With the omnipresent threat of Zika, mosquito control districts shifted efforts toward understanding and controlling the container breeding Aedes albopictus and Ae aegypti mosquitoes. Targeting the small and hard-to-reach habitats became the new challenge to best protect the public from diseases like Zika virus, yellow fever, chikungunya, and dengue. During the 2015 Zika outbreak in Brazil, many infected people arrived in Florida, and during 2016, at least 777 cases in Florida appear to have been caused by local mosquito vectors. Due to the outbreak and what is known about these mosquitoes, controlling container breeders became a critical component of vector control.
Unknown to most, long before Zika became an issue in the United States, the Florida Keys Mosquito Control District (FKMCD) began controlling container mosquitoes when they had the first endemic Dengue outbreak in 70 years.
Andrea Leal, the director of FKMCD, said that back in 2010, getting to the Ae aegypti larval breeding spots was a tremendous challenge. Leal noted every household is a potential breeding area for larvae – every tire, flower pot, toy, and rain barrel. The first plan of attack was public outreach and education so that the public understood the situation and how to best comply with mosquito personnel. Though strategic outreach and education programs helped encourage residents to comply with mosquito control personnel, workers were still unable to get onto about 10 percent of residences due to the massive amount of manpower needed. To combat this need for manpower, FKMCD looked to other forms of vector control.
One option to quell the new threat was to spray larvicide from the air to cover large areas and get at the hard-to-reach larvae which proved highly effective against salt marsh mosquitoes and mosquitoes breeding in rice fields.
FKMCD decided to go with Valent BioSciences’ (VBC) VectoBac WDG, a formulation of an environmentally safe soil dwelling bacteria Bacillus thuringiensis subsp.israelensis Strain AM65-52,commonly known as Bti. Bti was selected as the active ingredient due to its history of proving highly effective against Ae aegypti larvae.
Because the District was not yet certified by FAA for aerial spraying of the larvicide, VBC introduced them to Helicopter Applicators, a private company out of New Jersey to do the initial testing.
Peter DeChant and Candace Royals of Valent BioSciences worked with Steve Bradshaw of FKMCD, Glenn and Kirk Martin of Helicopter Applicators and Bob Mickle of RemSpec (a spectrometry service out of Charlton, MA) to nail down the specifics of the application technology. The right droplet size, mix ratio, and the effective swath of a flyover were imperative to the success of the product and were major areas of concern for the team.
The water dispersible granule was the chosen formulation of VectoBac due to the major concern of FKMCD treating over populated urban areas, a key advantage of the granules over the use of an aqueous solution is that the final mixture of VectoBac WDG would contain no oils or surfactants that can damage the finish on a car or a boat. The granular form also has little or no dust that one would get with a wettable powder.
Although the lack of dust makes VectoBac WDG better for the people working with it, it takes some effort to get the granules dissolved in the water on a large scale. For small operations like backpack spraying, the granules can be easily mixed in a bucket of water using a stirring rod fitted into a standard hand drill but it is much more difficult to hand-mix 100 gallons for an aerial spray.
To eliminate the need for hand-mixing, VBC decided to try a Venturi mixing system for getting the granules completely dissolved and dispersed in the water. The Venturi system is pipe with a narrow section that causes the water flowing through it to move faster, named after Giovanni Battista Venturi (1746–1822), the Italian physicist who first described the effect. As water flows through the Venturi, it speeds up, creating a partial vacuum that can suck particles into the water stream. By placing a hopper of granules over an opening in the Venturi, the granules can be sucked quickly into solution and into a storage tank. From the storage tank, the water is recirculated by a pump back through the Venturi until all of the granules have been sucked in and dissolved. Once the system was fully operational, several hundred gallons cold be mixed in under a half an hour.
DeChant, technical specialist with Valent BioSciences, recommended the perfect droplet size at around 112 microns but the challenge was in keeping the droplet size consistent which is vital to the product’s efficacy. Helicopter Applicators used Bell Long-ranger helicopters equipped with Micronair AU5000 spray heads, and did field tests to check the final swath width and whether the droplet size was accurate when the droplets reached the ground. The helicopter flew perpendicular to a 1000-foot line of petri dishes the team set up on a field making one pass, and spraying out the VectoBac mixture. Afterwards, the petri dishes were sent to a lab where they were filled with water and mosquito larvae were added. Matching larval mortality with the petri dish position on the flight path, the team was able to determine the effective swath of one flight.
Once the mixing system had been worked out and the swath width and droplet size determined, FKMCD set up their own system. They used a fleet of four Bell Long-Rangers and used the same Micronair AU5000 spray heads.
After experiencing some frothing issues when mixing large batches, they made some modifications to the mixing system. They found that mixing smaller amounts, 100 gallons at a time in a 200 gallon tank, eliminated most of those issues. The 100 gallon batch was also perfect for one helicopter run. A new batch of four 25lb dry bags could be mixed up in under 15 minutes before the helicopter returned for a new load. A learned advantage of smaller batches was that if the weather suddenly became unfit for spraying, the unused granules could be kept for later.
The FKMCD team also modified the mixture hose of the system to introduce the mixture from the bottom of the tank, because when the hose was at the top of the tank, there was too much agitation and frothing. “The water flow and circulation from the mixing tank back through the Venturi has to be handled with care,” Andrea said. “You can’t be too aggressive, but there has to be enough water so that it can mix properly.”
They added an attachment with a hose to their pump and, in the event of any frothing issues, they would insert the nozzle into the valve and spray directly into the system. Strainers located between the system and helicopter make sure there is no clumping or residue which is why thorough cleaning of the system is absolutely mandatory. FKMCD also put the entire system on a truck, to make it the system mobile to more easily refill helicopter reservoirs.
Since Florida Keys were able to successfully control dengue with wide area spraying (WALS) with VectoBac WDG and a Venturi mixing system in 2010, several other districts have followed suit when the same mosquito species that transmits dengue began spreading Zika in 2016.
Broward County and Collier Mosquito Control District out of Naples, Florida use a similar system of helicopter applied VectoBac WDG and a Venturi mixing system. The Mosquito Research and Control Unit out of the Cayman Islands uses fixed wing aircraft, but the same basic mixing system. With the correct swath and droplet size, ground applications through backpack sprayers can be greatly reduced or even eliminated. The dispersal of Bti droplets through the air allows the material to reach even hidden or hard-to-reach water sources like gutters or tree holes.
Rotary wing aircraft AU5000 atomizers were found to most effectively create a droplet size of 112 microns while running at 4000-5000 rpm. With six atomizers on the boom, using a Bell Long-Ranger helicopter, the effective swath width is 200 feet, flying at 80 mph. The wider the swath, the better, because it decreases the amount of time flying over a metropolitan area which was a concern for FKMCD. One of the chief complaints from the populace was morning noise, initially, according to Andrea Leal, and fewer passes greatly alleviated that concern.
Wide area larviciding is becoming a very important part of the overall control strategy, which includes monitoring with traps, inspections, spot treatments, and occasional adulticiding. The integrated control system is proving extremely effective in controlling Aedes mosquitoes as the 2018 summer season resulted in zero local mosquito-borne transmission cases of Zika.