April fools day marks the beginning of the new Oak Processionary Moth season

This year the first oak processionary caterpillars hatched from their eggs on April 1st  in the Netherlands, and this is no joke! Even mentioned by the newspaper Parool, the oak processionary caterpillar is now considered being one of the country’s biggest summer tormentors.  Is that justified or do we program each other to have a compulsive dislike for this particularly interesting insect. Here is an outlook for the coming season of Oak Processionary Moth.

Egg hatching

For many years now, I have been hosting an insect station in Mill, North Brabant. I use it to monitor the activity of insects that may cause nuisance in public spaces and which may require special management actions.  On April 1 2021, the first egg batches of the oak processionary caterpillars hatched in Mill. Egg hatching was also simultaneously reported in Sittard and Hengelo. Likewise,  in Belgium the first egg hatches were reported in the provinces of Limburg and Antwerp. Each year, the hatching of the egg batches is predicted based on historical data and outdoor temperature. The weather forecast  in the month of March is normally the  most decisive factor. The unforeseen high temperatures on March 31 this year, with temperatures as high as 26˚C, caused the caterpillars to hatch earlier than April 10, which was the predicted egg hatch date. Not all caterpillars hatched on April 1 though, this will still take some time depending on the weather conditions. In fact, the shift into colder temperatures from April 2 onwards results in a gradual spread of the egg hatching activity because the colder weather slows down the development of the caterpillar. Only 10% of the eggs hatched by April 1 in the insect station in Mill. After a cold period in April the remaining egg plaques hatched between April 14th and 17th. This means that the initial egg hatching prediction date was heavily influenced by unforeseen weather conditions.

It has been widely proved that frost does not affect survival success of these caterpillars, and now we see that warmth might activate their development. Due to the few but yet surprisingly warm days this year, the first oak processionary caterpillars have hatched early. However, this was not the earliest record, which so far was on March 30, 2014.  Last year, 2020, the first caterpillars hatched on April 6.

After hatching, the caterpillars are still very small, about 2 to 3 mm, and have a strong orange pigmentation, including black hairs and a noticeable black head. They nourish themselves by grazing the buds of oak trees, usually being the leading bud of a twig the main target. At this stage, the caterpillars are a good food source for different predators, especially other insects but also birds.

At this point caterpillars do not yet have urticating hairs, these develop during the third larval stage. During the fourth larval stage, the number of urticating hairs sharply increases and the first visible nest formation occurs. This is also when the first nuisance often arises for both people and animals, which I certainly do not want to underestimate. Based on the long-term weather forecast for April, the fourth larval stage is expected to be reached by fourth week of May. Since the temperature fluctuations play a decisive role, other dates might also be plausible.

Data-based management

For any kind of management procedure, it is important to first determine whether any intervention in needed. Two factors are decisive in this case; the extent to which a population is successful in developing over time and the degree of risk at which this can lead to nuisance. This is the key to identify whether we are dealing with a plague or not. For this you need a picture of the local pest pressure, but certainly also of the local risks. This needs customization, but only successful if both the affected trees and the degree of damage have been registered. Oak procession caterpillars have been actively combated in the Netherlands since 1992, and since 2000 there are more and more digital registration systems that make it possible to record the information in an ordered manner. I find it disappointing to note that after so many years of oak processionary caterpillars and registration options, the majority of tree managers in the Netherlands still do not record or do not know the extent of the pest pressure in their own area. What is their management based on? Is it gut feeling, available budget or number of nuisance reports? In my opinion, these are not sufficient arguments to build up a management system. The number of nuisance reports may reflect the degree of nuisance, but they also say something about the willingness of the population to report. Therefore it does not necessarily say anything reliable about the actual pest pressure.


For a number of years now, at Terra Nostra we have been working with our own developed EPR registration and management system. We do this using GIS systems as much as possible. This way we can display the various risk zones in accordance with the Dutch Guideline for the control oak processionary caterpillars, including protected butterfly areas, locations of bird nests and bat boxes and other ecological management measures. Besides, all management-related information can also be included here. This involves data on spraying locations and include spraying reports, nuisance reports from citizens, registration of affected trees during inspections, pest control activities using associated pest pressure indicators and monitoring of oak processionary moths. The entire process can be followed in real time and can also be publicly available. This makes it a very powerful tool with which we can motivate future management based on facts. This gives the citizen, but also the press, the opportunity to follow what is really happening. In fact, it is often seen that complaints and reports decrease in places where communication is open and transparent.

The biggest advantage of such oak processionary registration and management system is that, by making data a structural part of your management, you can achieve the most environmental benefits with it. You have the option to set out specific management measures, based on exact pest pressure and precise risks. And to be clear, one measure can also be the acceptance of nuisance, which is in fact a sort of management.

Management methods

The oak processionary season starts when the eggs hatch. Preventive control with nematodes can start once the night temperature is above 4OC.  Nematodes have a contact effect and this must be in close contact with the caterpillars in order to penetrate them. The spraying of nematodes must be repeated at least once. The agent is not selective and has effects on soft-skinned insects/larvae. The advantages are that you can start spraying early in the season when only few other insects are present and that the nematodes only stay alive for 3 hours. Therefore only limited negative effects on non-target organisms can be expected. A 85% reduction of affected trees is usually achieved, although this can fluctuate when pest pressure is high.

