Damage by C. ohridella is primarily an aesthetic problem, and there is no evidence that infestation, on its own, causes dieback or a decline in tree health, or tree death. Consequently, there is no reason to fell and remove trees just because they are attacked by C. ohridella. Even severely infested trees will re-flush as normal in the following spring.
However, damage to trees in parks, gardens and in other urban situations can be reduced by removing fallen leaves during the autumn and winter, and this can help ensure that trees retain their vitality.
In the long-term, it is hoped that biological control will lead to a permanent reduction in the pest population.
Where the moth is established, the safest and most practical means of control is to remove fallen leaves during the autumn and winter. C. ohridella over-winters as a pupa in the fallen leaves, and commercial composting of leaves or burning them (if local regulations allow) destroys the pupae and reduces the moth population in the following spring.
Composting is less effective when leaves are collected into smaller heaps, as in gardens, because temperatures in small heaps are too low to kill the pupae. However, in these situations, covering the leaves with a 10cm layer of soil or 15-30cm layer of other plant material, and leaving the heaps undisturbed until the end of May, will prevent adult emergence in the spring and will help to reduce damage.
Leaf removal from streets in London
(Photo: M. Bellett-Travers)
Leaf removal is effective because it eliminates the first generation of moths in the spring and reduces infestation at the start of the growing season. This is when trees are growing most strongly and when any reduction in damage is of the greatest benefit. Trees will become re-infested later in the summer, as moths fly in from the surrounding area, but damage at this time, toward the end of the season, is much less important.
A variety of chemical insecticides have been shown to be effective in controlling C. ohridella and preventing leaf damage, but they are difficult to apply safely and efficiently, particularly to large trees and trees growing in urban areas, and they are relatively expensive.
Insecticide applications are also effective for only one, sometimes two, growing seasons and therefore they would also need to be applied almost every year in order to maintain low rates of infestation.
Leaf removal is a far safer and more practical and environmentally friendly way of managing the pest.
Aesculus species and hybrids resistant to C. ohridella would seem more appropriate for new plantings at the present time. Even amongst white flowering horse chestnut (Aesculus hippocastanum), individual trees appear to vary in their susceptibility to the pest. In Europe, a small number of A. hippocastanum trees have been observed to be relatively resistant to attack and to suffer low rates of damage.
The use of resistant planting stock of A. hippocastanum has not been widely explored, but were it to be developed, it might prove effective in limiting damage from C. ohridella in new plantings.
See also Host plants of Cameraria ohridella
In the long-term, a permanent reduction in C. ohridella populations is only likely to be achieved by either native parasitoid wasps or other natural enemies adapting to the moth and causing greater mortality, or by the deliberate introduction and release of specific parasitoids as part of a biological control programme.
However, the area of origin of C. ohridella is still unknown and there appears no immediate prospect of finding and releasing a suitable biocontrol agent within the next few years.
It is not possible to prevent spread through phytosanitary measures because of the high risk of movement by passive transport. Controls on planting material would have little effect as nursery stock is moved in the dormant season when leaves are not present.
More detailed information on the likely impact of C. ohridella in Great Britain, and on ways to reduce damage to horse chestnut, can be found in:
Kehrli, P. and Bacher, S. (2003). Date of leaf litter removal to prevent emergence of Cameraria ohridella in the following spring. Entomologia Experimentalis et Applicata 107, 159–162.
Kehrli, P. and Bacher, S. (2004). How to safely compost Cameraria ohridella-infested horse chestnut leaf litter on private compost heaps. Journal of Applied Entomology 128, 707–709.
Mabbett T (2006) Horse chestnut leaf miner. Quarterly Journal of Forestry 101, 39–41.
Pavan. F., Barro. P., Bernardinelli. I., Gambon. N. and Zandigiacomo. P. (2003). Cultural control of Cameraria ohridella on horse chestnut in urban areas by removing fallen leaves in autumn. Journal of Arboriculture 29, 253–258.
Straw, N.A. and Bellet-Travers, M. (2005). Impact and management of the horse chestnut leaf-miner (Cameraria ohridella). Arboricultural Journal 28, 67–83.