December 2001

Sprayers and Spray Application Techniques By John W. Bartok, Jr.

The most important goal in the application of agricultural pesticides is to get uniform distribution of the chemicals throughout the crop foliage. Underdosing may not give the desired coverage and control needed. Overdosing is expensive as it wastes pesticide and increases the potential for groundwater contamination.

Two general types of sprayers are available for greenhouseapplication of pesticides: hydraulic and low-volume. There are many variationsof these that fit particular crops or growing methods.

In the hydraulic sprayer, a pump supplies energy thatcarries spray material to the target (plant foliage). Water is the carrier andthe pump creates the pressure at 40-1000 psi. Spray material is usually appliedto “wet” or “drip.” Nozzles on the boom or handheld gunbreak the spray into small droplets and direct it to the foliage.

In a low-volume (LV) sprayer, spray material in a water oroil carrier is injected into a high-speed air stream developed by a fan, bloweror compressor. In most LV sprayers, a small pump is used to inject aconcentrate pesticide solution into the air stream. The speed of the air streammay be as high as 200 mph. To get sufficient coverage, the air within thefoliage canopy must be replaced with air that contains the pesticide. As thedroplet size is much smaller, good coverage can be achieved with less chemical.

Sprayer Differences

One way to distinguish between a hydraulic sprayer andlow-volume sprayer is by droplet size. Hydraulic sprayers produce a spray withmost droplets in the 200-400 micron diameter range (thickness of the human hairis about 100 microns). Low-volume sprayers develop a mist (50-100 microns) orfog (0.05-50 microns). Small droplets from a mist or fog applicator can resultin more uniform coverage and greater likelihood of contact with the insect ordisease. In contrast to the hydraulic sprayer, spray material is usuallyapplied to “glisten” as it is difficult to see the individualdroplets on the leaf.

One disadvantage to smaller droplets is that they evaporatequicker when the humidity is low and may not reach the target. Another is thatthe tiny droplets tend to bounce or skip on the leaf surface. This can beovercome somewhat by adding a spreader and sticker.

Types of Hydraulic Sprayers

A hydraulic sprayer contains the following components: tank,pump with agitator, pressure gauge, regulating valve, relief valve, controlvalves, piping and nozzles, power source and support frame.

Compressed air sprayer. The smallest sprayers arehand-carried, compressed air sprayers. They contain a 1- to 5-gallon tank withan air pump in the top and a wand with a nozzle for directing the spray. Theirbest use is for spot treatment of small areas. In operation, the tank has to bepumped up frequently to maintain pressure, and the tank must be shaken toagitate the chemical.

Backpack sprayer. The tank in this sprayer holds about fourgallons of material. A hand-operated pump pressurizes the spray material as theoperator walks along, and the wand with nozzle directs the spray to thetarget.Á Its use islimited to small areas that can be reached from a walkway.

Skid-mounted sprayer. With a tank size up to 200 gallons,these sprayers will fit onto an ATV or electric cart. They can also be mountedon wheels and pulled by hand or with a compact tractor. A small electric or gasengine powers the pump. The unit may contain a hose reel and gun or a boom withnozzles.

Irrigation boom sprayer. With increasing production in plugand cell trays, the use of the boom sprayer has become an important tool forgetting uniform watering. By installing three-way turrets with nozzles forirrigation, misting and pesticide application, one piece of equipment servesmulti-purposes. An alternate method is to add a pesticide application boom tothe same transport cart. An independent mixing tank, pump, filter and valvesare needed.

Central pesticide application system. In gutter-connectedranges, it is possible to install a piping system that will deliver pesticidesto any part of the greenhouse. Pesticide preparation and filtration are done ina mixing area. A single pump and piping that will handle the pressure developedare required. A hose can be easily attached to one or more outlets in each bayto apply the pesticide. The disadvantage is that the entire system must bedrained and cleaned before changing to a new chemical.

Low-volume Sprayers

Backpack Mist Blower. A small gas engine and integral fancreates an air stream with a velocity of 100-200 mph. Concentrate sprayinjected into the air stream by a special nozzle is carried to the foliage bythe air. The spraying technique is more complicated than with a hydraulicsprayer. The nozzle should be directed into the plant canopy to get goodpenetration and coverage, but it should be kept at least six feet away from theplants to avoid blast damage. The operator should visualize that all the airwithin the canopy must be replaced by the air from the mist blower.

Electrostatic sprayer. Compressed air, given a negativeelectric charge as it travels through the nozzle, forms spray droplets andcarries them to the plants. This helps to create more uniformly sized particlesthat disperse well because they repel each other. Charged particles areattracted to leaves, metal and some plastics; when they strike a surface, theseparticles create a momentary overcharge that repels other particles. Theseother particles land elsewhere on the leaf, so there is more uniformcoverage.

The simplest electrostatic sprayer is backpack-carried andcontains a tank and spray gun. It requires an independent air supply to chargethe tank. Other units are cart-mounted with an integral compressor powered by agas engine or electric motor. Electrostatic sprayers work best if the spraydistance is less than 15 feet.

