Based on this PRA, Ambrosia trifida was added to the EPPO A2 List of pests recommended for regulation as quarantine pests in 2019. Measures for A. trifida are recommended. 

Main conclusions 

Ambrosia trifida presents a high phytosanitary risk for the endangered area with low uncertainty. 

The likelihood of new introductions to the EPPO region occurring via contamination of seed (maize seed, soybean seed, spring crops (sunflower and sorghum) and contamination of grain (e.g. soybean and maize) is moderate and high respectively. 

Within the EPPO region, A. trifida seems to preferentially become established in crops and ruderal environments. It is found on riverbanks within the EPPO region and the likelihood of further establishment in natural habitats is considered high with a low uncertainty. It is also found in arable land and fallows, road networks, rail networks and domestic and non-domestic gardens. The likelihood of further establishment in the managed environment is considered very high with a low uncertainty. 

The potential magnitude of spread within the EPPO region is high with moderate uncertainty. A. trifida has both short and long-distance natural dispersal pathways. Human assisted spread facilitated by agriculture machinery and movement within the EPPO region as a contaminant of seed or grain can act to move seeds over long distances. 

The main impacts of the species at a global level (North America and the EPPO region) are the reduction of crop yields and human health impacts which are translated in the PRA into socio-economic impacts. The EWG consider the potential socio-economic impacts in the EPPO region very high with a moderate uncertainty. Potential impacts on biodiversity and ecosystem services are moderate with high uncertainty. The species has the potential to establish along rivers where it could compete with native vegetation and reduce access to water bodies for recreation. The high uncertainty reflects the lack of quantitative studies on the impact of the species in natural habitats. 

The species is particularly difficult to manage due to early and prolonged emergence and very rapid biomass growth. An established population is very difficult to control as the seeds of A. trifida can remain viable in soil for 4 to 21 years, depending on burial depth. Within the EPPO region, there is a lack of effective and economical control options, and chemical control options are becoming increasing restricted in Member countries in the EPPO region.

2021: The PRA was modified to clarify the phytosanitary measures recommended