Biocide use in the context of combating oak processionary caterpillars at a historic low
Significantly fewer biocides used in the management of the oak processionary caterpillar in the Netherlands and Flanders
Since the start of our project, never before have so few biocides been used in the management of the oak processionary moth in the Low Countries as this year. We can deduce this from the data we collect annually from our ambassador municipalities.
In the management of oak processionary caterpillars, the main methods used are biological pesticides or biocides based on the bacterium Bacillus thuringiensis. This bacterium attacks the digestive system of the caterpillar, causing it to starve and die. The product is sprayed preventively over the trees in the spring.
Since 2021, around thirty ambassador municipalities from the three Dutch and two Flemish provinces of the project area have been providing us with an annual overview of the preventive and curative actions they have taken in managing the oak processionary caterpillar. This allows us to estimate how biocide use has evolved over the years.
While biocide use in the ambassador municipalities at the start of the project was still around 34 kg of active substance, it has dropped to just 4 kg in 2024, a reduction of almost 90%. These results are confirmed by research by the Vlaamse Milieumaatschappij (VMM) on all Flemish municipalities, in which the amount of active substance has dropped by 85% from 2018 to 2023. Unfortunately, these data are not available for the Netherlands.
We see two important reasons for this decrease. The reduced presence of oak processionary caterpillars in our region certainly plays an important role, but undoubtedly the increased attention for the consequences of chemical and biological pesticides on biodiversity, partly due to our project, may also claim a share in this.
Positive news for biodiversity
The biocides used are unfortunately not species-specific – they do not only kill oak processionary caterpillars, but all oak-dwelling caterpillars and butterflies. This also includes harmless butterflies such as the purple hairstreak and the rare ilex hairstreak. We therefore expect this reduction to have a positive effect on the biodiversity of animal life in our summer oaks.
