The hairs of the caterpillars carry a toxin which can be a threat to human health, causing skin rashes, eye irritation and respiratory problems. In high numbers the caterpillars can also cause defoliation of oak trees.
In late April / early May pesticide spraying takes place on oaks in busy areas and those where they have been previously heavily infested. Careful surveying of the whole Park is undertaken by volunteer spotters to locate nests which are then tagged for later removal by specialist operatives using spacesuit-like protective clothing and equipment.
If you come across the caterpillars or their webbed nests please do not touch them and keep children and pets away. Report any sightings to the park office on 0300 061 2200.
Full information here from the Forestry Commission.