Dr Glynn Percival and Emma Schaffert of Bartlett Tree Experts give a rundown on HCLM and how to manage it
Since its introduction to Britain in the 1600s, the common white flowering horse chestnut tree (Aesculus hippocastanum) has become one of the commonest ornamental trees in towns, cities, parks and woodlands.
Over the past decade this species has been attacked by the leaf mining insect Cameraria ohridella. The horse chestnut leaf miner is a moth about 5mm long first seen in the UK in Wimbledon in 2002. Since then it has spread at a rate of 40 to 60km per year. It is the larvae that are most problematic for the horse chestnut foliage, however.
Symptoms to look out for
The larvae are leaf miners that tunnel into the leaves of the tree for food and protection, causing damage to leaf tissue and stunting future growth. Infected leaves are covered in small brown patches that spread rapidly across the entire tree, giving an autumnal appearance by July/August. Eventually the leaves die and fall prematurely and when new ones grow they are again infested.
The moth is able to live at temperatures as low as -23°C and can achieve as many as five generations each year. Adult moths appear from April and from May until August lay their eggs along or near the lateral veins of the leaves. A female can produce an average of 20-40 eggs, which hatch after two to three weeks. Larval development takes up to four weeks.
The long term impact of repeated infestations is still not fully understood and remains an area of debate between scientists. Many researchers consider HCLM a cosmetic pest as trees have reflushed following heavy infestation. However, scientists studying its effect on reproduction concluded that reduced seed weight may impair seedling growth, reducing the viability of the species.
Recent research in the journal Urban Forestry and Urban Greening found a 37% reduction in tree energy caused by leaf mining, reducing seed (conker) dry weight by 50% (Percival et al, 2011).
Control without the use of chemicals is limited but removing fallen leaves during the autumn and winter can reduce future infestations. HCLM’s natural enemies are parasitic wasps but few species exist within the UK.
Effective insecticides that can be applied as a foliar spray or soil drench exist, providing longer term control. However, the use of insecticides within an urban or amenity environment is a sensitive issue. Recent research has concentrated on the use of an insect growth regulator (diflubenzuron) specific to moths and caterpillars i.e. non-toxic to honey bees, ladybirds, beetles, spiders and sucking insects such as aphids, scale or leaf hoppers. A single spray of diflubenzuron has been shown to provide 80 to 100% control over a growth season (Percival et al (2012) Urban Forest Urban Greening).
Trees in urban landscapes provide important practical, aesthetic and ecological benefits. With HCLM induced defoliation of many horse chestnut trees observed by mid August, their aesthetic qualities are limited. For these reasons Bartlett Tree Experts recommends this single spray treatment for controlling the pest.
Dr Glynn Percival is plant physiologist and technical support specialist for the Bartlett Tree Expert Company and manages its research and diagnostic laboratory at the University of Reading.
Emma Schaffert is a Research Technician for the Bartlett Tree Research and Diagnostic Laboratory.