Mosquito Control Frequently Asked Questions

Mass Audubon receives many inquiries about mosquitoes and mosquito control practices. Below are some of the most common questions on mosquitoes, their associated health risk, control methods, and environmental impacts of mosquito control activities.

You can also learn about Mass Audubon's position on mosquito control practices and our ideas for improved policies.

What is Mass Audubon's position on mosquito control?

Mass Audubon supports a scientifically-based approach to mosquito control focused on protecting human health and minimizing environmental impacts. We also support the Massachusetts Department of Public Health's (DPH) plan for management of mosquito-borne diseases. The DPH plan calls for limited use of pesticides, based on the levels of mosquito and disease activity in a specific location. The DPH plan emphasizes:

  • Monitoring of mosquitoes and mosquito-borne diseases
  • Public education
  • Personal protection from mosquito bites
  • Elimination of artificial mosquito breeding sites around developed areas, and
  • Targeted applications of pesticides when necessary to protect the public from mosquito-borne diseases based on monitoring and risk thresholds.

Mass Audubon generally opposes most nuisance control practices, including spraying of pesticides to kill adult mosquitoes when there is a relatively low risk of mosquito-borne disease, because of the human health risks of pesticide exposures, harmful environmental effects, and unproven effectiveness of these activities.

Mass Audubon supports administrative and legislative reforms to improve the outdated and inefficient mosquito control laws in Massachusetts. For more information, see Mass Audubon's call for reform and recommendations.

Why is Mass Audubon concerned about mosquito control practices?

Some mosquito control activities, including pesticide applications and wetland ditching or draining, can:

  • Harm or kill beneficial creatures (e.g., bees, butterflies, dragonflies, frogs, and fish).
  • Degrade important natural wetland habitats; alter water levels affecting water flows and supplies.
  • Degrade water quality.

In addition to these negative environmental effects, there is insufficient monitoring and reporting to determine whether mosquito control activities as they are routinely conducted in Massachusetts are effective.

From a human health perspective, the risks of mosquito-borne disease must be balanced against the risks of human health effects of pesticides. Because mosquitoes breed so rapidly and in so many locations, most mosquito control practices have only local and temporary effects on numbers of biting mosquitoes.

Do mosquitoes threaten human health in Massachusetts?

Two known serious human diseases in Massachusetts can be transmitted to humans by mosquitoes: West Nile Virus and Eastern Equine Encephalitis. Other mosquito-borne diseases may occur in people who have traveled to locations where other diseases are endemic.  For information on emerging mosquito-borne diseases in the western hemisphere such as Zika and Chikungunya, see the Department of Public Health and Centers for Disease Control.

West Nile Virus (WNV) can cause a form of encephalitis (swelling of the brain). Symptoms can include headache, high fever, neck stiffness, and in extreme cases stupor, coma, paralysis or death. Most cases in humans are mild, causing only a flu-like illness that lasts a few days, or even no symptoms at all. However, WNV can be serious or even fatal in some cases.

WNV is fairly new to the US, first appearing in the summer of 1999 in the metropolitan New York area. In 2000 it was detected in Massachusetts and it has since spread rapidly across the continental U.S. and into Canada, Mexico, and the Caribbean. Since 2000 there have been 128 human cases confirmed in Massachusetts.

Eastern Equine Encephalitis (EEE) is a viral disease affecting the central nervous system and causing encephalitis. It is very serious and extremely rare. About 100 cases of EEE have been confirmed in humans in Massachusetts since 1938. EEE is fatal in one-third to one-half of the cases, and most victims who survive suffer lifelong disabilities.

EEE outbreaks historically tended to occur in ten to fifteen year cycles, with several years of activity followed by periods of lower risk. However, it appears that this pattern may be changing, perhaps due to climate change.

EEE in Massachusetts historically occurred in the southeastern portion of the state, which contains extensive freshwater swamps (wooded wetlands) that harbor the mosquitoes that may transmit the disease. The risk of contracting EEE in other parts of the state is extremely low, although in recent years the disease has been found in central and western Massachusetts as well as northern New England states.

The Department of Public Health monitors mosquitoes, other animals, and humans for WNV and EEE. People should take precautions to prevent mosquito bites. Young children, the elderly, and people with depressed immune systems are at greatest risk.

For more information including statistics and maps, visit the Massachusetts Department of Public Health website and the US Geological Survey Disease Map page.  

Do mosquitoes threaten the health of wild and domestic animals?

WNV has been documented in over 250 species of birds (including various exotic, captive species as well as wild birds) and 18 species of mammals in North America since 1999. There was initially a very high mortality rate in many native species of birds when the disease first arrived, but it is still unclear whether the disease will have long-term population effects on native birds.

