Volume 33 / March 2019

Adapting to Vehicle-Mounted WALS

Districts Gaining Expertise in Calibrating Truck-Mounted Sprayers for WALS Applications

truck mounted WALS

The first large-scale, aerial WALSTM applications in the U.S. took place nearly ten years ago in the Florida Keys, when aerial applications of VectoBac WDG® (Bti strain AM65-52) proved effective in response to the area’s first local dengue transmission in 75 years. When Zika struck Miami-Dade and other Florida locales in 2016, aerial WALS applications were again employed and gained more widespread notoriety as a result.

But an aerial WALS approach isn’t always the most prudent. Instead, many programs have begun to rely upon vehicle-mounted WALS for ongoing operations. This shift has created a need for specialized equipment evaluation and optimization, and as the co-innovator of the WALS strategy, Valent BioSciences (VBC) has become invested in supporting configuration, calibration, and characterization of vehicle-mounted WALS systems in districts across the U.S. In 2018, VBC cooperated with a host of US mosquito abatement programs and private entities in an effort to fine-tune and increase the utility of vehicle mounted WALS applications. One such effort was in collaboration with the Florida Keys Mosquito Control District (FKMCD), the same district who helped introduce the WALS strategy back in 2010.

As the need for WALS treatments and frequency in the Keys increased, its operations team wanted an alternative to aerial applications. They were looking for a system more economical for smaller areas and also for areas with limited aerial access. FKMCD decided to purchase a Super Duty A1 Mist Sprayer®. The unit is equipped with a Micronair® AU 5000 and for initial evaluations, was mounted on a trailer pulled behind a pick-up. Working alongside VBC technical specialist Leanne Lake, the evaluation team began by properly calibrating and characterizing the equipment, then moved into an operational field study. 

The Field Study

For the study, the Super Duty A1 Mist Sprayer was calibrated to deliver three gallons per minute using a 12% VectoBac WDG solution. Characterization was completed by collecting data on the droplet size, density, and larval mortality at different locations. The drop spectrum was evaluated by collecting droplets using Kromekote cards at the different stations. Card analysis was completed with a specially configured Scansnap® scanner and BacDrop™ larvicide droplet analysis program created by VBC. The program provides data including volume median diameter (VMD), number median diameter (NMD), drop density and swath analysis. To evaluate the efficacy of WALS using the 12% VectoBac WDG solutions, larval bioassays jars were placed out at equal distances apart across the various stations. The FKMCD laboratory conducted larval bioassays on the jars to determine mortality for each station.

After the equipment was properly calibrated and the droplet spectrum was determined to be in the recommended range, and once positive mortality results were obtained during the characterization, FKMCD conducted an independent field study using WALS and VectoBac WDG with the new truck-mounted sprayer. The parameters for the field study were set to be the same as for the characterization. 

The study was conducted on June 28th, on three consecutive streets in the Key Colony neighborhood. Bioassay jars were situated throughout the spray block. The test area included 14 homes – each with either two or three bioassay jars placed under three types of cover: hidden (H), open (O) or moderate (M). Prior to application, 10 controls jars were placed at a sampling of homes then collected just prior to the application being made. Twenty minutes after application, all bioassay jars were collected and larval assays were conducted by the Florida Keys laboratory using percent mortality of 3rd and 4th stage Aedes. The results of the larval assays are seen in Figure 1. 

In addition to overall mortality, data was broken down by distance from the spray application to assess the efficacy behind buildings at various distances. For the jars located in the open covered areas, all bioassay jars reached 100% mortality within 24 hours out to 200 feet and approximately 92% mortality within 24 hours out to 300 feet
(See Figure 2).

For the jars located in the moderate covered areas, all bioassay jars reached 100% mortality within 24 hours out to 300 feet (Figure 3). 

For the jars located in the heavy covered areas, all the bioassay jars reached 100% mortality within 24 hours out to 300 feet (Figure 4).

Across all jars, the data averaged to approximately 97% mortality within 24 hours in all types of cover out to 300 feet. This field study was completed multiple times to validate the results, each resulting in equally successful data.

Given successful characterization and field trails, the FKMCD is implementing vehicle-mounted WALS into their regular operations and integrated vector management (IVM) program. 

Truck Mounted WALS Figures