Oriental chestnut gall wasp was discovered in a woodland in Kent, England, in June 2015, the first time this pest has been confirmed as present in the UK. A second site has now been confirmed with the gall wasp present in a small number of sweet chestnut trees in a street in St Albans. Forestry Commission staff are investigating neighbouring woodland and trees to establish the extent of the outbreaks, and have taken swift appropriate action to fell and remove the affected trees. Surveys are also being conducted around the country for evidence of any other outbreaks, but to date none has been found.

The invasive wasps were found by an Observatree tree health volunteer Amanda Yorwerth and her daughter Hannah in St Albans. Amanda said:
"It was quite exciting to get my first proper call to action when it was reported that Oriental Chestnut Gall Wasps had been found in Kent. Obviously it wasn’t good that another tree pest had arrived in the UK, but I set out looking at trees with renewed purpose. There aren’t many sweet chestnut trees around St Albans but people know where they are as they are one of the only wild trees around providing a free harvest."

"I immediately thought of a short row of sweet chestnut trees on a main road I often cycle past. So I persuaded my daughter, Hannah, to stop off with me for a moment to inspect the trees. She stood there whilst I started at one end of the row, peering up into the canopy, looking for puckered leaves and distinctive galls. It took a little while as there are lots of leaves on a sweet chestnut tree and Hannah began to get bored. The galls look smooth, a bit plastic, like a green or pinkish blob on a wrinkled leaf. It was Hannah who found the first one."

Amanda took photos and submitted a Tree Alert report. Whilst she was certain that Hannah had found OCGW galls, she was quite sure that it was being found all over and so what she had discovered wasn’t very exceptional. She changed her mind after numerous urgent phone calls from the Forestry Commission. The speed at which they went to look at the infested trees started to bring the importance of the discovery home, even receiving an email saying that the outbreak “had been discussed by the minister”. Within days the entire row of sweet chestnut trees were felled and every bit of the trees removed.

Oriental Chestnut Gall Wasp - top 5 facts:

This is the first time the pest has been confirmed as being present in the UK, more information

It does not attack horse chestnut or any other widely grown trees in Britain

As well as reducing nut production there is some evidence that it can affect timber production

It is the only insect that causes galls on sweet chestnut trees, so if you see something like this it’s likely to be an infestation

This is a quarantine pest which means that the Forestry Commission have the authority to contain or eradicate it

​Ask the experts – Dr Chris Malumphy, Senior Entomologist, Fera, answers some questions, more information is available from this CABI information leaflet 

1. How long do ‘gall forming’ symptoms last?

This depends where the gall is formed. The majority of galls found in Kent were found on the foliage which drops in the autumn or possibly earlier. In Italy galls often form at the base of the petiole (leaf stem) and become woody. They can remain attached to the tree for a few years. Only a couple of last year’s woody galls have been found in Kent.

2. What colour are the galls as they develop?

The galls are green while they develop at bud burst (from March onwards) and most turn reddish as they mature by June. The galls gradually dry out and turn brown over the summer and leaves may be dropped prematurely.

3. After the wasp emerges, is there anything left behind that leaves an indication of their presence?

Old galls will dry out, harden, turn brown and shrink. However galls that have been on foliage are likely to drop off the tree.

4. Can I get stung?

The wasp does not sting or pose any other risk to human health or to pet animals and livestock.

Information taken from the Observatree website which is jointly funded by Forestry Commission, Defra and Woodland Trust.