From the moment that trees have developed at least 40% of their foliage, preventive control with the bacterial preparation Baccillus thuringiensis "Aizawai", known under the product name XenTari, or Baccillus thuringiensis "Kurstaki" known under the product name Dipel can be used. This bacterial cocktail must be ingested by the caterpillar and is lethal. The agent is completely broken down by UV radiation within 1 to 7 days. A big advantage is that the agent is very selective because it only affects caterpillars of the Lepidoptera family. Not that I do not care about those other insects in the Lepidoptera family, but I do want to prevent and disprove the saying that "everything dies with it." That is precisely the easy but also incorrect argument that we regularly have to deal with when organizations representing nature’s interests jump into the discussion. Once again, this is written by someone who substantiates his advice on the basis of facts and data analysis, and who aims to get the greatest environmental benefits. I still propose that we should generate support from the population to accept nuisance. As long as residents overload municipalities with complaints, they will act within their regulated frameworks and try to limit nuisance as much as possible. This is the so called “effect of the citizen”.

As soon as it is no longer possible to spray or that there is insufficient reason to spray, nests that potentially may cause nuisance can be hoovered away. Both the protection of the workers and the filter packages on the vacuum cleaning equipment are highly important when vacuuming. In cases where vacuum removal is not possible, a nest can be manually removed  with the help of adhesive products.  For specifications of these methods, see the Dutch Guideline for the control oak processionary caterpillars.  

Cold water fear with natural predators

We should all work towards the development of natural management systems. As an example, the oak procession caterpillar researcher and entomologist Silvia Hellingman carried out with exceptional patience the project Stichting Boermarke in Westerveld. The use of natural predators has been developed and monitored here with untameable energy and monks work. This project was completed in 2019 with positive results, but one may notice that we are struggling with cold water fear because of working in uncharted territories. The different pilot projects in the country are still in their starting phase. A study by WUR students shows that the uncertainty about results, the yet unknown management costs and the expected time span of visible effects are the most determining factors that hold back tree managers for choosing natural management options.

The use of natural enemies is custom work that specifically need to be developed in each specific biotope/plant community. I advise everyone to work with this in places where an initial, possibly limited result, does not immediately lead to inconvenience. Including the landscape design of the working area info the bigger picture is also an asset. In my opinion, the need to convert oak trees is already necessary if 20% of a tree population consists of the oak genus. Also remember that planting a hawthorn is at least as effective as installing a great tit box. This also applies to maintaining old oak processionary caterpillar nests, which contain many natural predators that are protected by letting them hang in the trees. There are of course conditions attached to prevent unintentional nuisance in such cases.


The OakShield project from Maastricht University is among the most innovative development projects in the field. They make use of the RNA silencing effects within the E. coli bacterium for switching off a certain set of essential genes within the oak processionary caterpillar as soon as they ingest the agent containing modified E. coli.  The principle is the same as with the use of Bacillus thuringiensis, but with the big difference that the modified E.coli does not kill other caterpillars. The product is currently still under development, but does offer a hopeful perspective with regard to selectivity. However, playing around with genes is a process that leaves a lot of open questions and which leads to stricter acceptance rules for pesticide use under European regulations.

To my surprise, there is a company that sells some kind of foam on the internet for injecting nests under the same “OakShield”. I write "some kind" because: 1) the composition is a kind of soap, and 2) we do not know at all which organization is behind this, there is not even a telephone number to be found. The claim is: "With this approach the caterpillars and their urticating hairs are encapsulated in an environmentally friendly, biological foam cocoon". However, I completely miss the reasoning of the working mechanism behind the product and how it has been determined what exactly this does to caterpillars and stinging hairs. I will not worry too much about the anonymity of the organization in this matter, but unfortunately the product name corresponds completely with the aforementioned project name of Maastricht University. These kind of organisations with only commercial interest make me angry: Shame on you!

Gyro Gearloose

It is hard to keep up with the different innovations in the field of oak processionary moth control. An insane number of methods have been introduced to control nests since 2019, the year that almost all of the Dutch population suffered from itchiness. Let’s not mention any product names because the ‘gentlemen’ of a given catch trick with plastic bags could just threaten me again with another lawyer. Besides, where do these "friendly" innovators end up? Have they been so smart that they have made millions out of it? Until now, scientific justification or at least a good practice registration with promising results are almost always missing from any Gyro Gearloose solution. However, the claims are chosen very cautiously, such as “urticating hairs are encapsulated or broken down”, and all of this provided with a few popular catchy concepts such as “environmentally friendly” and “sustainable” to convince the friendliness of the product and the social commitment of the underlying organization. You can find a list with the reliable assessment of various Gyro Gearloose innovations in the information sheets on the website of the Processionary Moth Platform.

I am not against innovation at all, I think there are interesting products here and there. But dear innovators, please go into the market well-founded and work with pest control companies before introducing something. Currently more harm is done than good, and this just radiates badly on the whole sector.

Still, the most challenging development focuses on open communication and data management of the oak processionary caterpillar. It will become more and more  important to switch to stimulating natural predators and gradually transforming tree stands where oaks are dominant.

For now, enjoy nature, don't take any news headline too seriously and remember that acceptance makes life a lot easier.

This website uses cookies for optimal functioning. OK Allow Refuse For more information, read our privacy statement privacy Cookie settings This field is not filled in The entered text is too short The entered text is too long