Mosquito control: Trench warfare and beyond

In the early 1900s, California’s northern peninsula was a haven for wealthy Americans, who inhabited summer homes overlooking the San Francisco Bay. They were there for only a few months because the scenic setting could quickly become anything but restful. “The entire sky would go from day to night when the salt marsh mosquitoes came off the Bay,” says Robert Gay, district manager for the San Mateo County Mosquito Abatement District. “People couldn’t live here any other time of the year because of the mosquitoes.”

They had reason to flee. Across the United States – well before the use of window screens, air-conditioners and pest control science – mosquitoes brought the scourges of yellow fever and malaria. In a particularly dramatic epidemic in 1793, Philadelphia lost 15 percent of its population, when more than 4,000 residents died during a yellow fever outbreak. From New York to New Orleans and Houston to Norfolk, Va., thousands more would die before 1905, when yellow fever was finally controlled.

The earliest mosquito control organization was the National Mosquito Extermination Society, formed in 1903 in New York. Ten years later, the New Jersey Mosquito Extermination Association was formed, followed by other similar state organizations. Their efforts to ward off sickness were successful, indeed; today, mosquitoes are widely regarded by Americans to be nuisances rather than disease-carrying varmints.

The diseases are still here, however. In the United States, mosquitoes transmit a variety of forms of encephalitis – including West Nile Virus (see the story on page 48) – which can afflict humans, wildlife and horses. They transmit heartworms to dogs. And they carry malaria and dengue fever, which is found in the United States annually.

Therefore, mosquito control remains a modern necessity. Working with the USEPA, the medical and scientific communities, and state and local agencies, mosquito control districts are employing surveillance, treatment and education to keep nuisance and disease at bay.

Building the dossier

Successful mosquito control begins with thorough surveillance. “It is the sine qua non of any mosquito program,” says Joseph Conlon, technical adviser for the American Mosquito Control Association, based in New Brunswick, N.J. “You’ve got to find out what mosquitoes you’re dealing with so that you can target your control measures.”

According to EPA, there are 200 species of mosquitoes in the United States, each with unique characteristics. “Different species are found in different places; they bite at different times of the day or night or both; they feed on different things (e.g., some feed on birds, some on humans, some on cattle, etc.); they breed at different times,” Conlon says.

With surveillance, districts are able to identify the species in their areas; learn when and where they will become active; and better direct their resources at attacking only those species that present a problem. The process includes larval and adult monitoring, and it incorporates four main field exercises:

– Dipping. Because all mosquitoes breed in water, personnel are dispatched to area water sources, where they take samples of the water with dippers. (For large sources such as ponds, samples are taken at multiple points.) The larvae are counted and identified, and the results are compared to the treatment indices established for those areas. If, at a given site, the number of larvae captured exceeds the threshold for that area, the site will be treated.

– Trapping. Light traps are used to monitor adult mosquitoes. They consist of a light source, which attracts mosquitoes, and an impeller that sucks the insects into a bag. The trap is placed overnight, and the capture is counted. Species are identified, and, in some cases, live specimens are tested for diseases.

– Counting. Personnel are dispatched to strategic sites, where they allow mosquitoes to land on them. Accompanying personnel count the number of insects that land on the “victims” in a given amount of time, and the landing rate gives the district an indication of how many human-feeding mosquitoes are in each area.

– Disease monitoring. “Just because you have mosquitoes doesn’t mean you have disease,” Conlon notes. To check, control districts place chickens in strategic locations, where adult mosquitoes are present, and leave them overnight. The mosquitoes bite the chickens, and the birds’ blood is then tested for mosquito-borne viruses. A positive result indicates the presence of disease-carrying mosquitoes.

In addition to those tests, districts rely on service requests to help them determine mosquito activity. “We actually base our adult control efforts primarily on the landing rate counts and the service requests,” says Henry Lewandowski, director of the Chatham County Mosquito Control Commission in Savannah, Ga. “Both of them produce real-time information.”

The Chatham County Mosquito Control Commission services 450 square miles and employs 30 people, including entomologists, surveillance technicians, two pilots, an aircraft mechanic and heavy-equipment operators. It uses all the mosquito surveillance techniques.

