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6. Chemical control of larvae - OPM manual

A number of pesticide products are listed in the UK Pesticides Guide 2012 published by the British Crop Protection Council and CAB International as being approved for professional use against caterpillars on outdoor amenity vegetation. This includes OPM (and brown-tail moth) in amenity situations.

Other products are also approved for professional use on amenity vegetation by the Health & Safety Executive (HSE) and the Chemicals Regulation Directorate (CRD) (formerly the Pesticides Safety Directorate). However, the status and availability of chemicals can change from year to year, so it is important to check the manufacturer’s label to ensure that there is current approval for the use intended.

Bacillus thuringiensis var kurstaki (BT) and diflubenzuron are most effective against the very young (L1 – L3 stage) larvae, so timing of application is crucial. Surveys and monitoring of egg hatch are important to getting this right.

BT and other growth-regulating insecticides are less effective against older (L4-L6) larvae, whereas deltamethrin can still be applied, provided there is good coverage and spraying takes place while larvae are outside the nests. It is therefore the preferred insecticide for dealing with these stages as an alternative to removal of larvae or nests (see Section 7 - Nest and larvae removal). 

BT and diflubenzuron are often favoured for pest control because of the way they target larvae, and because of their relatively low environmental impacts.

However, where eradication is the objective, such as in the Control Zone, a single dose of BT or diflubenzuron will not be fully effective, and a second, follow-up spray must be applied either seven to 10 days later in the case of BT, or two to four weeks later for diflubenzuron. In the Core Zone, should you decide to spray simply to suppress the numbers of larvae, a single treatment should be sufficient.

Deltamethrin is less selective, but kills on contact and is fast acting. It might be a more appropriate insecticide to use for the more mature larvae, which will have become tolerant to BT and diflubenzuron. However, deltamethrin is highly toxic to bees and aquatic life, and particular care must be taken when using this product (and diflubenzuron) on or near flowering plants and rivers, streams or other bodies of water. Always check the product label for full information.

Where other susceptible oaks are within 50 to 100 metres of an infested tree, and you are responsible for them, it is strongly recommended that you consider spraying these, too, as a preventative measure.

There will be special restrictions if the trees are in a Site of Special Scientific Interest (SSSI). SSSIs are protected by law to preserve their special wildlife or geology, and permission from Natural England will be needed before any work is carried out. If you think this might apply, you should speak to your local SSSI adviser or contact

Because OPM larvae live in the canopies of oak trees, particularly in parks and built-up areas, spraying will have to be targeted at the taller, more mature trees, often in areas where the public have free access, and along roadsides and between buildings.

Applying insecticides to tall trees is difficult, and coverage is not always complete. OPM larvae tend to gather near the tops of trees, particularly during the early part of the season, and it is important that the insecticide reaches the very top. Motorised sprayers will generally only reach heights of about 8 metres, while tractor-mounted equipment can spray to about 15 or 20 metres. Full coverage of very large trees will require the use of hydraulic platforms ('cherry pickers'). Because of the health and safety requirements when working at height, this work should only be carried out by properly qualified and trained operators.

There have been reports of dead and dying older larvae falling from recently sprayed trees, resulting in increased contact between people and the hairs, and associated health problems. Therefore, if it is decided to apply insecticide to treat L4 – L6 larvae, it is recommended that access to the area around the treated trees is restricted for some time after spraying to allow time for most of these larvae to fall to the ground. Laying plastic sheeting beneath the trees can help to assess when this has happened. It will also help with collection and disposal of the larvae. Alternatively, larvae can be cleared from the ground using specialised vacuum equipment. Warning notices advising of spraying operations will minimise the risk of accidental contamination.

Where pesticide use is needed, it must be done by appropriately qualified and equipped professional operators. If, however, you are responsible for a large number of affected oak trees, you might find it cost-effective to buy or hire the appropriate tools and equipment, including personal protective equipment (PPE), and attend or send your staff to relevant training courses. A range of courses are available from a number of organisations, and an Internet search on ‘pesticide training’ will locate these. You may also contact us for generic advice on training, at

We also recommend that you take out insurance, including public liability insurance, for the work.

Last updated: 20th July 2018