News from Forest Research: December 2006
It’s been a busy year for Forest Research’s Tree Health Diagnostic and Advisory Service. Several pest and disease problems that have been building up over the past few years have recently come to the public and media’s attention, and other new arrivals look set to cause further problems in the future.
Horse-chestnut trees have looked particularly bad this year, with bleeding canker causing dieback and stem damage, and leaves browned and disfigured by leaf-miner infestation and blotch diseases. This has caused an almost exponential rise in enquiries to Forest Research, as well as numerous media requests for information.
Owing to the high demand for advice, Forest Research’s Tree Health experts held several workshops to present and discuss horse-chestnut problems, along with other pests and disease issues, with the International Society of Arboriculturists (ISA), County Council and community tree officers, and the London Tree Officers Association.
Horse-chestnut leaf miner (Cameraria ohridella) and bleeding canker
This has been present in the London area since 2002, but this year, for the first time, severe damage spread to East Anglia and the Midlands, making the steady colonisation of the country by the moth much more obvious. However, despite damage to foliage, especially in late summer, the effects of the leaf miner are primarily aesthetic.
Much more serious is infection by bleeding canker, which can cause dieback and death of mature trees.
Gypsy moth (Lymantria dispar)
A potentially serious defoliator of oak and other broad-leaved trees, this pest had been confined to an area in NE London, but in 2005 a second and much larger population was discovered by local residents in Aylesbury, Buckinghamshire. Both populations are being monitored by Forest Research, using pheromone traps that catch the adult male moths. Although the Aylesbury outbreak does not seem to have spread further, the number of moths caught increased from 324 in 2005 to 415 in 2006.
It had been hoped that the small population in NE London was dying out, but pheromone trap catches there also showed an increase in 2006, coupled with a worrying increase in the area over which moths were caught.
Oak processionary moth (Thaumetopoea processionea)
Oak processionary larvae mass
Haruta Ovidiu, University of Oradea
This moth is a serious defoliator of oak trees in Europe and is often found in association with gypsy moth. It is widely distributed in central and southern Europe, but has been expanding its range northward, in all probability as a response to climate change, and it is now established in north France and The Netherlands.
However, during 2006, larvae of oak processionary were found for the first time in Britain at several locations in SW London, in some cases associated with trees imported from the continent.
In addition to its potential as a defoliator, oak processionary caterpillars also pose a significant public health risk. They are covered in thousands of small hairs that can cause skin irritation, respiratory problems and allergic reactions.
Forest Research staff met with other interested parties recently to discuss action against the oak processionary moth and how to best provide information about its impact on oak and threat to health. A wider forum on tree health problems will be held in London in April 2007.
What does 2007 hold?
With citrus and Asian longhorn beetles continuing to be intercepted throughout the UK (both species potentially very damaging), southern European cicadas in Tunbridge Wells, and plane lace-bug (Corythuca ciliata) recently found by Defra plant health officers breeding and damaging plane trees in Bedfordshire (another new arrival!), 2007 looks set to be an even busier year than 2006.
The good news, however, is that policy makers world-wide are now aware that pest and disease problems are rising sharply, along with increased global trade, and are putting an increasing amount of effort into improving biosecurity measures to combat these threats.