Sprayers and Spray Application Techniques

December 28, 2001 - 14:25

Learn more about the advantages and disadvantages of low-and high-volume sprayers to help you choose the best one for your greenhouse.

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 greenhouse
application of pesticides: hydraulic and low-volume. There are many variations
of these that fit particular crops or growing methods.

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

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

Sprayer Differences

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

One disadvantage to smaller droplets is that they evaporate
quicker when the humidity is low and may not reach the target. Another is that
the tiny droplets tend to bounce or skip on the leaf surface. This can be
overcome 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, control
valves, piping and nozzles, power source and support frame.

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

Backpack sprayer. The tank in this sprayer holds about four
gallons of material. A hand-operated pump pressurizes the spray material as the
operator walks along, and the wand with nozzle directs the spray to the
target.  Á Its use is
limited 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 mounted
on wheels and pulled by hand or with a compact tractor. A small electric or gas
engine powers the pump. The unit may contain a hose reel and gun or a boom with
nozzles.

Irrigation boom sprayer. With increasing production in plug
and cell trays, the use of the boom sprayer has become an important tool for
getting uniform watering. By installing three-way turrets with nozzles for
irrigation, misting and pesticide application, one piece of equipment serves
multi-purposes. An alternate method is to add a pesticide application boom to
the same transport cart. An independent mixing tank, pump, filter and valves
are needed.

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

Low-volume Sprayers

Backpack Mist Blower. A small gas engine and integral fan
creates an air stream with a velocity of 100-200 mph. 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. The nozzle should be directed into the plant canopy to get good
penetration and coverage, but it should be kept at least six feet away from the
plants to avoid blast damage. The operator should visualize that all the air
within the canopy must be replaced by the air from the mist blower.

Electrostatic sprayer. Compressed air, given a negative
electric charge as it travels through the nozzle, forms spray droplets and
carries them to the plants. This helps to create more uniformly sized particles
that disperse well because they repel each other. Charged particles are
attracted to leaves, metal and some plastics; when they strike a surface, these
particles create a momentary overcharge that repels other particles. These
other particles land elsewhere on the leaf, so there is more uniform
coverage. 

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

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

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

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

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

Mechanical Fogger. Also called a cold fogger, this device
uses a high-pressure pump (1,000-3,000 psi) and atomizing nozzles to produce
fog-size particles. Distribution of the spray material is through a hand-held
gun or external fan unit. With the fan unit, the distance and amount of area
that can be covered depends on the capacity of the fan. Multiple units or
settings may be needed to cover large areas.

As with other foggers, penetration and coverage may not be
as good as with a mist or hydraulic sprayer. Droplets in the 30-micron size
drop out of the air fairly quickly but droplets in the 5-micron size may
evaporate or float in the air currents for hours. Small particles don’t
have 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 a
high-pressure pump. Keep hands and arms away from the nozzle because at high
pressure, spray particles can penetrate the skin very easily.

Calibration and Operation

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

Calibrating sprayers is important to get good control
without applying excess material. Low-volume sprayers can provide more uniform
coverage with less spray material. Where applicable, most labels have
recommendations for low-volume equipment.

When making a tank mix, both the dosage and water
requirement need to be considered. The dosage, the amount of chemical that
should be applied to a given area, is listed on the label. With most
pesticides, a range such as 4-12 oz. per 100 gal. is given. Selection of the
rate should be made based on the level of infestation, type and maturity of the
crop, past experience and other variables. If this is the first time that the
pesticide is being used, a dosage in the middle of the range is a good starting
point.

The amount of water needed to cover the growing area depends
on 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. per
10,000 sq. ft.

Most chemical labels state the amount of pesticide to be
diluted in 100 gallons of water and not the amount of spray concentrate that is
to be applied to a given area. Some labels now list the amount of Á
chemical that should be applied per acre. The instruction manual that comes
with each sprayer contains charts or tables that help determine how much spray
material 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 to
get good coverage. It should be developed to fit the type of equipment that is
to be used. With a hand-held gun, a sweeping motion over the foliage will allow
the spray material to penetrate and get to the underside of the leaves.

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

Observation should be made to ensure that the sprayer is
operating correctly and that sufficient coverage is obtained. A good way to
monitor coverage with a hydraulic or mist sprayer is to use strips of
water-sensitive paper (available from Spraying Systems Co., Wheaton, Ill., style="mso-spacerun: yes">   or from a local spraying
equipment supplier). The paper is folded over and attached to representative
leaves in the plant canopy. When exposed to spray droplets, stains will appear indicating
the size and number of particles that were received by the leaf.

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

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

Proper selection, calibration and operation of spray
equipment is important in achieving optimal pest control, as well as in meeting
environment and safety requirements.  

About The Author

John W. Bartok, Jr. is an agricultural engineer and extension professor-emeritus in the Natural Resource Management & Engineering Department at the University of Connecticut, Storrs, Conn. He may be reached by phone at (860) 486-2840 or via E-mail at jbartok@rcn.com.

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