Trees can be afflicted by various pests and diseases that can threaten their health and survival. Being able to identify and manage these threats is crucial for protecting our tree populations. This guide provides an overview of some of the most common tree pests and diseases found in the UK.


Major Tree Pests


Oak Processionary Moth

  • Insect pest, larvae feed on oak leaves and can completely defoliate trees.
  • Larvae have toxic hairs that can cause rashes and breathing difficulties in humans and animals.
  • Found in London and surrounding areas, range expanding northwards.
  • Control involves nest removal, pheromone traps, and insecticides. Prevent spread by reporting sightings.


Asian Longhorn Beetle

  • Large wood-boring beetle that tunnels into hardwoods like maple, birch, and willow.
  • Can kill trees by girdling trunks and branches as larvae bore large tunnels.
  • Accidentally introduced via wood packaging from Asia. Outbreaks in Kent controlled by tree removal.
  • Monitoring survey traps help detect and track spread. Public reporting also critical.


Emerald Ash Borer

  • Metallic green beetle that bores into ash trees, native to Asia.
  • Larvae feed under the bark, disrupting water and nutrient transport in infested trees.
  • Has killed hundreds of millions of ash trees in North America since accidental introduction.
  • Recently discovered in the UK for the first time (2022). Potentially catastrophic for UK ash trees.
  • Survey traps being deployed across UK to monitor spread. Public vigilance critical in reporting sightings.
  • Control focuses on rapid removal of infested trees and chemical treatments to protect high value ashes.


Elm Zig-Zag Sawfly

  • Leaf-eating insect that defoliates elm trees, native to Europe.
  • Larvae feed in large groups, skeletonizing leaves leaving only veins behind. This can weaken and potentially kill trees.
  • Mainly affects wych elm but also English elm. Widespread across the UK.
  • Outbreaks tend to be sporadic when populations spike, previously in the 1990s.
  • Control is difficult due to camouflaged larvae. Options include insecticides, removing alternate host plants, or simply tolerating defoliation.
  • Elm leaves usually re-grow after attack, so healthy trees can recover well. Monitoring helps predict outbreaks.


Pine Processionary Moth

  • Insect pest affecting various pine species, larvae feed on pine needles.
  • Larvae nest in silken webs and have toxic hairs that can cause allergic reactions.
  • Native to southern Europe, range expanding northwards due to climate change.
  • Small established population in London, but most outbreaks still found on imported trees.
  • Control involves nest removal and destruction, insecticide injection, and pheromone trapping.
  • Public reporting of nests important to allow control and avoid contact. Do not touch caterpillars.
  • Larvae pupate in soil so can spread via soil movement. Plant quarantine helps limit inadvertent spread.


Major Tree Diseases


Acute Oak Decline

  • Affects mature oak trees, causes black weeping lesions on stems and dieback.
  • Caused by bacteria working together with insect pests like the oak jewel beetle.
  • Widespread in southern and central UK, no cure available.
  • Management involves improving tree health and removing heavily infected trees.


Ash Dieback

  • Fungal disease that has killed millions of UK ash trees over last 20 years.
  • Causes leaf loss, crown dieback and bark lesions, spreads via spores on wind and insects.
  • No cure, focus is on slowing spread, breeding resistance, and replacing lost ashes.
  • Public help needed to report suspected cases and prevent movement of diseased ash plants.


Phytophthora Diseases

  • Devastating fungal-like pathogens that cause root and stem base decay.
  • Particularly affect larches (P. ramorum) and alders (P. alni) but also beech, sweet chestnut, etc.
  • Spread by water and human activity. Spores can travel long distances via waterways.
  • Control by prevention – strict hygiene, sourcing clean stock, restricting plant movement from infected areas.


Canker Stain of Plane

  • Fungal disease caused by Ceratocystis platani that affects plane trees.
  • Causes dark staining and canker lesions on bark, leading to wilting, dieback and potential death.
  • Mainly affects London plane but also oriental plane. Widespread in southern England.
  • Spreads via root grafts and infected pruning tools. Prevention focuses on hygiene and avoidance of winter pruning.
  • No cure, so infected trees need to be removed. Resistant cultivars being bred as replacement trees.
  • Public reporting of suspected cases important to monitor spread. Do not transport plane tree material from infected areas.


Sweet Chestnut Blight

  • Fungal disease caused by Cryphonectria parasitica, infects sweet chestnut trees.
  • Causes canker lesions on bark which can girdle stems, killing parts or entire trees.
  • Accidentally introduced to Europe from Asia in early 1900s, now widespread across UK.
  • Spreads via spores on wind, rain, insects, birds, and mammals.
  • No cure, so involves controlling spread by pruning out infections and using resistant cultivars.
  • Has devastated sweet chestnut populations in UK, now listed as an endangered native species.
  • Public help needed to monitor spread and prevent movement of infected sweet chestnut plants/timber.


Larch Tree Disease

  • Fungal disease caused by Phytophthora ramorum, affects European and Japanese larch.
  • Causes needle loss and bark lesions leading to dieback. Can kill trees within one growing season.
  • Accidentally introduced to UK, first detected in Southwest England in 2009.
  • Spreads via wind and waterborne spores. Now widespread across larch range in UK.
  • No cure so infected trees must be rapidly felled to control spread. Replanting sites with other species.
  • Restrictions on larch movement from infected areas. Public reporting helps track and slow spread.
  • Has killed many millions of larches across UK, dramatically altering larch landscape. Ongoing monitoring and research into resistant strains.


Shoot Blight of Cedar

  • Fungal disease caused by Kabatina thujae, affects Western red cedar.
  • Causes dieback of shoots and foliage turning brown or straw-coloured. Can deform growth.
  • More prevalent in wet seasons. Widespread in the UK wherever cedar is grown.
  • Weak pathogen but can disfigure trees over time. Main impact is aesthetic damage.
  • No chemical control needed. Prune out affected shoots to maintain form. Avoid overhead irrigation.
  • Resistant cultivars available. Disease rarely life threatening to established cedars.
  • One of the most common foliage diseases of ornamental cedar trees in gardens, parks and landscaping.


If you suspect your trees are affected by any of the pests or diseases above, contact Arbex today.

You can get more information on tree pests and diseases from the Arboricultural Association here:

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