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What can you do in the fight against the oak processionary caterpillar?

From glue boards to poison or aluminum foil, there are countless ways that are used to combat the caterpillar. But not all of them are allowed, says Kars Veling of the Butterfly Foundation.

For example, the Butterfly Foundation successfully objected to a trial with Vertimec: a means of injecting a tree and killing the caterpillar from the inside. Vermitec would kill not only caterpillars, but other animals as well. The drug was therefore banned.

Another means is Siltac. “They are testing it. The caterpillars get a layer of glue on them and cannot move anymore, but it also affects other insects. I don’t think Siltac will survive.”

Glue strips

And then there are the forbidden glue strips. You can use them indoors – for example in a greenhouse – but not outdoors. Unless you get an exemption from the province. “Such a strip is at the bottom of the tree, while oak processionary caterpillars are mainly at the top. But natural enemies, other caterpillars or even woodpeckers are also trapped by it.”

But what is allowed then? The best way to deal with the caterpillar is to simply cordon off areas with ribbons, Veling says. “People just have to walk around a bit, and you don’t have any collateral damage.”

But in places where that is not possible, the suction of nests is a good alternative, Veling explains. “People with suits suck the nests from the tree with a kind of vacuum cleaner. Then you really only remove the nests from the oak processionary caterpillar.” Nests are picked in inaccessible places, writes the Knowledge Center Oak Processionary Caterpillar on its website.

According to the Knowledge Center, there is also a method in which nests are burned with a burner. Accessible, but the method can spread hair and is therefore strongly discouraged. A better idea is to place bird nesting boxes or to plant flutes, which attract natural enemies of the caterpillar.


Finally, municipalities can choose to spray so-called bacterial preparations or nematodes on trees. They crawl into the caterpillar and kill it from the inside. A biological pesticide that is much cheaper than the labor-intensive suction, and that is allowed, says Veling.

But as far as he is concerned, that means is used on a much too large scale. “It is really a wild west of municipalities that determine what they do and what not. In Utrecht there are a lot of oaks, but the product is occasionally used, while in Epe almost all trees are sprayed.”

And that is not without consequences. “This means kills all kinds of caterpillars. As a result, birds have too little food, and they also die.”

That is why the Butterfly Foundation sent a letter to Minister Carola Schouten asking for regulations. “We want the use to be limited so that it only happens in places where there really is no other option.”

How is caterpillar control in the Netherlands arranged?

In the Netherlands, the owner of a tree decides how to handle the oak processionary caterpillar. In most cases this is the municipality.

There are two ways to control the caterpillar. Preventive, if the caterpillar has no stinging hairs, or curative, if it is and the stinging hairs have to be made harmless. In both ways, there are all kinds of resources on the market that have to comply with different rules.

Admission resources

Before such a product is put on the market, the Board for the Authorization of Plant Protection Products and Biocides (Ctgb) assesses its effectiveness and safety. Both chemical and biological agents may only be used if the Ctgb has approved them.

One requirement is clear: all means must not be harmful to protected animals and plants. In some areas, therefore, certain resources should not be used.

Source: Oak Processionary Caterpillar Knowledge Center


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