• Home
• Lessons
  • 1. Review of PRA and Introduction to Risk Assessment
  • 2. Resources Used to Conduct Pest Risk Assessment
  • 3. Conducting Pest Risk Assessment for Trade
  • 4. Practice
  • 5. Module Conclusion
• Topics
  • 1. Initiation
  • 2. Pest Categorization
  • 3. Estimating Likelihood of Introduction
  • 4. Estimating Potential Consequences of Introduction
  • 5. Developing Conclusions and Describing Uncertainty
  • 6. Lesson Summary
• Resources
  • Help
  • Glossary
  • Documents and Videos
  • Acknowledgements

Lesson 3: Conducting Pest Risk Assessment for Trade

Topic 3: Estimating Likelihood of Introduction

Introduction of the Pest

In this step of the pest risk assessment stage, the analyst considers pests categorized as regulated pests and estimates the likelihood of introduction and spread of those pests into the PRA area.

As you might recall from earlier in the module, introduction includes entry and establishment. Let us consider entry. In order for a pest to enter an area, a series of events must occur in sequence, each with some reasonable probability. What about establishment? After a pest has entered the new area, several conditions must be met in order for establishment to occur. For example, a host must be available and the climate must be compatible with the pest’s needs. So introduction of the pest is dependent on the outcomes of many events.

This step of PRA also considers the spread of the pest. One way to analyze spread is to map the pathway. Let us look at that next.

Mapping the Pathway

A useful exercise at this stage of the PRA is to map the pathway for introduction and spread. Mapping the pathway helps the analyst and the audience to visualize the events that must occur for pest introduction. It makes the assessment more transparent and helps to reveal key events in the pathway, as well as areas of uncertainty. Furthermore, mapping helps to quantify events in the pathway for a few modeling techniques. Mapping the pathway is done routinely in PRA; for example, New Zealand uses this approach in its routine import risk analyses (MAF, 2006), even when the assessment is qualitative or semiquantitative.

The figure below provides an example of a pathway description. It depicts the pathway for the importation of fresh pear fruit from China into New Zealand. The visual map of the pathway focuses the assessment on key points where risk can be introduced (for example, through infestation by a pest) or where risk may be reduced (for example, by removal of infested fruit or treatment). This type of depiction is useful at both the risk assessment and the risk management stage.

Example map of potential fresh pear fruit pathway from China to New Zealand (excerpted from MAF, 2009)

Elements to Consider When Estimating Likelihood of Introduction

Now let’s look at defining the events that determine whether there is any likelihood of introduction and spread. ISPM No. 11 provides a detailed description of the elements that should be analyzed. It directs the analyst to examine the likelihood the pest(s) will:

• Be associated with pathways that could lead to introduction
• Survive existing pest management procedures
• Remain with the pathway at origin
• Escape detection (either pre- or postentry)
• Survive transit or movement to the PRA area
• Find a favorable location in the PRA area
• Find suitable host material in the PRA area
• Overcome biotic and abiotic resistance (find a suitable environment)
• Be able to reproduce and spread

Each of these elements must be considered when determining the likelihood of introduction.

Assessment of Likelihood

Recall that introduction includes both entry and establishment. Thus, we often see the assessment of likelihood broken down into those two components, where entry and establishment are analyzed separately and then combined. Some PRAs consider spread in connection with introduction; that is, the assessment covers entry, establishment, and spread together in that order. In other cases, spread is considered together with consequences, since degree of spread can directly influence the severity of consequences.

There are several different ways that entry, establishment, and spread are treated in PRA models. Some models consider these elements individually, through separate questions, other models consider them together, and still other models split these elements into sub-elements and address those separately.

In some cases, we may have quantitative data for this assessment. For instance, we may have data on the prevalence of the pest in a particular pathway at origin or its ability to survive a particular treatment. In these cases, we can assess likelihood of introduction using quantitative methods. More often than not, however, we lack specific data on pest prevalence or probabilities related to specific events. In these cases, we may opt to use a narrative approach or use a rating system.

Whatever method we decide to use, we must be clear about the model we are using. What events in the pathway(s) are we considering? What assumptions we are making in the assessment? What are the sources and level of uncertainty in our assessment? We must answer these questions in the assessment. We should also link our pathway description back to the model we are using to analyze risk.

Let’s return to our assessment of Pestis horribilis. After pest categorization, our next step is to estimate the probability that Pestis horribilis will cross Importland’s borders and become established. In other words, what are the chances that this pest will get into the country and become a permanent problem? For this assessment, you need more detailed information than you needed to categorize Pestis horribilis as a quarantine pest.

You must consider these three factors:

• Probability of entry
• Probability of establishment
• Probability of spread after establishment

Pest entry includes both introduction and establishment and requires the assessment of all possible pathways. When conducting pest-specific risk assessments such as this organism assessment for Pestis horribilis, you may need to assess multiple pathways.

1. Probability of entry

The probability of entry is based on the evaluation of the following factors for each pathway assessed:

• Probability of pest being associated with the pathway at origin
• Probability of pest surviving during transport and storage
• Probability of pest surviving existing pest management practices
• Probability of pest transfer to a suitable host

Click on the box below to explore further.

2. Probability of establishment

The probability of establishment is based on the following factors:

• Availability of suitable hosts, alternate hosts, and vectors in the PRA area
• Suitability of environment in the PRA area
• Cultural practices and control measures
• Biological characteristics of the pest affecting the probability of establishment

The probability of Pestis horribilis establishment in Importland is high. There are many suitable plant hosts for the pest, and the environment is similar enough to Exportland’s that it should thrive. Furthermore, if the pest cannot be controlled in the fields with pesticides or other management practices, it will be hard to eliminate once it has invaded.

3. Probability of spread after establishment

The best way to evaluate probability of spread is by comparing biological, environmental, and cultural information for an area where the pest is currently established with similar information for the PRA area. If there are parts of the PRA area that have similar characteristics as areas where the pest is already established, then those parts of the PRA area have the potential to be identified as endangered areas. The IPPC defines endangered area as “an area where ecological factors favour the establishment of a pest whose presence in the area will result in economically important loss” [FAO, 1995].

Consider these factors when evaluating the probability of spread after establishment:

• Suitability of the natural and/or managed environment for spread of the pest
• Presence of natural barriers
• Potential for movement with commodities or conveyances
• Intended use of the commodity
• Potential vectors for the pest in the PRA area
• Potential natural enemies of the pest in the PRA area

You already know that Importland’s environment is suitable for natural spread of Pestis horribilis. However, many other factors could either increase or decrease the probability of spread. For example, Pestis horribilis has a flight range of only 0.6 km, so diligent control of transported goods could greatly reduce the chance of spread. A natural method of control might exist. Perhaps Importland has a bird or other type of predator that is not present in Exportland. A resident insect might compete aggressively for food and other resources that Pestis horribilis needs to thrive. Consider all the possibilities you can think of before deciding how high the probability of spread is.

Conclusion of the Probability of Entry and Spread

You can express a conclusion using either a quantitative or qualitative method or a combination of both, depending on your data format and method of assessment. However, a detailed discussion of these methods is beyond the scope of this module.

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