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PESTS & DISEASES

OAK PROCESSIONARY MOTH

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OPM pose a threat to human health – it is very important that you DO NOT try and remove the caterpillars or nests yourself and seek the services of a skilled professional.

If you suspect you have a tree which may be affected by oak processionary moth (OPM), 
PLEASE CONTACT US USING OUR ONLINE ENQUIRY FORM OR CALL US ON 01256 212908 
and we would be delighted to assist.

You should also report any sightings of OPM nests or caterpillars to the Forestry Commission to help track outbreaks via TreeAlert or by email to opm@forestrycommission.gov.uk.

What is OPM?

Oak processionary moth (Thaumetopoea processionea), also known as OPM is a native species of southern Europe which was accidentally introduced to England. Left uncontrolled, it would spread across much of Britain. 

The caterpillars (larvae) of OPM are defoliating pests of oak trees, feeding on their leaves. The damage which their feeding does to oak trees is quite distinctive and noticeable, because they tend to leave the leaves skeletonised, with the main veins remaining.

Large populations can strip trees bare, leaving them vulnerable to other pests and diseases, and less able to withstand events such as drought and flood. They will usually only affect other broad-leaved tree species (such as sweet chestnut, hazel, beech, birch and hornbeam) if they run short of oak leaves to eat. However, they generally cannot complete their development on other tree species.

A public information leaflet covering key facts, identification and control of OPM is available here from the Forestry Commission website.

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THREAT TO HUMAN HEALTH 

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OPM caterpillars have thousands of tiny hairs which contain an irritating toxin called thaumetopoein. Contact with the hairs can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye irritations.

The Forestry Commission advises the following health precautions are taken in respect of OPM:

  • do not touch or approach OPM nests or caterpillars;

  • do not let children or animals touch or approach nests or caterpillars; 

  • do not try removing nests or caterpillars yourself; and

  • avoid or minimise time spent under or downwind of infested oak trees, especially on windy days in summer.

You should seek medical assistance immediately if you or your pets/livestock have come into contact with OPM and begin to suffer symptoms. 
 

LIFECYCLE

EGGS

LARVAE/CATERPILLARS

ADULT MOTHS

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OPM moths lay their eggs between late July and sometimes as late as late September. The eggs are laid in masses, or plaques, about 2-3cm long on high branches and twigs. The egg plaques remain on the trees during the following autumn and winter.

Caterpillars typically emerge from the eggs between aboutmid-April and mid-May the following year. The caterpillar stage lasts for several weeks during which theypass through six developmental stages, known as ‘instars’, numbered L1 to L6 on the below table (L = Lifecycle Stage). 

The caterpillars descend lower down the trees as they get olderand bigger and build nests on the branches and trunks anywhere between ground level and high in the tree. They do not buildnests among the leaves.

From about late June to early August, the caterpillars retreat into the nests and moult to the pupal stage. The pupae remain in the nests during this period until they are ready to emerge as adult moths.

The final, adult stage in the life cycle is the moth. The first moths emerge from their pupae, shedding their pupal cases as they do so, about the middle of July, and the last ones can emerge late in September. OPM adult moths live for only three to four days. 

 

The moths are difficult to distinguish from some other species of moth but have white and grey markings providing an effective camouflage against the bark of Oak trees. 
 

CONTROL

Before consideration of the correct treatment method for OPM, the initial step is to survey the trees to confirm the presence of infestations. 

Surveys for oak processionary moth (OPM) can be carried out at any time of the year but are recommended, from late March to May, between the caterpillars’ emergence from the eggs and the time they start building their nests. The caterpillars are only about 2mm long when they emerge and tend to remain high in the trees until they are older and larger. 

OPM caterpillars, or larvae, can be present in all parts of the tree – on the trunk, branches and leaves, and also on the ground.
If OPM is found to be present, there are a variety of treatment methods for the affected trees:

 

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Early-stage (L1 to L3) caterpillars are the primary targets for spraying treatments (bio-pesticide or insecticide applications) as these are less effective against older, larger caterpillars. The timing of spraying treatments is critical - this should be at the start of the year catching the caterpillar in juvenile form prior too nesting. 

Due to the nature of spraying, this method will not only target OPM, but other species within the vicinity. An element of spraying coupled with removal of the nests is preferred as a complete treatment. 

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The older caterpillars spin bigger silken nests and spend more time in them during the day as they grow larger, and the nests protect them from chemical treatment products. Therefore, the main method of control for L4, L5 and L6 is to remove them and their nests. This can be done 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.

Removing caterpillars and nests can be very effective in reducing OPM populations. However, this alone is unlikely to lead to eradication, because it might not be possible to find and destroy every last caterpillar and pupa as different nests or groups may be at different stages of their life cycle. It is therefore best used as a follow-up supplement to insecticide or bio-pesticide treatment.
 

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Pheromone trapping has proven to be an effective method of surveying the presence of male OPM, identifying possible outward spread of the pest and enabling prompt action to be taken to combat it.

Trapping is carried out by installing traps in the affects areas from approx. mid July to the mid September. The traps work by containing a lure comprising a synthetic chemical cocktail which mimicks the sexual pheromone emitted by the female to attract a mate for breeding.

Data is then compiled from throughout the years survey and treatment to ascertain overall effectiveness year on year. In theory, the number of moths caught should drop whilst undergoing treatment. 
 

The below table from the Forestry Commission shows the different treatment options for OPM and when these are recommended to be implemented according to the lifecycle stage of OPM. Please note that the timings of the various growth stages are approximate, reflecting seasonal and local factors and the table should only be used as a guide. In some years, especially in a warm spring, the L1 stage might appear as early as late March, and L4 by the first week of May.

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L = Life cycle stage.

If you would like to find out more about our specialist treatment programme, please do not hesitate to get in touch.

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