Lewandowski notes that, like many mosquito districts, his staff supplements the light traps with carbon dioxide sources. “We put about two pounds of dry ice in an insulated thermos, which has holes drilled in the bottom to allow the dry ice to gas off,” he explains. “It’s a source of carbon dioxide, which is attractive to mosquitoes, and it collects more numbers and more different species [than a trap without carbon dioxide]. It gives us a very good indication of relative populations.” The technique also is used with the county’s sentinel flocks.

Taking the offensive

With an understanding of their survey data, control districts can attack. “Surveillance should drive your control measures,” Conlon points out. “You don’t do any control measures unless your survey tells you to.”

If control measures are in order, districts have several choices. First and foremost, they can reduce the number of breeding sites, Conlon says. “Source reduction is probably the most important measure because it’s permanent,” he explains. “It needs upkeep, but, if you’re going to leave mosquito breeding habitats all around you, you are going to be fighting a rear-guard action the rest of your life.”

In urban areas, source reduction can range from storm drain maintenance and filling tree holes to removal of trash tires and cans. In settings that are more rural, it also might include ditch and pond maintenance, as well as habitat modification.

“A lot of source reduction involves public education,” Conlon says. “Many of the mosquito problems we have are with mosquitoes that breed around houses. [Tell residents to] empty the water out of the tires and cans in the backyard; fill ditches where they’re allowed to; clean the vegetation out of ditches; if they have a pond, reduce the vegetation on the margins, where mosquitoes breed.”

Beyond the backyard, there are additional steps districts can take to reduce breeding sources. For large areas of water, they can increase flow and reduce vegetation or modify drainage to eliminate breeding habitats, Conlon says.

That is exactly what is happening in Chatham County, where the Mosquito Control Commission constructs ditch systems to enhance drainage. “If water is spread over a very large area after a rain, we’ll cut ditches that follow the natural contours of the area, and we’ll try to get the water to drain to a county canal or ditch structure,” Lewandowski says. “If that’s not possible, we’ll create a small pond within the ditch system that we’re creating, and we will [stock it with fish that eat mosquitoes].

“If the area floods again, the water spreads out from that pond, and the fish spread out over the flooded area. The canals then work to channel that water, so the water is contained primarily in the ditches and in the pond. A large area that held water is now a large area drained. In the long run, it’s a very economical way to control mosquitoes, and it reduces the pesticides that we use.”

Opening the war chest

Of course, all breeding habitats cannot be reduced or eliminated, meaning that mosquito control districts must be prepared to kill mosquitoes. For that job, they again turn to a variety of tools:

– BTI (bacillus thuringiensis israelensis). Ingested by the mosquito larva, the bacterium works with the mosquito’s biology to produce a toxin that kills it. It is available in liquid, granules and time-release briquets, and it is distributed at the breeding site. Because it is a food source, it must be applied before the mosquito pupates and at a stage when the larva is large enough to eat it. Additionally, effectiveness drops in highly polluted water, where it has to compete against other food sources.

– Methoprene. Methoprene is an analog of juvenile mosquito hormones. Applied to a breeding source in liquid, granules, pellets or time-release briquets, it alters the hormonal development of the mosquito larva, preventing it from pupating. “There is a timing issue,” Conlon explains. “You can’t just go out and spray the water with methoprene and expect it to do any good, because it may not. You have to apply the analogs when the larvae are in a certain stage.”

– Monomolecular surface films. Applied to the water surface, the chemical solution spreads and forms a film one molecule thick. According to EPA, the film makes it difficult for larvae, pupae and emerging adults to attach to the water’s surface, causing them to drown. It degrades within 14 days. (Some districts use oils that are derived from petroleum distillates and work similarly to the surface films.)

– Organophosphates (malathion and naled). Organophosphates are used as adulticides and work by affecting nerve transmission in mosquitoes. They have come under fire from activists who fear that the toxins are unsafe for non-target species and the environment. Although the chemicals remain registered and, according to EPA, safe and effective, public concern could affect their availability in the future. “I can foresee a time when malathion will be pulled off the market just because it’s too much of a pain legally to keep it registered,” Conlon says.

