For the next blog in our invasive species series we are moving from flora to fauna – the oak processionary moth (Thaumetopoea processionea).

While there is no current legislation concerning this species, the GB Non-Native Species Secretariat nonetheless name it as an Alert species, and requests that any sightings are reported.

Native to Central and Southern Europe it has been found in the UK since 2006 and has now been found in several locations in the west and south-west of London. It is thought to have been brought into the country through imported living trees, which had been infected by eggs. All oak trees imported into GB from other EU countries now require a “plant passport.”

The Oak Processionary Moth

It is thought that male moths have been found in the UK for a lot longer, compared to the female of the species they are very strong flyers and can fly over the Channel to reach southern England. However it wasn’t until females were introduced that this species became a problem. A female can lay 100-200 eggs per season, so each infected tree can host a high load count of moth larvae.

The moths lay their eggs at the end of the summer and these then hatch in spring. When fully grown, the caterpillars have a dark stripe along their back and a white stripe down each of their sides. In addition, they have reddish/orange warts from which long, white hairs protrude, down the length of their body.

Damage to tree health

The caterpillars eat the foliage of the host tree, mainly targeting species of oak, but other species of tree have also been impacted by this pest. They are a major cause of defoliation of oaks in Europe, and while they do not normally kill the host tree, they will cause considerable damage to the tree’s health and in some places (such as Germany) they have been associated with oak decline.

The larvae tend to stay together when feeding, and when resting they stay in communal nests made from a white silk webbing, normally attached beneath a tree limb or on the trunk of a tree. It can be the size of a tennis ball or much larger. The common name of processionary moth arises from a behaviour seen in the caterpillars, when travelling between their nest and feeding areas they will form a “procession” following one another in a long line.

Health risk

This invasive species poses a direct risk to public health and the NHS has issued a warning – the caterpillars short hairs contain a defensive toxin that is potentially harmful to humans. It can cause skin rashes, conjunctivitis and respiratory problems. Even without handling, the caterpillar hairs may break off and be carried in the wind, therefore old nests containing shed skin need to be disposed of properly as they pose a health hazard. The hairs can affect other animals too such as cats, dogs and horses, so it is important to keep them away from known infected trees.

If you come across an infected tree it is important you report it to the relevant authority. Then the infestation can be treated and the spread of this invasive species monitored. An accurate postal address with the full postcode, and/or a clear description of the tree’s exact position, is helpful. Digital photos sent with emailed sighting reports can also help experts to confirm the sighting.

Since this is a new species to the UK and is yet to become established, every effort should be made to eradicate them. Should their range increase and become more prevalent it could pose a real risk to both human and tree health in the UK.