Irradiation as a Quarantine Treatment: Research Protocols (Guideline I)
Irradiation as a Quarantine Treatment for Commodities including Fresh Fruit, Cut
Flowers and Durables against Insects, other Arthropod Pests and Nematodes. Related
methodologies: Research and Development of a Single Component Quarantine Treatment
or Multiple Components Applied Concurrently (Guideline II), Multiple Quarantine Treatments (Guideline III), Supporting services and facilities (Guideline IV) and Check list (Guideline V)
Irradiation for quarantine (phytosanitary) purposes, must have a scientifically
demonstrated level of efficacy whether it is used as a single treatment, part of
a multiple treatment, or combined with other pest mitigation measures as a component
of a systems approach. The research necessary to demonstrate the efficacy of any
plant protection quarantine (PPQ) treatment begins with graded dose efficacy tests.
These provide results that can be analysed statistically to establish the dose required
to achieve a treatment efficacy that will give the desired level of quarantine security.
Quarantine treatments with irradiation mostly utilise gamma rays from commercially
available radioactive sources such as 60 cobalt, 137 caesium or X-rays (bremsstrahlung)
generated electrically by an electron beam striking an intervening high density
material eg lead, to deliver ionising radiation. An electron beam itself can also
be used deliver an ionising radiation treatment. However, electron penetration is
much poorer than that of gamma rays or X-rays and can only be used to treat produce
loads of no more than 5-10 cm in depth, such as rotating single layers of fruits,
a thin stream of grain or shallow boxes of cut flowers on a conveyor. Any of these
radiation sources can be used experimentally provided that the absorbed dose range
can be controlled and measured. Irradiation will be used most often as a single
treatment although a combined treatment such as with cold storage or heat may be
Doses of irradiation required to kill every pest present, in less than a day, are
usually too high for most produce to tolerate without unacceptable effects on commercial
quality. However, much lower doses (20-500 Gy) can cause total mortality after some
time or result in total sterility, which is equivalent to mortality in preventing
the next generation. In practice, the goal of an irradiation quarantine treatment
is usually to cause mortality of immature stages before development to the adult
or, where the stage present is an adult, to prevent further reproduction.
The research procedure involves, firstly, accurate determination of the pest species
of quarantine significance for the host commodity, found in the geographic location
from which exports are intended. The taxonomic identity of the pest species and
of the commodity must be agreed upon with the proper authorities of the importing
country before developing research plans for disinfestation treatments. There must
also be concurrence on the exclusion of any species from host or pest testing. Some
pests vary with respect to hosts or may occur as recognisable geographic races and
are liable to be redesignated by taxonomic subdivision eg. oriental fruit fly. Voucher
specimens from the test population should be lodged in a secure tenure museum against
the possibility of future dispute.
As with any quarantine treatment, the life stage of the pest representing the highest
risk and occurring on or in the commodity at export, must be determined, and research
concentrated on that stage. Usually this is the life stage most tolerant of the
treatment. For irradiation, the most tolerant stage is usually the most advanced
but this needs to be agreed by the importing country or trials to show this will
be necessary. Because irradiation does not kill all individual pests immediately,
experimentally treated samples must be held under conditions favourable for pest
survival until all are dead. Unirradiated (control) samples must be held similarly
until the irradiated individuals die, to verify that the irradiation caused the
death or sterility. Development and reproduction in these controls must be normal.
1. Develop a general outline of the proposed experiments their objective(s) and
how the treatment is to be carried out. Include the location(s), dates stages will
be achieved, types of equipment, types of treatment(s), and the identity of the
principal scientist(s) who will manage the project.
2. Tests may be required to determine differences within the host and pest populations
i. For hosts, sound evidence exists that treatment doses to provide quarantine security
might differ from one cultivar to another.
ii. For pests, conditions such as diapause may exist, causing some populations to
respond differently from normally developing populations, even where there is acceptance
that the most advanced life stage will be the one most tolerant of irradiation.
3. The mode of infestation to be used must be decided. It can be:
i. Collection of naturally infested commodity from the field and sampling to determine
the stage(s) present.
ii. Laboratory infestation by permitting natural infestation of the commodity by
the pest organisms held in laboratory colonies or cultures, then holding the commodity
until the pest develops to the life stage required for treatment.
iii. Infested by transplanting or inoculating pest organisms of an appropriate developmental
stage at appropriate location on or in the commodity. The organisms may be of field
or laboratory origin.
iv. Treatment of pest organisms unassociated with the commodity ie. in vitro. Such
tests are unlikely to be acceptable alone to support a quarantine treatment proposal
but can be of value to corroborate in vivo results.
4. The irradiation source must be calibrated and dosage geometry determined using
an appropriate dosimeter in accordance with a recognised international standard
(eg the American Society of Test Methods, ASTM). This is of critical importance
as irradiation cannot be delivered as a uniform dose so the maximum and minimum
levels must be determined with accuracy by a method of known reliability. Source,
target dose, dose rate, max/min ratio mean dose, distribution of doses, temperature,
relative humidity, medium, atmosphere and ventilation should all be documented for