The Forestry Commission has advised people to stay away from the nests of the Oak processionary moth (OPM) which contain thousands of caterpillar hairs.
These hairs can cause unpleasant skin rashes, eye and throat irritations and, occasionally, breathing difficulties in humans and animals.
People can also be affected if they touch the caterpillars or if the hairs are blown into contact by the wind. The animals can also shed the hairs as a defence mechanism.
OPM have started to build their silken web nests on oak trees in Berkshire and large populations can strip whole oak trees bare of leaves, leaving them vulnerable to attack by other pests and diseases, and less able to withstand adverse environmental events such as drought and floods.
Currently the catarpillars are only known to be present in parts of Salham Woods near Pangbourne and it is thought they are close to eradication.
Report sightings of OPM or their nests to your local council.