The Oak Processionary Moth (Thaumetopoea processionea) was first identified on oak trees in west London in 2006 and can severely defoliate oak trees. From mid-May to the end of July, when the caterpillars molt, there is a significant health risk to humans. Each caterpillar has over 600,000 hairs that cause itchiness, inflammation and respiratory problems when humans are exposed to them. In the forest the caterpillar is contained by natural predators but on lane trees it can propagate into a real plague.
The Oak Processionary Moth most likely came into Britain as over-wintering eggs on semi-mature trees imported for planting in landscaping projects. It began breeding in several locations including the Royal Botanic Gardens Kew, and Richmond Park.
The Oak Processionary Moth has been largely limited to a core outbreak zone in west London, namely the boroughs of Ealing, Brent, Hounslow, Richmond Upon Thames and Hammersmith & Fulham. An outbreak was also discovered in Pangbourne in Berkshire in 2010.
The Forestry Commission recorded 700 nests in the core outbreak zone in 2007, 500 in 2008, 2500 in 2009 and 2100 in 2010, and most were destroyed before adult moths emerged. The large increase in 2009 might have been due to the very favourable conditions for caterpillars of all species that year, and surveyors’ growing expertise in finding them.
Despite the past two years’ increased numbers, no evidence was found that the moth had spread beyond the five boroughs. Eradication from this area is now considered impractical, so the Forestry Commission’s policy is to work to contain it. The commission is therefore surveying a 10km buffer zone around the infested area.
Oak Processionary Moth Lifecycle
The caterpillars pupate in their nests in late June and early July and emerge as moths between one and four weeks later. The moths lay their eggs in oak trees in July and August, and the eggs hatch caterpillars the following spring. The caterpillars feed in groups, and at other times congregate in communal nests.
The Oak processionary moth is a native of southern and central Europe and gets its name from the caterpillars’ habit of moving about in nose-to-tail processions.
This wobbly but insightful video by nextnex provides a good illustration of the how the caterpillars migrate:
Treating the Oak Processionary Moth
Management of an outbreak of the oak processionary moth includes treating the caterpillars either by vacuum or by insecticide at critical stages in their development during the spring, and removing and destroying their silken nests when any remaining caterpillars congregate in them in summer to pupate into adult moths.
Because of the health risk, the public should not try to deal with the caterpillars or their nests themselves. If you spot an Oak processionary moth nest call your local council. The picture right is of a Pine Processionary Moth nest which is a pest in parts of Europe. The Oak Processionary Moth nest with its silken webbing looks similar.
Our health advice is to stay away from caterpillars or their nests, and to consult a GP or the NHS about any unexplained skin or irritation to the skin, throat or eyes, particularly during spring and summer. The caterpillars and their nests are always dangerous to approach because of the presence of toxic hairs, which can remain present and harmful in old nests for some years, so do not attempt to remove the nests yourself.