Chapter 28

Pesticide Application and Equipment in Greenhouses

Methods of Pesticide Application

When applying pesticides, coverage is everything. Even the most effective new pesticide or fungicide is useless unless it comes in contact with the insect or disease it has been formulated to control. There are numerous methods of chemical applications available to greenhouse growers. The three primary types of sprayers used in greenhouses are: (1) hydraulic or high-volume hydraulic sprayers, (2) targeted low-volume sprayers, and (3) fog or ultra low volume sprayers. Each of these machines has their place in greenhouse chemical application. They all have advantages and disadvantages. Primary differences among sprayers are the quantity of water used, the operating pressure, and the size of droplets produced.

Hydraulic or High Volume Sprayers

A hydraulic or high-volume sprayer uses a high flow rate of water to wet the foliage to the point of runoff. A pump supplies energy that carries spray material to the target (plant foliage). This sprayer uses standard rates of chemicals and large volumes of water. The quantity of water to use depends on the specific sprayer and nozzle, the spraying technique of the operator, and the size of the crop. Calibrating the sprayer according to the operator and the crop is essential for mixing the correct amount of spray mix.

Types of Hydraulic Sprayers

  • Compressed air sprayer
  • Backpack sprayer
  • Skid-mounted sprayer
  • Irrigation boom sprayer
  • Central pesticide application system

Targeted Low-Volume Sprayers

A low-volume sprayer uses less water or carrier to apply insecticides, fungicides, disinfectants and even fertilizers to greenhouse crops. Targeted low-volume sprayers create droplets with an average diameter of between 40 to 70 microns utilizing high pressures between 1,000 and 3,000 psi. These high pressures create fine droplets with high velocity. This velocity is easily diffused once the spray cloud hits the target, creating a swirling spray cloud with a tremendous amount of turbulence in reaching the internal foliage.

Types of Targeted Low-Volume Sprayers

Backpack Mist Blower. A small gas engine and integral fan creates an air stream with a high velocity. Concentrate spray injected into the air stream by a special nozzle is carried to the foliage by the air. The spraying technique is more complicated than with a hydraulic sprayer.

Air-Assisted Low-Volume Sprayers. Air-assisted low-volume sprayers use air as the primary carrier of the chemical (See Figure 28.9). High-speed air strikes the stream of liquid chemical or chemical and water being injected into the sprayer and breaks the stream into small droplets. The air then carries the liquid to the plants.

Electrostatic Sprayers. Electrostatic sprayers produce fine droplets that 30 to 60-micron in size, which are electrically charged and then air-blasted into the crop foliage (See Figure 28.10). The negatively charged particles are attracted to any surface and can provide coverage that is as good as the coverage from a high-volume sprayer. In theory this can help to create droplets that are more uniform in size, which disperse more evenly because they repel each other, since all droplets carry a like charge.

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Fog or Ultra-Low Volume Sprayers

These are the machines most commonly called foggers. Ultra low-volume (ULV) systems generally use very small quantities of water and generate spray droplets smaller than 25 microns in diameter. This is a true fog, often resulting in a slight haze in the greenhouse. Because of the small particle size, ULV sprayers need to be used in an enclosed space to contain the spray solution. For best results with this method, proper air circulation is necessary. Generally, ULV systems reduce application time by operating very quickly or automatically, eliminating the need for an applicator.

Types of Fog or Ultra Low-Volume Sprayers

Mechanical Foggers.Mechanical (“cold”) foggers use turbulence in the fogging nozzle to shear liquid into small droplets (See Figure 28.12). The typical air source is a high velocity fan, usually integrated with the fogging nozzle. The high air pressure produced by the motor helps to spray the fogging liquid through the nozzle separating it into tiny particles. The liquid solution gets sprayed out of the nozzle in form of a fog or a fine mist. There is no need for liquid pressure. In most cases, the atomization nozzle doesn't have any small orifices, so nozzle plugging is not a problem.

Thermal Foggers. A thermal fogger uses a system like that used in jet engines (See Figure 28.13). The pesticide is injected into the extremely hot, fast moving air stream, where it is vaporised into fog-sized particles. Foggers generate very small drops, usually less than 10 to 50 microns in diameter, that are able to move long distances from the applicator. This means the spray is able to penetrate far into the structure rather than settling near the spray unit. The rate of application depends on the size of the metering orifice. Thermal foggers that are designed to treat larger areas have large tanks and require more time to dispense the greater volume of pesticide mixture. The defining characteristic of thermal foggers is their speed and ability to cover large areas quickly. The largest units can propel the fog hundreds of feet from the machine in seconds, treating areas as large as 50,000 sq. ft in less than 15 minutes. With some of the larger models, spray drops will travel more than 200 feet. Liquid flow rates also vary with the unit size. The area covered with these spray volumes will depend on whether a wettable or liquid formulation is applied.

Total Release Aerosol Canisters. Total release aerosol canisters can be set into a greenhouse, actuated, and allowed to fill the greenhouse (See Figure 28.14). Once a canister is activated, this propellant forces the spray solution through the nozzle atomizing solution into a cloud. The pesticide is delivered in small droplets, which drift throughout the greenhouse and finally settle on the plants and target pests. Because these systems have no lateral movement created by the canister, they must rely heavily on HAF fans to provide even coverage on all plant surfaces.

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