Rotary Disk sprayer. The spinning disk is used to impact andbreak a stream of water into droplets that are 60-80 microns in diameter. Avariety of sizes are available for greenhouse use.

Thermal Fogger. This machine requires a specially formulatedcarrier that is mixed with the pesticide to improve uniformity of droplet sizeand distribution of the spray material. The carrier also decreases molecularweight, allowing the particles to float in the air for up to six hours, adisadvantage if you have to get into the greenhouse to care for the plants.

In the operation of a thermal fogger, the pesticide isinjected into an extremely hot, fast-moving air stream that vaporizes it intofog particles. Moving from one end of a greenhouse to the other, a thermalfogger can cover in as little as 15 minutes. Air circulation from an HAF systemwill give more uniform coverage and better foliage penetration. Temperature andhumidity Á

also affect the spray droplets. Because of the noiseassociated with the jet engine, hearing protection is recommended.

Mechanical Fogger. Also called a cold fogger, this deviceuses a high-pressure pump (1,000-3,000 psi) and atomizing nozzles to producefog-size particles. Distribution of the spray material is through a hand-heldgun or external fan unit. With the fan unit, the distance and amount of areathat can be covered depends on the capacity of the fan. Multiple units orsettings may be needed to cover large areas.

As with other foggers, penetration and coverage may not beas good as with a mist or hydraulic sprayer. Droplets in the 30-micron sizedrop out of the air fairly quickly but droplets in the 5-micron size mayevaporate or float in the air currents for hours. Small particles don’thave the mass or velocity to move into heavy foliate; however, in most studies,good insect control has been achieved.

Safety is important when using spray equipment employing ahigh-pressure pump. Keep hands and arms away from the nozzle because at highpressure, spray particles can penetrate the skin very easily.

Calibration and Operation

Before selecting the sprayer to use, check the label and theIPM guidelines for the particular pesticide that will be applied. These givethe recommended application rate, the type of equipment best suited for theapplication, the style of nozzle and other information for best results. Basedon the type of crop and its size, select the sprayer that will give a goodbalance between droplet size and coverage.

Calibrating sprayers is important to get good controlwithout applying excess material. Low-volume sprayers can provide more uniformcoverage with less spray material. Where applicable, most labels haverecommendations for low-volume equipment.

When making a tank mix, both the dosage and waterrequirement need to be considered. The dosage, the amount of chemical thatshould be applied to a given area, is listed on the label. With mostpesticides, a range such as 4-12 oz. per 100 gal. is given. Selection of therate should be made based on the level of infestation, type and maturity of thecrop, past experience and other variables. If this is the first time that thepesticide is being used, a dosage in the middle of the range is a good startingpoint.

The amount of water needed to cover the growing area dependson the type of equipment used. With hydraulic sprayers, a rate of 25-50 gal.per 10,000 sq. ft. is common. Low-volume sprayers use only 1/4-2 gal. per10,000 sq. ft.

Most chemical labels state the amount of pesticide to bediluted in 100 gallons of water and not the amount of spray concentrate that isto be applied to a given area. Some labels now list the amount of Áchemical that should be applied per acre. The instruction manual that comeswith each sprayer contains charts or tables that help determine how much spraymaterial to mix with the water. This is usually based on a 10,000-sq. ft. area,so you will need to adjust the rate to fit the growing area to be sprayed.

Spraying technique

The technique you use to spray crops is very important toget good coverage. It should be developed to fit the type of equipment that isto be used. With a hand-held gun, a sweeping motion over the foliage will allowthe spray material to penetrate and get to the underside of the leaves.

With fixed-fan type units, an airflow pattern needs to beestablished so that all the plant canopy receives the pesticide; location ofthe unit contributes to good air flow. Using an air circulation system, such asHAF, will enhance the movement and distribution of the fog and mist particles.The fans should continue to operate for 30-60 minutes after the sprayingoperation is completed.

Observation should be made to ensure that the sprayer isoperating correctly and that sufficient coverage is obtained. A good way tomonitor coverage with a hydraulic or mist sprayer is to use strips ofwater-sensitive paper (available from Spraying Systems Co., Wheaton, Ill., or from a local sprayingequipment supplier). The paper is folded over and attached to representativeleaves in the plant canopy. When exposed to spray droplets, stains will appear indicatingthe size and number of particles that were received by the leaf.

For fog applications, evaluation of coverage can be madeusing a fluorescent dye in the water. Placing representative leaves under anultraviolet or black light will show droplet size and distribution.

Another tool that can help promote better technique andcoverage is to keep a log of the spraying operations and the results that wereobtained. This should include the date, time and location of application; cropand pest; pesticide used; tank mix; and an evaluation of the results obtained.Adjustments should be made in subsequent applications to try to improve theresults.

Proper selection, calibration and operation of sprayequipment is important in achieving optimal pest control, as well as in meetingenvironment and safety requirements.