Fact sheet on Plant Quarantine Organisms

Transcription

1 Fact sheet on Plant Quarantine Organisms Sampling for Plant Quarantine Organisms ISPM 31 Methodologies for sampling of consignments. Introduction One of the most used phytosanitary measures used to secure freedom of plant quarantine organism in traded commodities is visual inspection normally connected with sampling also for further laboratory testing. Inspections may lead to actions such as: -Issuing a Phytosanitary Certificate (PC) - Refusal to issue a PC for consignments (or parts of consignments) intended to be exported. - Refusal of entry, detention, treatment, destruction or removal from the territory of the importing country for a consignment (or part of consignment) at import. - Decision on ordered treatments/destruction of crops under production when quarantine organisms are found. It is therefore very important that the inspection methodology including the sampling procedures used by the National Plant Protection Organisations (NPPOs) are documented and transparent. The European and Mediterranean Plant Protection Organization (EPPO) published in 2006 its standard PM 3/65(1) on sampling, and the secretariat of the International Plant Protection Convention (IPPC) in 2008 published within its International Standards for Phytosanitary Measures (ISPMs), the standard on sampling ISPM 31, Methodologies for sampling of consignments (2008). Risk categorization of commodities ISPM 32 Categorization of commodities according to their pest risk (2009). Proper inspection and sampling can be a cumbersome and costly measure. It is therefore needed that NPPOs establish a risk categorization of the commodities they have to inspect and certify in order to set appropriate priorities. Basic elements are the intended use and the level of processing of the commodities. The following risk categories can be identified: Category 1: Commodities have been processed to the point where they do not meet the definition of a regulated article. Category 2: Commodities have been processed but may still harbour or spread regulated pests. The intended use may be consumption or processing. Pest Risk Analysis (PRA) may be necessary. Category 3: Commodities have not been processed and intended use is consumption or processing. PRA is required, as appropriate. Examples: fresh fruit and vegetables. Category 4: Commodities have not been processed and the intended use is planting. PRA is required. 1

2 Pest risk analysis for quarantine pests (PRA) ISPM 11, Pest risk analyses for quarantine pests including analysis of environmental risks and living modified organisms (2004). Pest risk analysis for quarantine pest is another basic step in order to prioritize the inspection work. Here the standard gives the ground for this work, which is worth a fact sheet of its own. Here it is enough to conclude that plants for planting including seeds in general are in the highest risk category. Then the plant health status of the exporting/importing countries indicate how risky a consignment should considered to be. The inspection efforts and priorities should be set after these analyses. Determination of the appropriate level of phytosanitary protection Article 5 of the SPS agreement, deals with this basic and important issue. Members of the World Trade Organisation (WTO) are bound also to the SPS agreement. They have to make sure that their measures are based on proper PRAs. At the same time they are bound to, when determining their appropriate level of phytosanitary protection, to take into account also the objective of minimizing negative trade effects. Basic elements in deciding on sampling level When deciding on how many samples to take from a consignment of course the number of units in the consignment is evident, but also the level of the phytosanitary protection in general and the outcome of the risk analysis for the actual commodity and the phytosanitary status of the exporting/importing countries are evident. There are two essential decisions to be taken: - Which level of confidence is expected. This mean that if 95% level of confidence is chosen, out of 100 samples taken, 95 is going reflect the true situation of the consignment, 5 not. - Which level of infestation do you want to find? It is easily understood that it will make a big difference if you are content to find a level of 5 % infection or rather 0,1%. Sampling plans To aid a harmonized way of working, NPPO should give instructions to inspectors through sampling plans, where confidence levels and level of detection are fixed. Used levels for detection are commonly between 0,1 % and 10% depending mainly on the commodity and its intended use. The lower levels are fixed for plants for planting (usually less than 1%) and for fruits and vegetables higher levels can be accepted (between 5 and 10%). Homogenous or aggregated distribution It is also important to know the normal distribution of the pest in the consignment. In large consignment it is generally considered that the commodity has been sufficiently mixed to be considered homogenous. In small consignments it is necessary to consider the possibility that the pest may be aggregated and if this is believed to be the case, raise the number of samples to be taken. The figures in the tables are total number of units and number of units to sample, dependant on the chosen detection level, together with the probability to find the indicated infestation level. 2

