12.7.1. The development of formal procedures
Harmonization is a key principle of the WTO-SPS Agreement. This means that when NPPOs implement phytosanitary measures in trade to mitigate immediate or poten- tial risk of pests to plant health within their jurisdictions, the measures must be based on available international standards (i.e. the ISPMs developed and adopted under the IPPC described earlier). If no international standard is available, or in cases where measures deviate from standards, measures need to be technically justified by
a scientific risk assessment as described under the WTO-SPS Agreement. Under the IPPC, a scientific risk assessment is referred to by the term ‘pest risk analysis’ (PRA).
The IPPC definition of PRA includes evaluating biological or other scientific and economic evidence to determine whether a pest should be regulated (pest risk assess- ment) and the strength of any phytosanitary measures to be taken against it (pest risk management).
The IPPC has published a range of reference and concept standards and has begun developing pest and commodity-specific standards. Therefore, most current decisions on phytosanitary requirements, such as allowing or prohibiting the entry of a commodity or requiring a quarantine treatment of a commodity, must be justified by a PRA as described in ISPM No. 2 Framework for pest risk analysis and ISPM No. 11Pest risk analysis for quarantine pests including analysis of environmental risks and living modified organisms. These two standards provide significant guid- ance for the international harmonization of PRA. All current ISPMs are published on the IPPC website, the International Phytosanitary Portal (IPP) at http://www.ippc.int.
One of the crucial steps in a PRA is the determination of whether or not the pest or pests of concern meet the definition of a ‘quarantine pest’ because, except in rare cases where ‘regulated non-quarantine pests’ are involved, only quarantine pests can be regulated in international trade. Only quarantine pests need to undergo complete risk assessment (i.e. evaluation of the likelihood of pest entry, establish- ment and spread, and of the potential economic and environmental consequences should the pest become established) and risk management (i.e. the identification and evaluation of pest risk management options to reduce the risk of introduction and spread of the regulated pest, followed by the selection of the most appropriate management option(s)).
As discussed earlier, an organism can only be considered a quarantine pest for a particular country if it is not present in that country or of limited distribution and under official control. Official control of regulated pests must be directed towards eradication or containment within the country and not merely a reduction or suppres- sion of the population, and the official control programme must be implemented nationally. In the USA, for example, if a pest is present in more than one state but a state quarantine programme is implemented by only one of those states, then this would not meet the definition of ‘official control’ for the purpose of declaring a pest a regulated pest at the national level. Therefore, the quarantine status of a pest can change over time, not only as populations become established or eliminated, but also as regulatory action is implemented at various levels.
PRA can be initiated for a variety of reasons, such as if a particular pest is inter- cepted at points of entry, a new pest risk is identified by scientific research, or a pathway other than a commodity import (e.g. natural spread, international mail, garbage) is identified. A PRA can also be initiated because a country makes a request to the NPPO of a second country for authorization to allow the importation of an agricultural commodity (e.g. fruits and vegetables for consumption or plant products for propagation) into that second country. The importation of an agricultural com- modity represents a pathway that pests can potentially follow to enter and establish in a new country.
In the preparation of a PRA initiated by a commodity import request, only quar- antine pests likely to be carried on the particular commodity require complete risk analysis (i.e. risk assessment followed by risk management). The pathway of most
fruits and vegetables for consumption is unlikely to carry plant-parasitic nematodes because these organisms usually only attack underground parts of plants. The excep- tions to this include plant-parasitic nematodes that attack foliage (e.g. Aphelenchoides spp.) or endoparasitic nematodes that may be carried in edible roots and tubers such as beets and potatoes (e.g. N. aberrans and M. chitwoodi, respectively). There is little evidence that nematode populations can become established if the host commodities are consumed after import as intended. Disposal of such commodities in compost, cull piles or garbage would need to be followed by dispersal onto susceptible hosts or their growing media for the nematodes to become established in the importing country.
By contrast, without the implementation of specific phytosanitary measures, the pathway of plants for propagation, such as potato tubers for seed, bulbs and nursery stock, is much more likely to carry plant-parasitic nematodes (e.g. PCNs, Ditylenchus dipsaci) and result in the establishment of these nematodes in the importing country.
