Common oaks are the most highly valued trees; caterpillars are rarely seen on other species of oak (e.g. American oak, pin oak). When there is a shortage of food, the caterpillars may be triggered to use other species of tree such as beech, birch, black cherry, etc.

The spread expands every year to the north and in Flanders towards the coast. Significant numbers of moths have been spotted this year in Wallonia too. The spread of the moths is also expanding in the Netherlands.

We are not sure. At each larval stage, there are different natural enemies, each with their specific conditions. That is one of the issues we will examine.

No, you will always have to make considerations within the context of public health. Evaluation of problem locations and monitoring of the OPC is the initial requirement. In some cases, the nests will have to be removed via suction and other pest-control methods will still be required.

Yes, but only in the last larval stage and that is short, so the earwig is less likely to be effective.

In the trials, we will try to take environmental factors into account. We will assess whether ‘distance to a farm’ has to be considered.

There is a collaboration with people from the LIFE PISA project, which is examining the control of the pine processionary. We will share our knowledge and experiences and there may be a new project concerning the pine processionary moth in our region in the future.

That’s right! Our landscape is a little one-sided, with many oak trees. The application of the 10-20-30 rule will allow us to progress in terms of diversity. However, governments must make choices and find the courage to transform our existing road structure. For the time being, that is simply not enough.

Very little is currently known about ground nests. We are closely monitoring research into this topic. 

Unfortunately, they are well-equipped to survive night-time frost. There is a type of anti-freeze in the eggs.

In our communications, we will certainly refer to the fact that we wish to control the problems that are caused by the OPC but that the species has a place in the food chain and that we are not trying to eliminate it.

Good idea. Enticing scents and odours may not be used in pest-control. However, research into odour trails is an interesting area for future studies.

The cuckoo is, indeed, a natural enemy. Its precise contribution to reducing the caterpillars is, as yet, unknown.

That is right. The structure of the website around these three techniques is based on the project actions whereby we will begin by investigating each method separately. We see this as the first step. We first need to see what works and how. Then, integrated application could be used.

We will only know in March, April and May 2021. We cannot yet make any forecasts. This also applies to capture using pheromone traps.


Biocides and pesticides are not the same; the overarching term for both is ‘pest control agents’. Pesticides are pest-control agents for agricultural use (i.e. crop protection agents or co-called phyto-pharmaceutical products; biocides are pest-control agents for non-agricultural use (i.e. disinfectants, preservatives, products for controlling infestations). Biocides and pesticides (crop protection agents) must be approved by the government in order to be traded on the market (in BE: FPS Public Health, in NL: Dutch Board for the Authorisation of Plant Protection Products and Biocides (Ctgb)).

See definitions in art. 3 (1) a) , b) and c), of the European Biocide Regulation no. 528/2012 of 22 May 2012: Biocides are substances or mixtures that: contain or generate one or more active substance(s); and which are intended to destroy, repel, neutralise or prevent hazardous or undesirable organisms ranging from bacteria and viruses to fungi or rats. BT falls under Biocides for Biological pest-control. In the Netherlands, the Ctgb is responsible for its approval. The Ctgb applies conditions for usage for each product and/or brand name.

A biocide must comprise one or more active substances; an active substance is a substance or a micro-organism that is effective against hazardous organisms; a micro-organism is a microbiological entity, cellular or non-cellular, that is able to replicate or transfer genetic materials, including lower fungi, viruses, bacteria, yeast mould, algae, protozoa and microscopic parasitic worms. To the extent that a bacterial preparation comprises such micro-organisms, and that nematodes are microscopic parasitic worms, and that they are intended to control a damaging organism, they must be regarded as a biocide.

These products fall under biocide regulations and the project’s aim is to reduce the use of these very substances.

Pheromone traps are biocides and, as such, permits are required (fall under biocide product category 19: lures – see Annex V of the European biocide regulation). The partners in Sittard-Geleen (NL) have been using pheromone traps for over 10 years but they do not seem to be especially suitable for the oak processionary moth. In their usual life cycle, in 95% of cases, mating will have already taken place before the moths fall into the traps. So, the traps are not effective for this type of processionary.

Research into the side-effects of biocides falls outside the scope of this project. We know that BT (which is found in Xentari) has no consequences for birds and bats, etc. Existing research has shown this quite clearly.


Bats eat nocturnal moths such as the oak processionary. This LIFE project does not include this aspect, however. We will examine whether we can work with this outside the project.

The LIFE project will scientifically monitor 360 nesting boxes. If nesting boxes are installed according to the trial guidelines for the LIFE project at other locations and these can also be monitored, we will integrate this data into our project dataset. In the province of Antwerp, this monitoring will take place with the help of work-placement students. Other nesting boxes will not be included in our analyses but we are, of course, interested in experiences and findings and will certainly support this type of activity.

The nesting boxes are dry-cleaned. All residues are removed and scraped off using a knife. There is a more detailed explanation in the flyer (also under ‘Outputs and Reports’).

Interesting question but, unfortunately, this will not be examined within the LIFE project.

Tits hunt for nesting boxes in January and February. Prior to the winter, however, they seek out a suitable resting place which then usually becomes their nesting place too. That is why it is best to install the nesting box between October and December.

We use 3 mm thick woven nylon thread (paracord). This does not break and does not have the same disadvantages as iron wire. If you use a special knot, the loop can also be enlarged each year. Instruction films can be found on our website ‘Outputs and reports’.

We know that other birds also eat the oak processionary caterpillar. It is clear, however, that tits eat high numbers of caterpillars and therefore offer an efficient natural control method.

