If you think you have spotted OPM you should firstly report any sightings to the Forestry Commission using their Tree Alert tool available from www.forestry.gov.uk/treealert or by email to opm@forestrycommission.gov.uk. They can then keep track of the outbreak.

Secondly, please do not try to remove the caterpillars or nests yourself. They need to be removed by an Arborist who has had the correct training and has the necessary equipment require. The nests must also be disposed of properly to avoid the pest spreading.

Thaumetopoea processionea

Larvae massing on a tree branch.

Oak processionary moth (Thaumetopoea processionea), also known as OPM, is a defoliating pest of many oak species including English, Sessile, and Turkey oaks (Quercus robur, Q.petraea, and Q.cerris). The characteristically hairy caterpillars also feed on the foliage of hornbeam, hazel, beech, sweet chestnut, and birch.

Affected Areas

Areas affected by OPM
OPM is already established or quickly becoming established in many London suburbs.

West and Southwest London are most severely affected with OPM-infested trees confirmed in the following boroughs: Barnet, Brent, Bromley, Camden, Croydon, Guildford, Ealing, Elmbridge, Epsom & Ewell, Hackney, Hammersmith & Fulham, Hounslow, Islington, Kensington & Chelsea, Kingston, Lambeth, Lewisham, Merton, Mole Valley, Newham, Richmond, Southwark, Spelthorne, Tower Hamlet, Waltham Forrest, Wandsworth, Watford and Westminster.

Visit the Forestry Commission web site for more information on the spread of oak processionary moth.



Eggs are laid from July to early September. Each female lays between 100 and 200 eggs on twigs and small branches in the canopy.


Oak processionary caterpillars can be found from April to June. There are six stages during the caterpillar feeding cycle with caterpillars getting progressively bigger during each stage.

Stages 1-3: Caterpillars are very small when they hatch, around 2mm long, and are still less than 1cm by the time they reach the third stage.

Stages 4-6: Caterpillars spin silken nests and the larvae eventually moult to the pupal stage within those nests. During this phase, larvae may also be seen massing on the trunks and branches of trees and moving in the characteristic processions that give the moth its common name.

Adult Moths

Adult moths have a wingspan of around 30-32mm and grey forewings suffused with white and some darker grey markings. This coloration provides an effective camouflage against the bark of oak trees on which the adults often rest.


OPM effect on tree health
Any type of defoliating insect can have a serious impact on tree health, and OPM is no exception.

Large populations of OPM caterpillars can completely strip the leaves from a tree. Loss of leaves is unsightly and also reduces photosynthesis, thereby reducing the energy a tree has to grow. In a weakened state, the tree will be more susceptible to other pests, disease, and environmental conditions.


Before treating OPM, the first step is to look for infestations and confirm the presence of caterpillars. If found, there are a variety of treatments available for affected trees, including:

Biological control

Biological control is a method of keeping pests below damaging levels using natural enemies (such as Bacillus thuringiensis) and usually involves an active human role in the process.

Insect growth regulator

Insect growth regulator is a chemical that inhibits the life cycle of the insect.

Synthetic insecticides

Synthetic insecticides are botanical pesticides based upon plant extracts that are used to manage pests.

Removal of OPM caterpillars or larval nests is another method to control the spread of this pest. Caution must be taken when coming in contact as there are many risks to human health.

Below is a table that shows the recommended treatment options for the life-cycle stages of development of the oak processionary moth. The table sets out the life cycle of OPM. Note that the timings of the various stages are approximate, reflecting seasonal and local variation. In some years, S1 may appear in mid-April and S4 by the first week of May.

Recommended treatment options for the life-cycle stages of development of the oak processionary moth

S = Life cycle stage. Timing can be heavily influenced by prevailing climatic conditions. The table provides an estimation and should be used only as a guide.

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