Frequently asked questions

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  • Can dogs and cats also get health problems from OPM caterpillars?

  • Dogs and cats are particularly at risk when they sniff at nests or take them in their mouths. This can lead to serious injury to the oral mucosa and tongue. Contact a veterinarian if you see serious injuries to the mouth of your pet. You should keep a close eye on your dog and maybe keep them on a lead if you are going to walk near oak trees that are affected.

  • Are any other trees affected by Oak Processionary Moth?

  • Usually, OPM will only use oak trees however, if there are too many caterpillars on it and the food source becomes scarce, they will move onto other nearby trees. No eggs have been found on other species though.

    Trees where OPM have been repeatedly identified:Sweet chestnut, hazel, beech, birch and hornbeam.

  • What should I do if I get a rash?

  • This depends. If you are particularly sensitive to the irritation or begin to feel unwell then you should consult your doctor as soon as possible. They are likely to prescribe an anti-histamine and, in some cases, a steroid cream. Long term use of steroid creams should be avoided as it will thin the skin. In the majority of cases the rash will calm down and eventually disappear after a few days to a week.

  • How long does the risk period last?

  • Oak Processionary Moth caterpillars are active from March to July, however, there still may be stinging hairs present after this period on branches and behind nests. It is important to have nests removed quickly before they are brought down by high winds and heavy rain which will prolong the problem.

  • Can I come into contact with stinging hair through the air?

  • Yes, the hairs are microscopic and can be carried through the air in the vicinity of the contaminated oak. They can cause health problems if they settle on the skin or end up in the eyes or respiratory tract.

  • Can ivy clad trees have OPM?

  • Yes, ivy provides good shelter for the caterpillars, especially in warm periods. Nests between the ivy are treacherous because they are not easily observable. There are known cases of serious injury while carrying out maintenance work in trees with ivy containing an old nest that was clearly not visible.

  • What are the direct health problems caused by OPM caterpillars?

  • Skin complaints:

    Burning pain / irritation; itching; bumps or blisters; red skin / spots; infections

    Nose, throat and airway:

    Running nose; tickle / irritation in the throat; cough; hard to swallow; shortness of breath

    Eye complaints:

    Burning pain; irritation; redness; swelling; inflammation

    Rare symptoms:

    Vomiting; dizziness; fever; malaise
  • Are OPM caterpillars harmful at all stages of development?

  • No, in the early stages of development (Instar 1 – Instar 2, April - May) the caterpillars don’t produce urticating hairs. At Instar 3 (early May) they begin to produce the hairs, but have still not fully grown but this changes quickly after this and by mid-May to June, each caterpillar can have up to 700,000 toxic hairs.

  • Are OPM moths harmful?

  • No, there are no urticating hairs on moths.

  • When do OPM moths fly?

  • Usually between July and September.

  • When do OPM caterpillars appear?

  • Caterpillars usually hatch in April but at this point are very small and confined the top most tips of the tree, near the leaf buds. May – June is the time that caterpillars are usually spotted as they begin to descend the tree and gather in groups.

  • What does the development of the OPM caterpillar look like?

  • The first caterpillars hatch at the beginning of April. Within a period of approximately three months, the OPM grub eats the oak leaves and goes through five stages of development. Only after the third moulting does the caterpillar produce the stinging hairs that make this moth so dangerous to people and animals. The caterpillars build the recognisable nests on the tree so they can shed safely and turn into a moth. The nests that are full of stinging hairs stay in the tree when the moths have flown, and can continue to be a hazard. The moth flies from the end of July to September and then lays its eggs on oak trees, near the tips of the branches. The eggs then hatch in early April of the following year.

  • The moths have already flown out, is it still necessary to have the remaining nests removed?

  • Yes, but only if the nests present a risk to health. The remaining nests are full of stinging hairs from the caterpillars. If nests are left on the tree, they will eventually fall and the stinging hairs can spread through the environment. If there is little or no risk to health then old nests are a fantastic breeding ground for some natural predators such as parasitic flies.

  • Do the nests stay attached to the trees?

  • No. Strong wind and heavy rain will eventually dislodge the old nests. This leads to more people and pets coming into contact with the toxic hairs as they are barely visible and easily dispersed by the wind or grass cutting into busy areas. Pets are likely to become interested in fallen nests, leading to a range of problems (see ‘can dogs and cats get health problems’).

  • Are the nests always high in the trees?

  • No, most nests tend to be lower down, often on the underside of large limbs. Although nests can be found at almost all heights, including ground level. Nests at head height and ground level pose significant risks to the public and pets.

  • Does the weather have an effect on the caterpillars?

  • A little. It seems that certain conditions such as an early, warm spring can cause numbers to increase. A hot, dry summer can also have a similar effect, but if it’s very hot, the caterpillars can change their habitat and build their nests at the base of the tree and even into the ground to keep cool. Cold winters appear to have no effect on the eggs that have been laid on the upper branches of the tree. Tests carried out in Holland by Henry Kuppen found that putting eggs in a freezer for prolonged periods had little or no effect.

  • I hold outdoor events and I'm worried that the nearby oaks are affected. What should I do?

