The horse chestnut tree was introduced to England around 1600, and it has become an accepted part of the landscape. In many parts of England the trees are beginning to look a bit sad as their leaves turn brown due to moth attack – will natural British predators (and parasites) eventually control this new threat?
Horse Chestnut Trees
The tree (Aesculus hippocastanum) originates from the Balkan mountains, but it is now one of the most familiar trees in England.
- The large, terminal buds ('sticky buds') will magically burst out with big hand-like leaves when twigs are cut off and placed in a jar of water – an exciting prospect for youngsters, and for many children their first natural history lesson.
- Older children often collect the fruit ('conkers') in autumn and pierce the largest to attach a string for the traditional seasonal game of conkers. Two players take turns to bash their prize conker into their opponent's one, and the conker that survives is the winner.
This moth was also originally restricted to the Balkans, but it has recently spread throughout Europe. The eggs are laid on horse chestnut leaves, and the young caterpillars emerge to burrow their way into the leaf causing dead brown patches. There are several generations each year, and the last fall off the tree when the leaves are shed in autumn. This last generation is very tough and will overwinter in the dead leaf, emerging in spring to start the attack once more.
Since the moth is an introduced species in England it is not bothered by specialist parasites or predators and is running out of control. There are concerns for the future of the horse chestnut tree in England, and there is an organisation (conkertreescience) tracking the spread of the moth.
Native English Predators and the Horse Chestnut Leaf-miner
England has many animals that can harm moths - but how effective are they against this new leaf-miner?
- Some of our parasitic wasps are able to attack (parasitise) the moth, but they are not very good at it – they are 'generalist' predators and their behaviour is not yet adapted to this new host.
- Ladybirds (ladybugs) and Lacewings, while very good at controlling greenfly, seem to have little effect on leaf miners.
- Blue Tits (Parus or Cyanistes caeruleus), Great Tits (Parus major) and Marsh Tits (Parus or Poecile palustris) will feed on the larvae, pecking them out from within the leaves. They have a small but significant impact (currently eating an estimated two or three percent of the caterpillars), but they will probably get better at it as time goes on since they are inquisitive creatures and learn quickly.
Sweeping Up Dead Leaves and Burning Them
When horse chestnut trees are planted in urban environments they are often surrounded by pavements (sidewalks) or other hard surfaces. In these conditions the fallen autumn leaves are usually blown away over winter, so the cycle of re-infection is broken. In more natural surroundings the leaves can stay close enough to the tree to allow the moths that emerge in spring to fly back up into the tree and lay eggs. If fallen leaves are gathered and burned, the cycle is interrupted, or at least the level of initial infection in spring is greatly reduced.
Waiting for natural predators to adapt, along with burning autumn leaves, will eventually get the horse chestnut leaf-miner under control. An alternative would be to consider the use of insecticides, but while this would undoubtedly be effective it would be harmful to all other insects and would interfere with the ecosystem in unpredictable ways.
Sources:
- 'Sustainable control of the horse chestnut leaf-miner, Cameraria ohridella (Lep., Gracillariidae), a new invasive pest of Aesculus hippocastanum in Europe.' EU Project: QLK5-CT-2000-01684.
- 'What's happening to our conker trees?', conkertreescience.
- 'Horse Chestnut Leaf-miner Cameraria ohridella', UKmoths.
