The Oak Processionary Moth (Thaumetopoea processionea) is here and, 10 years after it was first spotted, it looks like it might be here to stay.... and it could be heading your way if it's not there already.
The moth is native to southern Europe where natural predators and climatic factors have tended to keep numbers in check. Unfortunately trade in plant material has led to a rapid expansion of its range over the last 20 years (as with a number of pests and diseases) and it is now established in the Netherlands and north Germany and has a foothold in the UK. The Forestry Commission now advise that it is considered impossible to eradicate the pest and that resources will concentrate on containing its spread. Some years ago I heard one London tree manager expressing deep frustration that he could not get the necessary budget and coordination to attempt eradication when the pest was first discovered in south west London in 2006. It was perhaps an opportunity missed and, some say, demonstrated a lack of policy and coordination in the management of urban trees at the time.
Despite control efforts, the spread seems to be inexorable and is now a major problem at some important sites such as Richmond Park, Kew Gardens and Syon House where enormous effort and expense is expended in removing nests. From the initial outbreaks found in several boroughs in West and South-West London and the Elmbridge and Spelthorne districts of Surrey in 2006, it was then found in the Pangbourne area of West Berkshire in 2010; Bromley, Croydon and Lewisham Boroughs in South London in 2012; Hackney, Waltham Forest, Tower Hamlets and Newham Boroughs in East London in 2014 and the Guildford District in Surrey, in 2015. A small number of nests were found in Watford this year and at the time of writing the critters are suspected in southern parts of Buckinghamshire and Hertfordshire.
This is not just bad news for Oak trees which are defoliated by the caterpillars, but also to us and to our animals. The caterpillars have thousands of hairs containing an irritant, thaumetopoein, which can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems. This can happen if people or animals touch the caterpillars or their nests, or if the hairs are blown by the wind. The caterpillars can also shed the hairs as a defence mechanism, and lots of hairs are left in the nests, which is why the nests should not be touched. The greatest risk period is May to July, but they can be present on old nests, and could be blown or touched at any time of year.
The moths are named after their most distinctive identification feature, namely the manner in which the caterpillars form nose to tail processions when moving around the tree, or across the ground to move from one Oak tree to another. This video footage show one such column...they could almost be goose-stepping!
Whilst the implications to human and animal health posed by this caterpillar are currently the principal cause for concern, it could be increasingly damaging to the Oak trees. The larvae munch through the leaves and can strip a tree bare. This leaves them vulnerable to attack by other pests and diseases, and less able to withstand adverse environmental events such as drought and flood. It is possible that repeated heavy attacks could kill trees. Although the nests have only been reported on Oak, if they run short of oak leaves to eat, the larvae have been observed on hornbeam, hazel, beech, sweet chestnut and birch trees. Both of Britain’s native species of oak, pedunculate and sessile oak, and several other oak species grown here are susceptible to attack. In a very broadly descending order of susceptibility, they are:
Turkey oak (Quercus cerris), English or pendunculate oak (Q. robur), chestnut-leaved oak (Q. castaneifolia), white oak (Q. alba), Turner's oak (Q. x turneri), Holm oak (Q. ilex), Algerian oak (Q. canariensis), Hungarian or Italian oak (Q. frainetto), sessile oak (Q. petraea) and cork oak (Q. suber).
If you see this pest on your Oak trees you should report it to the Forestry Commission using the tree alert facility on their website . To be most effective control measures should be carefully timed and carried out by professionals with appropriate training and equipment. Do not try to remove caterpillars or nests yourself, because of the health risks.
The Forestry Commission advises:
Control efforts have currently seen the establishment of 'control zones'. It is intended to contain infestation within the 'Core Zone' which includes boroughs of Ealing, Richmond, Hammersmith & Fulham, Hounslow, Kensington, Chelsea and Kingston, as well as parts of Brent, Merton, Wandsworth, Westminster and Elmsbridge. The 'Control Zone' is a bufferzone of surrounding districts and where some presence has been found. The 'Protected Zone' is currently (largely) unaffected areas. Government policy is to contain spread from the West London 'Core' outbreak, and to eradicate it elsewhere. In general terms, Forestry Commission England will take charge of all aspects of treatment of privately owned infested trees in the Control and Protected Zones in 2016, and there will be no costs for the owner. In the 'Core Zone' control by owners is 'strongly advised' but it would appear that no help is offered. This can only be due to the scale of the problem and cost of control. But without determined efforts in the Core Zone, there will surely be a perpetual need for vigilance and control in the Control and Protected Zones. The current efforts must certainly have restricted its progress, but if containment isn't completely effective, there must surely be an exponential expansion of its presence across the country. Lets hope this has taught us to act swiftly and thoroughly when the next pest arrives because, despite Brexit, it surely will.
For information about regulations and requirements applying to the importation of oak plants, or to the movement, handling and disposal of oak material in affected areas, contact the Forestry Commission's Plant Health Service.
T: 0300 067 5155
Rather unhelpfully, the Pride of India tree comes from China (where it has often been a symbol used to mark the graves of scholars). Equally confusingly, its other common name, Goldenrain tree, is shared with Laburnum; so I tend to stick with the Latin - Koelreuteria paniculata.
Koelreuteria is another tree that has to be on the short list for smaller sunny gardens. It has attractive foliage, provides a striking mid-summer floral display, and has fiery autumn colour.
It belongs to the Soapberry family (Sapindaceae) which has lately been expanded to include the Maples and Horse Chestnuts. It is hardy, tolerates a wide range of soils, drought and air pollution, making it suitable for many locations - including as a street tree. The wood is quite brittle though and some shelter can help. The tree was introduced in 1763 and is said to have first been cultivated in Croombe, in Worcestershire. It is named after J. G. Koelreuter, a professor of botany at Karlsruhe (1733-1806).
The tree can attain 30 - 60 feet in height and can be rather gaunt in habit when young, becoming more compact with age.
The phytochemistry of Koelreuteria paniculata is known to be quite diverse and it has a medicinal reputation that includes antioxidant and anti-tumor activity.
The tolerance of Koelreuteria to a wide range of conditions and its ability to set viable seed mean that it is becoming invasive in Florida and Hawaii where it can out-compete native vegetation. Who knows, a touch more global warming and it will begin a march through our woodlands?