3 How to take samples To obtain a statistically valid estimate of the level of infestation in the consignment or lot, and that this does not exeed the allowed infestation level determined by the NPPO, samples should be taken at random, preferably using random-number tables. Inspections that only target the accessible part of the consignment (tailgate inspection) should be avoided. For quarantine pests, it is important to maximize the chance of detection by targeting the sampling wherever possible to those plants or units which are, or units which are most likely to be carrying the organism (e.g. by focusing on wet patches of potato bags or on wet sawn wood). Inspectors own experience should of course be taken on board. It should anyhow be noted that you can t prove the absence of a pest by sampling! What you can do is to demonstrate that the frequency of infestation is below a specified level or within a specified range, with a known level of confidence, but then you have to verify this by inspection reports indicating clearly the methodology, which has been used. 3

4 Table 1. Table of minimum sample sizes for 95% and 99% confidence levels at varying levels of detection according to lot size, hypergeometric distribution Number of units in lot P = 95% (confidence level) P = 99% (confidence level) % level of detection efficacy of detection % level of detection efficacy of detection * * * * * * * * * * * *

5 Values in table 1 marked with an asterisk (*) have been rounded down to a whole number because scenarios resulting in a fraction of a unit being infested (for example, 300 units with 0.5% infestation corresponds to 1.5 infested units in the shipment) are not possible. This means that the sampling intensity increases slightly, and may be greater for a shipment size where the number of infested units is rounded down than for a larger shipment where a larger number of infested units are calculated (for example, compare results for 700 and 800 units in the lot). It also means that a slightly lower proportion of infested units might be detected than the proportion indicated by the table, or that such infestation is more likely to be detected than the confidence level shown. Values in table 1 marked with a dash (-) refer to scenarios presented that are not possible (less than one unit infested). 5

6 Table 2: Table of sample sizes for 80% and 90% confidence levels at varying levels of detection according to lot size, hypergeometric distribution Number of units in lot P = 80% (confidence level) P = 90% (confidence level) % level of detection efficacy of detection % level of detection efficacy of detection * * * * * * * *

7 Values in table 2 marked with an asterisk (*) have been rounded down to a whole number because scenarios resulting in a fraction of a unit being infested (for example, 300 units with 0.5% infestation corresponds to 1.5 infested units in the shipment) are not possible. This means that the sampling intensity increases slightly, and may be greater for a shipment size where the number of infested units is rounded down than for a larger shipment where a larger number of infested units are calculated (for example, compare results for 700 and 800 units in the lot). It also means that a slightly lower proportion of infested units might be detected than the proportion indicated by the table, or that such infestation is more likely to be detected than the confidence level shown. Values in table 2 marked with a dash (-) refer to scenarios presented that are not possible (less than one unit infested). Table 3: Table of sample sizes for 95% and 99% confidence levels at varying levels of detection, according to efficacy values where lot size is large and sufficiently mixed, binomial distribution % efficacy P = 95% (confidence level) P = 99% (confidence level) % level of detection % level of detection

8 Table 4: Table of sample sizes for 95% and 99% confidence levels at varying levels of detection, according to efficacy values where lot size is large and sufficiently mixed, Poisson distribution % efficacy P = 95% (confidence level) P = 99% (confidence level) % level of detection % level of detection COMPARISON OF HYPERGEOMETRIC AND FIXED PROPORTION SAMPLING RESULTS 1 Table 5: Confidence in the results of different sampling schemes for a 10% level of detection Hypergeometric-based sampling (random sampling) Fixed proportion sampling (2%) Lot size sample size confidence level sample size confidence level This appendix is not an official part of the standard. It is provided for information only. 8

9 Table 6: Minimum levels that can be detected with 95% confidence using different sampling schemes Hypergeometric-based sampling (random sampling) Fixed proportion sampling (2%) Lot size sample size minimum level of detection sample size minimum level of detection Source of information Agreement on the application of sanitary and phytosanitary measures (SPS agreement), WTO. EPPO Standard on Phytosanitary Procedures PM 3/65 (1) International Standards of Phytosanitary Measures: ISPM 11 Pest risk analyses for quarantine pests including analysis of environmental risks and living modified organisms (2004). ISPM 31, Methodologies for sampling of consignments (2008). ISPM 32, Categorization of commodities according to their pest-risk (2008). 9