It started in the cotton fields of Arizona
and southern California. Farmers and agricultural pilots wanted an aerial spray system
that could place insecticides on the underside of plant leaves, where white flies lay and
hatch their eggs. Dennis Gardisser, an agricultural engineer for the Cooperative Extension
Service, University of Arkansas, says work began decades ago to develop a system that
could defy gravity. It would use an electric charge to attach spray droplets to all parts
of the plant, not just to the top side of leaves where the droplets landed.
James Carlton, a U.S. Department of Agriculture engineer stationed at Texas A&M, now
retired, developed an electrostatic spray system that does just that. USDA patented the
system, and now an Texas-based company, Spectrum Electrostatic Sprayers Inc., has the
exclusive rights to manufacture and market it.
Recently, company executives traveled to ag pilot Marc Mullins' airstrip near Pine Bluff,
Arkansas, to test the system. They worked with Gardisser and Hal Tom, a pilot and USDA
researcher stationed at Texas A&M. Tom explained how the system works. "It uses a
power supply that charges rings around the spray nozzles with a very high voltage DC
current."
It's capable of putting out about 17,000 volts, although you normally operate the system
at between 7,000 and 9,000 volts. "The charge is applied to the rings that the spray
goes through, not the spray itself. The spray picks up the charge as it passes
through." Tom noted that the spray boom mounted under one wing of the aircraft is
charged negative, while the boom on the other wing is charged positive. "That
neutralizes any effect the charge might have on the aircraft." The positively and
negatively charged spray particles are attracted to one another and to the plant, like
iron filings to a magnet. The plant's tissue has a neutral charge, like a human, so both
charges are attracted to it, according to Gardisser. "When the charged
particles hit the plant, they're attracted to both the top and underside of the leaf,
which means better coverage with the spray and potentially less drift." Gardisser
says the electrostatic spray system's main selling point is its ability to wrap the spray
material around both sides of the plant's leaves, but that's not the only benefit.
The system makes the pilots and their airplanes more productive because they can cover
much more acreage per trip. "This system applies small droplets at low volumes,"
says Gardisser. "The droplets have to be small because it's more difficult for the
system to fully charge larger particles. "If you use large droplets, gravity will
overcome the charge and drag the droplets to the ground. With the small droplets, gravity
is no longer the driving force. Once the charged particles get close to the plant, they're
attracted like a magnet."
Gardisser says the electrostatic system works best when the spray materials can be applied
at volumes of one gallon per acre or less. Anything under a gallon per acre we consider
low volume. Any amount under half a gallon we call ultra low volume.
If pilots can apply an insecticide or fungicide at a rate of one gallon per acre or less
and their plane has a 500 gallon hopper, they can cover 500 acres or more in one trip. If
they have to apply 10 gallons of material per acre, they can only spray 50 acres per trip.
Gardisser says scientists are looking at the drift characteristics of pesticide materials
applied at low volumes with the electrostatic system. "For sure, when the charged
droplets get close enough to the canopy to be attracted to the plant tissue, you get more
thorough coverage. With gravity, the droplets only hit the top of the leaves." Gardisser believes the electrostatic spray system has potential in Arkansas, especially
for applying insecticides and disease-control products that can be used at low and ultra
low rates.
