They are toxic to humans and animals and they can destroy trees. As we approach summer, a warning has been issued to beware of a caterpillar which is emerging in forests across the south.
The larvae of the oak processionary moth lives in and feeds on oak trees.
The caterpillars have thousands of tiny hairs that contain a substance called thaumetopoein. The hairs can be carried on the wind and can cause itching skin rashes and occasionally, sore throats, breathing difficulties and eye problems.
The Forestry Commission says the first ones of the year were reported last month and that controlled treatment of affected trees with approved insecticide to kill the caterpillars is under way.
It added that those which survived treatment will now be getting big enough and descending low enough in the trees to be seen.
Outbreaks have previously been confirmed in wooded areas in Pangbourne in Berkshire and Guildford in Surrey.
Watch Emma Wilkinson's report.
Caterpillar pictures courtesy of the Forestry Commission. Interview with Craig Harrison from the Forestry Commission.
The oak processionary moth (Thaumetopoea processionea; OPM) is a pest which has established itself in oak trees in parts of southern England in recent years, after being accidentally introduced from mainland Europe. They damage oak trees, and can cause itching skin rashes and other health problems in people and animals.
OPM caterpillars can damage our precious oak trees by feeding on the leaves. Large numbers can strip whole trees almost bare of leaves, weakening the trees and making them vulnerable to other threats such as drought and disease.
The caterpillars emerge about April every year and develop thousands of tiny hairs which contain an irritating substance called thaumetopoein. This can cause itching skin rashes, eye irritations and sore throats in people and animals that come into contact with them. In rare cases they can cause breathing difficulties and allergic reactions.
It is important to avoid contact with the hairs, to teach your children to avoid them, and to protect your pets from them. Curious pets might need to be restrained from approaching nests and caterpillars.
However, if you are affected, the symptoms, although unpleasant, are not usually medically serious and will pass in a few days. You can ask a pharmacist for something to relieve the symptoms.
If you do have a serious allergic reaction, call NHS111 or see a doctor. Similarly, consult a vet for badly affected animals
Campaigners say roadside litter's 'getting worse' - despite the drive to 'Clean for the Queen'Read the full story ›
Walkers are being warned about a large crack which has appeared in cliffs near Weymouth on the Dorset coast.Read the full story ›
Windsor Castle is to undergo a multi-million pound makeover.
The Royal Residence in Berkshire is sharing a £37m pound investment by the Royal Collection Trust. The work will open up more of the castle and create a special learning centre.
The works, known as the Future Programme, will start next year. The castle will remain open to visitors throughout the development.
An archaeological project in Berkshire will be aiming to uncover a King's burial site.
The 'Hidden Abbey Project' will try to discover the extent and significance of Reading's Royal Abbey, founded by King Henry I in 1121. The Abbey was his final resting place, alongside his Queen Adeliza.
Archaeologists will use a ground-penetrating radar (GPR) to find the boundaries of the Abbey Church. They will also try to discover areas for more investigation, including the High Altar where Henry was buried, the Ambulatory and the Lady Chapel.
Remarkably, this will be the first time the Abbey Ruins have undergone a comprehensive and recorded archaeological survey for over 150 years.
We know that Henry I was buried in the royal Abbey that he founded in Reading. We hope to show the lavish scale of what, in the Middle Ages, was one of the major Benedictine Abbeys in western Europe, and a regular place for royal visits and events.
This project has the potential to bring huge cultural, historical and economic benefits to the Abbey Quarter and the town as a whole."
A farmer who lost 116 sheep and 70 unborn lambs in the country's worst ever attack by a dog says he felt like crying when he saw the pile of dead animals. Gordon Wyeth is worried that his insurance company may refuse to cover his £17,000 loss. Kerry Swain has been to see him.
A pioneering scheme for people with learning difficulties has become the latest victim of council cuts in our region.
The Nature Corridors for All project in Lewes has been told by East Sussex County Council that the £50,000 a year it takes to fund it - will be stopped.
And as Andy Dickenson reports, its organisers have been left asking just where the cuts will end?
He speaks to Ebrima Furlong from Priory School, Jackie Poole, a participant of the scheme, and Dr John Parry, it's organiser.
Villagers living in a picturesque part of Kent say they're outraged about plans to build a development of giant warehouses on their land. The earmarked area near Ashford, would cover the equivalent of 31 football pitches. John Ryall reports.
Spring might still be a few weeks off, but the lambing season is already well underway. And sadly, that means many will fall victim to dogs allowed to run out of control.
Farmers say at least 100 have been killed so far this year and signs are being put up at fields where sheep are grazing to make dog owners more aware.
Richard Jones' report includes interviews with Farmer Andy Jackman, Terena Plowright from National Sheep Watch and James Osman, from the National Farmers' Union.