By law, the U.S. Secretary of Agriculture is authorized to enforce measures that prevent the introduction of exotic pests and plant diseases into the United States. Consequently, the United States, like most other countries, has regulations requiring that plants, plant products, and other regulated articles entering the country pose, at most, a minimally acceptable risk for introducing new or not widely distributed pests into the country. In many cases, the risk to quarantine security posed by a commodity is determined by USDA to be unacceptably high. Therefore, that commodity is denied entry unless the risk of pest introduction or spread can be practically and effectively eliminated by an approved treatment. This makes appropriate treatments vitally important to ensure safe agricultural trade.

For about 50 years, the United States has required that the efficacy of commodity treatments for certain pests, especially fruit flies, meet or exceed a Probit 9 statistical standard. To meet this standard, treatment must kill or sterilize 99.9968 percent of the pests in a test of at least 100,000 individual pests. Although heat, cold and irradiation have limited uses as Probit 9 treatments, fumigation with methyl bromide is presently the most practical option for many commodities. Its loss will significantly affect the ability of the United States to manage the risk of introducing pests associated with exporting and importing agricultural commodities. Anticipating the loss of methyl bromide, USDA scientists from the Agricultural Research Service and the Animal and Plant Health Inspection Service (APHIS) are studying whether a less rigorous standard might be more appropriate for certain low-risk commodities.

"Probit 9 was adopted as a one-size-fits-all standard to provide adequate quarantine security for the highest risk commodities," says Kenneth W. Vick, ARS national program leader for postharvest entomology. "While this single standard has a long history of usefulness, given the ready availability of methyl bromide (and earlier, ethylene dibromide), it may now dictate an overly severe treatment for rarely or minimally infested commodities that have a very low probability of carrying exotic pests."

Historically, the Probit 9 standard was applied without considering the rate of pest infestation, the survival and reproductive capacity of the pest, or the effects of harvest, processing, and distribution on the pest's ability to survive and establish itself. No consideration is given to packaging and shipping conditions or to the season of shipment.

According to Vick, these factors are all important in determining the risk a particular commodity presents to the importing country.

"Our objective is to make the severity of the treatment proportional to the risk posed by the commodity," he says. "For many commodities, a less severe treatment is more appropriate, compared to the present standard. Recognizing that treatments may range in severity depending on the risk allows expanded use of controlled atmospheres, systems approaches, and other treatments which have not in the past met Probit 9 requirements. A less severe treatment means less commodity damage and longer shelf life."

Vick and ARS research entomologist Nicanor J. Liquido are working with APHIS to redefine the applications for Probit 9. Liquido heads the ARS Tropical Fruit and Vegetable Laboratory in Hilo, HI.

"In cases where the natural rate of pest infestation in the field is low and the chances of survival and reproduction are poor, the Probit 9 standard could be too stringent and therefore detrimental," Liquido says. "We're proposing a less severe treatment combined with modifications in packing, distribution, and inspection. The risk of pest survival and establishment would be based not only on quarantine treatment, but on biological, ecological, marketing, and distribution data as well."

He defines risk, in this case, as the probability of at least one reproductive pest being present in a particular commodity shipment.

"We'd welcome effective alternatives to the prescriptive requirements of Probit 9," says APHIS' Robert L. Griffin, who is in charge of APHIS' Risk Analysis Systems in Riverdale, MD. "We're seeking a more sophisticated definition of quarantine security based on pest risk. Most of the simple treatment solutions have been addressed over the past 40 years. The present need to identify alternatives provides a valid basis to evaluate increasingly more diverse and complex pest risk- management proposals. This requires working closely with our ARS counterparts to more precisely evaluate these proposals and consider whole systems, from production to consumption, rather than just postharvest."

This systems approach considers factors beyond simple treatment mortality in estimating the probability of live pests being present. A sliding scale based on pest risk, combined with a holistic approach, provides a much more versatile and technically valid way to evaluate whether a pest is present and how best to manage it, Griffin says.

"This increases the opportunity to identify new treatments and new approaches to treatment, which will result in improved ways to achieve quarantine security," he notes.

One new approach that APHIS is considering is to allow irradiation as a quarantine treatment for papaya, carambola, and litchi grown in Hawaii. While this treatment would provide for interstate movement of fruit, it would also protect other parts of the United States from Hawaiian pests.

"We're proposing several amendments to regulations that now govern irradiation procedures and facilities and the handling of treated and untreated fruits from Hawaii, Griffin reports.

Many Hawaiian-grown fruits and vegetables cannot be shipped to the U.S. mainland because of fear of introducing the Mediterranean fruit fly, the melon fly, and the Oriental fruit fly. Irradiation sterilizes these pests. But, growers have been unable to ship irradiated papayas under existing regulations simply because there is no irradiation facility in Hawaii.

Griffin says that an amendment is being considered that will allow papaya, carambola, and litchi to be shipped from Hawaii for irradiation treatment at approved facilities on the mainland in areas where the fruit flies would not become established. At approved locations on the mainland, irradiation would mitigate the risk of any pest eggs or larvae reaching maturity and escaping from the fruit.

In addition to papaya, carambola, and litchi, the proposed regulation would potentially allow shipment of rambutan, atemoya, and other exotic fruits from Hawaii if they are free of other plant pests. Treatment efficacy other than Probit 9 may provide quarantine security for non-fruit-fly pests in these commodities.

"Some postharvest quarantine treatments designed to meet Probit 9 standards damage commodities to the point where they're unmarketable," Liquido says. "If a commodity is a poor host to a pest and can tolerate a treatment requiring a mortality rate lower than Probit 9, then it may be possible to export that commodity without any significant threat of introducing a new pest."