Pine Processionary Moth: Assessing the threat

Pine processionary moth (PPM) is native to much of southern Europe, Northern Africa, and parts of the Middle East. In recent years, the international live plant trade has brought them further north. They are now found in Switzerland, Hungary, and the Paris region of France. Unlike oak processionary moth (OPM), PPM is not known to be in the UK. However, individual moths, and at one time a transient population, have been recorded in England.

The pine processionary moth (PPM) is a focal species for Observatree due to the severe damage they can cause to pine trees and several other conifers. They are also a potential health hazard because their small hairs contain a protein that causes irritation to humans and animals. This protein, known as thaumetopein, gives rise to their scientific name “Thaumetopoea pityocampa”. Some sources claim that “Thaumetopea” comes from the Greek word for “wonder” (thauma) and “admiration” (poieo) at the nose to tail processions they form as caterpillars. There appears to be less debate over their species name, the latin “pityocampa”, which translates as “caterpillar of the pine”.

The caterpillars feed on living pine needles and in large numbers can completely defoliate trees. This weakens the trees, leaving them vulnerable to other pests and diseases. Commercially important European larch, lodge pole pine and our native Scots pine are particularly susceptible, but the process may also occur in other conifer species found within the UK.

Signs and Symptoms:

- Defoliation is one of the predominant symptoms of this pest. Complete defoliation may occur in high levels of infestation.

- For this pest you may see colonies of caterpillars form white, silken nests throughout the tree from late January onwards. The nests degrade slowly, discolouring to a muddy green hue. Hairy brown and orange caterpillars with blue bands start to appear in early spring.

Although related, there are several differences between OPM and PPM. These often come down to behaviour. For example, PPM are more likely to form single file processions rather than the “wedge shaped” formation commonly seen with OPM. Habitat is also a key indicator, as OPM are typically found nesting in the trunk or stems of oaks, whereas PPM nests are mainly found in the foliage of pines. You can find out more about these differences on the Forest Research website.

Behaviour and life cycle:

The most important feature of PPM behaviour are the characteristic nose-to-tail caterpillar processions, which are usually observed between dusk and dawn, as caterpillars often feed at night. After their spring emergence, caterpillars pupate in the soil until summer, when they emerge as dull coloured moths. The pupal stage may be extended until the following year, increasing the life cycle by one or two years. Adult PPM live for only 24 hours, giving them just enough time to mate and lay eggs.

International trade and warming climates increase the risk of PPM establishing breeding populations in the UK. If, after studying the identification guidance here, you think you have seen evidence of PPM, please report it immediately to the plant health authorities.

• Report suspected sightings in Great Britain to us using our Tree Alert pest reporting tool.

• Report suspected sightings in Northern Ireland to the Irish plant health authorities using Tree Check, the all-Ireland tree pest reporting tool.

Note that TreeAlert and TreeCheck require clear, well-lit digital photographs of the symptoms to be uploaded.