On 4 August 2010, a Community Tree Officer with Sheffield City Council surveyed a semi-mature English oak tree that had been imported from the Netherlands and planted in March 2010 alongside another imported oak tree outside City Road Cemetery, Sheffield.
Under a branch he noticed a nest and cast-off caterpillar skins which he suspected might be those of oak processionary moth (Thaumetopoea processionea) (OPM). The tree officer sent photographs to the Entomology Branch of Forest Research on 5 August.
A Forestry Commission plant health inspector inspected both of the English oaks that had been planted as well as six smaller Cypress oaks that had also been imported in 2010 and planted about 4km from the cemetery.
One small OPM nest was found on the infested tree, with no evidence of OPM found on the other semi-mature English oak or the six Cypress oaks.
The inspector removed the nest and, because OPM is a tree pest not normally found in Great Britain, it was incinerated on site.
Because OPM over-winters at the egg stage the tree could not have been infested in Sheffield, so the infestation must have occurred in the Dutch nursery. As a result of this, exports of oak trees to the UK from the Dutch nursery in question have been suspended.
Given the low numbers of caterpillar skins, it is probable that there were not many eggs on the infested tree to begin with.
There were 24 caterpillar skins, but only three viable pupae in the nest when it was examined in August, which is three weeks into the flight period that has been recorded in the OPM outbreak in London.
It is therefore possible that 21 adult moths had emerged before the nest was destroyed, but no exit holes were observed on the nest. As a precautionary measure the imported trees were treated with the insecticide Deltamethrin in May 2011, when OPM larvae (caterpillars) emerge from their eggs.
Additionally, all known oak trees within a 1km radius of the infested tree were surveyed by Forestry Commission plant health inspectors in 2011 and 2012, and no symptoms of OPM infestation were found on either occasion.