Within the Florida citrus industry, the most frequently used pesticide-application equipment for pest control in the tree canopy is the airblast sprayer. Airblast sprayers come in a wide range of shapes, sizes, and capacities as well as being engine or power-take-off (PTO) driven. These sprayers are designed to use air and liquid to deliver the pesticide to the tree canopy, where the targeted pest is suppressed and/or controlled. The air stream from the airblast sprayer is generated by a high-speed fan driven by a separate engine or from the PTO of the tractor that is pulling the sprayer.
Each type of sprayer has its advantages, disadvantages, and costs to operate. Sprayers have a series of nozzles that are arranged in a specific configuration to allow the air stream to pass over them as each nozzle releases the spray liquid. As the air stream passes over these nozzles, the released spray solution is shattered into droplets and propelled toward the tree canopy within the airflow. By adjusting airflow, spray pressure, or nozzle sizes selection, spray droplet size is modified. Once the spray material is released onto the air stream, it is directed toward the tree canopy by deflectors that are strategically placed, guiding the airflow in a specific, targeted direction.
Spray volume generated by airblast sprayers is generally measured in gallons per acre (GPA). This volume can be modified or adjusted by changing ground speed of the application traveling through the grove, by using different nozzle types or sizes, by modifying pressure, or by changing the number of spray nozzles by supplying product into the air stream.
For taller citrus trees (greater than 14 feet tall), the common practice is to direct more (approximately one-half to two-thirds) of the spray volume toward the upper half of the tree. This volume adjustment is easily done by directing the deflectors and nozzles in the desired direction.
For pest management purposes, spray volume is adjusted depending on the targeted pest as well as the location of the pest within the tree canopy. Application volumes can range between 25 gallons to 500 gallons per acre, with most applications in the 100 GPA range. For pests that reside on the tree trunk, higher spray volumes are required to adequately reach the target pest. The spray volume should be adjusted to consider the targeted pest and intended market for the fruit. With some of the newer low-volume sprayers, this spray volume may be decreased to as low as 2 to 5 gallons per acre. When selecting a spray volume to be applied, be sure to check the selected pesticide label, as some pesticides may have restrictions on the application volume allowed. Remember the label is the law!
With the spray being carried in the air stream to be deposited on or into the tree, the volume of air being created by the airblast sprayer is important. The sprayer must have sufficient air output to effectively transport the spray droplets to the targeted site within or on the tree canopy. A high spray volume may create runoff from the tree canopy, thereby wasting costly pesticide products, whereas too low of a volume may not effectively cover the target site.
All airblast sprayers consist of the following basic items: tank, agitation, pumps, strainers, pressure regulators, flow cutoff valves, and nozzles.
Tanks vary in size, generally ranging from 50 to 1,000 gallons and are made of corrosion-resistant materials. A good tank design minimizes corners or angles that could increase the chance for materials in the spray solution to settle prior to application. The tank should have an opening large enough for filling or cleaning and a drain in the bottom of the tank.
Agitation in the tank is essential to keep materials in suspension, thus allowing all applied material to be at the same concentration during application. Inadequate agitation may cause powders, emulsified concentrates, or oils to separate or layer within the tank, resulting in poor pest control or tree damage.
The pumps selected must be resistant to wear or corrosion and have adequate capacity in both gallons per minute and pressure to achieve the desired application rate. Frequently used pumps include centrifugal, roller, or piston pumps that vary with manufacturer and application design.
Strainers on most airblast sprayers are usually in at least three locations: the fill-point within the tank, line, and nozzle. The capacity of the strainer increases from the fill point to nozzles, where the smallest strainers are located just before the nozzle. The strainers are identified by the number of openings per linear inch of screen, such that a 20-mesh strainer has large openings and will not block the small material caught by a 50-mesh screen.
Pressure regulators provide two functions on every machine. These devices regulate system pressure and can relieve excessive pressure back into the tank when the valves for application are closed.
Pressure gauges should be located where the operator can easily see the gauge during all pesticide applications. These gauges should be located between the pressure regulator and nozzles and have an operating range greater than the peak pressure that can be generated by the pump. If the pressure gauge will not return to zero when not in use, it should be replaced.
Nozzles come in many shapes, sizes, and materials and are designed to apply a specific amount of material at various pressures. Nozzles may be manufactured from aluminum, plastic, brass, stainless steel, tungsten carbide, and ceramic. Tungsten carbide and ceramic will last the longest and have good corrosion resistance.
Many of the newer sprayers are now being delivered or retrofitted with sensors that control pesticide delivery to the individual tree canopy based upon actual tree-canopy size. These sensors adjust valve opening or closing to apply spray material only into the section of the air stream where foliage is present. These new sensing systems can greatly decrease the amount of spray material being applied, saving the grower valuable resources while not impacting the effectiveness of the product. Additionally, lower application amounts of pesticides can reduce potential adverse impact to the environment.