Increasing numbers of oak processionary caterpillars in the Netherlands, biologist says

The number of oak processionary caterpillars in the Netherlands may be increasing again, according to biologist Arnold van Vliet of Wageningen University (WUR). In some places in the country, more nests have been counted in recent months than in previous years. The nuisance caused by the caterpillar does not seem to be greater this year than in previous years.

May, June, and July are the months in which the caterpillars live in nests on oak trees. Every now and then, they leave the nest in procession to eat. If they feel threatened, they release their stinging hairs, about 700,000 per caterpillar, which can cause severe itching in humans.

The number of nests seems to be increasing in some places, especially in the north. Van Vliet mentions Hardenberg in Overijssel as an example. 680 oaks were recently viewed there, and nests were seen in 512 of them. Conclusions about a possible increase in caterpillars can only be drawn once the butterfly count is complete. The caterpillars will turn into butterflies from this week onwards and fly out. The butterfly counts will be completed this autumn.

In 2019, there was a peak in the nuisance of oak processionary caterpillars in the Netherlands, Van Vliet explains. That was also when the Processionary Caterpillar Knowledge Platform was founded, of which he is a member. Since then, the caterpillar has been successfully combated, which has reduced the nuisance. But it is impossible to get rid of the caterpillar completely, says Van Vliet. According to him, the Netherlands offers "a fantastic living environment" for the insect. There are many oaks, while due to human intervention, the natural enemies of the caterpillar, such as other insects and birds, "have declined sharply." Moreover, there are limits to combating it because other caterpillars and insects are also affected by it.

According to Van Vliet, there is still "no indication" that the nuisance is higher this year than in previous years. The number of nests is not systematically recorded, but people can help map it.

Butterflies lay eggs in oak trees. They hatch in the spring. Some of the caterpillars do not pupate in the same year but crawl underground. In the peak year of 2019, for example, fewer butterflies were counted than expected. The caterpillars can remain underground for years, crawl into the tree, and form a nest. Combating the underground caterpillars is difficult, especially because finding out where they are all is difficult.