Frequently Asked Questions

  • 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 nests are full of stinging hairs because they have remained in the nest with the moulting and pupation of the caterpillars. If the nests linger, they will eventually fall from the tree and end up in the environment. The stinging hairs can then 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.

  • 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.

  • Can dogs and cats also get health problems?

  • 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 it on a lead if you are going to walk near oak trees known to be affected.

  • 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.

  • Are the 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 the moths harmful?

  • No, there are no urticating hairs on the moths.

  • When do the moths fly?

  • Usually between July and September.

  • When do the eggs hatch?

  • The caterpillars will usually hatch close to bud burst around mid-April. Although, sometimes not all eggs will hatch in the same year. This is one of the problems we encounter in controlling the pest as the spray used to kill the caterpillars will have no effect on the eggs.

  • How many eggs does a moth lay?

  • They can lay up to 300 eggs per plaque, but a female will often lay less then this in one place, preferring to use more than one location. The eggs are usually laid on twigs that are 2-3 year growth, enabling the newly hatched caterpillars’ easy access to emerging leaves. The caterpillars will usually hatch close to bud burst around mid-April. Although, sometimes not all eggs will hatch in the same year. This is one of the problems we encounter in controlling the pest as the spray used to kill the caterpillars will have no effect on the eggs.

  • Does the weather have an effect on the caterpillars?

  • A little. It seems that certain conditions such as an early, warm spring can help to increase the numbers. 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 cooler. 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.

    Ideally, you need to prepare at least a year in advance. This would enable for a plan to be prepared. In the first instance, pheromone trapping in the summer would help to determine the presence of OPM within the area. If OPM is found, then we would advise spraying the trees in the spring followed by remaining nest and caterpillar removal in May and June, prior to any summer events taking place.

    By using this method of planned preventative maintenance, you could avoid large nest removal costs if the nests were only noticed just before the event. It is far cheaper and more effective to monitor and spray the trees than to remove nests.

  • 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 01462 420851 or info@maydencroft.co.uk for prices and availability.

  • How do I get trained to deal with OPM?

  • Contact Maydencroft for details of our training courses.

  • Can I spray the nests myself?

  • No, not unless you’re prepared to invest thousands of pounds in an ultra-low volume (ULV) mist blower spray unit fitted with an electrostatic control. These types of machine are able to coat even the top most leaves of the tree from the ground. Attempting to climb around the canopy of the tree with a knapsack sprayer would be extremely difficult and frankly very inefficient. The chances of you being able to coat almost every leaf of the tree in this way are very slim. Contact Maydencroft for more information on info@maydencroft.co.uk or 01462 420851.

  • 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?

  • Although any garden insecticides that kill caterpillars would work on OPM, there would still be problems here. Firstly the dead caterpillars would fall to the ground but the hairs will still be toxic and you and your family are likely to be affected by them. The caterpillar bodies would still need to disposed of as clinical waste by a specialist so you may as well pay the to remove and dispose of the caterpillars in one operation as it will most likely cost the same. Secondly, these insecticides are non-selective and will kill almost everything else it comes into contact with, including all the beneficial species that might help us to control OPM.

  • How many hairs are on a caterpillar?

  • Up to 700,000 per caterpillar. They are microscopic, barbed and contain a protein called Thaumetopoiene which causes your body to send histamine to the area, causing the irritation.

  • 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 our specialist waste carrier for incineration at high temperature 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.

  • Can the moths be trapped?

  • Yes. We can use pheromone traps but these only trap male moths. This method is mostly used to help determine the amount of moths in a given area, although figures can sometimes be slightly skewed. This can be caused when there is large number of moths in the area of both sexes as the males seem to be more attracted to the real pheromone on the female moths than the artificial type. Either way, for every moth caught in a trap, that’s one more that won’t breed that year. Whilst it’s one of the tools in our arsenal, used on its own it’s unlikely to have any real impact on caterpillar numbers.

  • 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 wind, mowing, children playing, traffic etc. will all help to disperse them. Pets are also likely to become interested in fallen nests, leading to 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 its also very common to find nests at almost all heights, including ground level. Nests at head height and ground level pose significant risks to the public and pets.

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

  • No, although it depends. Usually, it’s enough to remove the nests and spray the tree in the spring to help to prevent a re-infestation. Sometimes, if safe to do so, it will be beneficial to do nothing as old nests are fantastic breeding grounds for parasitic flies that predate on the caterpillars. In some extreme cases, it will be better to remove the tree and replant with another species. This will add to the biodiversity of the area and will also rid you of the OPM problem in your garden (providing there aren’t other infected trees outside your property).

  • What will happen if my dog touches an OPM nest or caterpillar?

  • 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.

  • Are all oak trees affected by OPM?

