International trade in fruit and vegetables within the Asia-Pacific region (and internal trade within Australia) is severely constrained by quarantine barriers erected to prevent the spread of fruit flies. The barriers can only be removed when an effective treatment exists for use on fruit after harvest. Existing treatments are based on the use of chemical fumigants such as ethylene dibromide (EDB) and methyl bromide (MB). However, it became apparent that these chemicals would be withdrawn or their use severely restricted for reasons of consumer resistance, human health concerns or, in the case of MB, because it is an ozone-depleting substance. Eventually, postharvest chemical treatments could be unacceptable to the major fruit-importing countries (USA, Japan) that take fruit from Southeast Asia.
Physical treatments, such as exposure to heat or cold, were increasingly seen as the best solutions to the problem as they leave no chemical residues on the fruit. The main difficulty was that these treatments can either damage fruit or adversely affect its development or shelf-life in ways that the chemicals do not. Furthermore, the existing physical treatments, and their systems of application, were more complex to apply and more costly than the systems used for chemical fumigation.
There was thus a need to develop cheaper and simpler techniques for physical disinfestation that would not affect the characteristics of the fruit. This was particularly important in many Southeast Asian countries where development of the horticultural sector was seen as a high priority. The push to increase exports of fruit was constrained by compliance with quarantine regulations and the need to extend the storage life of the fruit to cope with sea transport.
This project compared three different treatment systems - and combinations of components of those systems - to identify the best parameters on which to base a simpler, cheaper treatment method suited to Southeast Asia and Australia. Partner countries were Thailand, Vietnam and Australia.
This project investigated the suitability of alternative, cheaper ways of treating fruit to remove pests such as fruit flies. The main techniques studied were heat treatments that, with minimal damage to fruit, could remove most major pests but which cost much less than chemical treatments and were easier to apply. An important objective was to create facilities and expertise for fruit disinfestation in Vietnam.
The work was related to several other projects. The major species of fruit flies were exposed to various temperature and humidity combinations for different time periods and responses noted. Fruit was also exposed to a similar set of variations and researchers tested for quality deterioration, ripening time and storage life. The possibility that some fruit could be pretreated to impart greater resistance to high temperature exposure was also considered.
In another strand of the project, the current systems and practices used in Australia and overseas for fruit disinfestation were assessed. The recorded responses of fruit and pests to temperature disinfestation were used to build pilot-scale demonstration units to evaluate simplified heat treatments. Prototype units were initially tested in Thailand and an opportunity was sought to establish a demonstration unit in Vietnam.
A primary objective of the project was to build the capacity in Vietnam to undertake the development of quarantine treatments for fruit - to facilitate the export of products prone to fruit fly infestation to countries with quarantine barriers. Insect rearing rooms and disinfestation laboratories were established within the MARD Plant Protection Department in Hanoi. Equipment and the special materials to undertake the work were provided. Initial training of Vietnamese scientists covering insect culturing, disinfestation treatment development methodology and treatment methods took place in Bangkok. Colonies of oriental fruit fly (OFF) and melon fly were established for treatment development experiments. The Hanoi laboratory reached full independent operational status. Thermo-tolerance data and most tolerant stage data for OFF were collected and analysed. Insect diet studies identified inexpensive locally available diet materials suitable for fruit fly rearing. Development of heat treatment conditions for dragon fruit progressed.
A postharvest laboratory with a range of equipment for undertaking fruit quality evaluation and storage and handling experiments was provided to the MARD Research Institute for Fruit and Vegetables (RIFAV), Hanoi. Staff received intensive training in use of the equipment and in postharvest experimental methods. The research leader received additional specialist training in Australia and later became the inaugural Vietnamese recipient of a John Dillon Fellowship which supported management training in Australia. The local experiments focused on storage and evaluation of major crops such as banana and lychee, to provide staff with the range of necessary background experience in the field.
In Thailand, scientists established disinfestation heat treatment conditions for mangosteen, to satisfy efficacy requirements for export to Japan, and commercial shipments commenced. Treatment development for pomelo and longan, two difficult fruits to work with, commenced.
Some of the Australian research focused on technical options for reducing damage risk while maintaining treatment efficacy. They tested the hypothesis that slow air heating using low temperature and relative humidity in the early stages of treatment could reduce the incidence of fruit injuries. They tested mangosteen, tomato, mango and papaya. Pretreatment conditioning at 38C for 12-18 hours was found most effective in reducing injury development induced at higher temperatures, but the treatment efficacy also fell under these conditions. The scientists investigated an alternative approach to pretreatment using atmosphere modifications.
Insect mortality was high when a low oxygen pre-treatment stage was incorporated in the total treatment process using milder temperatures. Elevated carbon dioxide appeared to have little effect. An improved treatment system, completed within a complementary project (funded by Australian agencies) by fabricating a dedicated low oxygen treatment chamber, overcame some problems that had produced inconsistent and difficult-to-interpret results. Treatment conditions for fruit fly Bactrocera jarvisii that did not injure the fruit were developed to disinfest 'Kensington' mango. Similar work for papaya was well advanced.
The Plant Protection Department in Hanoi received an experimental version of a new treatment system developed in Australia. This unit could treat up to 10 crates, each containing up to 10 kg of product. The unit, which could operate fully automatically when needed, was used for data collection to determine the most tolerant stage of B. dorsalis in dragon fruit. The Plant Protection Department in Ho Chi Minh City received a similar unit through a parallel AusAID CARD project.
Towards the end of the project a small pilot commercial unit was installed at RIFAV. In Australia the project also provided some technical support to commercially scale-up the treatment technology.
The use of disinfestation heat treatment has become a preferred disinfestation technology in the Asian and Pacific region. Harmonisation of treatment development and broad agreement on experimental data development methods and interpretation are of significant importance to trade in the region. Some of the countries of the region attended an ACIAR sponsored meeting in 1995, where participants finalised a document titled Guidelines for the development of quarantine disinfestation heat treatment for fresh horticultural produce for fruit flies.
Links:
[1] http://www.aciar.gov.au/country/Thailand
[2] http://www.aciar.gov.au/country/Vietnam
[3] http://www.aciar.gov.au/programarea/Horticulture