FAQs about aerial spraying for West Nile virus mosquitoes

A single plane can treat 40,000 acres in a night, an area that would require approximately 40 ground-based trucks. Photo courtesy CDC, Janet McAllister.

Dallas County recently decided to offer aerial mosquito spraying services for Dallas area communities.  The decision was made in response to the threat of record numbers of West Nile virus (WNV) cases in the north Texas area.  As with any important decision, there are differing opinions on what should be done to battle this latest mosquito crisis.  Because of concerns about aerial applications of insecticides over urban areas, I thought it might be useful to address some of the common questions from a science, applied entomology and public health perspective.

Q. Aren’t there other actions we can take that can stop West Nile virus other than pesticides?

Yes! Pesticides are just one tool among several that have been developed over the past century to manage mosquitoes.  Most cities in the north Texas area, including Dallas and Dallas County rely on multiple tactics to manage mosquitoes throughout the year. Mosquito management starts early every year with source reduction and surveillance.

Source reduction means looking for places where mosquitoes breed and treating or eliminating those sites.  This job is usually one of the tasks conducted by city code compliance and enforcement departments.  Trained staff respond to complaints as well as proactively look for and map potential mosquito breeding sites.  The Department of Code Compliance for the City of Dallas, for example, maintains a database of approximately 1500 mosquito trouble spots that they visit and treat on a rotating basis. When a site is found to be infested with mosquito larvae, the water is either drained, mosquito-eating fish are introduced, or the site is treated with a low-toxicity insecticide.

Unfortunately, however, when you deal with large areas, the number of mosquito breeding sites can quickly become overwhelming.  Consider that a single container the size of a soda can is capable of producing dozens to hundreds of mosquitoes a month. Mosquitoes may have dozens or hundreds of potential breeding sites on a single city block–far more than can be handled by a small city staff.  This is where education comes in.  Most cities and counties maintain a public education program to encourage citizens to patrol their own yards and protect themselves when venturing outside during mosquito season.

Surveillance is where city/county/mosquito district employees collect, identify and test mosquitoes for the presence of West Nile virus. These surveys tell health workers where the biggest risks are located and where education and control efforts should be directed.  Surveillance is what directs neighborhood truck spray teams and the less visible efforts to eliminate and treat breeding sites.

Dallas County and it’s partner cities have integrated mosquito management programs that incorporate all these elements.  However, despite their best efforts, this year WNV infection rates have climbed to what some officials consider unacceptably high levels.  Consequently, the Centers for Disease Control, Dallas County Medical Society, and the Department of State Health Services have all supported aerial spraying to save lives and stop the growing number of WNV cases.

Q. Are the chemicals being used harmful to people?

In 2003 the Centers for Disease Control (CDC) issued a report on acute health risks of several mosquito insecticides over a four year period and found only 133 reports of human illness following mosquito spraying.  These cases were from a population in the treated areas covering 118 million people (one illness per 887,000 people).  They concluded that serious human health effects due to public health insecticide applications were uncommon and generally not severe, but recommended ways to reduce risks even further. Cities are advised to use integrated pest management strategies (as described above) and inform the public when and where spraying will occur and how to reduce chances of exposure. Also, persons with exposure-related health concerns should consult their health-care providers prior to aerial spraying.  Although risks of harm  should be low, everyone is advised to stay indoors during evenings when spraying is scheduled.

The low doses of insecticides used in mosquito abatement programs explain the low risk of mosquito sprays. The label application rates for Duet, the product being planned for aerial spraying, range from 0.41 to 1.23 fl.oz (1-2 tablespoons, or approximately 0.0625 lbs) DUET per acre (5-6 typical home properties).  Such rates are far below doses that would be expected to pose a health risk to humans, pets, fish and other large non-target animals.  An estimated LD50 dose (likely fatal dose by ingestion or skin exposure) for sumithrin and prallethrin is more than 0.75 lbs for a 150 lb human.

