Oak processionary caterpillars double this summer, triggering health complaints
Oak processionary caterpillars have doubled in number this summer compared with last year in the Netherlands, with every oak tree infested in some municipalities and causing health issues from their stinging hairs.
The caterpillars produce more than 700,000 stinging hairs each. Contact can result in itchy bumps on the skin with redness and sometimes pain. The hairs can also irritate the eyes, throat, and respiratory system. The complaints can last two to three weeks. Animals can suffer similar effects.
This year, the caterpillars appeared in 13% of more than 122,000 oaks examined, Nature Today reported. The Plaagdrukkaart infestation pressure map, as of July 14, 2026, shows large areas of heavy infestation, especially in the north. In the municipalities of Hoogeveen, De Bilt, and Montferland, all oaks are affected.
The rise comes after several quieter years following severe outbreaks in 2019 and 2020, when thick white nests covered trees across the country. Experts predicted the increase because three times as many butterflies were observed last summer, especially in Groningen, Drenthe and Overijssel. Those northern provinces are now heavily affected.
Heat and drought are key factors. The warm weather has caused powdery mildew on oaks, making leaves higher in the trees less suitable as food. As a result, caterpillars have left the trunks and created nests in the ground. Inventories in Drenthe, Overijssel and Flevoland showed relatively many caterpillars emerging from the soil in June. Some appear to have waited several years for favorable conditions.
“Any control measures taken earlier in the year have had no effect this way,” Nature Today wrote.
Summer storms with strong winds can blow nests out of trees, increasing the spread of stinging hairs. In Zevenaar, the presence of protected butterfly species made prevention more difficult, according to a municipal spokesperson.
A major outbreak in Germany recently led to the closure of parks and other public spaces, with authorities working to control the toxic caterpillars, CNN reported.
