The Oak Processionary Moth at Bethlem Royal Hospital, Beckenham

by Ishpi Blatchley, March 2013.

Fallen nest of the Oak Processionary Moth,
Thaumetopoea processionea, in July 2012.
Photo by Ishpi Blatchley.

Last year, I was asked to lead some public nature walks associated with the Bethlem Royal Hospital's Summer Fayre held in early July. As I had only visited the site once previously, I accompanied Alister Hayes and the gardener on a recce which included small areas of woodland and two large meadows. Towards the end of the walk as we were nearing the hospital buildings, a strange-looking 'lump' on the grass caught my eye and when I got closer I could see it was moving.

I originally thought it was a dead rabbit which was host to hundreds of maggots, but on closer inspection I saw that the moving objects were caterpillars, not maggots. I had never seen anything like this before. On moving into the adjacent woodland glade we came across caterpillars processing along the ground, up trees and even going around in a circle, several caterpillars deep, doing an imitation of an F1 circuit.

Oak Processionary Moth larvae,
Thaumetopoea processionea,
on a tree trunk in July 2012.
Photo by Ishpi Blatchley.

And on looking up into the canopy of the trees we saw that many were severely defoliated. A processing caterpillar feeding on turkey oak — this must be the Oak Processionary Moth, Thaumetopoea processionea!

Alister and I were fascinated and spent some time finding new congregations, sighting the cobwebby nests in the tree branches and some on the trunks of the trees which had some pupating larvae.

It was not until I got home and looked up Oak Processionary Moth on the web that I realised we were dealing with a notifiable pest. The Forestry Commission were contacted and Bromley tree officers were alerted. The millions of hairs produced by the later instar larvae contain a toxin, thaumetopoein, which can cause rashes, sore throats, irritable eyes and even respiratory problems such as asthma. Neither Alister nor I suffered any bad effects but the gardener complained of a facial rash which lasted a couple of days.

A processing circle of the Oak Processionary Moth,
Thaumetopoea processionea, in July 2012.
Photo by Ishpi Blatchley.

The Oak Processionary Moth (sometimes just called the OPM) is a native of Europe and was first found in this country in the Richmond (SW London) area in 2006. It has been proved that the moth entered the country on imported nursery stock (does this remind you of anything???). Eggs hatch just as the oak tree comes into leaf and the larvae feed on the buds.

By early May when the caterpillars reach instar stages 3 and 4 they construct silken nests from which they emerge at night to feed. This is when the animal becomes more obvious — 'nests' can be seen high in the canopy and defoliation becomes obvious. As they mature, nests become bigger and can be found on larger branches and on trunks of trees. At this stage pupation occurs, and the adult moth emerges in July or August.

The preferred trees are our two native oaks, Quercus robur and Q. petraea. At Bethlem it was also on turkey oak, Q. cerris. However, it can feed on other oak varieties and if these are in short supply, hornbeam (Carpinus betulus), hazel (Corylus avellana), sweet chestnut (Castanea sativa), silver birch (Betula pendula) and beech (Fagus sylvatica) can also act as hosts.

The London Borough of Bromley's Arboricultural Officer, Julian Fowgies, has provided this up to date information (March 2013) about the LBB infestation:

Bethlem Royal Hospital: 4000 nests (approx) removed and destroyed.

Approximately 100 outside Bethlem grounds. Around 50 of these were found just over the border in Croydon, some of these were found in the grounds of a primary school. The remaining 50 were found in several private gardens, Langley Park Golf Course, private sports grounds and railway sidings within a 2km zone of the Bethlem site.

The winter survey work is near completion, which has identified another 3 nests to date.

Do look out for signs of this moth particularly in the Bromley and Croydon area where this infestation seems to have started. If you find any, report it to Julian Fowgies, julian.fowgies@bromley.gov.uk. Here is a link to photos of the adult and other stages on the UKMoths site: Oak Processionary.