Thus, compared to commodities for consumption, seeds and plants for planting pose a greater risk for the introduction and/or spread of nematode pests.
In addition to the type of plant commodity being imported, other factors typi- cally used in assessing pest risk include, among others: (i) the biology of the pest (e.g. host range, feeding habits, life cycle, habitat, symptoms produced, overwinter- ing/dormancy ability, dispersal ability, interaction with plant pathogens); (ii) the economic and environmental impacts of the pest in other parts of the world; and (iii) the host and environmental conditions in its geographical distribution, as well as those conditions in the importing country. The more knowledge of these factors one has, the more accurate the estimate of risk. Unfortunately, biological informa- tion is often lacking for many new pests intercepted in international trade, includ- ing most new nematodes. Furthermore, soil temperature data, so necessary for nematode assessments, is often non-existent and has to be extrapolated from air temperatures, a technique that has disadvantages, as discussed by Baker and Dickens (1993). Not only is soil temperature influenced by air temperature, but also by ground cover, soil texture, wetness, sun angle and day length. In addition, rainfall data often cannot be used because of the added complication of irrigation at monitoring sites.
During the final stage of the PRA, i.e. pest risk management, the estimated pest risk helps determine the most appropriate phytosanitary measures needed to reduce risks. There are several important WTO-SPS/IPPC principles that should be followed when deciding whether or not to allow importation of a commodity and, if so, what phytosanitary measures should be implemented to manage the risks associated with that commodity (many of these principles are embodied in the WTO-SPS Agreement).
The first of these principles is that of ‘minimal impact’, also known as ‘least restrictive measures’, which states that measures must not restrict trade more than that required to achieve the appropriate level of protection for the importing country.
In other words, the selected risk management measure(s) should be ‘proportional to the risk identified in the pest risk assessment’ (ISPM No. 2).
The principle of ‘equivalence’ means that an importing country must recognize that different phytosanitary measures can potentially be used to achieve their appro- priate level of protection. For example, if an irradiation or hot water treatment is equally effective in eradicating a particular plant-parasitic nematode pest on a par- ticular commodity compared with a chemical management treatment, then exporting countries should be allowed to use one of the alternatives.
The WTO-SPS principle of ‘non-discrimination’ states that importing countries should not discriminate between countries that have the same phytosanitary status;
that is, countries that have the same pests for a certain commodity should not be treated differently in terms of what phytosanitary measures are required. For exam- ple, all countries with G. rostochiensis should be required to meet the same phy- tosanitary requirements for commodities (that may carry the nematode) to enter a given country.
Finally, the principle of ‘transparency’ requires countries to provide information regarding their risk analysis procedures, including information justifying why certain phytosanitary measures were selected.
By following the key principles outlined above, and ensuring that decisions are based on scientific evidence through risk analysis, countries benefit from a more pre- dictable, transparent, and fair trading system while at the same time enhancing their ability to protect their own agriculture and natural environments.
12.7.2. Practical problems
The risks of B. xylophilus and M. chitwoodi presented by different trade pathways have been assessed (Tiilikkala et al., 1995; Braasch et al., 1996; Evans et al, 1996).
Initially, the trade patterns involving M. chitwoodi concentrated on the most impor- tant economic host, potato, but subsequent research has highlighted other pathways involving other root vegetables, such as carrots, and ornamentals including flower bulbs. The knowledge that M. chitwoodi can be transported inside potato tubers forms the basis for most inspection procedures, but in the field it is often difficult to detect low populations which produce few symptoms on potato skins, and immature females, most often found in fresh tubers, are opaque and difficult to see during inspections, necessitating laboratory testing in suspect cases.
Many species, such as M. chitwoodi, are not host-specific and pose real problems for quarantine specialists developing phytosanitary control measures. Such measures work best where the species in question has a narrow host range, a slow rate of population increase and a rapid rate of decline under non-host crops. PCNs and T. semipenetrans are good examples of such species.