Mosquito Control Methods
The District utilizes three basic methods for mosquito control operations: Source Reduction, Larval Control and Adult Control.
The purpose of source reduction is to reduce the number of larval habitat areas available to mosquitoes. The domestic control program targets two species of mosquitoes: the yellow fever mosquito, Ae. aegypti; and the Southern house mosquito, Culex quinquefasciatus. These mosquitoes are most easily controlled by treating or emptying containers that hold water, since the eggs of both species are laid in water-holding containers. Citizens can do much to prevent mosquito breeding on their property by the elimination or water in garbage cans, flowerpots, other containers and flushing bird baths on a weekly basis. Items that can hold water but are no longer used, such as old tires, should be disposed of properly. Boats should be covered or otherwise prevented from holding water.
Visiting Residents Homes
Homeowner inspections are one of our largest and most important methods in helping to eliminate mosquitoes around homes and drains in residential communities. Inspectors work to remove the mosquitoes by ridding the property of standing water thereby removing places for the mosquito to lay her eggs. Drains are inspected and treated or mosquito-eating fish may be introduced to combat mosquitoes where appropriate. The most problematic mosquito is the Aedes aegypti that has the potential to spread dengue fever or one of the other viruses which it is capable of transmitting. Homeowners are educated on effective methods of mosquito prevention and control including:
Larval Control Program
Larval control targets the immature mosquitoes living in water before they become biting adults. A naturally occurring soil bacterium, Bacillus thuringiensis israelensis (Bti), is applied from the ground or by air to larval habitats. This bacterium is used because it virtually has no effect on non-target organisms when it is applied properly. The District also uses Spinosad as a larvicide for container breeding mosquitoes. Spinosad is another naturally occurring soil bacterium called Saccharopolyspora spinosa that is a 2010 Presidential Green Chemistry Award winner. Both of these larvicides have the highest ratings for safety and environmental compatibility. Larval control is, by far, the most effective method of mosquito control and is the method primarily employed by the District. The vast majority of larval control is accomplished by the use of Bti which is coated onto corncob granules and applied by helicopter. Small areas of less than an acre usually are treated with liquid, granular, packet or pellet formulations of Bti or methoprene utilizing hand-held applicators. Containers may either be emptied or treated with spinosad tablets. Small fish (Gambusia) may be used for the control of mosquito larvae in small ponds cisterns, storm drains and like places.
Adult Surveillance Program
Landing Rate Counts: Landing rate counts are the primary method of adult mosquito surveillance employed by the District. Inspectors visit the same count stations daily and count the mosquitoes that land on them and attempt to bite for one minute. There are over 350 count stations visited on a daily basis throughout the Keys. The information gathered by these inspectors allows the District to better assess the biting population of mosquitoes throughout the Keys and respond appropriately.
Adult Trap Collections: A secondary method for monitoring mosquito populations is to employ adult mosquito traps. The District utilizes a variety of traps to sample the adult mosquito population. One of the trapping methods utilized is the CDC light traps baited with carbon dioxide. These traps attract host-seeking female mosquitoes in search of a blood meal. The District monitors over 50 CDC light trap sites throughout the Keys on a weekly basis. Another collection method used by the district is the BG Sentinel trap. This particular trap has both visual and chemical attractants to lure host seeking mosquitoes into the trap. The BG Sentinel trap is specifically designed to attract the yellow fever and dengue carrying mosquito, Aedes aegypti. Because this mosquito breeds almost entirely in man-made containers, this trap is particularly useful in areas of higher human population density, such as Old Town, Key West. There are over 40 BG traps sites monitored on a weekly basis throughout the Keys. Other methods of adult collection include aspirators, and other specially designed traps.
Adult control targets the flying, blood-seeking female mosquitoes. Adult control is conducted from the ground via truck-mounted spray systems, and from the air via helicopters and fixed-wing aircraft. Adulticides can be applied only when meteorological conditions are exactly right. In particular, if the wind speed is too high the spray will move out of the target area too rapidly, or if too low, effective dispersal of the spray will not be achieved. Spray trucks are deployed in the evening usually starting one half hour after sunset, to catch the mosquitoes as they begin their evening flight activity. Aircraft are deployed in the early morning, when temperature inversion conditions ensure the tiny spray droplets will reach the ground in order to contact mosquitoes as they finish their nightly flight activities. In both cases, ultra low volume (ULV) spray technology is used. This allows a very small amount of adulticide, amounting to fractions of an ounce per acre, to be used. It also eliminates the use of diluents such as diesel oil which would be an undesirable environmental contaminant. All insecticides used by the District are approved by the US Environmental Protection Agency and the Florida Department of Agriculture and Consumer Services and are specifically selected for use as adulticides because of their safety, effectiveness and environmental compatibility.
