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5. Surveying trees and timing of control and treatment - OPM manual

Surveys can be carried out any time of the year, and by anyone with reasonable eyesight. Spring and winter are particularly useful times to check. The number and the height of the trees can influence the time of year when surveys are carried out, the number of times each tree is surveyed, and the life stages being looked for.

Where there are only one or two oak trees in a garden it is worth checking for symptoms fairly frequently, from the ground and perhaps also from any upstairs windows. Binoculars will be useful.

OPM do not build nests among the leaves, so when looking for nests, concentrate on the branches and trunk at all heights. Check the ground beneath the oak trees as well, because nests can be dislodged and fall to the ground.

Larvae of oak processionary moth (Thaumetopoea processionea) in classic nose-to-tail procession across the ground, LondonLarvae (caterpillars) can be present in all parts of the tree – on the trunk, branches and the leaves, and also on the ground, as in the picture.

Egg masses

Egg identification

Looking for egg masses (pictured above) during the winter months might be useful, although these are so small (2-3cm long) that they are often difficult to spot. It might be impracticable to look for them where oak trees are growing in large numbers.

Egg plaque

Egg plaque close up

Spent nests

Surveying for spent nests can also be carried out during winter, when the absence of leaves can make them easier to see

Pupal nest dislodged

Intact nest after falling to the ground

Pupal nest 

Rear view of unemerged nest showing pupal chambers

Finding egg masses or old nests will provide early warning of the likely presence of larvae the following spring, and will help Forestry Commission England plan insecticide spraying, which will have to be done in most cases if the trees are in the Control Zone.

If the affected trees are in the Core Zone it will be for you to decide whether to treat them. Any egg masses found can be marked for regular inspection from late March onwards, and used as indicators of egg hatch and appearance of the first-stage larvae. These are the primary targets for insecticide applications, the timing of which is critical.

The table below 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, L1 may appear in mid-April and L4 by the first week of May.

OPM life cycle chart


Surveys can be carried out at any time, but should be concentrated in the Spring, because the most effective method of controlling OPM is to apply insecticides against the early-stage larvae.

1st instar larvae 

First-stage (L1) larvae (pictured above) are very small: about 2mm long when they hatch in April and/or May. They are still less than 1cm long in the L2 stage, and are most likely to be seen between mid-April and mid-May. 

2nd instar - also known as L2

Second-stage (L2) larvae (pictured above) are still less than 1cm long when the reach the L3 stage, seen below feeding on oak leaves.

3rd instar

The first three larval stages are the most susceptible to the insecticides approved for use against OPM, especially the biological and growth-regulating insecticides Bacillus thuringiensis (BT) and diflubenzaron, which have a lower overall environmental impact.

Fourth-stage and older larvae also remain mainly within their silken nests during the day, where they are protected from chemical sprays. As a rule of thumb, therefore, insecticide applications will be most effective when applied to larvae smaller than 1cm. Most larvae will have reached this size by the end of May, and by mid-May in warmer years.

4th instar - also known as L4

Fourth-instar larvae massing on oak trunk

The main method of control for larger larvae (stages L4 – L6) and pupae is to manually remove and destroy them and their nests, either by using specialised vacuum equipment, or by hand.

Both of these techniques should only be carried out by properly trained and equipped professional operators. These can be commercial contractors, although some owners of large numbers of oak trees might find it cost-effective to train and equip their own tree or ground-care staff to do the work.

The larvae spin bigger silken nests and spend more time within these nests during the day as they grow larger.

L5 in procession

Fifth-instar (L5) larvae are seen above in characteristic procession on an oak trunk, which can be seen from May until the beginning of July.

Nest of oak processionary moth. London

They leave white silken trails leading from their nests (pictured above), and these become discoloured and harder to see over time.

L6 creating nest

L6 larvae creating their nest, from June to mid-July

Eventually, the larvae moult to the pupal stage, again within the nests. By this stage the nests tend to have become tougher and usually brown (they are whitish when first formed), containing cast skins and shed hairs.

Removing nests immediately after they are discovered will reduce further damage to trees, and minimise the health risks from dislodging the irritating hairs.

However, delaying nest removal until the larvae have completed feeding and have moulted to the pupal stage has the benefit of increasing the chances of destroying all of the larvae and pupae within the nests.

Large, old nests need to be removed with considerable care to reduce the exposure of people and animals to the hairs, which are inevitably shed, especially from the cast skins adhering to the nests. During this phase of the life cycle, larvae might also be seen massing on trunks and branches, and moving in the characteristic nose-to-tail processions.

Removing larvae and nests manually, by vacuum equipment or by hand, can be very effective in reducing OPM populations. However, this action alone is unlikely to lead to eradication, because it might not be possible to find and locate every last larvae and pupae, so it is best used as a follow-up supplement to insecticide treatment.


Oak processionary moth (Thaumetopoea processionea) adult

Adult moth (imago)

Adult OPM (pictured above) emerge and fly from around the middle of July to early September. Males are strong fliers, the females less so. Deployment of pheromone traps baited with the OPM female sex attractant pheromone can help to provide an indication of population size and distribution.

However, the traps only capture males and, because they are strong fliers, it is uncertain whether the distribution of captures in the traps is an accurate reflection of the local distribution of the breeding population. Consequently, captures soon after initial adult emergence will tend to provide the most accurate measure of the distribution of OPM in the local area. However, traps are unlikely to yield any meaningful information in the Core Zone. See Section 8 - Pheromone trapping .

Reporting OPM

If you do find OPM in your trees, regardless of which zone they are in, you must report it to the Forestry Commission to enable the distribution and spread to be monitored. In the Control Zone, reporting will also enable the Forestry Commission to treat infestations on private land.

You can report it by:

  • using our Tree Alert on-line pest reporting tool (preferred).

If you cannot use Tree Alert, you may:

However, please use Tree Alert if you can.

When reporting a sighting, please indicate the exact location of the affected tree/s as precisely as you can.

    • A 6-figure Ordnance Survey grid reference is ideal.
    • Otherwise provide a full address, including the street or road name, the property number and/or name, and the full postcode
    • If that is not appropriate, describe the location in terms of distance and direction from an easily identifiable feature, e.g. “25 metres northwest of the park gate in Smith Street”.



Last updated: 20th July 2018