Oak processionary moth – management in Windsor Great Park
Published by
The Crown Estate
May 12 2025
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A flexible, evidence-led approach to oak processionary moth (OPM)
The oak processionary moth is an invasive insect pest of oak trees.
OPM are not native to the UK and were accidentally introduced in 2005 when live oak trees were transported to the country from central and southern Europe.
Since then, the invasive insect has spread to the south-east of England and was first sighted in Windsor Great Park in 2018.
Oak processionary moth caterpillars.
The OPM lifecycle
Caterpillars are most active between May and July when they move down to lower parts of the tree in a nose-to tail procession to feed and build balls of white silken nests on oak tree trunks.
From July to August, caterpillars pupate inside the nest before emerging as a moth. The moths mate and lay their eggs on the high branches of oak trees.
In the spring, the young caterpillars emerge from the eggs and feed solely on the leaves of the oak tree.
A procession of oak processionary moth caterpillars.
An oak processionary moth nest.
An oak processionary moth.
Health precautions
Though caterpillars can strip oak trees of their leaves leaving them vulnerable to other pests and diseases, the caterpillars also pose a health risk.
The hairs of the OPM caterpillars are a defence mechanism containing a chemical that when touched, can cause skin rashes, eye irritations and respiratory problems in humans and animals.
The caterpillars can also shed the hairs when threatened or disturbed. The hairs can be blown by the wind and can accumulate in the nests of the caterpillar.
Oak processionary moth caterpillar damage
Oak processionary moth eating the leaves of an oak tree.
Protecting yourself
The greatest risk of exposure to these hairs is between May to July when the caterpillars are active. Hairs can however be left in old nests which can fall to the ground once the moths have emerged.
Below are some simple precautions which will help to minimise coming into contact with OPM caterpillars:
- Do not touch or go near nests or caterpillars
- Do not try to deal with the nests or caterpillars yourself. This requires specialist expertise and careful timing
- Do not let children or animals touch or approach nests or caterpillars
- Keep dogs under control and on lead where required
- Keep to footpaths
- Avoid spending prolonged time under trees during the months of May to July
A family keeping to the footpath.
UK management
OPM is not native to the UK, so many of the insect’s natural predators that would control the population are not present.
Now at a point where the UK cannot eradicate the outbreak, a government programme undertaken by the Forestry Commission has been in place since 2012 to control the spread of OPM and manage the risks.
As an Estate of approximately 6,400 hectares (15,800 acres) with a high proportion of oak trees in comparison to other Estates, we have an evidence-led, flexible approach based on the level of risk. This allows us to balance safety with our role in biodiversity and tree conservation.
OPM spraying
Spraying the leaves of oak trees with insecticide is one method we use to manage OPM in Windsor Great Park:
- The insecticide we use comes from a bacteria (Bacillus thuringiensis) that occurs naturally in soil and is safe for humans, dogs, birds, fish and most beneficial insects
- Spraying is highly targeted. We only spray the leaves of oak trees. We use tree surveys to spray only where caterpillars have been sighted and in high public access areas
- The insecticide is targeted. Insects have to eat the leaf to be affected. The spray does not affect insects on contact, so other insects that live on or under the tree and feed off other matter remain unaffected
- We spray only when OPM caterpillars are most active. This reduces the impact to other insects that feed on oak leaves. To date, Bacillus thuringiensis is the most targeted spray, that impacts the lowest number of non-target species
- In private areas of Windsor Great Park, we do not spray for OPM at all. This maintains the biodiversity of the area and sustains levels of food sources for nesting birds
- Tree surveys indicate that dense woodland remain unaffected by OPM. This means our ancient trees are largely free from spray and helps to protect the rare moths that rely on these older trees including the heart moth, triangle moth and moon moth
Our Arboriculture Team spraying oak trees.
Nest removal
Where spraying has not been fully successful in managing OPM found on an oak tree, a further response is the removal of OPM nests:
- This removes any residual caterpillar hairs inside OPM nests
- The removal of nests reduces the subsequent population of moths and future caterpillars next year
Trials at Windsor Great Park
As we continue to balance our commitments to public safety and biodiversity, Windsor Great Park are carrying out trials in collaboration with other organisations to explore more species-specific responses.
- In 2014, a predator of the OPM was first discovered in the UK, the tachinid fly (Carcelia iliaca). In a trial with Newcastle University, the team are releasing the fly in Windsor Great Park to respond to the spread of OPM
- The team are exploring the use of dispensers filled with the pheromones of the female oak processionary moth to act as false sources. The dispensers disorientate the male who are unable to find the female and in doing so reduces mating numbers.
These trials are still in their infancy, with a few years of data now collected, and particularly for areas of high public footfall allow for little control in managing OPM number on their own. However, we hope these trials will lead to a mixed control method that will contain OPM numbers in the future.
The tachinid fly.
Brown-tail moth
A second caterpillar that the team manage in Windsor Great Park is the Brown-tail moth caterpillar.
Similar to the OPM, the caterpillar has hairs that can cause breathing difficulties and rashes when touched.
Unlike the OPM, the brown-tail moth caterpillar spin dense web-like nests on trees and hedges in late summer.
The Brown-tail moth is native to Britain and predators like birds, hedgehogs and beetles are able to maintain population numbers. Although the webbing may look alarming, the plants themselves generally survive with only minor checks in growth.
The Forestry Commission have no requirement to manage numbers of brown-tail moth however Windsor Great Park does take action against this insect in areas of high footfall.
Butterflies and moths are an important part of our ecosystem so where contact is low, we tolerate populations of this caterpillar.
A brown-tail moth caterpillar.
A brown-tail moth nest.
Remaining vigilant
OPM and brown-tail moth caterpillars can be found across south-east England.
If you suspect you have come into contact with OPM caterpillars, or a nest, then please seek medical advice from a pharmacist or GP.
Similarly, if you suspect your dog has come into contact with a nest or caterpillars then please seek advice from a vet.
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