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| Significant Research Projects |
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The attractant-baited barrier line, sometimes referred to as removal trapping, concept for mosquito control was developed as an alternative control method to the aerial adulticide applications normally used by the Collier Mosquito Control District (CMCD). This concept was first used to control the tsetse fly in parts of Africa and has been extensively modified by CMCD. The majority of adult mosquito control in the District occurs through aerial applications of insecticide. These applications can negatively impact non-target organisms in environmentally sensitive zones surrounding treatment areas, especially when the treatment area is very small. The barrier line system eliminates the potential for non-target impact by utilizing attractants to lure mosquitoes into a "kill zone," or "trap out" area. Basically, the mosquitoes are drawn toward targets that are treated with a residual insecticide. The mosquito lands on the target, contacts the insecticide, and dies. The CMCD studied the attractant barrier line method of mosquito control for several years.
The system that the CMCD used for the attractant-baited barrier line utilized carbon dioxide and octenol (a derivative of oxen's breath) as attractants. The carbon dioxide was supplied from CO2 tanks connected to a manifold. The CO2 traveled through the manifold to a regulator. The regulator controlled the pressure of the CO2 as it traveled from the tanks to a control panel. Inside the control panel were all the instruments necessary to maintain the flow of CO2 at a constant pressure and volume. The control panel housed pressure gauges and flow meters that allowed a technician to observe the pressure, and indirectly, the amount of CO2 flowing through the system. The control panel also helped to identify problems, such as leaks. The CO2 then traveled from the control panel to a 1" polyethylene pipe. The pipe was positioned along the outer perimeter of the area to be protected from mosquitoes. Targets were situated alongside the pipeline and were connected to the pipeline by a small rubber hose. The CO2 traveled through the hose to the target. The octenol was placed near the area of the target where the CO2 was released. The targets were treated with insecticide on a regular basis. The CMCD has performed several studies using the attractant-baited barrier line. Each study tested various target designs, target spacing, or other parameters in order to find the most efficient system.
The first study began in the summer of 1996 on Key Island in Naples, Florida at the site of the old Keewaydin Club. The goal of the project was to create a perimeter line around a designated area of the island and test whether the area was protected from adult mosquitoes. The barrier line was constructed from 1" PVC pipe. The line consisted of three sections, north, main, and south. The north line measured 161 m, the main line measured 432 m, and the south line measured 165 m. Targets were placed along the pipeline and treated with insecticide every two weeks. Carbon dioxide was released through the targets at a rate of 200 ml/min. CDC surveillance traps were used to collect mosquitoes in order to determine the efficacy of the system. Six CDC traps were placed inside the barrier line in the area to be protected, and twelve traps were placed outside the line in the unprotected area. These traps were collected daily and the mosquitoes were counted. In order to determine differences in mosquito populations inside and outside the protected area when the line was off and on, the system was turned on for two-week intervals, followed by two weeks off.
The data from the first study showed significant differences in the ratio of mosquitoes collected inside and outside of the line when the system was on compared to when it was off. For example, on test day 56 the system was on and an average of approximately 4,250 Aedes taeniorhynchus were collected outside of the line, while an average of approximately 1,000 Aedes taeniorhynchus were collected inside the line, a ratio of 0.24. While the system was off on test day 69, approximately 2,000 Ochlerotatus taeniorhynchus were collected inside the line and approximately 3,000 were collected outside of the line, for a ratio of 0.67. With the line on there was a reduction in the ratio to almost one third of the line-off level, indicating that the system was working. Although this example is species specific, there was a dramatic difference in the ratio of mosquitoes collected inside the line when the system was on, in comparison to when the system was off. Ratios comparing the total number of all species of mosquito when the line was on and off, for the duration of the study, showed extreme differences in number for every species. The data showed that the line was effective in reducing the number of mosquitoes, as well as sand flies, inside the protected area. However, the level of control that was achieved was insufficient for operational use.
From 1998 to 2000, the CMCD studied the use of the attractant-baited barrier line at a condominium complex on Marco Island called Stevens' Landing. The area was an excellent candidate for the study because it is surrounded by salt marsh, and therefore inundated with the salt marsh mosquito Aedes taeniorhynchus. The goal of this study was to determine whether the barrier line could be effective in protecting a populated area from adult mosquitoes.
The system design was very similar to the design used on Key Island. Approximately 2,400 ft of 1" polyethylene pipe was placed around the entire community. CDC traps were used to determine the number of mosquitoes inside and outside the line. Four traps were placed outside the line, and four traps were placed inside the line. The traps were collected daily and the collections were counted and identified for the number of Aedes taeniorhynchus and other mosquitoes.
The study was conducted for three years, testing variables such as target spacing, target shape, rate of CO2 released, and octenol release method. It was determined that the system was most efficient when the targets were spaced every 20 feet around the perimeter. The target shape was finalized as well. The most effective target was "bucket" shaped. It was composed of regular weed block cloth hanging from the lid of a five-gallon bucket. The target was hung from a metal pipe that was placed near the line. This target was either dipped in or sprayed with insecticide bi-weekly for control purposes. During the final year of the study, liquid octenol was replaced with "stick" octenol to facilitate handling and reduce costs.
The three years of data collected from Stevens' Landing showed that the line provided the residents with relief from mosquitoes. Statistical analysis proved that there were fewer mosquitoes inside the barrier line when the system was on than when the system was off. The people within the community also noted the differences in mosquitoes when the line was on and when it was off.
The system worked so well that the community at Stevens' Landing voted to have it installed permanently. During the winter of 2001, the CMCD, in collaboration with Stevens' Landing, made the barrier line a permanent facility at the condominium. However, several changes were made to the system to make it more efficient. The target design was changed slightly and all pipelines carrying carbon dioxide to the targets were buried under ground. The control panel was also refitted with copper tubing rather than plastic tubing. The system now uses cryogenic carbon dioxide held in 500 lb tanks. The tanks are replaced weekly in order to provide the system with enough gas to attract mosquitoes. The CMCD purchased the materials for the pipeline and the targets and installed the entire system; however, the condo association is responsible for the costs of maintenance and operation. The permanent attractant-baited barrier line system has been running at Stevens' Landing since the summer of 2001. The District no longer needs to aerially apply insecticide to the area, reducing the potential impact on non-target organisms in the mangrove environment surrounding the community.
While this control method has been proven effective at Stevens' Landing, it is not a method that can be applied in all circumstances.
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Projects