Control of adult mosquitoes is necessary when mosquito populations cannot be treated in their larval stage. Use of mosquito adulticides is an integral part of a modern, integrated mosquito management program. It also is the most effective way to eliminate adult female mosquitoes that are infected with human pathogens. Depending upon the size of the area that needs to be treated, the District will utilize either trucks for ground adulticide treatments or aircraft for aerial adulticide treatments. Justification for adulticide treatments are based upon citizen requests, landing rate counts and trap collections.
Ground Adulticide Treatments
When mosquitoes are concentrated in small areas, the District will deploy truck- mounted ULV spray systems. Ground treatments are done in the evenings beginning approximately one hour before sunset. Trucks are dispatched to those neighborhoods with high numbers of biting mosquitoes, allowing area-specific treatments to control problem populations. Each truck is driven at speeds specified by the pesticide label which usually do not exceed 10mph while the insecticides are being dispersed. Wind plays an important role during spray missions because of the minute size of the insecticide droplets. If the wind is too great, the insecticide will not be distributed properly throughout the air column and will be blown away from the target area. Therefore, spray missions will be conducted during times when wind conditions are optimal, usually between 2 to 10mph. The District uses permethrin, a synthetic form of pyrethrum, to control adult mosquitoes when dispensed by truck-mounted sprayers.
Aerial Adulticide Treatments
In instances when mosquito populations are high and cover a large area, the District will employ aircraft to spray for adult mosquitoes. This is done with either a Bell 206B Jet Ranger helicopter or one of the District’s fixed-wing aircraft. These aircraft are equipped with ULV spray systems that are designed to distribute insecticides over large areas. Again, aerial missions are conducted only during optimal weather conditions. Flights are conducted at an altitude of 100 to 150 ft. when winds are less than 15mph on the ground.
The insecticide naled is used as a part of the District’s integrated pest management program to control migrating adult mosquitoes. It is a non-persistent insecticide which breaks down rapidly when applied in the manner of application employed by the District.
Mosquito Control has an extensive public education program throughout the Florida Keys. School programs are presented to various grade levels at all the schools throughout Monroe County. Speaking engagements for local civic groups and public outreach booths at festivals throughout the Keys are a significant part of the District’s outreach program. Mosquito prevention tips are included in weekly newspapers and on the radio. Homeowners receive informational door hangers and timely literature as part of the educational campaign. The District also distributes a 12-minute DVD titled A Florida Keys Homeowners Guide to Mosquito Control which details the steps they can take to rid their premises of container breeding mosquitoes.
New Challenges for Mosquito Control in the Florida Keys
The most recent challenge for the District is the recurrence of dengue fever in 2009 and 2010, a viral disease that has returned to Key West after an absence of 75 years. Dengue is the most common vector-borne viral disease in the world, causing an estimated 50-100 million infections and 25,000 deaths each year. Dengue is carried by Aedes aegypti, the same mosquito that transmits yellow fever and chikungunya. The viruses of dengue fever include serotypes 1, 2, 3, and 4. Recovery from an infection of dengue-1 provides lifelong immunity to that serotype, but does not provide immunity to serotypes 2, 3 or 4. In fact, if a person has had dengue -1 and later contracts a dengue -2, -3, or -4 infection it likely will be more serious and may result in the more severe hemorrhagic form which can be deadly. There presently is no vaccine available for the treatment of dengue. Aside from personal protective measures, the most effective preventive action that can be taken is to eliminate the mosquito vector that causes the disease. Residents can eliminate mosquito breeding around their homes by getting rid of, or treating, all water-holding containers and plants such as bromeliads.
There are other emerging mosquito-borne diseases, much worse than dengue, that are a real threat to Florida and the Keys in particular, that also are transmitted by the Aedes aegypti mosquito. One of which is a viral disease known as chikungunya that was discovered in Africa in 1952 and has caused epidemics in Africa, the Indian Ocean, India and Italy in 2007. Chikungunya virus infection is characterized by fever, severe headache, arthritis affecting multiple joints, joint pain, insomnia and an extreme degree of prostration. Thirty-six cases of chikungunya have been identified among travelers in the United States. One of these cases occurred in Volusia County, Florida in 2007 and Miami in February of 2010.