Oak processionary moth

What is oak processionary moth?

Oak processionary moth has caterpillars that are covered in irritant (urticating) hairs that can pose a health risk to humans and animals. The moth is not native to the UK but has become established in London and some surrounding areas since it was first found in south west London in 2005.

Adults moths are brown with a wingspan of 25-35mm and are similar in appearance to several other moth species found in the UK. They emerge from pupae in mid to late summer and only live for a few days in which time they mate and lay eggs usually high up in oak trees. The moth overwinters as eggs. The eggs hatch in spring, between March and late April. The caterpillars usually initially feed high up in oak trees, and at this stage often go unnoticed.

By late spring to early summer the caterpillars become larger (20-25 mm) and are dark in colour covered with many white hairs. At this stage they become easily recognised as they have the distinctive habit of moving around in nose to tail processions, which give the species its name. The caterpillars also construct white webbing nests on trucks and branches of oak trees. At this time the caterpillars spend much of the day in the nests leaving in processions to feed at dawn and dusk.

When the nests and processions of caterpillars become visible it is at this stage that the irritant (urticating) hairs have developed which pose the potential of causing rashes and respiratory problems in humans and animals. 

Symptoms

Whilst the caterpillars can defoliate parts of oak trees the most obvious signs of the presence of this moth are the dark hairy 25mm long caterpillars which are present in late spring and early summer. They are easily recognised as they have moving around in nose to tail processions, these processions usually leaving white silken trails behind.

In addition the caterpillars construct white webbing nests on trucks and branches of oak trees. The white silk often discolours brown. During late spring and early summer the caterpillars spend much of the day in the nests leaving in processions to feed at dawn and dusk, occasionally they will process between oak trees. The empty nests can remain present throughout the winter. 

The problem

The caterpillars of oak processionary moth can defoliate oak trees however it is the irritant (urticating) hairs that have the potential to cause health problems to humans and animals. For medical advice see NHS choices.

The moth is native to southern Europe but has recently become more widespread in northern Europe. It appeared in South West London in 2005 and has since become widespread in West and South West London. There have also been outbreaks in East London, Hertfordshire, Berkshire and parts of Surrey.

It is the subject of a control and containment programme co-ordinated by the Forestry Commission. For current information on the moths known distribution see the Commissions maps.

Control

Before reporting an infestation please check the information on the Commissions website. Key identification points;

• Oak processionary moth feeds almost exclusively on oak trees (Quercus)
• Oak processionary moth caterpillars can be distinguished by the distinctive habit of moving in nose-to-tail processions and clustering together, often in silken nests
• The nests are never woven in or on leaves but are always on the bark of oak trees. As empty nests can retain the irritating hairs these should also be reported
• Be aware that many other species of harmless moth caterpillar also produce webbing. In gardens these include the cotoneaster webber and small ermine moths

If you are still convinced they are indeed oak processionary moth caterpillars, what should you do?

• Do not approach or touch the caterpillars or nests
• Suspected cases should be reported to the Forestry Commission and where possible the landowner. If the landowner is aware that this moth is present in the location they may already have a strategy for its containment and control. Please follow any warning signs and safety information provided
• Due to the irritant hairs control of this insect should only be carried out by professional contractors. More information is available from the Forestry Commission

See also...

Angle shades moth
Box tree caterpillar
Brown tail moth
Buff tip moth
Cotoneaster webber caterpillars
Elephant hawk moth
Encouraging moths in your garden 
Forestry Commission information on oak processionary moth 
Holm oak leaf-mining moths
Lackey moth
NHS information on UK bugs that bite or sting
Oak decline
Oak gall wasps
Small ermine moths
Swift moth caterpillars
Vapourer moth
Winter moths