Plants or parts of plants (pathway vector)

International Common Names

• English: Bonsais; Cut flowers; Cut flowers/cuttings; Fruits; Leaves; Plants for planting/nursery stock; Seeds; Stems; Timber/logs/fresh wood; Vegetables
• Spanish: Plantas y partes des plantas
• French: Plantes ou parties de plantes

Local Common Names

• Germany: Pflanzen und Pflanzenteile

Plants and parts of plants is a very large concept, and is the most important pathway of introduction for alien plant pests and diseases. It includes over 90% of all introductions of plant pests, which are themselves the most commonly introduced alien organisms. Most herbivorous insects, mites, nematodes and plant diseases are introduced via the trade of their host plant. The commodity involved in the introduction can be fruits, vegetables, seeds, plants for planting, cut flowers, logs, etc. In most cases, pests and diseases also use their host plant as the main means of dispersal to spread within the invaded region. The plant trade can also carry other invasive organisms such as other plants, which can be introduced as seed contaminants, or non-herbivorous invertebrates such as predators or parasitoids.

Summary of organism types or species introduced

By far the most abundant organisms introduced with a plant or parts of plant are invertebrates and pathogens using this plant as a food resource or habitat. In particular, most plant pests (insects, mites, nematodes, etc.) and diseases (fungi, bacteria, viruses or viroids) have been introduced on their host plant. These are usually transported by cargo, on agricultural and horticultural commodities such as fruits, vegetables or ornamentals (McCullough et al., 2006; Kenis et al., 2007), or in baggage of individual travelers (Liebhold et al., 2006). In Europe, during the period 1995-2004, the most intercepted plant pests were the tobacco whitefly Bemisia tabaci (1502 interceptions; 22% of all insect interceptions), the leaf mining agromyzid fly Liriomyza huidobrensis (658; 10%), the cotton bollworm Helicoverpa armigera (447; 7%) and the western flower thrips Frankliniella occidentalis (222; 3%) (Roques and Auger-Rozenberg, 2006). These four species are all polyphagous greenhouse pests in Europe and are frequently carried on a variety of vegetables and ornamental plants. In general, insects are more frequently intercepted than other plant pests, mainly because they are more easily detectable. Among the non-indigenous organisms intercepted by USDA APHIS and recorded in the PIN database (1984-2000), 77.5% were insects, 13.1% plant pathogens, 0.8% mites and 0.1% nematodes. Over 95% were intercepted on plants and parts of plants (McCullough et al., 2006). Among plant pathogens, the most common genera were the fungi Cercospora sp. and Elsinoe sp., and the bacterium Xanthomonas sp.

Other kinds or organisms can travel as stowaways on traded plants or plant material, for example seeds of alien plants, non-herbivorous invertebrates or even small vertebrates. Although this kind of introduction is rather marginal compared to plant pests and diseases, they may sometimes have major consequences, such as the Asian tiger mosquito, Aedes albopictus, a vector of serious human and animal diseases found traveling inside bags watering lucky bamboos in Europe (Roques et al., 2009). 

Principle processes

In general, plants and parts of plants are transported in large quantities for commercial purpose, by plane, boat, road or train, mainly as food products (vegetables, fruits, grains, etc.), ornamentals (plants for planting, cut flowers, bonsais, aquarium plants, etc) or unprocessed wood (logs). Plants and plant parts, in particular fruits, vegetables, seeds and cut flowers are also frequently carried in baggage by individual travelers (Liebhold et al., 2006). Forest and orchard trees can theoretically represent another pathway of introduction but rather at short distances through the seedling trade. Between continents, trees are usually transported as seeds, that limits but does not entirely eliminate the chance of introducing plant pests and diseases. For example, it is suspected that the Douglas-fir seed chalcid, Megastigumus spermothrophus, a serious pest in European seed orchards, was introduced in seeds from North America (Mailleux et al., 2008).

Geographical routes and corridors

There is no particular route and corridors for this pathway since all regions in the world exchange plant products. There are, however, important variations over time in the respective importance of these routes. For example, it is clear that importations of Asian ornamentals or tropical fruits from the tropics to Europe or North America are increasing more rapidly than other imports. Some commodities and their related pests are strongly associated with specific exporting countries. In Europe, 96.7% of the alien insects intercepted on imported bonsai trees came from China, whereas 47.9% of the insects intercepted on fresh wood and bark came from Russia (Kenis et al., 2007).