Horses can contract EEE and WNV, and a vaccine is available for EEE for horses.  Certain other species can also be vulnerable, including emus and exotic game birds.  There have been a few cases of WNV documented in cats. Dogs may contract WNV but do not generally become ill. Contact your veterinarian for more information on the potential susceptibility of particular domestic animal species.

Dogs (and occasionally cats) can contract heartworm from mosquito bites. Dog owners should have their animals tested annually and treated with preventative medication to avoid the serious health effects of heartworm.

How do I know if I am at risk of infection?

The Massachusetts Department of Public Health monitors mosquito populations and tests for the presence of the WNV and EEE. The Department warns local boards of health, hospitals, and the public when it detects viruses in mosquitoes and updates their website with regional statistics. You can also find tips there on staying safe.

Do all mosquitoes bite people? Do all mosquitoes carry EEE or WNV?

Absolutely not to both questions. Of the more than 50 species of mosquitoes in Massachusetts, most do not usually bite humans or transmit disease to people. EEE is very rare because the great majority of mosquitoes do not carry the virus, and of those that sometimes do carry EEE, most are a species (Culiseta melanura) that bites birds almost exclusively. Exactly which species transmits EEE to humans by biting an infected bird and then a person is unknown. The Department of Public Health monitors populations of the three species of mosquitoes that are suspected EEE transmitters.

WNV is somewhat more easily transmitted to people, but it causes serious illness in a much smaller percentage of the total cases. A mosquito called Culex pipiens that commonly breeds in small areas of stagnant water such as catch basins, gutters, discarded tires, etc. is believed to be a primary transmitter of WNV but other mosquito species may also transmit this disease. Since it is fairly new to this continent the transmission routes and trends of WNV are not fully understood.

What is the recent history of EEE in Massachusetts? When was the last outbreak and what happened?

EEE outbreaks tend to occur in 10 to 15 year cycles, with several years of activity followed by periods of lower risk. However, it appears that this pattern may be changing, perhaps due to climate change.

A significant outbreak with multiple human cases of EEE also occurred in the early 1990s. In 1990, 800,000 acres in southeastern Massachusetts were sprayed with malathion. There were reports of fish kills and human health effects associated with that spraying operation. We published a report, "Learning from Experience" on the subject.

Subsequently, and consistent with our recommendations, the Department of Public Health worked with other agencies, experts, and environmental groups to prepare a phased response plan that established protocols for different levels of response depending on risk indicators.  The state has conducted wide-area aerial spraying with Anvil 10 + 10 (Sumithrin) in southeastern Massachusetts several times since 2006, when risk indicators were high.  Even after aerial spraying the Department of Public Health urges people to take personal protection measures since the spray does not kill all infected mosquitoes.

What methods are used to control mosquitoes? What are the effects of these activities?

Mosquito control methods include:

Source reduction involves removing areas of stagnant water where mosquitoes breed. For example, buckets, tires, and other artificial containers can be removed. Storm water detention basins should be designed to drain within a few days after each storm or to hold permanent ponds of water that can support mosquito predators like fish. Catch basins should be cleaned regularly. Roadways and storm water outfalls should be cleaned and maintained to minimize deposition of sediment into streams. Low Impact Development methods for treating stormwater, such as rain gardens and vegetated swales that do not collect stagnant water, should be preferred for new development. Other source reduction methods involve more complicated manipulations of water levels in wetlands, often with significant ecological impacts.

Open Water Marsh Management, used in ditched salt marshes to improve access by fish to mosquito breeding areas, can be effective and environmentally sound when carefully managed. There is no equivalent method for freshwater marshes and swamps, and few data are available on the effectiveness and impacts of inland water management practices.

Mass Audubon generally opposes construction, maintenance, and enlargement of drainage ditches in freshwater wetlands for nuisance mosquito control purposes. These activities can have harmful environmental effects including draining of wetlands, destruction of amphibians and reptiles by heavy equipment, erosion and sedimentation, and other, permanent or long-term alterations of productive wildlife habitat. However, we support stream restoration projects that improve fish habitat and access by fish to areas blocked by artificial structures such as undersized road culverts and obsolete dams.

The State Reclamation and Mosquito Control Board developed Best Management Practices (BMPs) for freshwater mosquito management as part of a Generic Environmental Impact Report in 2008. Mass Audubon supports the application of scientifically based BMPs and associated monitoring programs to document the effects of mosquito control practices. The primary focus should be on reducing public health risks and promoting a healthy environment. Proper design and maintenance of storm water systems is also important.

