Control of oak processionary caterpillar

The oak processionary caterpillar has been active in the Netherlands for quite a few years. You can find hairy caterpillars on oak trees in May, June and July. After contact with the hairs of this caterpillar, complaints may arise such as itching, skin rashes, irritation to the eyes or respiratory tract. 

Our approach consists of 3 components:

1. Ecological control (preventive, to avoid caterpillars).

We try to prevent nuisance by encouraging citizen initiatives to hang nesting boxes on our municipal trees; the municipality itself does not hang nesting boxes. In addition, we mow the roadsides less frequently (according to the color-coded model released by the Butterfly Foundation and Groenkeur), thus preserving flower-rich roadsides and making them more attractive to the natural enemy of the caterpillar. Among other things, cabbage and blue tits, parasitic wasps and lacewings that we lure in this way eat the caterpillars. Our contractor and its employees who manage the roadsides are trained to work according to the color-coded model.

In the coming years, we will continue to develop and monitor this ecological method. The aim is to improve the overall biodiversity in the municipality. We will do this by constructing (in cooperation with the IVN) bee and butterfly gardens at various locations. This in turn also stimulates natural enemies of the oak processionary caterpillar.

2. Control with pesticide (preventive, to prevent caterpillars)

Starting in mid-April, when the first leaves appear on the trees and the first caterpillars appear, we spray several hundred trees at so-called "hot spots" with biological control agent XenTari. This preventive spraying is possible until the caterpillars develop fire hairs (mid-May).

Many municipalities use this pesticide when controlling the Oak Processionary Moth caterpillar. Although this is a biological agent, it can also be harmful to other caterpillars (no better alternative is currently available). This depends on the larval stage the caterpillars are in. Therefore, we are reluctant to use this pesticide and only spray the trees in areas where the expected nuisance is greatest.

Because the nuisance was so great in 2018 and 2019, we chose to preventively mist all oak trees in the built-up area (about 8,800 trees) in 2020 and 2021.

3. Sucking away nests

In places where the caterpillar nests are a nuisance, we vacuum out the nests. In principle, these nests occur only on trees that the contractor has not treated preventively. Basically, we do this in built-up areas on all municipal trees, with the exception of trees in forests.

Approach in the future

We are updated twice a year by the Province and the Public Health Service on the most successful methods in controlling the oak processionary caterpillar and learn from other municipalities' approaches. We also follow the good examples through all the trade journals. The most optimal control has not yet been determined and many 100% poison-free methods do not yet have sufficient effect. If there were a 100% poison-free and ecologically friendly method that could also eliminate the worst nuisance, we would choose it.

Frequently asked questions and answers

Which nests does the municipality remove?

In principle, the municipality removes all nests in municipal trees within built-up areas, with the exception of:

  • nests in the forests
  • nests in places where equipment cannot reach (too high, or inaccessible)
  • places where occasional people come

When extreme nuisances occur, we make choices and concentrate on the places where many people come.
 

Can you give a schedule of when you will remove the litters?

Giving a schedule is difficult. In the period starting in May, we receive many reports and we have to continuously make choices and set new priorities.
 

Who removes the nests?

The municipality is having the nests sucked out by green space company Herman Vaessen. The people doing this work are trained for this purpose and use only certified equipment and protective devices.
 

What is the municipality doing to solve the long-term oak processionary caterpillar problem?

We make the habitat attractive to the caterpillar's natural enemies. Natural enemies are cabbage and blue tits, parasitic wasps and lacewings. They eat the caterpillars. To attract these enemies, we mow the verges according to the Color Label model released by the Butterfly Foundation and Groenkeur.

In addition, as a municipality we are trying to increase overall biodiversity; we have also set this as a goal in the 2020-2026 Green Management Plan. One way we do this is by planting planting species that are good for biodiversity. For the future, we also want to create a diverse tree population. At various locations in the municipality we are also laying out Bee and Butterfly Gardens in cooperation with the IVN to encourage biodiversity.

As an individual, what can I do to control the oak processionary caterpillar long-term?

As an individual, you can hang your own suet boxes in your own garden. And you can sow flower and herb mixtures in your own garden. There is no single list of the best species yet. Most suppliers have created their own mixtures. These are easy to find online.
 

Is preventive misting harmful to humans and animals?

We use the biological pesticide XenTari. The pesticide is safe for humans, pets, squirrels, birds, fish (in case the pesticide gets into surface water) and bees. However, the pesticide can be harmful to other caterpillars. This depends on the larval stage the caterpillars are in. Currently, this product is the least harmful product on the market. Should other agents be available in the future we will certainly prefer this one.