DOSES USED FOR QUARANTINE TREATMENTS AND EFFECTS ON FRUIT QUALITY Guy J. Hallman Weslaco, Texas USA

Irradiation has advantages, including applicability to packed product and broad tolerance by fresh commodities. Disadvantages include lack of independent verifica- tion of efficacy. That is because unlike all other commercial treatments efficacy of irradiation does not depend on acute mortality, hence live insects may be found. This is not an insurmountable problem, as certification of treatment should be reliable.

Same dose serves for several pests and/or commodities although not all combinations were tested Not a new concept; has been used with cold and fumigation treatments Especially applicable to irradiation for 2 reasons: 1 Host not considered to affect efficacy 2 Dose has to be broadly applicable because there is no independent confirmation of efficacy

A phytosanitary treatment should be effective when non-target regulated pests occur in a consignment If non-target pests are dead (fumigation, cold, heat) consignment may be accepted San Jose scale

Non-target pests will most likely not be dead after irradiation Therefore a generic dose that includes these non- target quarantine pests would prevent a consignment from being rejected mealybugs

In 1991 the International Consultative Group on Food Irradiation proposed 2 generic doses: 150 Gy for Tephritidae 300 Gy for Insecta In recent years the USDA adopted: 150 Gy for Tephritidae 400 Gy for Insecta not including pupae and adults of Lepidoptera

The 400 Gy generic dose is often used for fruits although 150 Gy would suffice for the key pests (fruit flies) because non-fruit flies may be found mealybugs Bactrocera invadens

Table 1. Minimum absorbed dose ranges that might yield quarantine security of various pest groups in increasing order of radiotolerance Pest group Measure of efficacy, prevent Dose (Gy) Aphids, whitefliesReproduction of adult Seed weevilsReproduction of adult Fruit fliesAdult emergence from larva WeevilsReproduction of adult ThripsReproduction of adult Borer larvaeAdult emergence from larva Scales, mealybugsReproduction of adult Borer pupaeReproduction from pupa MitesReproduction of adult NematodesReproduction of adult ~4000

A generic dose should be based on several criteria: Risk level for the group Proportion of the group has been studied and to what degree Variability of doses in ‘well done’ studies

Problem points: Clearly defined measure of efficacy Infestation technique proven adequate Adequate dosimetry reported Large number of individuals of most tolerant stage treated Non-irradiated ‘control’ insects respond normally

High risk group Many economic species studied (<20) Efficacious doses vary from Gy; e.g.: Bactrocera cucurbitae 150 Gy B. dorsalis120 Gy Ceratitis capitata 100 Gy Anastrepha ludens 70 Gy Rhagoletis pomonella 50 Gy Preferably generic dose should be > minimum dose needed for any species included B. cucurbitae

Excludes most radiotolerant part of group, pupae and adults of Lepidoptera High risk Although many species studied, group is huge, so a small fraction of possible quarantine species studied to any degree Dose could be as low as 250 Gy, but set at 400 Gy because of uncertainty

Irradiation is most broadly tolerated commercial treatment at doses that would control most quarantine pests For commercial application tolerance should be proven to at least 2.5 X minimum absorbed dose required for control Commercial experience has shown fruits such as mangoes tolerate higher doses than the literature has indicated

As a general rule, if a fruit is ready to eat when irradiated it tolerates the treatment well Some fruits that ripen post-irradiation may show problems typical of other treatments: spotting of peel, uneven ripening, off flavors Fruits that ripen after harvest may be picked later for irradiation compared with other treatments; that alone will result in a better quality fruit Irradiation may or may not increase shelf life

Check 3 properties when evaluating tolerance: 1. Appearance, inside and out 2. Organoleptic qualities 3. Shelf life Industry should do these studies Casimiroa edulis

Some fruit tolerances Apple700 Gy Peach700 Gy Strawberry 1000 Gy Grapefruit600 Gy Mango800 Gy Avocado100 Gy

More research is needed to support a radiation treatment than any other phytosanitary treat- ment for 2 reasons: 1. No independent confirmation of efficacy 2. Data must be recorded until organisms die Cryptomphalus aspera

Although it is generally accepted that host does not affect efficacy, this is not proven and some research indicates host may affect efficacy For sure low oxygen lowers efficacy and fruits can differ in internal oxygen deficit Some fruits have high levels of antioxidants which could theoretically lower efficacy

Does dose rate affect efficacy? Seems logical Faster dose rate may be more effective in overwhelming molecular repair processes Dose rate commercially may be hundreds of times faster in a machine source vs. isotope A given insect in the load is irradiated in a second with a machine source, while it may take several minutes or more of continuous radiation to absorb the same dose via isotopes

If it does, it would mean that studies done with slow dose rates would have increased efficacy when applied with a machine source, and research has historically been done with slow isotopic sources. But recently some research has been done with machine sources, and if treatments resulting from machine source research were applied commercially using isotopes, would efficacy be reduced? Needs answer.

On the other hand could commodity quality research done with a slow isotopic source show damage when applied with a fast machine source? isotope e-beam

Low oxygen during irradiation known to reduce efficacy for some insects Could irradiation of commodities from low oxygen storage reduce efficacy? Perhaps, but low oxygen also kills insects, although not very effectively at low temperatures This relationship needs research for commodities stored under low oxygen

Do populations of insects vary in radiosuscep- tibility? Taken at face value literature seems to support that possibility (e.g., control doses for Ceratitis capitata vary from 40 to >300 Gy) although other variables (handling, research technique, dose rate, dosimetry) could be responsible for differences observed C. capitata

Doses for mites Support for lowering generic dose for Insecta (less Lepidoptera pupae and adults) to Gy Generic dose for Lepidoptera pupae <250 Gy doses for other key organisms Effect of radiation on disease vectoring Panonychus citri Ostrinia nubilalis

Conclusion: The one conclusion I would like to stress is that radiation phytosanitary treatments require greater quantity and quality of research effort and more stringent control of the application process than any other treatment precisely because there is no independent measure of efficacy (dead insects)