Oak Processionary Moth (OPM).

Oak processionary moth (OPM) is a defoliating pest that poses a threat to oak trees in the UK. These caterpillars are covered in toxic and irritating hairs that can cause skin irritation and respiratory distress in humans. OPM infestations were first identified in London in 2006 and have since spread to other parts of the country. Effective control and management of OPM populations are critical to preventing further damage to oak trees and reducing the risk to public health. In this article, we will explore the life cycle of OPM, its impact on oak trees and human health, and strategies for controlling and managing OPM populations.

Oak Processionary Moths: A Threat to Human Health and Public Safety.

Oak processionary moths (OPM) are defoliating pests that target English, Sessile, and Turkey oaks, as well as other trees such as hornbeam, hazel, beech, sweet chestnut, and birch. In 2006, OPM infestations were first identified in London. These caterpillars represent a human health risk, unlike other tree-feeding caterpillars. The third to sixth instar caterpillars are covered in toxic and irritating hairs that can cause skin irritation (dermatitis) and allergic reactions (rashes, conjunctivitis). Inhalation of these hairs can cause respiratory distress such as asthma or anaphylaxis. Even if the larvae are not handled, the hairs can easily break off and disperse in air currents, making abandoned nests risky to handle. Since OPM is found in urban trees, forest edges, and amenity woodlands where the public often visits, concerns have been raised.

The life cycle and monitoring of Oak Processionary Moths in the UK

In the UK, OPM eggs are typically laid from July to early September, which overwinter on branches in a single layer, covered with greyish scales. The first instar larvae hatch in April, and branch inspections during the winter months are recommended to locate and mark egg masses for inspection from late March onwards. This practice can serve as indicators of hatch times and the appearance of the first stage larvae, which are the primary targets for insecticide applications. Oak processionary caterpillars are typically observed from April to June, with six stages during their life cycle. The larvae feed in groups and congregate in communal nests made of white silk webbing spun under branches or on trunks. When not feeding, the larvae follow one another head to tail in processions to and from the nest and from one feeding position to another. Adult moths can be observed from July until early September. They have a wingspan of around 30-32mm with grey forewings suffused with white and darker grey markings that provide excellent camouflage against the bark of oak trees. The use of pheromone traps, baited with the female sex attractant pheromone, can provide an indication of population size and distribution. However, since the moths are strong fliers, it is uncertain whether the distribution of captures in the traps accurately reflects the local distribution of the breeding population.

Surveying and Control of Oak Processionary Moths.

Surveying for the presence of OPM is a key criterion for control, but it is a time-consuming process that requires patience, experience in identifying OPM symptoms, and a good pair of binoculars. Symptoms to look for include skeletonised leaf feeding, clusters of larvae on shoots, the underside of scaffold branches and on stems, and nest formation during later instars. Nest removal is labor-intensive, time-consuming, and costly, although essential. Nevertheless, some nests are missed despite vigorous searching, leading to ineffective control. Nest removal is done either by MEWP or rope and harness, which exposes arborists to the health risks of OPM and the stress and fatigue of working in the summer heat in full personal protective equipment (PPE). Full PPE is required because exposure to the OPM toxin is sensitising, meaning that the greater the exposure, the greater the effect on human health. Nests should be removed before the moth pupates and flies from July to early September. Nest removal alone, however, is not an effective control option as some nests are bound to be missed, leading to re-infestation the following year.

Baldock, K. C. R., & Pye, A. J. (2021). Integrated management of oak processionary moth. Arboricultural Journal: The International Journal of Urban Forestry, 43(2), 73-87.

CABI. (2021). Oak processionary moth (Thaumetopoea processionea). Retrieved from https://www.cabi.org/isc/datasheet/53093

Forestry Commission. (2021). Oak processionary moth. Retrieved from https://www.forestryengland.uk/oak-processionary-moth

Goulson, D., & Hughes, W. O. (2019). The Oak Processionary Moth: A Pest Species That Threatens the Health of Oak Trees and Humans in the UK. Annual Review of Entomology, 64, 1-20.

Hart, A. G., & Jiggins, F. M. (2020). Understanding the biology and management of the oak processionary moth. Forestry, 93(4), 429-441.

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Horse Chestnut Leaf Miner (HCLM; Cameraria ohridella).

This section provides essential information about the horse chestnut leaf miner (HCLM), a notorious pest that has wreaked havoc on horse chestnut trees in the UK. The text delves into the life cycle of the HCLM, the damage it causes to trees, and the potential long-term impact on the survival of the horse chestnut species. Additionally, the text explores various control measures, including both non-chemical and chemical options, while highlighting the challenges of implementing these measures in urban and amenity environments. If you're looking for comprehensive insights on the HCLM and how to control its spread, you've come to the right place.