  • If there are oak trees within or around the event area, it would be advisable to contact a specialist to at the very least survey the trees for OPM.

  • Can the caterpillars also be on objects near the tree?

  •  Yes, especially when caterpillars or their nests have fallen out of the tree. They may form a group on an object in the vicinity such as a letterbox or bin for short periods of time before moving into the tree again. They may even form nests on other objects and walk from there to the tree to feed.

  • Can you come and give an OPM awareness presentation for my organisation?

  • Yes, one of our experts would be delighted to come and talk to your organisation about OPM. Please contact us on 0800 157 7707 or

    for prices and availability.

  • How do I get trained to deal with OPM?

  • Our experts are also available to give training on OPM. This can be provided at two separate levels; OPM awareness training and Working with OPM training with the latter being tailored to those who work in the sector and are at higher risk.

  • Why don't we just cut all the oak trees down?

  • Oak is the food source of the oak processionary caterpillar and is directly linked to the genus oak. However, oak is an extremely important tree species with a high ecological value. Thus, felling all oaks is not a realistic solution. However, it is considered necessary that woodlands and landscapes have a varied structure so that the effects of attacks are limited. A diverse tree population stimulates an improved biotope for natural enemies of OPM.

  • Are there natural enemies that you can put out?

  • Gauze fly larvae and larvae of the 2-dot ladybirds are available on the market. Other natural enemies must appear naturally by implementing appropriate measures. These larvae only eat the young caterpillars. They are not specialists. If there are aphids in the area or butterfly eggs, they also eat them.

  • What will happen if I just leave the nests?

  • If the nests are in an area close to people such as a garden, park or recreational area then this is likely to put people at risk, so it would be best to have them removed. Old nests falling from trees throughout the year into your garden wouldn’t be something you’d want. If there are nests on trees in woodlands where there is little or no public access, then leaving them could be very helpful in our long-term fight to manage this pest. This is because there are a number of beneficial species that would use the old nests to breed and overwinter, helping to increase the number of OPM predators for the following year.

  • Can I put old nests and/or caterpillars in the bin?

  • No, you will be spreading the toxic hairs across an even larger area and putting others, such as refuse collectors, at risk.

  • Can I use standard garden insecticide on the caterpillars?

  • There are issues in using garden insecticide on caterpillars. Firstly, even though this would kill the caterpillars, the toxic hairs would still be present. The caterpillars would still need to be disposed of as clinical waste so it is best to pay a contractor to remove and dispose of the nest. Secondly, garden insecticides are non-selective meaning they would kill almost everything that comes into contact with it, including any beneficial species that might help to control OPM.

  • How are nests and caterpillars disposed of?

  • All waste from our OPM removal works, including nests, caterpillars, all associated Personal Protective Equipment (PPE) are treated as clinical waste.

    Everything is triple bagged and taped on site and stored in sealed boxes for transportation. Once at the depot, the team will again don full PPE to enable them to transfer the waste into sealed UN steel drums. When these drums are full, they are taken away by a specialist waste carrier for incineration in a sealed incinerator fitted with special filters. All climbing equipment used for the removal of OPM is never used for any other purpose, is stored in separate sealed containers and is destroyed in the same manner as the waste at the end of the year.

  • Do I need to remove my tree if it has OPM?

  • Not in most cases. Often nest removal is enough to control infestations. Sometimes where there is a small chance of people or pets being in close proximity it can be beneficial to leave old nests as they are fantastic breeding grounds for parasitic flies that predate on the caterpillars. Only in extreme cases is it advisable to remove the tree and replant with another species.

  • What can be done about OPM in the long term?

  • The best long-term management solution would be a collaborative effort to make changes to the landscape, planting and the way we manage land. Maydencroft have an experienced consultancy team who will be able to put together a tailored management plan for OPM.

  • Does anything predate on OPM?

  • Yes, a number of UK species predate on OPM.

    These include starlings, sparrow, nuthatch, tits, woodpeckers and parasitic wasps and flies. It is important we support these species to control OPM numbers.

  • Who is responsible for the preventive treatment and/or removal of caterpillars and nests?

  • Legally, the tree owner is responsible for the costs of controlling OPM, unless otherwise agreed. Owners and managers of oak trees have a duty of care to prevent damage to other people and animals and to another person’s property. Our advice is to consult with the tree owner in the first instance.
  • What should I look for when employing a company to remove OPM nests?

  • It is recommended that you ask a contractor offering OPM nest removal the following:

    How do you dispose of your waste? Unless the company is planning on incinerating the waste on site and have gained permission from the local council then regulations dictate that it is taken off site and treated as clinical waste to be disposed of via a specialist waste company. The company should be able to provide proof of correct waste removal.

    Have staff been trained correctly? Full training is vital to protect the operatives and public from risk. All OPM staff at Maydencroft must undergo vigorous training and certification programme. It’s important to ask any company offering OPM nest removal for evidence of training.How do you ensure that your operatives stay safe? All operatives should be fully suited so that no clothing or skin is left uncovered, be wearing eye protection and use a respirator.

  • Can I send a photo of a caterpillar or nest for confirmation of OPM?

  • Yes, you can. Please email any queries to