Sunday 12 September 2021

Watch out for this caterpillar species.

Thaumetopoea processionea (Oak processionary moth). 

In late summer the oak processionary moth will lay its eggs on branches and twigs high in the canopy of Quercus spp (oak) trees in masses, or plaques of about two hundred eggs covered in scales that are greyish they spend the winter in this phase.

The eggs hatch in late April the larvae will eat the foliage of the oak trees always in groups.

When they are not feeding they spend most of their time in a group nest which is made of silk webbing and shed skin.

They get their name OPM (oak processionary moth) due to the way they travel as larvae following each other nose to tail in long processionary lines when moving between their nest and feeding areas.

The larvae can moult up to five times shedding their skin in the nest the older caterpillars spend most of their time in the nest.

Caterpillars pupate when they have been feeding for 9-12 weeks after this they remain in the nest for up to 2 weeks more before leaving the nest in July-August as adult moths which starts the whole process again.

They are harmless in their adult moth stage and quite beautiful.

The mature caterpillars have hairs emerging from warts and due to a natural chemical in them can be harmful to humans and animals. They can cause health problems such as conjunctivitis, respiratory problems, rashes, and allergic reactions.

This is especially a problem as even an empty abandoned nest is still just as bad due to the larvae shedding their skins in the nest up to five times each and the hairs will keep their toxicity. They should be treated the same as live larvae with great care and by professionals.

The best way to deal with abandoned nests is with controlled fire which burns all the harmful hairs.

Chemicals can be effective at the early stages of the larvae stage when most of their time is spent in their nest either individually targeted nests or if possible spray the whole tree.

The other ways of controlling and removing are vacuuming

The whole tree can be removed in some cases cut down carefully and bagged up to be burnt in a controlled and safe area.

Removing from the tree by climbing and bagging the nests carefully to be transported away and burned.

But all these and any other methods should be carried out by professionals with the correct training.

The Oak processionary moth is mostly found in London.

To report a sighting or suspected sighting email to
or telephone to 0300 067 4442.