Q. Do the pesticides used in mosquito control cause cancer?

All insecticides pose different risks–risks which are influenced by both the toxicity of the product, but also by the level of exposure.  According to the National Pesticide Information Center, the common mosquito insecticide permethrin (the active ingredient being used in Dallas for  ground spraying) was classified by the International Agency for Research on Cancer (IARC) as “not classifiable as to its carcinogenicity to humans” in 1991. This means that IARC could not decide whether or not permethrin can cause cancer. The U.S. EPA decided that permethrin was “likely to be carcinogenic to humans” if it was eaten. Neither prallethrin nor sumithrin (the active ingredients in the aerial spray) are thought to be carcinogenic at any dose.  Most environmental scientists would consider cancer risks from mosquito control programs, from any of these or other insecticides labeled for mosquito control, to be low.

Q. What effect will pesticides have on wildlife?

The impact of aerial spraying on wildlife is difficult to measure accurately because of the vast number of different living organisms in the environment. It is likely that at least some insect species, for example, may be harmed, at least temporarily, by mosquito spraying. These sorts of impacts are assessed when an insecticide is tested by the U.S. Environmental Protection Agency (EPA).  The standard used by the EPA is that no insecticide should pose an “unreasonable adverse effect on people or the environment”. This includes looking at the fate and impact of mosquito sprays in the environment.  The most commonly used insecticides in public health sprays are all relatively short-lived in the environment (<24 to 48 hours), and pose little risk to most birds and mammals. In addition, mosquito spraying is done at night, when many beneficial insects are likely to be sheltered.  In this way honey bees and other pollinators, beneficial predators, dragonflies and butterflies are less likely to be harmed.

Q. How will aerial spraying affect bees?

Insecticides used in mosquito control programs are toxic to honey bees.  Left unprotected, some bee colonies will suffer some mortality from aerial sprays, particularly those bees that cluster outside hives at night.  For this reason it’s important for beekeepers to take steps to protect their colonies on nights when aerial sprays are scheduled.  Practical methods include moving hives out of the treated area or covering hives with loose fabric, like wetted burlap, to keep bees in the hive and protect from insecticide drift.  For more information see http://edis.ifas.ufl.edu/aa145

Wild bees generally nest in protected locations and should be mostly protected because they are in the nest when sprays are applied at night. Wild bees are relatively common in north Texas, although numbers may be lower this year due to drought.  Although the full impact of aerial applications on honey bees and other wild bee pollinators is unknown, wild pollinator populations would not be expected to be harmed for long-term after sprays are discontinued.

Ground based vehicles allow for precise applications, but cannot reach all possible mosquito habitat. Photo courtesy CDC, Janet McAllister.

Q. What will happen if we don’t spray?

The number of West Nile virus cases typically peak in late August or September. Given the high numbers of cases of West Nile virus already seen, and the increasing percentage of mosquitoes still testing positive for the virus, health officials expect to see the number of disease cases increase.  As of the second week in August there have been ten deaths and over 190 cases of West Nile disease in Dallas county alone.  The statistics fail to show the full impact of WNV on people.  Survivors of the most severe form of the disease are often left with lingering health problems that can last for years. The death rate for the neuroinvasive form of the disease this summer is ranging from 8-9%, so the number of deaths will also be expected to increase.

Q. Will aerial spraying help?

Data from Dallas in 1966, as well as numerous other studies conducted in urban areas in California, Louisiana, Colorado, and Michigan show that aerial applications can make a substantial difference in the infection rate of both mosquitoes and people.  In Dallas, the rate of Eastern equine encephalitis infection declined from one infected mosquito per 167 mosquitoes tested to one infected mosquito per 28,639 mosquitoes tested after aerial spraying (a 170 fold reduction).  In Michigan, treated communities had a 10X lower rate of human infection than untreated communities. Significantly lower infection rates were also seen in Fort Collins, CO and Sacramento, CA following aerial spraying.

Aerial spraying is considered to be more effective than truck-based spraying.  While they can provide precise targeting of or avoidance of sites, truck-based sprayers have difficulty treating behind fences and shrubs and other obstructions.  Aerial applications, like the mosquitoes themselves, are not limited by areas without roads, and can treat areas otherwise unreachable by ground-based sprayers.

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