Radio frequency puts the heat on plant pests

Agricultural Research, Feb, 2003 by Alfredo Flores, Jan Suszkiw, Marcia Wood

To prevent influx of pests that could create agricultural problems, produce-importing nations enforce strict rules--depending 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.

Electromagnetic waves of radio frequency can make molecules vibrate and heat up--like microwaves heat food. The trick is to kill pest insects without killing the taste or texture of the food they infest.

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 Texas--It'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.

"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 center--essential to killing all fruit flies that might be present--a 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 spots.

"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 nuts--and 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 for them.

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 worm--a larva hatched from a codling moth egg.

And Hansen wants these apples intact--no 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 market--or 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 fresh produce.

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.