HANG ON TO YOUR CHESTNUTS!

That's the message from the Forestry Commission, which is urging people not to cut down their horse chestnut trees if they appear to be dying or in poor condition this summer, without having a reputable arboriculturist or tree surgeon check what the problem is first.

That's because the tree might not be dying at all.

Horse chestnuts across southern and central England are being attacked by a pest that can cause the trees to lose many of their leaves in late summer.

Far from being dead, though, the trees are probably still healthy.

The premature leaf loss is a result of horse chestnuts being infested with the larvae of Cameraria ohridella, or horse chestnut leaf miner, a moth that is spreading its wings across the southern half of England. The larvae mine within the leaves, which eventually shrivel and drop off as early as the middle of August.

The worry is that people will think about having the trees felled, when there might be no need.

Nigel Straw, a scientist with the Commission’s Forest Research agency, is monitoring the spread of horse chestnut leaf miner. He said:

“Infested trees do not look pretty, but they are well able to withstand the damage, and will reflush normally next year. And the owners of horse chestnut trees can help to limit the effects of this pest by disposing of the leaves. In autumn, the leaves can be collected and burned, or composted under 10cm (4 inches) of soil or 15cm (6 inches) of other plant material. This should reduce the degree of attack at the beginning of the next year and will help to maintain the health of the tree.”

“As well as the moths flying about and spreading, they can often hitch a lift on cars and lorries. This probably explains why, for instance, we have an isolated outbreak in Newport, south Wales.

“At the moment there is nothing we can do to stop the spread in terms of controlling the moth, and we have to live with the damage for now. Other countries have lived with the problem since the 1970s, and there is research being done to develop some form of control. We are looking to establish whether there will be any long-term impact on the trees.”

NOTES TO EDITOR / BACKGROUND:

1. HORSE CHESTNUT FACTS: Horse chestnut (Aesculus hippocastanum) is a native of the Balkans region of south-eastern Europe. It is believed to have been introduced to Britain in the 1500s.
2. ORIGINS OF LEAF MINER: Cameraria ohridella is a small moth, first noticed in Macedonia in northern Greece in the 1970s. It was described as a species new to science in 1985. The moth is not native to northern Greece, but appears to have been introduced from another part of the world. However, its area of origin is unknown at present.
3. ARRIVAL IN GREAT BRITAIN: The first report was in Wimbledon in July 2002. Since then it has spread throughout South-east England from Margate and Folkstone in the east to places as far west as Oxford and Bournemouth and as far north as Norfolk and the Midlands. There is also an isolated report in Newport, Wales, but there have been no reports in Scotland so far.
4. SPREAD: The moth will fly from place to place assisted by the wind, and will also “hitch-hike” on motor vehicles. Transportation by vehicles appears to be responsible for the sudden appearance of the moth in towns and cities a long way from known outbreaks.
5. METHOD OF ATTACK: Larvae of C. ohridella mine within the leaves of horse chestnut, and the damage caused by large numbers of larvae can be striking. Up to 700 leaf mines have been recorded on a single leaf under favourable conditions.
6. EFFECTS ON TREES: Severely damaged leaves shrivel and turn brown by late summer and fall early, well before the normal leaf fall in the autumn. There is no evidence from Europe, where the moth has been present for 15-20 years, that horse chestnut trees decline or are killed by the moth alone. Most of the damage by the leaf miner occurs too late in the season to have a significant impact on trees' vitality. However, infestation might further weaken trees that are already suffering from some other problem. In these cases, defoliation by the moth might hasten the decline, resulting in the trees' death sooner than if the disease had acted alone.
7. EFFECTS ON CONKER PRODUCTION. There might be an effect on conker production, but we don’t know enough yet to say where, why or by how much.
8. LONG-TERM EFFECTS ON TREE NUMBERS: We do not believe leaf miner will cause any noticeable reduction in the number of healthy horse chestnut trees in Great Britain. It is possible that some trees might suffer after several years of repeated attacks, but we think these will be a minority. We are undertaking research to answer this question. It is estimated that there are almost half a million horse chestnut trees in woodlands, copses, shelterbelts and other landscape-feature groups across Britain, as well as thousands of individual trees in streets, parks and gardens.
9. PREVENTION: It is not possible to prevent infestation now that the moth is here. It will spread naturally to all parts of Britain over the next few years, and damage will occur every year. The only long-term, permanent solution is to develop biological control using native parasites and predators, or natural enemies from the area of origin, to suppress the moth’s populations. However, because we do not know where the moth originates from, and do not know how it is controlled naturally, development of a biological control programme might be a long time away.
10. CONTROL MEASURES: Owners of horse chestnut trees can help to protect individual trees. In autumn, once all the leaves have dropped off an infested tree, they can be swept up and composted or burned. Composting is effective if the leaves are covered with a 10cm (4-inch) layer of soil or a 15cm (6-inch) layer of other plant material. This prevents the adult moths emerging the following spring, and thereby delays the return of severe infestation. This is a reasonably effective control in individual gardens. Trees in parks could be similarly protected, but this is a much larger task and not always a practicable option.
11. OUTLOOK: We cannot stop the spread of the moth or prevent damage to horse chestnut. It is possible that damage will be less severe in Scotland and northern England because there might be fewer generations of moths produced in a season. However, the problem is mainly aesthetic: severely infested trees look bad, but they are generally well able to withstand attack and should show no serious, long-term effects. A permanent solution to the problem might only eventually come about through biological control, but this might take many years to become established. Until then we will have to learn to live with the damage. We believe there will be a lot of reports this year, and hot dry weather means the moths will spread quicker and will produce more generations in a season, making damage more visible.
12. OTHER PROBLEMS OF HORSE CHESTNUT: Horse chestnut trees can suffer a disease known as "bleeding canker of horse chestnut". The symptoms are "bleeding" lesions on the trunks, and for many years the main cause was either of two known organisms, Phytophthora citricola and Phytophthora cactorum. Some trees die, others recover, and some trees weakened by the disease have to be felled for safety reasons. The rate of infection remained at acceptably low levels for many years, but has increased in recent years, and scientists are not finding Phytophthora in most of the new cases. They therefore believe that another, still-unidentified organism, possibly a bacterium, is to blame for most of the recent increase. It is estimated that between 35,000 and 40,000 horse chestnut trees are affected by bleeding canker, and between 2000 and 3000 have had to be felled because of it. Research is continuing to identify the new organism. For further information visit www.forestresearch.gov.uk/fr/INFD-6KYBGV.