Dear @kakkalama,

Caterpillars come from butterflies wich laid their eggs on this vegetation.
After periods of eating and growing these caterpillars will become buterflies.
These butterflies are going to mate and lay eggs wich will hatch next year.

Yes it is harmfull for this plants. All their leaves are going to be eaten.
There is a big change the plant is going to die.

I think you have seen the oak processionary (Thaumetopoea processionea), this kind of caterpillars are causing human health problems. They are a human irritant because of their poisonous setae (hairs), which can cause skin irritation and asthma.

Lot’s of love,
Ray #lifeisforliving