Forced Hot-Air Treatments Moving Forward for Citrus

Naval oranges have recently been added by USDA’s Animal and Plant Health Inspection Service (APHIS) to the list of citrus from Mexico and areas of the United States infested with fruit fly pests that can be treated with high-temperature forced air as an alternative to methyl bromide fumigation. The forced hot-air treatment was approved for grapefruit, tangerines, and oranges other than naval oranges in December 1998.

The approved treatment calls for the temperature at the center of largest fruit to be raised to 44° C (111° F). If it takes less than 90 minutes for the fruit center to reach the target temperature, the fruit must remain at that temperature for whatever additional time is needed to reach the 90-minute mark plus another 100 minutes. The heating is monitored by temperature probes placed inside the largest fruit at the coolest part of the treatment chamber.

Cooling time after the treatment is irrelevant because the amount of time at the increased temperature ensures the killing of any fruit fly larvae, so APHIS is not requiring any specific cooling period. A cold water spray may be used to speed cooling.

Mexico is a major supplier of citrus to the United States, providing one-third or more of all the oranges imported. In 1998, Mexico exported 9,100 metric tons of oranges and 3,100 metric tons of tangerines to the United States.

AMECAC, a cooperative of Mexican growers, is close to having the first forced hot-air chamber for citrus ready for APHIS certification. The chamber is located in Montemorrelos, Mexico, and is expected to have the capacity to treat eight tons of citrus at a time. The Mexico Citrus Exporters Association estimates that use of the new facility once certified could lead to the export of an additional one million pounds of oranges to the United States a year. One million pounds was about 5 percent of the total Mexican orange imports into the United States in 1998.

In the regulated parts of Texas, more than 19 million pounds of citrus were treated with methyl bromide in 1997, with about 90 percent of it shipped to California. APHIS expects that Texas citrus producers will be closely monitoring Mexican experiences with the new treatment.

Forced hot air has been approved as an option for quarantine treatment. Methyl bromide for quarantine use continues to be exempt from the coming ban. But the fumigant does cause some loss of quality in citrus, sometimes causing bronzing—which damages the appearance of the fruit. In addition, as the supply of methyl bromide produced each year drops in response to phased-in reductions, the cost of methyl bromide is likely to increase.

The forced hot-air system not only provides as effective a treatment as methyl bromide against Mexican and Mediterranean fruit flies, but it is less phytotoxic and less damaging to fruit quality. A side benefit of the forced hot air is that it also kills the fungi that create green mold, one of the major pathogens that diminishes the shelf life of citrus.

The concept of using forced hot air for quarantine treatment is not brand new. The treatment has been used for several years to kill fruit flies on papaya being shipped from Hawaii. What is novel about forced hot air for citrus is that it specifies a fruit-center temperature profile so that it can be used for multiple commodities. Both the papaya and citrus treatments are the result of work by USDA’s Agricultural Research Service.

Entomologist Robert L. Mangan and plant physiologist Krista Shellie with the ARS Crop Quality and Fruit Insects Research Unit in Weslaco, Texas, developed the treatments that APHIS approved for citrus. Shellie and ARS entomologist Donald B. Thomas have continued to explore the possibilities of forced hot air.

In a recent study, Shellie and Thomas found that the rate at which the fruit is heated affects the time it takes to reach “probit 9,” the point at which 99.9986 percent of insects have been eliminated. “Using a very fast ramp up to the target temperature, you get probit 9 at 95 minutes after the fruit center reaches 44° C, versus 104 minutes to reach probit 9 with a slower ramp,” Shellie says. “In other words, if you heat it fast, you don’t have to heat it as long to get your effect.”

APHIS requirements are designed to ensure that no matter what speed of temperature increase is used, treated fruit spend sufficient time at 44° C to ensure a lethal dose to insect larvae. “But our results show that it is important to consider the link between approach time and holding time,” Shellie adds.

Thomas also found that, when a slow ramp was used, a high percentage of the fruit fly larvae produced heat-shock proteins. These proteins, which insects produce in response to stress, can help protect the insects from future exposure to stress. When heating to 44° C took 120 minutes, 76 percent of the insect larvae tested had detectable levels of heat-stress proteins. With a ramp of 20 minutes, only 42 percent of the larvae showed heat-stress proteins. “With the fast rate, the insects die before they have time to produce the protective protein,” Thomas says.

Understanding the biology of how insects react to forced hot air will allow the development of more refined treatments and APHIS requirements, he adds.

Last Updated: October 1, 1999