Irish trees are struggling against several tree diseases caused by bacteria or fungi, including Dutch elm disease, ash dieback and horse chestnut bleeding canker. Another tree species is now facing a threat that could become established in Ireland.

The native oak tree species are vulnerable to the oak processionary moth species (Thaumetopoea processionea).

Although this non-native insect is not present currently in Ireland, vigilance is required from the public to stop this moth from becoming a pest here.

Native oak

Two species of oak, common or pendun culate oak (Quercus robur) and sessile oak (Quercus petraea), are native to Ireland. The two species are very alike in appearance and an easy way to tell them apart is to look at their acorns. The common oak’s acorns have long stalks, while the sessile oak’s acorns have no stalks. The name of the sessile oak, meaning “stalkless” highlights the difference between both species.

Oak trees were widespread across Ireland for centuries, but now are more often found in small areas of woodland. Oak trees are also planted in urban parks and streets. They can live for centuries and grow to over 40 metres high, with their impressive scale securing them pride of place in old estate house gardens. Oak leaves are dark green in colour and have wavy edges.

The flowers of the oak appear before the leaves and are known as catkins. Catkins are long, dangling clusters of small flowers found along tree branches. The oak seeds are the widely known acorns. Acorns are covered in a tough shell which sits into a cup-like cupule to protect the seed.

Large amounts of acorns are poisonous to cattle and horses, but not to pigs, which are allowed to forage acorns in forests across Europe.

Invasive species

The oak processionary moth is originally native to central and southern Europe, but has been introduced into the UK, Sweden, the Netherlands, Germany and other countries through imported oak saplings. Ireland is the only country in the European Union without an established oak processionary moth population.

In July 2020, a nest of oak processionary moth caterpillars was found in a Dublin park and was professionally removed.

No other incidents of oak processionary moths have been currently reported in Ireland. Careful examination of oak sapling imports into Ireland is necessary to spot any oak processionary moth present before the saplings are planted out. Oak processionary moths are most easily noticed in the caterpillar stage.

The adult oak processionary moth is nearly indistinguishable from other moth species. The caterpillars are covered in stiff, long white hairs with grey bodies and black heads. They move together in head-to-tail processions along oak trees and can strip an oak tree near bare of leaves.

Oak processionary moths are most easily noticed in the caterpillar stage

Afterwards, the oak tree is left susceptible to other diseases and environmental changes such as drought. The caterpillars are most likely to be seen on oak trees in late spring and early summer.

The guard hairs on the caterpillars can cause skin irritation, breathing difficulties and allergic reactions in humans and other animals. Oak processionary moths pose a risk to human health if the guard hairs make contact with skin, eyes, mouth or nose.

Oak processionary moth caterpillars or abandoned nests should not be handled or approached by the public. The caterpillar hairs can easily break off and be carried through the air.

Forestry experts at Teagasc or the Department of Agriculture, Food and the Marine should be contacted to remove the caterpillars from the oak trees at

Removal of caterpillars before they mature into moths and can reproduce is essential to stop the introduction of a breeding population of oak processionary moths into Ireland.

Did you know?

The Irish word for oak is dair, which appears in many Irish place names, including two counties.

  • Kildare or Cill Dara translates as “church of the oak”.
  • Derry or Daire means “oak grove”.
  • The oak is also mentioned in sayings used to predict the weather. For example, “Oak before ash, in for a splash. Ash before oak, in for a soak.” Both trees produce leaves about the same time in spring, but ash trees start leafing in longer hours of daylight, while oak trees are influenced by warmer temperature changes. The accuracy of these sayings is debated, but they, along with other associations of the oak, are a part of a rural cultural heritage.

    The oak tree is frequently used in furniture-making and cask-making and has influenced Irish nature, crafts and culture, which heightens the damage the oak processionary moth could cause in Ireland if it becomes established.

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