Kirkham told a Barcham Trees seminar last week that the moth, which has hairs that cause humans severe rashes, "is running away at present and we will lose control of it this year. Within five years it will affect everyone within the M25."
He added that Thaumetopoea processionea could be joined in the UK by a pest from mainland Europe - the pine processionary moth. "OPM may not have killed a tree yet, but rest assured pine processionary moth will," he said.
OPM reached the UK from its native Spain and southern France on a consignment of fastigiate oaks from the Netherlands in 2006. On oaks it is confined to the species cerris and robur, but it has also been found on Fagus, Castanea and Betula - and at Kew, Kirkham has even found it on Ilex.
Each of its egg "plaques" holds around 200 eggs. A plaque is the size of a human little fingernail and is usually found on pencil-thin branches high up in the tree canopy. The eggs are laid in July and August ready for hatching the following year. Caterpillars emerge on 18-20 April, then burrow into buds, forming a web over them before starting to eat the forming leaves.
Only at the fourth instar do they start to become a threat to human health. Kirkham said he believed that because OPM is much more of a threat to humans than to trees, its eradication should be a matter for the Government rather than for arborists.
Full-grown caterpillars are aggressive, 40mm long and have around 62,000 hairs, which they can eject at will. Moths live two or three days in July or August. Males can fly 50km and females 1.6km.
Kirkham said tackling the problem included using the army in Belgium. Rigs that vacuum up nests and incinerate them operate 24 hours a day during summer.
Four years ago at Kew just three nests were discovered, but in 2009, 800 nests were destroyed. Spraying took place on 10 and 11 May, when 400-500 oaks were selectively sprayed. By spraying late like this, the caterpillars fall from the trees to the ground, which is potentially dangerous.
Each caterpillar hair has a barb to ensure it stays put. The hairs cause a severe, red, blotchy rash, rather like that from a stinging nettle. It lasts about six weeks.
This necessitates a post-spraying clean-up. The work is undertaken with a vacuum cleaner used for the removal of asbestos in industry. Personal protective equipment is used by operatives.
The hairs can remain active for up to five years. This year at Kew they have been dealing with the problem at the second instar. The key is to keep the caterpillars in the tree. Decis (Deltamethrin) has been 100 per cent successful in dealing with OPM this year and Kirkham said he will continue to spray at instar two annually at a cost of around £8,000.
He warned that it will become a huge problem in Richmond Park. Spraying will not be an option because it is a site of special scientific interest. "The pest doesn't like a forest - it prefers individual trees - so Richmond Park will be a playground for it,", he predicted.