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The number of oak processionary caterpillars in the Netherlands is on the rise again. More nuisance is expected from the critters this year, so residents should be on the lookout for nests.
Peak oak processionary caterpillar season begins
This year, thanks to warmer-than-usual weather, the first oak processionary caterpillars appeared earlier than ever before, hatching from their eggs in March. Now they have entered their fourth larval stage, where each caterpillar develops about 700.000 stinging hairs, which can cause health issues in both pets and humans.
Their numbers were higher last year than in previous years, and are expected to increase even more this year. At the end of October 2025, a threefold increase in the number of oak processionary moths was measured, reports Nature Today, giving us an indication of what to expect for this season.
The number of nests counted so far also indicates a population surge. Not only are there many nests high up in trees where moths lay eggs, but lower down on tree trunks as well.
"Oak processionary caterpillars can be in an extended dormancy in the ground," biologist Arnold van Vliet explains to RTL Nieuws. "They can remain there for several years, until conditions are favourable enough and they crawl out." These lower nests weren’t taken into account in the initial counts, so there are likely even higher numbers than first assumed.
Monitoring of pesky caterpillars in Dutch cities
To control the population of oak processionary caterpillars, municipalities need to get an accurate estimation of how many there are. The Oak Processionary Moth Knowledge Centre tracks their numbers with the help of volunteers in hotspots.
There are some areas where the caterpillars are more likely to be found in the majority of oak trees, such as Almere, Steenwijkerland, Ede and Nieuwegein. Residents are advised to report any sightings of nests and are urged not to remove nests themselves as this could cause stinging hairs to be carried by the wind or cause health issues by accidental contact.
Stinging hairs from the toxic caterpillars can cause skin rashes, breathing problems and severe allergic reactions. By mid-July, the caterpillars should transform into moths and the nuisance should lessen.
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