The oak processionary may well make its nest behind (usually) the nesting box or, indeed, inside it. We have, nevertheless chosen to deliberately install the nesting boxes in the trees to be studied so that we can more effectively measure the impact and eliminate any side-effects from adjoining land use. There is also often a lack of alternative locations for nesting boxes with respect to rows of oak that run along roads.

Each tree will have one nesting box. Blue tits and great tits are not territorial with respect to one another and have very small territories so they can occupy alternating nesting boxes.

This will be done. We are studying the ecological management of road verges. In order to conduct more accurate measurements, the issues are investigated individually. In practice, a combination of the various methods will probably be the most effective approach.

Tits eat other caterpillars and insects. Tit eggs hatch just as the oak processionary caterpillars are emerging. The oak processionary is therefore a substantial part of their diet.

Tits usually nest without much fuss. In October/December, they find a suitable sheltered area. This usually becomes their nesting point thereafter. That is why the nesting boxes must be hung early. There is no guarantee that birds will nest in the boxes provided. Other environmental factors also play a role. You can therefore never be certain that the nesting boxes will be used.

This will not be specifically examined within the LIFE project but we will try to follow this issue up in our expert group.

You are advised to hang the nesting box on the NE side of the trunk (side with least rain) and you can turn it slightly to the north or east in order to find the side with least likelihood of disturbance. Tits can put up with some disturbance during breeding, e.g. near walking routes.

For the time being, this will not be considered. The trial locations are currently located in principally rural areas. There will be a pilot project in a more urban setting, when your questions will certainly be suitable for further research.

There is a difference between biocides with a long-term, negative effect on the ecosystem and which are unilaterally applied by humans, and an indigenous bird species which is already part of the natural food chain. The tits will adapt their breeding behaviour to the abundance of food. So, if the quantities of oak processionary caterpillars reduce, the breeding success will also reduce.

Observation has shown that tits prefer oak processionary caterpillars if they are offered ‘nearby’. However, the tits also eat other caterpillars.

Yes, the north-east side of the tree is best, at around 2.5 metres. Further instructions can be found on our website ‘Outputs and reports’.


Each year, we will try to examine this issue by cultivating 250 caterpillar nests to assess the parasitic content.

Indigenous, long-term herbaceous vegetation, primarily the parsley family. The emphasis of the experiment, however, not only lies on sowing, but also on the comparison of the various control methods (e.g. grass-cutting frequency) and comparing vegetation with and without brambles. Overall, we will review 6 different vegetation types.

The parasites that live off caterpillars will be reviewed by examining the nests. In the province of Gelderland, we will take a broader look at all the insects that are found at the trial locations. Trial fields in Sittard-Geleen will also be monitored.

Very little is currently known about ground nests. We will follow this up.

Within the LIFE trial, we will examine the effect of normal, ecological verge management. Given that the aim of ecological verge management is to provide effective management for a broad group of desirable insects, we do not expect special measures for natural enemies of the OPC to be required.

We are doing this in a trial outside the LIFE project in Belgian Limburg, Antwerp and Gelderland.

Yes, we will place old nests into parasite nest crates. The parasites will be able to leave but the oak processionary moths will not. This will take place as additional research alongside the LIFE project.

We are removing a limited number of nests; most will be left alone. The time at which we remove them will also play a role.

We have noticed that ichneumon wasps are parasitic with respect to eggs too. Attracting natural enemies to the verges is not just about attracting parasites that attack caterpillars but potentially egg parasites too.

Ivy is actually less than ideal along roadside verges as the trees cannot be checked effectively for trunk/branch breakages. At other locations, ivy is valuable for insects. In the LIFE trial, we will look at the role of brambles at the sites.

The life cycle of the pine processionary is different; pheromone traps are more effective with respect to the pine processionary. Expertise will be shared with researchers who are working with the pine processionary.


We don’t yet know but it doesn’t appear to be a preferred food source for the tits. The beetle is quite large (3-4 cm).

The oak processionary is the forest caterpillar hunter’s favourite but they also eat caterpillars from the gypsy moth and the black arches. It prefers hairy caterpillars.

3-4 cm, like a stag beetle.

The forest caterpillar hunter is found along the edges of woodland.

There is no guarantee. Firstly, on the basis of a habitat map which the Institute for Nature and Forestry Research (INBO) will create for this project, we will find the most suitable locations for the forest caterpillar hunter. They will probably survive at these locations. If this is not the case, there is an option to continue cultivating the species and releasing it at locations which are struggling with issues caused by the oak processionary. We hope to know more about this subject area by the end of the project.

We will work with populations from South-east Europe, which are genetically identical to populations that were present in our regions until recently.

According to literature, around 50-250 metres. We would like to test this by means of transmitters.

As a tree beetle, its natural enemies are usually birds. As a larva or when spending the winter underground, they are eaten by small mammals such as mice and hedgehogs.

There is a very small risk. So far, these beetles have never caused any problems.

As indicated earlier, the Research Institute for Nature and Forest (INBO) will develop a habitat model for the selection of potentially suitable habitats for the beetle. Good suggestions are, of course, always welcome!

The small forest caterpillar hunter also plays a role; we are seeing numbers increase gradually and will monitor the situation. The small forest caterpillar hunter is a slightly smaller beetle which lives in the same biotope and probably benefits from the more favourable conditions created by climate change. Observations tend to be made in May and June, with a peak in May. They principally eat winter moths but when the oak processionary caterpillars are small (they grow quickly in May), they can serve as a food source.

It would be great if it became self-sustaining once again. But this is not something we can assume. The beetles may well have to be released on an ongoing basis.