A letter from Anthos and LTO said: "Dutch oak exporters to the UK should be aware of high risks when exporting oak trees to the UK because of the recent findings of the oak processionary moth.
"The English Forestry Commission advises the business community to carefully check the oak imported from the Netherlands and in the event of a find to report it to Treealert. inspect and inspect again is the advice of the committee. The commission also states that the UK importer should not blindly rely on the promises of the Dutch suppliers, or the results of the inspections carried out in the Netherlands.
"Based on this Royal Anthos, LTO and the Netherlands Inspection Service for Horticulture Naktuinbouw have warned the Dutch tree nursery companies about the high risks associated with oak trade with the UK. They therefore advise them to be extremely cautious when exporting them to the UK."
Defra biosecurity minister Lord Gardiner says he is considering tougher action against imports and is angry that a series of interceptions have found OPM on trees arriving from Holland in several English counties outside Greater London.
Defra said on 28 June that action is being taken by the UK Plant Health Service to eradicate late June findings of OPM in Hampshire, Warwickshire and Gloucestershire, including surveillance, tracing work and destruction of both the caterpillars and infested trees.
The Plant Health Service has also announced an urgent review of import controls on oaks. On June 14, Defra called for vigilance after imports of OPM-infested oaks reached Hampshire.
UK nursery Boningale's Tim Edwards subsequently called for a moratorium on oak imports and said it would no longer import oak. Majestic Trees' Steve McCurdy called for quarantines on imported oak to stop OPM spreading outside Greater London.
Anthos/LTO added: "Companies that nevertheless send oaks to the UK are advised to take into account the very stringent import inspections. Especially since the trees will also be thoroughly inspected at the final destination. After a possible find they will be destroyed. Apparently in such a case the entire lot is destroyed, so including the uninfected trees."
A mild winter and warm spring has meant OPM numbers are up Europe-wide this year. In the Netherlands, the infestations have also increased compared with 2018, with the oak-rich provinces of Noord-Brabant, Drenthe and Overijssel especially affected.