Radio Frequency Puts the Heat on
Entomologist Jim Hansen infests
apples with codling moth larvae
while technicians Millie Heidt
(left) and Michele Watkins
(center) remove larvae from a
To prevent influx of pests that could create agricultural
problems, produce-importing nations enforce strict rulesdepending
on the commodity and the infesting insect. The rules often require vulnerable
produce to be treated in some way that ensures destruction of pests.
For several decades, methyl bromide has been a mainstay
treatment to kill a wide array of quarantined pests as well as those
encountered in orchards, packinghouses, and food plants. But this potent
chemical fumigant is being phased out because of evidence linking it
to damage to Earth's ozone layer.
Although the effectiveness of using radio waves to kill destructive insects in agricultural products has been known for 70 years, the technique has never been applied on a commercial scale. A recent cooperative effort by four ARS research laboratories and two universities aims to overcome the technical barriers for the use of radio wave heating to control pests on a commercial scale.
Technician Carlos Solis, of
the ARS Kika de la Garza
Research Center, in Weslaco,
Texas, places oranges in a
water bath heated with radio
waves to kill fruit flies.
The water is circulated to
heat the oranges uniformly
and prevent fruit damage.
Electromagnetic waves of radio frequency can make molecules
vibrate and heat uplike microwaves heat food. The trick is to
kill pest insects without killing the taste or texture of the food they
Since 2000, a team led by Juming "Jimmy" Tang of Washington State University (WSU) in Pullman, involving four ARS laboratories and the University of California-Davis (UC-Davis), has been working on a 4-year study to see whether radio waves would be an economical, environmentally friendly alternative to methyl bromide and other chemicals to effectively rid fruits and nuts of live, quarantined insects.
In TexasIt's Chiefly Citrus
In Weslaco, Texas, ARS entomologist Guy J. Hallman is checking out use of radio frequency treatment of citrus against the Mexican fruit fly. He's in the Crop Quality and Fruit Insect Research Unit at the Kika de la Garza Subtropical Agricultural Research Center. Hallman is developing a device to simulate what's needed to heat-treat citrus fruit with radio waves commercially.
Technician Michele Watkins
infests an apple with
codling moth larvae. The
apples will then be subjected
to radio waves to determine
exposure times that will
kill the larvae without
affecting fruit quality.
"We're trying to bridge the gap between the laboratory
and real world," says Hallman. "Once we know how to treat
fruit in a commercial situation and how much it will cost, any producer,
shipper, or packinghouse operator can use the information to decide
whether radio wave pest control is a viable option."
In Hallman's system, citrus fruit would pass through a conveyor between
a series of radio frequency heaters. To simulate a commercial system
in the laboratory, the fruit are conveyed in a circulating water bath
to keep them moving during heating. This would prevent the fruit's overheating
from extended contact with any one area of the bath. And to ensure continuous
heating from the peel in to the fruit's centeressential to killing
all fruit flies that might be presenta bumper would dunk any fruit
that bobbed above the water surface. This prevents dark-black rings
from forming around the fruit at the water's surface because of an energy
concentration where the water meets the air.
A method using just hot air to treat fruit in boxes or bins has been
tried commercially in Mexico with mixed results. It takes hours to complete
and puts a strain on the fruit's skin, sometimes causing heat damage.
Radio frequency heating can be done in less than half an hour and is
less damaging, since the fruit is heated uniformly throughout.
Hallman has focused on grapefruits but is also working with other citrus,
including oranges and tangerines. The larger the fruit, he notes, the
harder it is to heat uniformly and the more likely to form hot and cold
"This multi-lab project is making a serious effort to take a look at things that haven't been looked at with radio frequency heating," says Hallman. "I think we stand a good chance of finding out how radio frequency disinfestation can be done to a large volume of fruits or nutsand if it can be done on a commercial scale." He predicts that by this summer he'll have a good idea of what the treatment will cost.
In Washington--Apples and Cherries
If you're in Wapato, Washington, don't be enticed by the apples floating
in the tub in James Hansen's laboratory. You don't want to go bobbing
There are several reasons, says Hansen, an entomologist with ARS' Yakima
Agricultural Research Laboratory. The first is that this particular
tub is filled with salt water. Second, if you were to latch onto one
of the apples with your teeth, you might bite the proverbial worma
larva hatched from a codling moth egg.
And Hansen wants these apples intactno tooth marks, please. Such
a mark might skew the results of tests he is conducting on use of radio
waves to rid the fruit of live, pesky insects like the codling moth
before marketor shipment to trading partners like South Korea
and Japan, where such pests might not already occur. Japan is particularly
stringent about what phytosanitary methods it will accept for disinfesting
This rule also applies to sweet cherries, a tree fruit commodity that
generates over $145 million in yearly national export sales, notes Hansen.
In cooperation with the team led by Tang, a professor in biological
systems engineering at WSU, Hansen plans to "bathe" tubs full
of apples and cherries with radio waves to determine exposure times
that will kill codling moth larvae without affecting fruit quality.
From two lines of researchone focusing on the insects, the other
on fruit qualityand three major disciplinesengineering,
entomology, and plant physiologythe collaborators hope to position
radio wave treatment as a technology that can be readily adopted by
commercial packinghouses or quarantine operations in lieu of methyl
"Without suitable alternatives to methyl bromide, we're going
to be up a creek," ARS horticulturist Stephen R. Drake says of
the U.S. fruit industry's fight against quarantined pests. He is with
ARS' Tree Fruit Research Laboratory in Wenatchee, Washington.