Aesthetic damage caused by the horse chestnut leaf miner.

The horse chestnut tree (Aesculus hippocastanum L.), a common ornamental tree in the UK, has been under attack from a leaf mining insect known as the horse chestnut leaf miner (HCLM; Cameraria ohridella) for the past 7-8 years. HCLM is a moth that tunnels into leaves, causing significant aesthetic damage by physically destroying leaf tissue and stunting future growth. Infected leaves are covered in small brown patches which spread rapidly across the entire tree, giving an autumnal appearance by July-August. Eventually, the leaves die and fall prematurely, and when new ones grow, they become infected again.

HCLM is of particular concern because once established, the moth appears to maintain exceptionally high rates of infestation without any historical evidence of decline. The full impact of repeated HCLM infestations on long-term tree vitality and growth is still an area of debate among scientists. Although many researchers consider HCLM a cosmetic pest based on the fact that trees have re-flushed on an annual basis following heavy HCLM infestation with no significant crown die-back, other scientists studying the repeated effects of HCLM defoliation of horse chestnut on reproduction have concluded that reduced seed weight may severely impair future growth and survival of horse chestnut seedlings, endangering the long-term persistence of this species throughout Southeast Europe.

Controlling HCLM: Challenges and Options for Horse Chestnut Trees in Urban Areas

Non-chemical control of HCLM is limited, and natural control measures based on bio-control are unfeasible at present due to the lack of parasitic wasps that exist within the UK. Effective insecticides exist that provide longer-term control of HCLM that can be applied as a foliar spray or soil drench, but their use within an urban or amenity environment is reluctantly employed. Recent research has concentrated on the use of an insect growth regulator (diflubenzuron) that is specific to moths and caterpillars, which has been shown to provide 80-100% control over a growth season when used as a canopy spray. However, as aesthetic characteristics such as flowers, bark, berry, and leaf color are important considerations for trees planted within town and city streets, public recreation areas, parks, etc., the aesthetic qualities of HCLM-infested horse chestnut trees are now highly questionable. For these reasons, the use of insecticide control strategies may be warranted.

Santini, A., & Faccoli, M. (2017). Impact of invasive insects on urban trees. Journal of Applied Entomology, 141(8), 581-583.

Day, K. R., & Leather, S. R. (2017). Applying an ecological framework to manage insect pests: the influence of environmental and societal contexts. Journal of Applied Ecology, 54(1), 9-15.

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If you have concerns about the management of trees affected by pests or any other issues related to tree care, we are here to help. Contact us today to discuss your needs and we will be happy to assist you with any questions or queries you may have. We offer a range of consultancy services which may be of use in the managment of your site or grounds.

Managing Massaria: A Fungal Disease Threatening London Plane Trees.

London Plane trees, particularly the Platanus x hispanica species, are at risk from a fungal disease called Massaria, caused by the Splanchnonema platani fungus. Massaria can kill the bark and cambium of the tree's branches, leading to branch drop and posing a potential hazard to public safety and traffic. With London Plane trees being a major species planted within the London Boroughs, managing Massaria has become a major concern, and ongoing efforts are being made to monitor and manage the disease. This article delves into the challenges of managing Massaria, its symptoms, detection difficulty, and potential management options.

Massaria: The Fungal Disease Threatening London Plane Trees

Massaria is a fungal disease that can have severe consequences for the health of London Plane trees, causing branch drop and posing a potential hazard to public safety and traffic. The disease is caused by the fungus Splanchnonema platani, which attacks the bark and cambium of the tree's branches, leading to their death. Due to the significant number of London Plane trees planted within the London Boroughs, Massaria branch breakage has become a major concern, resulting in an increased maintenance cost. Tree inspections need to be conducted at least three times a year to monitor the health of these trees.

Detecting Massaria: The Difficulty of Observing Symptoms from Ground Level

One of the key challenges of managing Massaria is its difficulty to observe from ground level. The disease is characterized by a strip of dead bark that starts at the base of the branch and extends along the top. The width of the strip can vary but usually tapers to a distinct point. However, this symptom is not always present in larger branches greater than 20cm in diameter. Therefore, fallen limbs must also be examined for a clear zoned area of decay, and a white fungal mycelium present at the break point can also indicate Massaria infection.

