The process of controlling the oak processionary should be as environmentally-friendly as possible. The current (2020) recommendations are set out below. We hope that this LIFE project will emphasise the need to move towards ecological controls and a complete ban on the use of biocides in the long-term.

Health impact

The oak processionary caterpillars have microscopic, irritant hairs that are formed from the third larval stage (from May) and offer an active defence mechanism for the species.

These hairs stay active for up to 7 years and are also found in nesting material and shed skin.

The hairs contain a thaumetopoein protein which causes an allergic reaction in humans and animals. Contact with these hairs leads to irritation of the skin, eyes and the upper airways. These effects can be very serious indeed.

The problem is most prevalent in May to June when the caterpillars are fully grown.

Control measures where?

We recommend that control measures are only used in urban areas or other busy public spaces such as walking or cycling routes. In natural or wooded areas, the biological balance is usually maintained by natural enemies of the caterpillar and they do not pose an undue risk to humans. Taking no control measures at all is the most environmentally-friendly option.

Removing caterpillars and nests by means of suction is preferable over using fire. If the above methods do not offer a solution, it may be worth considering the use of a recognised pesticide.

Bottlenecks for control

Control measures to tackle oak processionary caterpillars aim to eliminate the problems caused by them for humans as much as possible. The starting point in all this is pesticide-free control according to the principle of the three-step strategy (See Action Plan).

A distinction is made between preventative controls and curative controls:

  • Preventative control focuses on preventing the caterpillars reaching the stage at which they can spread their hazardous hairs.
  • Curative control focuses on removing existing caterpillars that have developed hazardous hairs and any residues thereof.

There are various issues with this:

  • The period for preventative control is limited in time (around 3 weeks).
  • Preventative control is weather-dependent.
  • The control measures are expensive. Curative controls, in particular, are labour-intensive and costly.
  • In Belgium, in 2020, just one product was approved for preventative controls, i.e. NeemPro®tect. There is also a temporary permit for the use of Foray®ES at locations where NeemPro®tect may not be used according to the approval conditions.
  • The preventative measures also affect other insect species that feed on the leaves of the oak and could also be damaging to aquatic life. As a result of pesticide ‘drift’, it may also end up in the surrounding area and could impact upon other species too.

Plan of action

The basic principle when tackling the issues caused by the oak processionary is that methods should be pesticide-free and applied according to the three-step strategy:

  1. Preventing health problems, e.g. by raising awareness
  2. Applying alternative, pesticide-free techniques
  3. Using biocides to tackle the issue

The oak processionary is an indigenous species and has its place in our ecosystem. That is why they are only controlled at locations where there is a risk to health. In most cases, therefore, they can be tackled without biocides, using other methods. As well as managing the oak processionary, communication with the general public and monitoring are two other important actions that should not be overlooked.

Communication and raising awareness

It is important to provide accurate information to the general public and local authorities, in their capacity as a contact point for the public at large and also controller of the oak processionary. By keeping the general public informed about the genuine health risks and the advantages and disadvantages of the control measures applied, you create support whereby control measures will become more acceptable within the municipality.

Raising awareness can also lead to behavioural changes which reduce the numbers of people being exposed to the hazardous hairs and generate a degree of tolerance with respect to limited control measures.

Informatiebord in Maaseik

Control measures at important public locations for health reasons

The preference is to only apply control measures at public locations where there are signs of the caterpillars: in urban areas, near businesses (agriculture, industrial estates, meadows) or other important public spaces, e.g. along walking and cycling routes, play areas, cemeteries, schools, etc. Old and new nests, which are a source of irritating hairs, will be removed from these areas as far as possible.

This vision will be followed up in order to limit health risks and reduce the negative impact on visits and the natural environment. It is not the intention, and also impossible and undesirable, to eliminate the species altogether. In natural and woodland areas, natural enemies (certain flies, wasps, beetles and also viruses and fungi) maintain an ecological balance. At these locations, the oak processionary poses less of a risk to health.

Proactive control with the right resources, at the right place

To decide where controls should be applied and which resources to use, it is best to consider the degree of infestation, the social problems caused and the ecological impact, taking account of the location in relation to protected areas, watercourses and the prevalence of protected moth/butterfly species.

The choice of the right resources or control methods also depends on the development stage of the caterpillars themselves. The provinces communicate start and end dates for possible pest-control measures via their newsletters. Provincial experts follow the situation in the natural environment very closely. Monitoring and evaluation are vital in order to determine the degree of infestation and the best possible approach for that year and the subsequent year.


Thorough monitoring is essential in order to determine suitable controls and evaluate the methods used, even when biocides are not used.

Summary of control methods

Taking no control measures at all is the most environmentally-friendly option.

