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| Significant Research Projects |
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Presently, the most effective method for the control of adult mosquitoes over large areas is the aerial application of insecticides. Aerial applications of insecticide allow mosquito control districts to treat large problem areas more efficiently and more effectively than do ground applications. However, the size of the droplet of insecticide is the determining factor as to how well this method controls mosquitoes. Large droplets, those with a greater diameter, are heavy and fall to the ground quickly. They do not stay suspended in the air long enough to land on flying mosquitoes. Therefore, large drops are wasted, and more insecticide is available to affect non-target organisms on the ground. Ideally aerial application equipment should produce a large quantity of small droplets in a narrow size range. Greater numbers of small droplets can be released into the atmosphere creating a wide swath of insecticide for the mosquitoes to fly through. Small drops stay in the air longer because they weigh less than large droplets. This also helps to reduce the risk of the insecticide reaching the ground and affecting non-target organisms.
In 1995 Dr. Michael A. Shirley of the Rookery Bay National Estuarine Research Reserve (RBNERR) noted increased mortality of fiddler crabs within the reserve after mosquito control applications over adjacent residential areas. The fiddler crab mortality evoked concerns of negative environmental effects that resulted in several research projects to study the problem and determine how to correct it.
  The first of these research projects was directed by Dr. Shirley and was co-funded for several years by grants from the CMCD and the National Oceanic and Atmospheric Administration (NOAA) to RBNERR. Dr. Shirley documented the environmental conditions under which mosquito control applications had a negative impact on the fiddler crab population in the study site. He found that under certain combinations of wind speed and direction, insecticide was drifting into the reserve and depositing on the ground in sufficient quantities to negatively impact the crabs.
Once the cause of the problem was understood, the CMCD took steps to change specific application criteria to eliminate, or greatly reduce, the impact on fiddler crabs in the reserve. However, the District wanted to better understand the problem of insecticide deposition and then find a way of reducing it to levels that would not be environmentally harmful. This quest for solutions led to further cooperative research.
With the help of two research grants from the Florida Department of Agriculture and Consumer Services (DACS), researchers from Florida A&M University's Public Health Entomology Research and Education Center began a study on the downwind drift and deposition of fenthion (an insecticide used for mosquito control) following ultra low volume (ULV) applications for mosquito control. The CMCD, DACS, Florida Department of Environmental Protection, U.S. Environmental Protection Agency, the Canadian military, numerous mosquito control districts, and private industry assisted the University in the study. The primary goals of the study were to reduce the quantity of insecticide affecting the fiddler crabs while maintaining the ability to control adult mosquitoes.
The study compared two different nozzle systems mounted under the tail of a DC-3 aircraft. The first system was the conventional flat fan nozzle that was used by most mosquito control programs throughout the world for making Ultra Low Volume (ULV) applications of insecticide. The experimental system employed high-pressure hydraulic nozzles developed by Mr. Jim Robinson, Director of the Pasco County Mosquito Control District. The conventional flat fan nozzle operated at 60 psi and created aerosol droplets with a VMD range between 30-60 microns. The experimental system operated at 3000 psi and generated aerosol droplets within a 20-40 micron VMD range. The volume median diameter (VMD) is a measurement used to describe the droplet spectrum produced by a ULV spray system.
The study was conducted at two sites over two years. The first site was in the southern Golden Gate Estates area of Collier County, FL. A DC-3 aircraft, flying at an altitude of 300 ft., applied fenthion at 0.52 fl oz. per acre in one swath at the east end of the test area. Sampling stations were set up downwind at 500 ft. intervals along a three-mile stretch of road. Each station consisted of hanging yarn, ground level filter paper, and caged adult mosquitoes. Residues were collected on the yarn and filter paper to measure the amount of insecticide remaining airborne and impacting the ground at each station. The caged mosquitoes were used to determine control efficacy. Caged fiddler crabs, used to determine non-target impact, were located at three points within the test area. Caged mosquitoes and fiddler crabs were also located outside the test area to serve as controls.
Data from single swath replicates using the flat fan nozzles showed that 50% of the applied insecticide had deposited on the ground within 0.5 miles of the application point. This figure approached 100% deposition by 3 miles. The hydraulic nozzles, with the smaller droplets, deposited 7% and 47% at the same respective distances. This means that approximately 55% of the insecticide should have remained airborne and available for mosquito control. The heavy deposits from the flat fan nozzles resulted in 80-90% mortality in the exposed fiddler crabs, while the hydraulic nozzles resulted in no fiddler crab mortality. The hydraulic nozzle system also produced better mosquito control than the flat fan nozzle system over a greater area.
The second test site was adjacent to the Rookery Bay National Estuarine Research Reserve, and monitored operational mosquito control spray applications of eight swaths. The same type of data was collected during this test as for the previous test; however, the sampling station was slightly different. Each station was placed downwind at either 500 ft. or 1000 ft. intervals along five miles of the drift zone for both nozzle systems.
The adulticide released by the flat fan nozzles produced significant ground deposition within 0.6 miles downwind of the application point. The residue concentrations collected approached 3000 micrograms per square meter. The residue deposits for the high-pressure hydraulic nozzles reached a maximum of 50 micrograms per square meter. Between 1.5 and 3 miles downwind, mosquito mortality was comparable for both systems, with major differences beyond 3 miles. The smaller aerosol droplets produced by the high-pressure hydraulic nozzles provided enhanced mosquito control levels, averaging 80% mortality at five miles downwind. The quality of control was not monitored beyond that point.
The data from this study indicates that not only will a reduction in droplet size improve adult mosquito control, but also radically reduce ground deposits and non-target species impact. The smaller droplet size results in a greater quantity of droplets released, for the same amount of insecticide. This greater quantity of droplets also stays airborne longer, making it easier for a mosquito to be contacted by the insecticide, therefore improving mosquito control. Because the droplets stay airborne longer and less insecticide is actually applied, less insecticide reaches the ground resulting in less impact on non-target species.
The CMCD has put the results of this research to good use. The application equipment on all District aircraft has been changed so that the droplet spectrum produced corresponds to that of the high pressure test system. The District has also changed the way in which aerial applications are made. Spacing swaths farther apart and using the wind to carry the insecticide have reduced the amount of insecticide actually applied over the District. This leads to reduced insecticide and application costs and at the same time further reducing impacts on non-target organisms. And best of all, none of these changes have reduced the high level of mosquito control that residents of the District have come to expect.
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