When to monitor oak processionary caterpillar: complete guide

What is the oak processionary caterpillar?

The oak processionary caterpillar (Thaumetopoea processionea) is a caterpillar species living mainly on oak trees. It is not just any garden pest - it is a health hazard. The caterpillars have hairs with toxin that attack skin, eyes, breathing, and lips. One contact can cause itching, swellings, and serious allergic reactions.

How do you recognise them? They live in white silk tents (nests) high in oak trees. They are grey-yellow and walk in long processions (hence the name). Around May-June they become dangerous.

When to monitor: calendar

You check your oaks for oak processionary caterpillars in these periods:

April-early May: This is the first check. In April the first silk tents appear in oak trees. They are small still. This is the time to spot them before they grow large.

May: This is crucial. Late May the caterpillars reach their most dangerous stage. They form massive nests and leave the tree in long processions. This is the period to be very careful and keep distance.

June: Mid-June most caterpillars are already in the soil (pupating). But nests remain in the tree. Caution remains important.

So: April check for detection, May for intensive monitoring, June for late check.

How to monitor for oak processionary caterpillars

Look up. Oak trees with nests have visible silk tents - white, fluffy, visible from far. They hang in branches, usually high in the tree. Do not put your hand to your eyes, just look upward with binoculars.

See holes in leaves? This can be a sign of caterpillar activity. Not all holes are caterpillars, but many small holes can suggest feeding.

Resting in soil under tree. In June-July, pupating caterpillars can be in the soil. Ensure children do not play or dig here.

Ask for help. If uncertain, contact your municipality or nature organisation. They help free with identification.

Danger: how do you recognise symptoms?

If you or your family come in contact with caterpillars or nests:

• Skin: Itching, red patches, bumps
• Eyes: Red, painful, tearing
• Breathing: Coughing, throat irritation
• Allergic reaction: Facial swelling, breathlessness (very rare but serious)

Symptoms appear within minutes to hours. What to do:

1. Wash everything that had contact (clothes, skin, hands) well
2. Tell children not to touch nests
3. In serious symptoms: call doctor

What NOT to do

Do not remove nest yourself. This is dangerous. The hairs spread. Wait for professional.

Do not burn. Many want to burn the nest. This spreads toxin in air. Very dangerous.

Do not spray with your own products. This usually fails and is dangerous.

What to do

Report to municipality. In the Netherlands oak processionary caterpillar is reportable (some municipalities). Call your municipality. They send professional to remove nest.

Keep distance. Until nest is removed, maintain minimum ten metres distance. Tell children they cannot climb under the tree.

Keep monitoring. Check regularly if the tree becomes reinfested.

Step-by-step

Step 1: April - first inspection

Walk around your oaks. Look up at silk tents. Uncertain? Take photos and ask municipality.

Step 2: May - intensive monitoring

Check weekly. See leaf holes or nests? Report immediately to municipality.

Step 3: June - late check

Check nests are gone. Ensure children do not dig in soil around tree.

Step 4: July-August - rest

Monitoring can relax a bit. Most caterpillars are gone. But stay alert.

Step 5: September-October

No oak processionary caterpillars until next spring. But check in October tree looks healthy.

Oaks and oak processionary caterpillars

Pedunculate oak (Quercus robur): Very susceptible. This tree attracts caterpillars.

Sessile oak (Quercus petraea): Slightly less susceptible, but still risk.

Red oak (Quercus rubra): Less affected, but it can happen.

Common alder is not susceptible - caterpillars are true oak specialists.

Prevention

You cannot fully prevent caterpillars, but:

• Healthy tree: A strong, healthy oak tolerates caterpillars better
• Do not prune in May: Pruning spreads the hairs
• Monitoring: Early detection means faster removal
• Distance: Ensure oaks are not crowded together

Frequently asked questions

Are oak processionary caterpillars deadly?

No, very rarely. Most contacts cause only itching and redness. Serious allergic reactions are very rare. But hospital is needed if breathlessness.

Can adults also be allergic?

Yes. Adults usually have fewer symptoms than children, but it can. Pregnant women must be careful.

Does insecticide help?

Only in early stages (April). Professional control uses this. You yourself: no idea. Ask municipality.

Will my tree die?

Rarely. Oak processionary caterpillars reduce leaf cover, but tree does not die. Next year it recovers.

Discover your own garden design

At [gardenworld.app](https://gardenworld.app) you can upload your front yard and see how oaks fit - with realistic growth shapes and safety for your family. Plan your garden with health in mind.