The following control measures are possible and also effective:

  • Sucking up and removing nests
  • Manual removal of the nests
  • Removal using fire
  • Leaf-spraying (temporary) with approved products under specific conditions

The following method will also be used; this appears to be successful but must be investigated further:

  • Treatment with nematodes

In the current LIFE project, we will test the effectiveness of a number of alternative techniques on a large scale and measure the results in a scientifically backed study. Small scale studies have already shown that they lead to a reduction in the numbers of oak processionary caterpillars. At the end of the project, we would like to be able to estimate the precise level of reduction.

  • Attracting tits as predators, by installing nesting boxes
  • Attracting natural enemies through adapted road verge management: increased provision of nectar and/or sheltered spots attract more parasitoid wasps and flies.
  • Release of the Calosoma sycophanta or forest caterpillar hunter, a natural enemy of the oak processionary.

Choice of control methods, criteria and conditions

The various control methods and preconditions for their usage will be discussed below:

No control measures – warnings or closure of zones


  • Warnings via signs, cordons/barrier tape, information signs
  • Temporary closure where necessary

The most ecological approach is to use no controls at all. If the caterpillars are observed in a region where the infestation level is very low or at locations that are rarely used by members of the general public, it is best not to use control measures, as the health risk is very low. If there is a health risk, the local population can be warned using signs and/or barrier tape. The locations which pose a potential health risk could be announced in municipality newsletters, for example. If necessary, zones could be temporarily closed.

barrier tape oak processionary caterpillar
barrier tape oak processionary caterpillar

Suction or manual removal of nests


  • The materials gathered must be incinerated by a recognised processing company for animal remains.

Vacuum cleaners can be used to control the as yet undeveloped caterpillars without the hazardous hairs (up to the third larval stage, but this is difficult to determine visually). Special precautionary measures will then be unnecessary.

Caterpillars with the hazardous hairs (from June to August) can be sucked up with a heavy-duty, industrial ‘hoover’ or a slurry tank with a vacuum pump, partially filled with water at a pressure of at least 3 bar. This sucking and burning process must be repeated several times throughout the season. Old cocoon nests can be cleared away using this method throughout the year. To ensure the impact on the surrounding area is kept to a minimum, this process should be carried out during wet conditions.

Suction should begin in spring, just after the eggs have hatched. The caterpillars gather on the underside of the thicker branches and look like dark, hairy balls. The fact that they do not yet have hazardous hairs and are also quite small means they can be processed quickly and efficiently. This suction process should take place in the morning, as young caterpillars move in procession later in the day. From the beginning of June, the older caterpillars sit on the branches and can therefore be accessed easily for suction.

Nests can also be removed manually.

Using an industrial vacuum cleaner to remove oak proecssionary caterpillars

Removal using fire


  • Burning may be used from the third development phase
  • Burning must be applied by experienced personnel to ensure the bark of the tree is not damaged
  • The method must be repeated several times during the caterpillar season
  • Nests and old nests are also a source of irritating hairs and must be removed

The suction method is preferable to burning given the risk of damaging the trees, causing verge fires, and the unadulterated spread of hairs by burning.

Once the caterpillars gather on the trunk, they can be burnt off. Good instructions are vital for the process, as the fire must be used extremely accurately. Damage to the tree bark must be avoided by not directing the flame directly at the trunk, but by ‘brushing’ up and down like a painter with a brush. Only burn using the yellow part of the flame. Start at the top so that the falling caterpillars are caught by the flame again. To prevent caterpillars falling out of the tree and crawling up the trunk again, the ground should be burnt too. Be careful to avoid starting verge fires. During the season, you must repeat the method several times so that the caterpillars which crawl up to the crown at a later stage can be eliminated too.

There are various propane burners on the market. To burn at a height of two or three metres off the ground, you are advised to use an extension piece.

Nowadays, it is only the fire-service that will help with this burning process. There are very few businesses that offer this service.

This method could damage the cambium layer of the oak and there is also a risk of verge fires. It is not a pleasant job in hot weather and any clothing must be disposed of once the job is complete. Some of the hairs will not be burnt and may spread in the hot air created by the burning process.

Bladbespuiting met biociden (Belgische richtlijnen)

In Belgium, there is just one product that is recognised as a biocide for use against the oak processionary, i.e. NeemPro®tect. The negative impact of this product on biodiversity and, in turn, the natural enemies of the oak processionary means there are strict conditions for use. At locations where the use of NeemPro®tect is not permitted, a temporary permit may be obtained to use the less damaging Foray®ES.

In order to use NeemPro®tect, a permit from the Flemish Environment Agency (VMM) must be requested via Procedure 3. In order to use Foray® ES, it must be announced in advance via procedure 1. More information and an outline of the steps to be taken can be found on the website “Zonder is Gezonder” (in Dutch).

The Control Guideline for the Oak Processionary 2020 drafted by the Province of Antwerp can be found here (in Dutch).

At the end of this project, a new guideline will be one of our most important outputs.