical sense in light of their claim that the product provides protection for 21 days (usually). Some pyrethroids can last for 90 days if they are on shaded surfaces. One possible reason (in my view) that pyrethroids retain their ability to kill is that morning dew and guttation (see photographs) might re-suspend the pyrethroids on plant surfaces, and then insects walk through or drink these droplets (see photographs).

Signage and notification requirements

All of the above information is of course useful to those interested in getting their yard fogged, but it’s also important for neighbors of treated properties. The reason for this is because the aerosolized pesticide always drifts onto neighboring properties a bit (sometimes a lot). So if a neighbor has a patch of milkweed or a vegetable garden right along the property line, pyrethroids will kill monarchs and coat the vegetables. Homeowners should always check with neighbors before hiring a spraying service and should always personally inform neighbors when the spraying will actually take place.

Pesticide application sign OhioThere doesn’t seem to be a Federal law requiring either signage or neighbor notification when a yard is sprayed with insecticide, and state laws are variable (here’s a good review; here’s another). Many states have laws that require applicators to leave signs at properties that have been sprayed, and some require notification of neighbors prior to spraying. For example, most counties in New York require that neighbors be notified 48 hours before spraying (details). In some states (e.g., Maine) you can get your name on a notification registry that requires any applicator to contact you in advance of spraying. That’s also an option for beekeepers in some states, too; in that case the pesticide applicators need to search a database of hives in a town, then notify hive owners when spraying will happen.

I live in Pennsylvania, one of the states that doesn’t require signs or notification. But PA residents do have the option of requesting prior notification as well as the brand name of what is going to be applied. So if you see a sign (e.g., Mosquito Shield) on your neighbor’s yard, just send a written request to that company asking for prior notification on subsequent treatments. They’ll likely be shocked to get the request so you might clarify that your request is based on PA Code 128.81c. Illinois has a similar rule. Another rule in Pennsylvania allows for people to be placed on a Pesticide Hypersensitivity Registry. Once on it, pesticide applicators will know you have a medical issue with pesticides and they are obliged to inform you of future spraying (so you can leave the area). Also on this registry are beekeepers, apparently.

If you want to know what rules might apply in your state, Googling “pesticide notification laws regulations residential pennsylvania” (replace with your state) will get you started. If that doesn’t help just contact your state agency that regulates pesticide use. Towns and cities will sometimes have their own rules but some states prohibit such rules.

Is spraying effective?

Although spraying pyrethroids might be a good way to kill mosquitoes near the ground and in low shrubs, some species spend most of their time high up in the canopies of trees and are untouched by sprays. For example, many species in the genus Culex (transmitters of West Nile virus, St. Louis encephalitis, eastern equine encephalitis, western encephalitis, avian malaria, etc.) are tree dwellers and only occasionally come down to feed on humans and pets. This limitation isn’t just theoretical: one study showed spraying didn’t reduce numbers of Culex at all. So if you have trees and birds, don’t expect that spraying will protect you from the diseases these birds carry.

But I don’t want to imply that spraying is ineffective. Pyrethroid fogs will probably kill a large percentage of Anopheles spp. (vectors of malaria) and Aedes spp. (vectors of dengue, yellow fever, Zika, etc.) because these species are more likely to be lower in the vegetation. There are more genera of mosquitoes, of course, so depending on where you live and what is growing in your yard, pyrethroids might vary in effectiveness. I think most companies suggest that their sprays might miss 10% of the mosquitoes on a property.

Prevention is cheaper

Preventing mosquitoes from breeding in the first place is entirely achievable if you do two things regularly during the spring and summer:

  1. Open buckets of water waiting for mosquitoesEliminate all places where water pools (unused kid toys, wheelbarrows, clogged gutters, open buckets of water, etc.). Mosquitoes can breed in very small volumes (bottle caps, e.g.).
  2. Regularly add Bti granules to ponds, water features, and bird baths that can’t be eliminated or drained. Bti (Bacillus thuringiensis israelensis) is a bacterium that kills only mosquito larvae and can be purchased at local hardware stores or online (e.g, Amazon sells Mosquito Bits and Mosquito Dunks).

The Bti isn’t free, of course, but it’s a lot cheaper than the $800 you’d spend on a spraying subscription. If you want to be the block hero, spend your savings on Bti granules for all your neighbors. Just leave the Bti on porches in a gift bag along with a nice card that explains the game plan.

How local governments can help

All towns and cities should maintain a web page that provides mosquito information and relevant pesticide laws to residents (I designed a template if your town needs one). In addition to maintaining the page, local governments should push prevention by sending to residents timely, regular reminders — with visuals — about how to prevent mosquitoes from breeding. Such tips might be especially important for towns that strive to be pollinator-friendly (e.g., Bee City USA, Pollinator Friendly Alliance, Mayors’ Monarch Pledge). Towns can also enact ordinances on third-party pesticide applications. For example, a town might require that franchises alert neighbors 48 hours before spraying is done. Or, at the very least, towns can remind residents that neighbors can (usually) ask franchises to provide alerts before the next spraying so that cats are inside. Finally, local governments should consider banning automatic pesticide foggers (pipes that deliver pyrethroids on a timer, just like those automatic misters in the grocery aisle).

Logo that says

Image from Mosquito Squad

State governments can can also make sure that pesticide franchises are not making false claims. E.g., if companies claim that their pyrethroid-containing sprays are “EPA approved,” “environmentally friendly,” “bee friendly,” “kid friendly,” or “pet friendly,” sue them. Almost all sites I’ve looked at have misleading wording, especially the claim that pyrethroid-containing sprays “target” mosquitoes and ticks (completely false; they do no such thing).  Some sites also use misleading graphics and logos (like the one on the right, from a Mosquito Squad site in Florida) to suggest that the EPA has approved the products on a page. I could be wrong, but I’m fairly sure that the EPA neither approves any of these chemicals nor provides that logo. But a typical homeowner might be fooled into thinking otherwise — that’s why state oversight is needed.

Further information

I highly recommend “Ecologically sound mosquito management in wetlands” produced by The Xerxes Society. If you’re looking for a comprehensive guide to ridding your yard of mosquitoes, please see my “Mosquito control tips” page. If you prefer your mosquito news and killing tips via Facebook, I post updates on the Mosquito Control page.

Please share this post with your friends (especially beekeepers and monarch fans) and with groups that might care about biodiversity. And with neighborhood groups if you have those — most folks simply don’t know what these sprays actually do, and the franchises are not reliable sources of information.

If you notice death of any type of animal after a spraying event, report the incident to the EPA. Once you’ve done that send me photographs.

If you have questions. please send me an email.