Assessment of 5 years of control by the Walloon Ambrosia Observatory

Monitoring ragweed in the Walloon Region (Belgium): The Walloon Ambrosia Observatory takes stock of 5 years of control

In 2020, in order to curb the spread of ragweed in the Walloon Region, the Belgian authorities funded the creation of the Walloon Ambrosia Observatory, run by the Biodiversity, Ecosystems and Landscapes unit of Gembloux Agro-Bio Tech (ULiège). At the time, this plant, still rare in the region, was hunted down relentlessly and was the subject of intensive communication. Five years on, it's time to take stock.

The case of ragweed in the Walloon region is atypical, given the species' emerging status at the time. By 2020, only 20 populations had been recorded in the entire region. To make up for this lack of data and improve monitoring, the Observatory relied on citizen participation through participatory science tools. Videos, training courses and a number of popular articles have been distributed to encourage people to report their observations.

The results are in: today, over 120 homes are monitored, with around 50 new sightings per year, spread over almost the entire Walloon Region (with the notable exception of the Upper Ardennes).

Although the number of sightings increases every year, the majority of populations recorded only involve a few dozen individuals. However, some populations have reached thousands of plants, indicating a worrying expansion. This progression has not prevented the Observatory from taking effective action: nearly 40 populations have been considered eradicated, i.e. no presence of the plant has been detected for three consecutive years. These results demonstrate that active, meticulous management can bear fruit.

The emergence of ragweed in Wallonia has also enabled us to identify its main pathways of introduction. The Observatory revealed that seed mixtures for birds and poultry are the main source of contamination. A study carried out in 2021 showed that a third of the products sold commercially contained ambrosia seeds, contributing to the contamination of the soil beneath bird feeders. Although European regulations aim to limit this problem, vigilance remains essential to prevent our gardens from becoming hotbeds of dispersal.

After five years of control, the results remain mixed: while some populations have been eradicated, the total number of outbreaks continues to rise. Citizen involvement and awareness campaigns remain crucial to contain this invasive plant before it becomes a major problem, as is already the case in several European countries.

Author : Adrien Delforge, Arnaud Monty (ULiège)