Tougher action will be needed to stop a caterpillar threatening England鈥檚 oak trees from continuing to spread across the country, say researchers who found efforts to control the pest have failed to slow its steady growth.
The oak processionary moth (Thaumetopoea processionea) 鈥� or OPM 鈥� has spread across most of London, and occasionally even further afield, since it was first detected in south-west London in 2006 after being accidentally imported.听
The invasive species鈥� caterpillars eat oak leaves. In great enough numbers they can strip trees bare, leaving them more vulnerable to disease. The species is also a risk to human health, as the tiny hairs its caterpillars shed can irritate skin.
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There have been concerted efforts to physically remove the pest鈥檚 nests from oaks in public parks and woods, and insecticides have been used to kill the caterpillars, with a helicopter being used to apply the poison in one case. But 鈥渇urther controls are needed to reduce the infestation rate鈥�.听
at Newcastle University and colleagues collected information on the number of nests removed from London鈥檚 Bushy Park and Richmond Park between 2013 and 2020 and used it to build a mathematical model of the species鈥� advance. The team found OPM鈥檚 reproduction number 鈥� a measure usually used for infectious disease such as covid-19听鈥� has been consistently above one, meaning it is still spreading.
听鈥淲e can see the infestation rate鈥檚 been kinda steady since 2013,鈥� says Wadkin, whose . 鈥淚t shows the controls are keeping it down to a certain extent, because it鈥檚 not been increasing off the charts. But still, we are going to need something more to stop the steady creep outwards.鈥�
OPM is just one of a host of pests and pathogens putting pressure on England鈥檚 trees, alongside a fungus sweeping ash trees and a . While some have been caught and contained, , the UK government鈥檚 aim with OPM since 2011 has been to slow rather than halt its spread.听
From south-west London, the species spread at about 1 to 2 kilometres a year until 2014, but . The rate is slower than in the Netherlands, where it is around 20 kilometres per year.
鈥淚t seems we have have had quite a bit of success in dampening the footprint of OPM and maybe that鈥檚 helped it not expand as quickly as we would have expected,鈥� says at the Forestry Commission, who leads the 拢1.9 million-a-year effort to control the pest鈥檚 spread.
Asked whether Wadkin鈥檚 research shows authorities should be escalating those efforts, Hoppit says OPM is challenging to tackle. That is partly because we aren鈥檛 entirely sure where in the UK landscape all of its hosts, the oak trees, are. But the big difficulty is there is only what he calls a 鈥渧ery small window of opportunity鈥� to kill the caterpillars with insecticide spraying, which is only effective when they emerge from their eggs around April and before they pupate and fly off in July. Moreover, the insecticide is chosen to avoid harming other species, meaning it only works well when the caterpillars are small.听
鈥淚 would say it [the control programme] is a success when you compare it to what鈥檚 happened on continental Europe,鈥� he says.
Reference: bioRxiv,