Human-mediated history

The food trade has a very long history. Grains and seeds have been transported over long distances for centuries. The transport of fresh fruits, vegetables and timber has expanded since the late 1800s and early 1900s, when faster means of transport were developed. In contrast, the ornamental trade is rather recent and is presently developing very rapidly in all continents.

Decision support systems for selecting the most appropriate risk management measures against a quarantine plant pest are usually included in PRA schemes. Phytosanitary measures against alien plant pests and diseases can be taken both in the exporting and in the importing countries. These measures largely vary according to the commodity and the related pests and cannot be described in details here. As usual, prevention is better than remediation, particularly for plant pests and diseases which, in contrast to many other invasive organisms, usually enter a new region accidentally and are often detected too late to be eradicated. Thus, every effort should be made to prevent the introduction and establishment of alien plant pests and diseases. Plants and parts of plants usually travel accompanied by a phytosanitary certificate. However, it must be said that, according to the International Plant Protection Convention (IPPC) and the agreement on the Application of Sanitary and Phytosanitary Measures (SPS agreement) of the World Trade Organisation (WTO), countries may require phytosanitary measures only for quarantine pests and regulated non-quarantined pests. These measures must be based on scientific principles and cannot be maintained without sufficient scientific evidences. The technical justification for phytosanitary regulations is usually provided by a Pest Risk Analysis (PRA), following the IIPC standard ISPM 11 (FAO, 2004). PRAs comprise mainly a risk assessment part, which assesses the likelihood of an organism of being introduced, establishing and spreading in the PRA region, and the potential impacts, and if needed, a risk management part which states which measures to take to manage the risk. PRAs are thus an essential component of an effective plant health programme, despite important weaknesses (Baker et al., 2009). One of the weaknesses of the classical species-initiated PRA approach is that many species are of minor importance or even unknown in their area of origin, becoming serious pests only when invading new areas. Therefore, international or regional phytosanitary measures are being developed for entire pathways, such as logs or wood packaging, which need to be free from bark and fumigated or heat-treated, in the hope that measures taken to prevent the introduction of well-known pests will also be efficient against other pests. Another example is the implementation of more stringent regulations on imported bonsais from China, following the accidental introduction of several tree pests such as the citrus longhorned beetle Anoplophora chinensis (see USDA APHIS, 2004, for the USA).

The most stringent phytosanitary measure is prohibition, which is usually the last resort when no other measure can prevent the introduction of serious pests. This is for example, the case for the international trade of potato tubers, particularly from South America, which is usually prohibited because of the large amount of pests of potatoes that may be introduced through this pathway (see OEPP/EPPO, 2004, for imports to Europe). In many cases, however, to encourage free trade, the tendency is to move away from one strict quarantine measure and to replace it by a combination of measures in a systems approach, as recommended by ISPM 14 (FAO, 2002). A systems approach requires that the place of production and exportation should develop and implement approved integrated pest management practices, such as pre-export treatments, preventive measures to ensure cleanliness of growing media associated with plants, sanitary practices for waste management, effective diagnostic procedures, inspection at growing sites and clean packing practices. When necessary, propagative material should be obtained from certified stock sources. A good example is the standard (ISPM 26) for systems approaches in the management of fruit flies that has recently been agreed at international level (FAO, 2006b).

Importing countries can lower the likelihood of establishment and subsequent spread of alien plan pests by improving inspection and developing pre-certification programmes and appropriate quarantine procedures, such as treatments, testing, quarantine periods, etc. In most countries, border inspections are still neither very efficient or effective. For example, in the USA it is estimated that inspectors inspect no more than 2% of cargo arriving at maritime ports, airports and border crossing (Work et al., 2005). Nevertheless, border inspections are very useful for their dissuasive effect.

When an alien plant pest or disease is detected in a new region, various options may be considered. If the pest is detected at a very early stage of establishment, an eradication programme can be implemented. The success of eradication programmes largely depends on the target species, the environment, the level of establishment and the eradication method utilized. ISPM 9 (FAO, 1998) provides general guidelines for pest eradication programmes. If an outbreak cannot be eradicated, containment measures can be taken, which aims to restrict the pest to a defined locations through the use of operational procedures, physical barriers and facility design. Although many national and regional plant protection organisations have developed directives describing measures to be taken following outbreaks of some well known quarantine pests, in general there is no generic decision support scheme to help guide eradication or containment actions for all quarantine pest. However, such a decision support scheme is currently being developed for Europe by an EU research project, PRATIQUE (Baker et al., 2009).