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Oriental chestnut gall wasp (Dryocosmus kuriphilus)

Oriental sweet chestnut gall wasp (Dryocosmus kuriphilus, gall wasp, chestnut gall, chestnut gall wasp, oriental chestnut gall wasp)Oriental chestnut gall wasp (OCGW) was discovered for the first time in the UK in a woodland in Kent, England, in June 2015. A second site was later confirmed with OCGW present in a small number of trees in a single street in St Albans, Hertfordshire.

Working with site owners and our partners in government, we took swift and appropriate action to reduce the population, and the risk of spread, as much as possible. We conducted surveys around the country for evidence of any other outbreaks, but found none.

Then follow-up surveying in summer 2016 found OCGW at several places in London and South-East England. The survey was mounted to find out whether OCGW was present at other locations than those found in 2015, and in compliance with our Protected Zone status for the species. (See 'Regulations and Powers' below.)

Confirmed finds locations map

Image courtesy of Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org

Identification and symptoms

OCGW is an insect of Asian origin which affects sweet chestnut trees in the Castanea family of trees. The only species of the Castanea family grown in significant numbers in Britain is the European sweet chestnut (Castanea sativa).

Oriental chestnut - gall wasp damage It is a pest of sweet chestnut trees because activity by its larvae (the 'grubs', or immature life stage) causes abnormal growths, called galls, to form on the buds, leaves, and petioles (leaf stalks).

The presence of these galls is the most obvious symptom of OCGW infestation.

The galls start out green when they develop at bud burst about March, and turn rose-coloured or red by June. They then gradually dry out and turn brown and woody over the summer, when the adult wasps emerge from them, and they can cause the leaves to drop early. Most galls fall off the tree when the leaves fall, although galls attached to the bases of petioles can stay on the tree for some years.

Galls can grow up to 4cm (1.75in) in diameter, although most of those seen in England have been between 1cm and 2cm.

They can occur at any height on the tree, and on any age of tree with buds.

No other organism is knoRose-coloured call caused by oriental chestnut gall wasp (Dryocosmus kuriphilus) on sweet chestnut (Castanea sativa) leaf stalkwn to cause galls on sweet chestnut trees, so their presence is a reliable indicator of OCGW infestation.

The adult wasp is tiny, being only 2.5-3mm long, with a black body, translucent wings and orange legs. It emerges in June and July, but its small size mean it is unlikely to be seen by most visitors to affected sites.

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Impact

OCGW is a low-impact pest of sweet chestnut trees. The wasp does not bite, sting or pose any other threat to people, pets or livestock.

It does not attack horse chestnut, or 'conker' trees, which belong to the Aesculus family, or any other species of tree widely grown in Britain.

However, in high numbers the galls can weaken sweeet chestnut trees and make them more vulnerable to other pests and diseases, especially sweet chestnut blight, which is caused by the fungus Cryphonectria parasitica. Severe attacks can result in tree decline.

Britain’s suitable climate and the presence of sufficient numbers of its host plant, sweet chestnut, mean that OCGW could get established here. The pest is also parthenogenetic, meaning it does not need male wasps to reproduce.

Sweet chestnut is valued as a timber species and is locally important in Britain, particularly in Kent, where the small chestnut coppicing industry has been enjoying a revival in recent years. (Chestnut trees quickly re-grow from their stumps after they have been felled. Therefore, rather than plant new trees after harvesting existing ones, chestnut timber growers use this re-growth to cultivate new crops in a sustainable cycle of felling and regrowth, called coppicing.) New markets for high-quality chestnut products have been developed.

Sweet chestnut trees are also the source of the edible variety of chestnut, popular in Britain as part of Christmas celebrations. Although there are some orchards, Britain is not a large producer of sweet chestnuts, and most of those consumed in Britain are imported, so supplies are unlikely to be affected by the current outbreaks.

We are working with those involved to minimise the impact as much as possible.

Survey and control

Dryocosmus kuriphilus galls & associated sweet chestnut leaf deathWe have carried out annual surveillance for this pest in nurseries, orchards and the wider environment since 2006, and found no evidence of it until 2015. 

Following discovery of the Farningham Woods outbreak we urgently surveyed chestnut trees in the surrounding areas and further afield in summer 2015 for any evidence of the pest elsewhere, and assessed what eradication or control action was practicable.

We served a containment notice on the owners to prevent sweet chestnut material being removed from the sites.

Sweet chestnut trees in the most heavily affected part of Farningham Woods were felled to minimise the risk of the pest spreading to other areas. All the sweet chestnut trees in the affected group of street trees in St Albans were felled and destroyed.

The adult wasps emerge, fly, and lay their eggs in early July, so to minimise spread of the pest in summer 2015 we needed to take action before that happened. We hoped that by destroying the worst-affected trees in Farningham Woods we would significantly reduce the population and likely spread, and preserve the possibility of eradicating it with later action.

