The processionary caterpillar is not an invasive species. It is native to the coniferous forests of south-eastern France.
Thanks to a milder climate, it gradually spread inexorably towards the north. In Yvelines, 70% of the 259 municipalities are affected, and one in five at level 1. The only authorized insecticide, BTK, requires closing the site to the public for one or even several days, and is also very controlled because it is harmful to other species.