Oak processionary caterpillars
Oak processionary caterpillars emerge in spring from nests in oak trees ahead of their transformation into moths.
The caterpillars are each covered in tiny hairs that can cause itchy and painful rashes if they come into contact with skin. They can also cause irritation to eyes and throats.
The hairs can be blown on the wind and left in the silken, web-like nests that the caterpillars build in oak trees.
The caterpillars cause damage to oak trees through defoliation.
Oak processionary caterpillars are spreading. They were first found in London in 2006. In 2007, the Forestry Commission collected and destroyed 708 nests. That figure rose to 4,410 in 2011.
Get more background information from the Forestry Commission.
What to do
The Forestry Commission urge the public to report infestations - but to steer clear of touching the caterpillars or their nests.
If you spot a nest or you think an oak tree is infested with these caterpillars you should contact the Forestry Commission on 0131 314 6414. For further information visit the Forestry Commission's website.
Anyone worried by an intensely itchy or painful skin rash, sore throat and irritated eyes, and who might have been near oak trees containing oak processionary caterpillars, should consult their GP or call NHS Direct on 0845 4647.
Further details are available from the Health Protection Agency