Outbreaks have been spotted after the bug was accidentally released into the UK
People across the UK are being warned about a caterpillar that can ‘send you to A&E’. The Oak Processionary Moth is a species whose adult form is harmless, but whose caterpillar stage is one of the most significant health hazards currently breeding in British gardens.
As Britain’s record-breaking May heatwave gives way to cooler, unsettled June weather, experts warn the extreme heat has accelerated hatching, pushing the high-risk caterpillar season earlier than usual and producing larger populations than a normal spring would have.
Garden expert Luke Newnes at Hillarys said: “The Oak Processionary Moth was accidentally introduced to the UK in 2005 via imported oak trees from the Netherlands. It is now established across London and surrounding counties, with the Forestry Commission running annual treatment programmes from mid-April.
“Outbreaks have been confirmed in Hampshire, Warwickshire and Gloucestershire. Active from late April through till summer, it’s the moth’s caterpillar stage that poses the biggest risk to people and trees. The Forestry Commission has specifically noted that warm spring weather causes OPM caterpillars to grow and hatch faster than in previous years.
“Following the recent record breaking heatwave heatwave, described by the Met Office as ‘an exceptional spell of warmth’ with highs approaching 35C, this has now compressed the hatching window and pushed caterpillar numbers higher than a cooler spring would have produced.
“Once hatched, the caterpillars are highly visible – moving in distinctive nose-to-tail processions on oak bark and building silken nests the size of a golf ball to a rugby ball on trunks and branches. Those nests are where the danger concentrates, but the hairs the caterpillars carry can spread well beyond the tree itself.”
Why are the caterpillars a health hazard?
Each OPM caterpillar carries thousands of microscopic hairs containing thaumetopoein, an irritant protein the moth’s larvae produce as a defence mechanism.
According to the British Journal of General Practice, contact with these hairs can cause urticating dermatitis within one hour, producing intense itching, skin rashes and eye inflammation. Where hairs are inhaled – which can happen simply by walking beneath an infested tree – symptoms can include coughing and breathing difficulties.
Children are especially vulnerable.
The caterpillars’ moving processions attract curiosity, and the hairs remain active in shed skins and old nests for years after the moth has completed its life cycle. Professional treatment per tree costs between £200 and £500 or more, and severe infestations carry a real risk of tree loss.
Luke said: “The Oak Processionary Moth itself is not the problem – it is what it produces. The caterpillar stage is when the risk to people begins, and that window is now, running through the whole summer. The heatwave we’ve just had is exactly the kind of warm spring the Forestry Commission warns about – more caterpillars, hatching sooner, at a time when families are spending more time in the garden. These caterpillars do not need to be touched to cause harm. The hairs become airborne and settle on anything nearby.
“Anyone with a mature oak tree in London, the South East, or who has planted an imported oak anywhere in the UK in the last few years, needs to be checking their trees from the ground right now and not waiting until they see a problem.”
Where OPM could be hiding – beyond the tree itself
- On the trunk and branches of oak trees
Look for white or grey silken nests ranging from golf ball to rugby ball size, typically in the fork of branches or against the trunk. Active processions of 20-300 caterpillars moving nose-to-tail down the bark are a clear sign that the moth has established itself in the tree.
- Garden furniture, toys and equipment left beneath oak trees
The caterpillars’ airborne hairs settle on any surface beneath or near an infested tree. Garden furniture, children’s toys, trampolines and paddling pools stored close to oak trees can carry hairs without any visible sign. Bring them indoors or cover them during the high-risk period from May to August.
- Washing dried outdoors near oak trees
The caterpillar hairs are light enough to travel on the breeze. Clothes, towels and bedding dried on a line within range of an infested tree can carry them indoors. If you have a suspected OPM infestation nearby, dry laundry inside until the moth’s caterpillars have been treated and cleared.
- Ground beneath the tree and surrounding grass
The caterpillars process across the ground as well as on bark, and shed skins containing active hairs fall and accumulate at the base of the tree. Children and pets playing or walking barefoot in the grass around an infested oak can come into contact with hairs without going anywhere near the moth’s nest.
- Sheds and outbuildings adjoining or near oak trees
The caterpillars will process along any surface on their route. Garden sheds, fences and outbuildings adjacent to oak trees can accumulate hairs on walls and surfaces. Items stored inside that were left with doors open during the active season should be handled with care.
Three things homeowners should do right now
- Inspect your oak trees from a safe distance
Use binoculars if necessary. Check trunks and main branches for silken nests and any nose-to-tail caterpillar processions. Do not touch nests, caterpillars or shed skins, and keep children and pets away from the area.
- Report any suspected nest immediately via TreeAlert
The Forestry Commission’s official reporting tool is treealert.forestresearch.gov.uk. Do not attempt to remove nests yourself. Professional treatment per tree starts at around £200.
- Clear the area around the tree and move items indoors
Remove garden furniture, toys and any items stored beneath the oak. Keep grass short to reduce caterpillar ground routes and avoid drying laundry outdoors nearby until the tree has been professionally assessed and the moth’s caterpillars treated.
