What are the small worms in German trees?

In Berlin, when you a see a tree covered in small worms, they are probably ermine moth worms (Gespinstmottenraupen) or oak processionary worms (Eichenprozessionsspinner).

Summary: If the worms have hair on their backs, don't touch them. They are dangerous.

Oak processionary worms

Oak processionary larvae (Eichenprozessionsspinner). You can find them in from May to July¹. They live in large groups on oak trees, and around them. They often move together in the same direction.

These worms are dangerous to you and your pets¹. Do not touch them. The hairs on their backs are toxic. They cause allergic reactions. They cause itchy, blistering skin rashes. The symptoms start after a day, and they can last for a few weeks^{1, 2}. In some cases, they can cause more serious problems like fever, dizziness and breathing problems¹.

The city sometimes puts warning signs in parks, and even block roads^{1, 2}. They also try to kill them with fire or vacuums¹.

Ermine moth worms

Ermine moth larvae (Gespinstmottenraupen) are around 3 centimetres long. They have smooth skin, and dark dots on their backs. You can find them from April to July¹. Just look for trees covered in silk.

Ermin moth larvae are not dangerous to you or your pets, but they often kill the trees they invade¹. They sometimes hang from trees by their silk, so you can run into them.