Oak Processionary Caterpillar

The oak processionary caterpillar is the larva of a moth. The moth lays its eggs mainly in the tops of oak trees, where the eggs overwinter. They hatch from mid-April through early May. After molting several times, groups of caterpillars gather together. They form large nests on the trunk and side branches of the oak tree. At night, they march in a procession in search of food in the tree’s crown—hence the name. Large groups of oak processionary caterpillars can completely strip a tree bare. The caterpillars molt three times, after which they develop stinging hairs. These stinging hairs cause health problems.

The caterpillar was first spotted in the Netherlands about 30 years ago. There are many oak trees in our country, so there is plenty of food for the caterpillar. In recent years, its population has exploded, likely because conditions were favorable.

Oak trees infested with oak processionary caterpillars can be identified by the spherical, white-gray nests on their trunks or thicker branches. The nests are dense webs made of molted skins, stinging hairs, and feces.

In the spring, all oak trees susceptible to oak processionary caterpillars are treated preventively. For this, we use a biological agent that works best once the trees have leaves. In addition, we adapt the surrounding environment wherever possible. We plant so-called screen and legume plants and hang birdhouses for titmice. This way, we ensure that natural predators of the oak processionary caterpillar—such as coal tits and blue tits, as well as parasitic wasps and lacewings—feel at home there. They eat the caterpillars. If the caterpillars have nevertheless taken up residence in a municipal tree, the municipality will hire a specialized company to remove them.

If there are oak processionary caterpillars in a tree, the tree owner must take action.

• Is the tree in your yard? If so, do not try to remove the caterpillars yourself. Call a specialized company (often a landscaper or arborist) or ask your local municipality for advice. Do not sit under or near the infested tree. Stinging hairs can be carried by the wind up to 100 meters. While the stinging hairs do wash into the ground when it rains, many of them also remain stuck to the grass. These remaining stinging hairs can cause problems again even when the weather improves.
• Does the tree belong to the municipality? Please report it to the municipality by calling 14 0168.
• Does the tree belong to another manager or owner, such as Staatsbosbeheer or a swimming pool operator? Please report it to that party.

• Do not sit under or near an infected tree.
• Cover your neck, arms, and legs if you're going to an area where oak processionary caterpillars are present.
• Do not touch caterpillars, (old) stinging hairs, webs, or shed skins. Explain to children why they should not touch processionary caterpillars.

The stinging hairs of the oak processionary caterpillar remain present well into August. Be aware of this and try to avoid areas with (old) nests as much as possible.

The stinging hairs of the oak processionary caterpillar are so small that you can't see them. They easily penetrate the skin, eyes, and airways. Within a few hours, this causes symptoms such as:

• itching
• bumps or blisters on the skin
• red skin
• red, swollen, and sometimes inflamed eyes

Sometimes the symptoms resemble those of a cold:

• a runny nose
• sore throat, coughing
• hard to swallow
• shortness of breath

Some people have a strong reaction to the stinging hairs and experience:

• vomited
• dizziness
• fever
• a general feeling of misery

The severity of the reaction varies from person to person. Do you come into contact with stinging hairs frequently? If so, you may have a more severe reaction. Animals can also be affected by the stinging hairs of the oak processionary caterpillar.

• Carefully remove any clothing containing stinging hairs and use adhesive tape to strip the skin as quickly as possible.
• Then rinse your skin and eyes with lukewarm water. Try not to scratch or rub them.

The symptoms usually go away on their own within a few days or weeks. Do you have severe itching? If so, an ointment may help. Ointments containing menthol, aloe vera, or calendula work especially well. If you’re having a lot of trouble, see your doctor.

The stinging hairs have barbs that cause them to get caught in clothing. Remove the stinging hairs by washing your clothes. Do this as soon as possible to prevent the stinging hairs from getting into other items of clothing.

A temperature of 60 °C is needed to break down the proteins in the stinging hairs. If you can’t wash your clothes at this temperature, wash them at a lower temperature and see if you experience symptoms again after wearing them.

Yes, in recent years there have been increasing reports of health issues in horses and dogs caused by the caterpillar. Farm animals are also likely to be affected.

Animals often ingest large numbers of stinging hairs. They may experience swelling of the tongue and lips, inflammation of the oral mucosa and eyes, and breathing difficulties.

The animals begin to drool and tear up. Dogs may have difficulty swallowing and may gag and vomit. The damage to the mouth can be severe; in rare cases, part of the tongue may die. Skin symptoms are rare because animals have a coat.

If your pet is showing the symptoms mentioned, take it to the vet right away. Treatment helps reduce swelling, pain, and vomiting. If there are stinging hairs in the mouth, it must be thoroughly rinsed with water. Treatment often takes several days. It can sometimes take a few weeks for all symptoms to disappear.

• Avoid areas where there are warnings about oak processionary caterpillars
• Prevent pets from coming into contact with the caterpillars and their remains, such as released stinging hairs and empty nests.
• If the animal comes into contact with the caterpillars or stinging hairs, do not scratch or rub it. Wash or rinse the mouth, skin, or eyes thoroughly with water. If the animal shows signs of an allergic reaction, always contact the veterinarian.
• Do not let horses or other farm animals graze under or near (within 100 meters of) oak trees with nests. Have the nests removed by a professional.
• Do not turn grass found under or near (within 100 meters of) oak trees with nests into hay. This will prevent animals from ingesting the stinging hairs through the hay.