"Ideally, you want to treat the fruit with radio frequency while
it's being packed and designated for a particular market," Hansen
explains. "Commercial packers can't afford to have produce sitting
there, so we want this treatment to kill the insects as soon as it can."
Tang, Hansen, Drake, and Lisa Nevens, an ARS entomologist at Yakima, first began working on the radio frequency project in 1996.
In California--Tree Nuts and Dried Fruits
Meanwhile, in California, entomologist Judy A. Johnson is also keenly
attuned to the potential of radio frequency energy to zap destructive
insects. Her primary targets? The wiggly larvae of the navel orangeworm,
Indianmeal moth, and codling moth. These insects are among the worst
enemies of walnuts, almonds, and pistachios and of dried fruits such
as figs and raisins. Johnson has newly added red flour beetle to her
list of culpritsa lesser pest of the nut and fruit crops but a
major problem in flour mills and food-processing plants.
Johnson is doing the radio frequency work at the ARS San Joaquin Valley
Agricultural Sciences Center at Parlier, near Fresno, in close association
with cooperators at WSU and UC-Davis. They have already developed a
preliminary picture of the target insects' ability to endure heattheir
The laboratory experiments that Johnson and her ARS and university
colleagues conducted are the first to extensively detail the thermal
tolerance of the navel orangeworm, Indianmeal moth, and codling moth.
For one test, Johnson and co-investigators drilled tiny holes in over
500 in-the-shell walnuts; enticed the slender, whitish navel orangeworms
to enter the shells; then plugged the holes to block the insects' escape.
The scientists then tried some novel combinations of radio waves and
hot forced air, that is, air that's heated and blown into the test chamber
holding the nuts. Radio waves, alone or combined with hot forced air,
were used to heat the nuts to 55° C (131° F) in about 5 minutes.
Hot forced air was then used to keep the nuts at 55° C for 5 or
"All the treatments killed 100 percent of the navel orangeworms,"
reports Johnson. What's more, tests led by co-researchers Tang at Pullman
and Elizabeth J. Mitcham at UC-Davis, showed that the treatments didn't
harm the quality of the nutseven in long-term storage.
"That's critical," Johnson points out, "because walnuts
are often stored for a year or more before they show up at your supermarket."
In addition, walnuts are rich in oil, so they're more vulnerable to
heat damage than some other kinds of nuts, such as almonds. "That's
why we think that if a treatment is okay for walnuts," Johnson
continues, "it will likely be okay for almonds."
Indianmeal moths proved to be more susceptible to heat than navel orangeworms
are. "Indianmeal moths are silvery, and wedge-shaped. If you open
a kitchen cupboard where you keep raisins or breakfast cereals and a
little moth flies out at you," Johnson explains, "chances
are it's an Indianmeal moth."
For this experiment, Johnson and her colleagues used metal heating
blocks, or plates, specially designed by Tang for the radio frequency
research. They placed target insects in the small gap between the plates,
which were then heated to specific temperatures and held at those temperatures
for precise periods.
She and her associates assembled some 15,000 Indianmeal moth larvae
for the study, exposing them to temperatures ranging from 44° C
to 52° C (about 111° F to 126° F) for 2 to 100 minutes.
"In general," Johnson comments, "the lower the temperature,
the longer it took to kill the larvae. These readings are the basis
for a new, math-based model for projecting the rate of kill at other
time-and-temperature regimens. Because of the large number of larvae
tested and the range of temperatures that we investigated, we're very
confident of the accuracy of the model's projections."
Johnson and her cooperators also used the special heat plates to discover
more about the amount of time and heat that it takes to kill navel orangeworms.
They exposed 15,000 navel orangeworms to temperatures ranging from 46°
C to 54° C (about 115° F to 129° F) for 1 to 120 minutes.
Using the new data, they created a mathematical model of the navel orangeworm's
The findings from the Texas, Washington, and California experiments
are an essential starting point for making the radio frequency energy
approach a success, Johnson says. The scientists have published their
findings in the Journal of Economic Entomology, Journal of Stored
Products Research, and Postharvest Biology and Technology.By
Suszkiw, and Marcia
Wood, Agricultural Research Service Information Staff.
This research is part of Methyl Bromide Alternatives, an ARS National
Program (#308) described on the World Wide Web at http://www.nps.ars.usda.gov.
Guy J. Hallman
is in the USDA-ARS Crop
Quality and Fruit Insect Research Unit, Kika de la Garza Subtropical
Agricultural Research Center, 2413 E. Hwy. 83, Bldg. 200, Weslaco, TX
78596; phone (956) 447-6313, fax (956) 447-6345.
Judy A. Johnson
is with the USDA-ARS Commodity
Protection and Quarantine Insect Research Laboratory, San Joaquin
Valley Agricultural Sciences Center, 9611 S. Riverbend Ave., Parlier,
CA 93468; phone (559) 596-2768, fax (559) 596-2721.
"Radio Frequency Puts the Heat on Plant Pests" was published in the February 2003 issue of Agricultural Research magazine.