  • In descending order of susceptibility:

    • Turkey oak
    • Pedunculate oak
    • Chestnut-leaved oak
    • White oak
    • Turners oak
    • Holm oak
    • Algerian oak
    • Hungarian or Italian oak
    • Sessile oak
    • Cork oak
  • Are any other trees affected by OPM?

  • 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.

  • When do the 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 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 will likely prescribe anti-histamine and in some cases, a steroid cream. Long term use of steroid cream should be avoided as it will thin the skin. The majority of cases of the rash will calm down and eventually disappear after a few days to a week.

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

  • The best way as a long term management plan will be for us all to work together to make changes to the landscape, planting and land management. By encouraging our native species to breed and populate the land, we’ll help to control OPM by natural predation. Removing nests and spraying trees is only a reaction and is not likely to have any long term effects.

  • Is it true that Brown Tailed Moth caterpillars also have stinging hairs?

  • Brown Tailed Moth caterpillars also have stinging hairs that, just like the Oak Processionary caterpillar, can produce negative health effects for humans and animals. It is also known that they can cause allergic reactions with repeated contact. They occur incidentally almost everywhere in the
    UK on many types of trees and shrubs.

  • 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.

  • 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 their own trees, unless otherwise agreed. In addition, there is a statutory duty of care;

    • As the owner of the trees, a site owner, local authority or other body has a duty of care to prevent damage to another person's property
    • Our advice is to consult this with the tree owner in the first instance.
  • What should I look for when employing a company to remove OPM nests?

  • The questions you should ask of a contractor offering to carry out OPM removal are:

    • How do you dispose of your waste? If the answer is anything apart from ‘treat it as clinical waste and use a specialist waste company to incinerate in a sealed, filtered incinerator’ then you should not employ them.
    • Likewise, the company should be able to provide proof of correct waste disposal.
    • Have your staff been trained correctly? Full, correct training is vital to protect the operatives and the public from risk. All OPM staff at Maydencroft must undergo a vigorous training and certification programme. You should ask for evidence of training.
    • What types of PPE do your operative use? If a company is removing OPM nests, the operatives should be fully suited, use a respirator, eye protection and have no skin or normal clothing uncovered.
  • How long does the risk period last?

  • The active period of caterpillars lasts until early/mid July. They will then pupate. However, stinging hairs will remain behind in the nests. It is important to have nests removed as quickly as possible. Old nests can be bought down by high winds and heavy rain causing a year round problem.

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

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

  • How far do the moths fly?

  • Females deposit 250-300 eggs after mating, but not all at once. Most eggs are laid in the tree or trees in the immediate vicinity. Then she flies away with the remaining 30 to 50 eggs. That can be a 20km away if the wind conditions are right, although in calm conditions, a maximum of 5km is usual. Males can even fly 50km away in extreme conditions but a maximum of 20km is usually the case.

    A general rule: moths tend to fly the shortest possible distance to lay its eggs on appropriate food source.

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

  • Yes, especially when caterpillars or complete nests with caterpillars have fallen out of the tree. They may group themselves on an object in the vicinity such as a letterbox, bin etc. after a short amount of time they will move into the tree again. They are able to form nests on the objects and then walk rom there into the tree to eat and return to the nest in the morning.

  • Why are pheromone traps used?

  • Pheromone traps are often used as monitoring traps. The Forestry Commission, Royal Botanic Gardens Kew, and other bodies have been monitoring for years to gain an insight into the amount of moths within the UK. Measures can be based on these catches. Every male who falls into a trap cannot mate anymore and indirectly contributes to a decrease in the population.

  • 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

    • Burning pain
    • Irritation
    • Redness
    • Swelling
    • Inflammation

    Rare symptoms

    • Vomiting
    • Dizziness
    • Fever
    • Malaise
  • Can I send a photo of a caterpillar or nest for confirmation of OPM?

  • Yes, you can. Please email any queries to info@maydencroft.co.uk

  • How can I deal with fallen nests?

  • The only way to process waste from OPM nests and caterpillars is to have it removed by a professional company. Strict protective guidelines have been drawn up for all parties in the waste processing chain. We strongly advise against touching the nests yourselves. If you need to wait for a control company then we advise you to temporarily shield the immediate surroundings of the waste from contact with humans and animals.

  • 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 to shed safely and turn into a moth. The nests with 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. The moth lays its eggs on oak trees, ear the tips of the branches. The eggs hatch in early April the following year.

  • What is Bacillus thuringiensis?

  • It is a bacterium that is naturally present in the soil and can produce substances that caterpillars cannot cope with, if they have eaten it. The bacteria develop in the gut of the caterpillar and damage the gut wall of the catterpillar, causing it to die. Controlling OPM with Bacillus thuringiensis is only effective if caterpillars eat sprayed leaves. Young caterpillars are generally more sensitive than older caterpillars.

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