Managing Massaria: Alternative Options and Ongoing Efforts to Monitor and Control the Disease

According to estimates from a London Tree Officers Meeting, around 23% of trees in the Royal Parks are infected with Massaria. The lack of registered fungicides or bio-control agents to manage the disease has led to discussions of alternative management options such as selective pruning to reduce water stress within the tree. Given the potential public and traffic hazard that Massaria branch breakage poses, it is essential that ongoing efforts are made to monitor and manage the disease, particularly in high-risk areas such as heavily planted London Plane tree locations.

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If you have concerns about the management of trees affected by pests or any other issues related to tree care, we are here to help. Contact us today to discuss your needs and we will be happy to assist you with any questions or queries you may have. We offer a range of consultancy services which may be of use in the managment of your site or grounds.

Ash Dieback in the UK: Understanding the Disease and Managing Its Impacts

Ash dieback, also known as Chalara dieback of ash, is a fungal disease that affects ash trees. It is caused by the fungus Hymenoscyphus fraxineus, which spreads through the leaves and twigs of the tree and can eventually kill the tree. The disease was first identified in the early 1990s in Poland, and has since spread across Europe, causing significant damage to ash tree populations. In recent years, it has become a major concern for foresters, ecologists, and the general public, as it poses a serious threat to the future of ash trees in many parts of the world.

Understanding Ash Dieback: The Fungal Disease Devastating Ash

Ash dieback is a fungal disease that is affecting the common ash tree and other species of Fraxinus trees. The disease is caused by a fungus called Hymenoscyphus fraxineus, which is native to eastern Asia. The disease was first identified in England in 2012, although research has shown that it is likely to have been present since at least 2005. Today, most parts of the country are experiencing the impacts of ash dieback, causing widespread decline of ash trees in some areas, and this is expected to continue. It is likely that the majority of our native ash trees will exhibit symptoms of ash dieback, but not all that do will die. A small percentage of ash trees will have a degree of tolerance to the disease, and others will exist in locations where they escape the worst impacts.

The severity of the disease varies locally, depending on the local conditions. Trees in woodlands with high proportions of ash are likely to decline more quickly due to higher inoculation loads. There is some evidence that ash trees growing in open, less humid locations such as streets and hedgerows may deteriorate more slowly or persist indefinitely, although it is not yet clear whether this will be a consistent pattern. Some trees with few symptoms could survive on these sites for many years, and a small proportion of trees may have a degree of genetic tolerance to the disease.

Identifying ash dieback:

Infection can lead to leaf loss and dead branches throughout the crown of ash trees, and clumps of new growth towards the centre of the crown. In areas of high infection, the fungus can cause lesions at the base of the tree, making it more susceptible to secondary infections, such as by Armillaria fungi.

Tree safety:

People who manage ash near roads, railways, buildings, and other publicly accessible land must consider the risks posed by infected ash. Trees or woodlands in these areas should be risk-assessed, monitored, and managed to reduce the risk. By law, the owner of land where a tree stands is responsible for the health and safety of those who could be affected by that tree. If you are unsure about health and safety risks, consult a fully insured tree management professional who holds a relevant qualification.

Guidance for land managers and tree owners:

It is important to consider the effects of ash dieback if you own or manage ash trees, even if you are dealing with low levels of infection. Felling diseased ash requires a felling licence from the Forestry Commission, unless the trees are dead or pose a real and immediate danger. Restrictions such as tree preservation orders must also be respected. Your local authority will be able to provide guidance.

Some ash trees may have genetic tolerance to ash dieback, meaning they may survive and reproduce to create the next generation of ash trees. Therefore, it is important to retain ash trees where they stand out as being healthier than those around them and it is safe to do so. Retaining a proportion of dead, dying, or felled trees will provide deadwood habitat and be beneficial for biodiversity.

Forestry Commission. (2022). Ash dieback. Retrieved from https://www.forestry.gov.uk/ashdieback

Defra. (2022). Ash dieback: managing the impact on England’s woodlands. Retrieved from https://www.gov.uk/government/publications/ash-dieback-managing-the-impact-on-englands-woodlands

Gange, A. C., Gange, E. G., Redfern, M., & Chen, Y. (2019). A meta-analysis of the effects of ash dieback on woodland arthropod communities. Insects, 10(10), 318.

McKinney, L. V., Nielsen, L. R., Collinge, D. B., & Thomsen, I. M. (2011). The ash dieback crisis: genetic variation in resistance can prove a long-term solution. Plant Pathology, 60(5), 934-942.

Talk to us about tree pests and diseases

If you have concerns about the management of trees affected by pests or any other issues related to tree care, we are here to help. Contact us today to discuss your needs and we will be happy to assist you with any questions or queries you may have. We offer a range of consultancy services which may be of use in the managment of your site or grounds.