Battling Mosquitoes: The Fog of Chemical Warfare

August 1, 2017

TipSheet: Battling Mosquitoes — The Fog of Chemical Warfare

As dusk settles on many U.S. neighborhoods in summer, people linger on screened porches over cool lemonade and the evening meal — accompanied by the whine of a mosquito-fogger (which is sometimes more welcome than the whine of a mosquito).

People hate mosquitoes. They make people’s lives miserable in many ways, starting with the itchy bite. Even nature-lovers want to kill or repel them.

There are plenty of reasons to want to get rid of mosquitoes. Beyond the itchy red swelling, the insect can host and spread serious diseases: Malaria, dengue, yellow fever, West Nile virus, Zika virus, chikungunya, filariasis, tularemia and a bunch of viral encephalitis diseases. Control of mosquitoes is a big part of controlling these diseases.

So it’s not surprising that spraying and other forms of mosquito control are standard practice in many U.S. communities. Yet some wonder if the remedy is worse than the problem.

Mosquito spraying often in the news

Mosquitoes get newsy when outbreaks of mosquito-borne disease, most recently Zika, are in the headlines. Governments often respond to public concern with spraying programs — which themselves often become news.

An Air Force Reserve aircrew flying a C-130 Hercules performs aerial spraying of mosquitoes
An Air Force C-130 Hercules performs aerial spraying of mosquitoes in South Carolina on June 15, 2013. Photo: U.S. Air Force

The 2016 attack on Zika in the Miami area generated news from both the spraying and the protests against it. And another news story came from the collateral damage — when Zika spraying in South Carolina killed millions of commercial honeybees.

But whether the patchwork of mosquito-control efforts amounts to an effective national defense against outbreaks is a legitimate (but neglected) policy issue.

There are some 176 species of mosquitoes in the United States alone. Different species spread different diseases (e.g., Zika is spread by Aedes aegypti and Aedes albopictus). The target of mosquito-control programs may vary according to disease concerns — and the control strategies as well.

For instance, mosquitoes are more abundant in places with lots of standing water, such as wetlands and marshes. So many localities around the United States have organized mosquito control districts, which are authorized by the state to collect taxes and conduct mosquito control within specific boundaries.

Is there a mosquito control district in your area? It’s a likely sign of potential mosquito issues. You can find lists of mosquito control districts here. But another reliable way to find local ones is to Google the search term “mosquito control district” and add the name of your state or county. They are typically the ones who operate the fogger truck.

Information critical to successful mosquito control

The most effective mosquito control programs include a lot of community involvement and information.

An example: Because mosquitoes breed in standing water, residents and businesses can do a lot to reduce mosquito breeding habitat. Emptying containers that collect rainwater is important and needs to be done often. Clearing lots and yards of the items that collect rainwater is better (turn that wheelbarrow over). Clogged gutters are often overlooked as breeding grounds, and cleaning them regularly helps a lot.

People have more motivation to do these things once they understand that mosquitoes don’t travel far: The mosquitoes they prevent are the ones that will plague their own decks and patios.

Community residents also need to know that controlling mosquitoes gets progressively harder as they progress through their life stages.

Eggs can survive in dry conditions, but will hatch in places that fill with water. Hatched eggs become larva (“wigglers”), which live in water but usually must breathe air from the surface. This makes them vulnerable to control by spraying an oil mist on water surfaces, which suffocates them. The oil can be comparatively non-toxic.

During the larval stage, mosquitoes can be controlled with Bt (Bacillus thuringiensis) pellet products (often called “dunks”). This biocontrol is derived from naturally occurring soil bacteria. It is especially useful in water that is hard to reach (exterior drains) or that cannot be drained (rain barrels).

Dangers of insecticides

Controlling mosquitoes in the early stages is wise prevention since once mosquitoes reach the adult stage, chemical insecticides are about the only way to kill them.

And all chemical insecticides have risks to health and the environment — though far less if they are used correctly. Nevertheless, at this point people are balancing the risks of mosquito-borne disease against the risks of pesticide sprays.

For instance, spraying of insecticides can result in human exposure. The organophosphate family (a relative of nerve gas) is much more dangerous than the pyrethroids (fairly common in household use).

Regardless, you may not want to breathe this stuff. If foggers and airplanes are going to spray, people need to know exactly when, so they can go indoors and close the windows. And they need to know what is being sprayed.

Adulticides are typically sprayed on areas, either from fogger trucks or from aircraft. Often they are diluted with a large amount of water and applied at low rates per acre (so-called ultra low volume or ULV application). But ULV may be little consolation if you are driving an open car behind the sprayer truck.

A U.S. Environmental Protection Agency overview of mosquito adulticides is here. An overview from the CDC is here. Commonly used adulticides include:

  • Naled: An organophosphate insecticide also known as dibrom that has been registered for mosquito control since 1959 and is used quite commonly. Some EPA information on Naled is here. The material safety data sheet, or MSDS, is here.
  • Malathion: An organophosphate insecticide that because of its higher toxicity is used only rarely in mosquito control spraying and is reserved for the most serious situations. Some EPA information on malathion is here. The MSDS is here. EPA is currently working on a draft health risk assessment for malathion, which can be found here.
  • Pyrethroids: Permethrin, Resmethrin and d-Phenothrin (Sumithrin®) are synthetic chemical relatives of pyrethrin, which is derived from plants. Some EPA information on pyrethroids for mosquito control is here. An MSDS for permethrin is here. One for resmethrin is here. Etofenprox is another pyrethroid used as a spray for mosquito control (sold under the trade name Zenivex®).  An MSDS for it is here. Still another is Prallethrin.

Some good sources of mosquito control information include the American Mosquito Control Association, your local mosquito control district, your state health department, the Centers for Disease Control and Prevention, state and county extension agencies, the National Pesticide Information Center, Mosquito World and the group Beyond Pesticides.

* From the weekly news magazine SEJournal Online, Vol. 2, No. 30. Content from each new issue of SEJournal Online is available to the public via the SEJournal Online main pageSubscribe to the e-newsletter here.  And see past issues of the SEJournal archived here.

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