As for naled, Conlon rates it as the “most effective mosquito adulticide in the world.” However, he notes that it is difficult to work with because of its corrosive qualities. “If you don’t keep your machinery very, very clean and well calibrated, it will foul up machinery. It’ll clog lines,” he says.

– Pyrethroids. Used to control adult mosquitoes, pyrethroids will kill the insects on contact. “You use it in minute quantities, and it’s pretty non-toxic, comparatively speaking,” Conlon says. Pyrethroids are sprayed at strategic times – usually in the morning and at dusk, when most adult mosquitoes are feeding and when temperatures will not degrade the droplets.

Successful mosquito control relies on a plan that combines surveillance and source reduction with a variety of larvicides and adulticides. Pesticide preferences differ from district to district, but all districts concentrate on ridding themselves of larvae first. “Once the mosquitoes become adults, it can be difficult to get rid of them because you’re trying to kill something flying around in a three-dimensional space,” Conlon explains. “In a small way, you’ve lost the war once they’ve gotten to that stage.”

In St. Paul, Minn., the Metropolitan Mosquito Control District – with a staff of 48 full-time and 170 seasonal workers – focuses almost exclusively on larva control. It monitors 60,000 breeding sites in 2,600 square miles, including approximately 250,000 acres of wetlands. “About two-thirds [of the wetlands] are capable of getting mosquito infestations,” says Joseph Sanzone, the district’s director. “So our major program is to monitor the breeding sites on a regular basis and to treat them with larvicides as needed.”

The district starts larva treatments in February, and it relies heavily on BTI. Using the granulated form, field crews distribute the pesticide by hand, or, in areas three acres or larger, by helicopter.

Additionally, the district uses some methoprene, in pellets and briquets. The briquets, in particular, are useful in areas that are difficult to reach during the spring and summer, Sanzone says. “A lot of [our sites] are secluded and hard to get to when it’s wet and everything is thawed out,” he explains. “So we [treat] in the winter, when we can walk out on those iced areas and put the briquets out. When it thaws, the briquets drop into the water and treat for mosquito larvae.”

If adult control is necessary, the district applies pyrethroids strictly by ground, either with truck-mounted or hand-held equipment. “Since a lot of our people live in concentrated areas, it’s so much easier to control [the adult mosquitoes] by ground,” Sanzone says. “Even in the less populated areas, we don’t try to control them by air because it’s too much treatment involved for such small returns.”

San Mateo County is similarly focused on larvae. “Our district is more urban,” Gay explains. “If you’re rural, you’ll typically spend more time controlling the adults because you’ll have them over such a broad area. In the urban area, we do a lot more larviciding.”

Servicing 166 square miles with 11 full-time and six seasonal staff members, the San Mateo County Mosquito Abatement District uses a combination of mosquito-eating fish, surface films, methoprene and BTI to kill the area’s mosquito larvae. The upscale county, with homes averaging $1 million, has nearly 4,000 backyard ponds, and the fish are particularly valuable for larva control in those areas, Gay says.

The district provides fish free to residents and re-stocks the ponds as necessary. “We don’t rear the fish ourselves, but we have other districts out in the valley that use many, many more fish than we do, so we just get fish from them,” Gay says.

Catch basins also present a problem for the district. “With all the automatic sprinkling systems in people’s yards, there’s enough water going down the streets on a daily basis, even through the summer, to keep [mosquitoes breeding in] all those catch basins,” Gay says. “The mosquito that lives there is the same mosquito that vectors West Nile Virus, and it readily comes inside your house and readily feeds on you.” Surface films, in particular, have helped the district combat catch basin larvae, he notes.

Of course, even with its aggressive efforts to reduce its larvae population, San Mateo County has adult mosquitoes; but, given the choice, the district prefers to not treat them. “There’re lots of mosquitoes out there, but we try to convince the local population that we’d like to not spray,” Gay explains. “When you go out and fog for the adult mosquitoes, you’re also killing the parasites and predators, so it allows for a pest population like mosquitoes to rebound very quickly.” When spraying cannot be avoided, the district chooses pyrethroids, applied on the ground.

