Several municipalities in the Netherlands have issued warnings for residents to be on the lookout for oak processionary caterpillars as their numbers start to increase again.
Oak processionary caterpillars make a comeback in the Netherlands
Back in 2019, there was an explosion of oak processionary caterpillars in the Netherlands, with more than half of all oak trees in the Netherlands covered in the critters. The toxic caterpillars can cause serious health issues in both pets and humans. Their stinging hairs can cause skin rashes, breathing problems and severe allergic reactions, among other things.
Over the years, oak processionary caterpillar numbers have died down, with 2024 seeing the lowest levels - the arthropods were found in just 4,5% of 135.000 trees inventoried. However, their numbers have increased this year: after inspecting 60.000 trees, the caterpillars were found in 6% of them, including trees where preventive measures have previously been implemented. This means the numbers could be higher than what the first inspection suggests.
According to Nature Today, the numbers could continue to rise in the coming years. Experts believe the reason for the increase could be that caterpillars in ground nests have become active after a dormant period for several years. Other possibilities are that the bad weather last year meant that the moths couldn’t fly far after fledging and deposited their eggs closer together, or that the oak trees have been thriving, providing good food conditions for caterpillar development.
Dutch municipalities warn residents about pesky caterpillars
There are certain hotspots where oak processionary caterpillars are more likely to be found in the majority of oak trees this year, such as Almere, Steenwijkerland, Ede and Nieuwegein. Several of these municipalities and others are asking residents to report any nests they see so they can be removed, either by suction or burning.
Residents are advised not to remove nests themselves as this could cause the stinging hairs to spread through the wind or cause health issues by accidental contact. Municipalities are also encouraging people to put up bird boxes or make their gardens attractive to birds and other insects which eat the caterpillars.
If you come into contact with hairs from the oak processionary caterpillar and develop a rash, it can be treated with an antihistamine cream. However, if you develop anything serious, like difficulty breathing or a more severe allergic reaction, contact your GP or nearest hospital.