If you have oak trees in your garden, now is the time to check them. Oak processionary moth caterpillars – one of the most significant garden health hazards in the UK – are emerging rapidly after May’s record-breaking heatwave.
While adult moths are harmless, the caterpillars are covered in thousands of tiny stinging hairs that can trigger itchy skin rashes, irritated eyes and sore throats. The risk is highest from May to July, but the hairs can linger in nests and on bark for more than a year.
Below, we explain why oak processionary moths are spreading earlier than usual, which areas are most affected, and the three quick checks homeowners should carry out now.
What is happening with oak processionary moth caterpillars in the UK?
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 also 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,’ explains Luke Newnes, garden expert at Hillarys. ‘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, with temperatures approaching 35°C, the hatching window has been compressed, pushing caterpillar numbers higher than a cooler spring would have produced.
‘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,’ adds Luke.
Why are oak processionary moth caterpillars a health hazard?
Once hatched, oak processionary moth caterpillars are highly visible, moving in distinctive nose-to-tail processions along oak bark and building white silken nests ranging from 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,’ says Luke. ‘Each oak processionary moth caterpillar carries thousands of microscopic hairs containing thaumetopoein – an irritant protein the moth’s larvae produce as a defence mechanism.’
Contact with these hairs can cause intense itching, skin rashes and eye inflammation. Where the hairs are inhaled – which can happen simply by walking beneath an infested tree – symptoms can also include coughing and breathing difficulties.
Where oak processionary moth caterpillars could be hiding
Want to know where they could be lurking? Luke shares where and what to look for.
- 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 forks of branches or against the trunk.’
- Garden furniture, toys and equipment left beneath oak trees – ‘Garden furniture, children’s toys, trampolines and paddling pools stored close to oak trees can carry hairs without any visible sign.’
- 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.’
- The 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.’
- Sheds and outbuildings adjoining or near oak trees – ‘Garden sheds, fences and outbuildings adjacent to oak trees can accumulate hairs on walls and surfaces.’
What to do if you find signs of oak processionary moth caterpillars
The key things are to inspect your oak trees from a safe distance, looking for silken nests on the trunks and main branches, as well as any nose-to-tail processions of caterpillars.
If you do find any of these signs, report the suspected nest immediately via TreeAlert, which is the Forestry Commission’s official reporting tool. Professional treatment starts at around £200 per tree, and it’s imperative that you do not attempt to remove nests yourself.
Additionally, move any garden furniture, toys and other items stored beneath the oak tree. Keep the surrounding grass short to reduce caterpillar ground routes, and avoid drying laundry outdoors nearby until the tree has been professionally assessed and the caterpillars have been treated.
Follow House Beautiful on Instagram, TikTok and Pinterest
The Garden Edit
Folding Bistro Set, 3 Pieces