Incorporating technology

As in other areas of local government, technology is playing an increasingly important role in mosquito control. Geographic information systems can assist control districts in mapping their breeding sites and planning treatment regimens.

Chatham County contains more than 7,500 acres of spoil sites – areas where sediment is dumped when channels are dredged – and the sites are hotbeds for salt marsh mosquito breeding. As a result, aerial application of pesticides is a must, and the control district uses GIS to improve the efficiency and economics of that process.

“[The sediment] is pumped into these contained sites, and, when that mud surface dries, it cracks,” Lewandowski explains. “The insides of the cracks stay moist, so mosquitoes lay their eggs on the sides of the cracks. The next time they pump material into the site, or when we get a significant rain, all the eggs that have been collecting for weeks – or even months – hatch off into hundreds of thousands of mosquitoes.”

When the district’s technicians survey the sites, they use existing orthophotographic maps or take digital photographs from a helicopter and map the targets for aerial treatment. (The district uses methoprene mixed with sand for aerial application of larvicide.) GIS allows the district to calculate the size of the targeted area, determine how many missions it will take to cover the target and decide how much material will need to be mixed.

GIS also is being used in San Mateo County and in St. Paul. Sanzone notes that global positioning technology has been especially helpful in guiding technicians through unpopulated regions. “Some of the areas are very hard to find, and you have to go deep in the woods, and you can become disoriented very easily,” he says. “The GPS will pinpoint those sites so our people can go right to them.”

Training and communicating

Occupied with the tasks of monitoring and battling mosquitoes, control districts also must educate the public, elected officials and their own personnel to ensure that their programs meet the specific needs of their communities. “No district goes out and just sprays pesticides willy-nilly,” Conlon says. “And it is important that everyone understands that.”

It is as important for staff as it is for the customer. “Ensuring that your people are trained is absolutely essential,” Conlon notes. “They have to know the importance of what they’re doing and the reasons they do things a certain way.”

Pesticide application requires certification, and, to retain certification, technicians usually have to participate in annual continuing education. Those hours can be obtained by attending state or professional association workshops. Some control districts, including the one in St. Paul, provide their own continuing education programs.

“We give our full-time staff two days of training every year,” Sanzone says. “We cover what we’re doing and why we’re doing it; safe handling methods; proper pesticide application; environmental and health issues; responding to the public; safe driving; the whole spectrum. We rotate the subjects so we have everything covered in a three-year cycle.” When seasonal staff come on board, they, too, receive training, but it is more focused to the specific tasks they will have.

In addition to training its staff, the St. Paul Mosquito Control District has programs to educate the public and to receive feedback from its customers. Like many districts, it uses printed material, school visits, public service announcements, public displays (e.g., at the local libraries), a Web site and a telephone hotline to give residents information about mosquitoes and health risks; ways they can protect themselves; and tips on reducing breeding sources. Additionally, it conducts a biennial survey to measure the public’s knowledge.

“We try to gauge what our customers – meaning the public and elected officials – want,” Sanzone says. “We want to get a good, basic knowledge of what people know of us or perceive us to be and what they would like to have from us.

“We have the survey done by an outside party, and they ask about 15 or 20 questions,” he explains. “We see if people are aware of what we do; if they approve of what we’re doing; whether they would like to see more control or less control; or whether they have concerns about environmental or health issues.”

In addition to educating the public, control districts must be prepared to educate elected officials and the media, who are the parties most likely to demand explanations about control measures. “Have a concept of what you’re doing and why you’re doing it,” Conlon warns. “That’s where public relations begins, and, in some cases ends.

“Get the local media and the elected officials on your side,” he says. “Convince them – don’t lie to them – but convince them of the reasons why you’re doing certain things and why they need mosquito control. Indeed, if you cannot convince them, or, if you don’t have a good reason to control mosquitoes or put out pesticides, then maybe you shouldn’t do it.”