OPM is an established pest in parts of London and is spreading beyond surrounding areas, but the rest of the country is designated as a Protected Zone (PZ).
OPM caterpillars feed on oak leaves and can increase trees’ vulnerability to attack by other pests and diseases, making them less able to withstand adverse weather conditions such as drought and floods.
Professor Nicola Spence, Defra Chief Plant Health Officer said: "Since 2012 we have invested more than £37 million in tree health research, including a dedicated programme of research and development on oak.
"We are working with local authorities and land managers to tackle OPM with a control programme of treatment and surveillance and in 2018 we introduced tighter restrictions on the importation of oak trees to England."
Dr Anna Brown, Head of Tree Health & Contingency Planning, Forestry Commission, said: "Those of us involved in importing or trading plants must maintain our vigilance against exotic pests and diseases such as OPM. There is a lot we can do such as buying British, only buying stock from reputable, responsible suppliers and inspecting imported plants.
The Forestry Commission, councils and land managers tackle the pest with an annual control programme of tree treatment.
Increased measures to protect the country from the spread of OPM were introduced in 2018. Restrictions on the import of most species of oak into England have been introduced as part of these regulations to protect native trees.
The new regulations ban the movement of certain oak trees into the UK’s OPM Protected Zone (an area of the EU declared free of the pest) unless specific conditions are met. They cover both imports from overseas and movement of trees from areas of the country where OPM is already present – in London and surrounding counties.
An oak processionary moth nest was believed to have been imported direct by an Italian nursery to a show garden designer at Chelsea Flower Show 2016. OPM was found at Chelsea shorty before 2018's event, and was eradicated, leading to extra checks ahead of 2019's show.
If you suspect OPM, you should not attempt to destroy or move infected material yourself as the nests and caterpillars can pose some risks to human health. For more on how to identify OPM, visit https://www.forestresearch.gov.uk/opm.