Adulticiding involves spraying chemical pesticides, such as malathion, resmethrin, or sumithrin (Anvil) to kill flying adult mosquitoes. This method has only short-term local effectiveness, as new mosquitoes soon fly into the area or emerge from breeding pools. Spraying of broad-spectrum pesticides also exposes people, pets, and wildlife to the chemicals. Butterflies, bees, aquatic invertebrates and fish are particularly sensitive to some of the commonly used adulticides.

Routine spraying for nuisance mosquito control also may lead to pesticide-resistant mosquitoes, which would be more difficult to control in the event of a public health emergency. Pesticide spraying may present some level of human health risk as well, particularly for certain sensitive individuals such as asthmatics. Mass Audubon opposes adulticiding for nuisance control of mosquitoes.

For information on the health effects of pesticides, contact the Department of Public Health's Bureau of Environmental Health Assessment at 617-624-5757.

Larviciding involves application of chemical or bacterial materials to mosquito breeding areas to kill mosquito larvae. Bacillus thuringensis israelensis (Bti) is a toxin producing bacterium that, unlike broad-spectrum chemical pesticides, narrowly targets mosquitoes, midges, and other closely related flies. However, Bti may disrupt the food web in vernal pools where amphibians breed.

Another commonly used larvicide is methoprene. It is a growth regulator, which acts by interfering with the normal metamorphosis process thereby preventing mosquito (and various other insect) larvae from reaching the adult stage. Methoprene briquettes are often used in catch basins, because they provide a much longer duration of control than Bti.

How effective are mosquito control practices?

The effectiveness of mosquito control in a rural or suburban landscape with large amounts of wetlands is questionable. There is little documentation of the effectiveness of mosquito control activities in Massachusetts, and most of the available information is on short-term effects within a few days after pesticide applications.

Pesticide applications and wetland ditching or draining may harm or kill beneficial species (including pollinators and mosquito predators), alter water chemistry; lower water levels, and degrade wetland habitats.  There is no organized program for monitoring of such potential effects.  The high reproductive rate and short life-cycle of mosquitoes may allow populations to evolve which are resistant to the pesticides, while local populations of mosquito predators (such as frogs, fish, and predatory insects) are less resilient.

The mosquitoes of greatest concern for WNV transmission breed primarily in small isolated areas, such as buckets, old tires, tree hollows, and stormwater catch basins. Therefore, local efforts to manage WNV should focus on prevention and removal of human-created breeding habitats, and on public education and personal protection from mosquito bites.

How can I protect myself and my family from mosquito bites?

In and around the home, mosquitoes can be excluded by the use of screens. Containers of standing water (e.g., buckets, old tires, clogged gutters, dirty birdbaths) should be removed or cleaned frequently. Some mosquitoes can grow from egg to larvae to adult in less than a week under favorable conditions.

Away from home, avoid marshy or swampy areas (particularly at dawn and dusk when mosquitoes are most active); wear protective clothing (long sleeved shirts and pants); and use insect repellent on exposed skin. For those concerned about the possible effects of DEET, the most commonly used insect repellent, one can apply the repellent to clothing rather than directly to the skin. Never use DEET repellents on infants.

Some people place bat and/or bird houses on their properties to attract these insect-eating creatures, or purchase dragonfly larvae because adult dragonflies eat mosquitoes. There is no proof that these practices have significant effects on local mosquito populations. Insect-eating birds may benefit from nesting boxes and the property owner may enjoy their presence. There is no harm in placing nesting boxes even though the mosquito control benefits are unproven.

How can I get my property excluded from a local spraying program?

Property owners may have their land excluded from routine nuisance control spraying at any time through the state's online system. If a public health emergency is declared in your area, you will probably not be able to get your property excluded (although exceptions might be made for sensitive areas like surface water supplies, beekeeping operations, fish farms, and organic farms).

How does Mass Audubon deal with mosquitoes on its wildlife sanctuaries?

Mass Audubon manages its wildlife sanctuaries as natural ecosystems of which mosquitoes are a part. We do not use or allow the application of pesticides (adulticides or larvicides) to Mass Audubon properties for control of nuisance mosquitoes. Mass Audubon does not interfere with the natural water regime of wetlands on our properties to control mosquitoes, because of the significant adverse effects on wildlife and the natural system as a whole.

Exceptions to this policy may be made:

  • In the case of a declared public health emergency.
  • For Open Marsh Water Management or other ecologically based management practices where modification of existing mosquito control ditches or degraded wetlands may have ecological benefits as well as reduce mosquito populations.

Bird-nesting boxes are also placed on some of our properties to attract and provide breeding sites for insect-eating birds such as tree swallows and bluebirds.