The felling affected only a small proportion - less than  four hectares - of the 79-hectare total woodland area. Access to the operational areas was closed for safety reasons during the operation.

Healthy, useable timber was recovered for the market. Some of the affected trees were already due to be felled in 2015’s chestnut coppice wood harvest.

Later in 2015 our Forest Research agency examined bud samples from sweet chestnut trees at Farningham for evidence of egg-laying by the wasp, but found none. 

We continue to survey for OCGW to monitor its distribution, and work with owners to minimise its impacts.

Treatment

Control options include insecticide treatment. However, insecticide treatment of widespread outbreaks in the wider environment is unlikely to be effective because the galls encase the larvae, protecting them from chemical treatments.

An option in localised outbreaks is to conventionally harvest affected trees by felling or coppicing them and burning, deeply burying or mulching the lop and top (branch and tree-top material). Mulching (grinding the material into small fragments to destroy the pest) was used at Farningham Woods. The timber from the trunks can then be used in a bio-secure manner, e.g. for fencing.

Action

In addition to the actions under 'Survey and Control' (above), we contacted interested parties and woodland owners to offer advice and guidance. We circulated a symptoms guide for circulation to woodland owners.

Good biosecurity practice is the best available means of minimising spread and impact. It is very important that visitors to, and workers at, affected sites do not remove ANY plant material, including branches, leaves, twigs, sticks or bark, because this could accidentally spread the insect. Children must not take galls away to keep or play with. So forestry workers, tree surgeons, ground-care professionals and others working at affected sites must brush all soil and plant material off their footwear, clothing, tools, machinery and vehicles before leaving affected sites. Chestnut logs should be free of foliage, including epicormic growths, before being moved off site.

Regulation and powers

OCGW is a quarantine pest, giving national plant health authorities powers to take measures to contain or eradicate it.

The UK has Protected Zone status for this pest, which means that:

• we have a legal status as being free from Oriental chestnut gall wasp;

• sweet chestnut plant imports must be certified as coming from an area declared to be free of OCGW; and

• we must do annual surveys to check that the UK remains free of OCGW.

A statutory notification scheme is in place which means that pending landings of sweet chestnut plants and planting material must be pre-notified to the UK plant health authorities. This helps to raise awareness, build intelligence about the trade, and facilitate targeted inspections. This requirement was primarily intended to protect the UK from introductions of sweet chestnut blight, it also helps to provide protection against Oriental chestnut gall wasp.

OCGW is listed on the UK Plant Health Risk Register.

Origin

OCGW is a native of parts of Asia which has been accidentally introduced in international trade to Europe and North America. In Europe it has been recorded in France, Spain, Italy, Slovenia, Croatia, the Czech Republic, Germany, Hungary, Portugal and Switzerland.

We do not yet know how it entered Britain, and this is part of our investigation. The main pathways for the pest to travel a long distance are in movements of sweet chestnut plants, by being blown by the wind, or by being carried in vehicles. It can expand its range a short distance each year by flying.

Lifecycle

The female wasp lays eggs in the trees’ growth buds during the summer, and the eggs hatch within 30 to 40 days. The early stages of the larvae then lie dormant in the bud over the winter.

Oriental chestnut - gall wasp damageLarval activity resumes in the spring, and this activity causes the formation of galls, inside which the larvae develop, in early summer. The galls are formed on young twigs, on leaf petioles or on the midrib of leaves. These green or rose-coloured galls start at approximately 5–20 mm in diameter, and can develop up to 4cm in diameter as the leaf tries to form.

Adult wasps emerge during June and July, leaving exit holes in the galls. The galls turn brown and woody over time, and can remain on the tree for two years or more.

Reproduction is achieved from unfertilised eggs without mating in a process known as thelytokous parthenogenesis: male wasps have never been recorded.

Reporting suspected cases

Tree Alert online application

Report a sighting of Oriental chestnut gall wasp using Tree Alert

Before reporting a sighting, please:

  1. check that the tree is a sweet chestnut tree – we do not require reports of galls on other trees and
  2. check the symptoms section above

If you are unable to use Tree Alert, you may report a sighting by one of the following means:

Please be sure to:

  • Provide a precise location of the tree/s.
    • A 10-digit Ordnance Survey grid reference is ideal
    • otherwise provide a full address, including the full postcode and/or
    • precise instructions for finding the tree/s, e.g. “35 metres north-west of the park entrance in South Street”
  • provide a telephone number where we can reach you during the daytime to clarify any points and
  • provide a clear, well lit, close-up photo if you can

You are unlikely to see the tiny wasps, which are less then 3mm long.

We do not require report of galls on other trees.

Further information:

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Last updated: 17th August 2016