Oriental chestnut gall wasp

The oriental chestnut gall wasp causes green to red galls which distort the foliage and can affect the chestnut production of sweet chestnut trees.

Oriental chestnut gall wasp (Dryocosmus kuriphilus) on sweet chestnut (Castanea sativa)

Quick facts

Common name Oriental chestnut gall wasp
Scientific name Dryocosmus kuriphilus
Plants affected Sweet chestnut (Castanea sativa)
Main symptoms Galls on growth buds and leaves
Most active Galls develop in the spring

What are oriental chestnut gall wasps?

Oriental chestnut gall wasps are small insects (2.5-3mm) with black bodies and orange brown legs. Originating from Asia, this species arrived in Europe in 2002 and was first recorded in the UK in 2015 in a woodland in Kent and later in the same year in Hertfordshire. In these cases infested trees were destroyed in an attempt to stop the insect becoming established in the UK. In 2016 however, the gall wasp was found in several areas of London extending into Surrey. In 2017 this spread has continued and it is becoming fairly widespread in south east England.

If you suspect a case of oriental chestnut gall wasp, please report via TreeAlert.

Symptoms

  • Green, rose-coloured or red galls up to 4cm in diameter on the buds, leaves, and petioles (leaf stalks).
  • Leaf distortion as a result of the growth of the galls.
  • Leaves affected by the gall wasp can drop early, or remain on the tree throughout the winter.

Control

Whilst this insect is unlikely to affect the long term vigour of sweet chestnuts it can affect appearance and the crop of chestnuts. Other than destruction of infected trees there is no control for this insect. Pesticide control would be ineffective as the larvae are protected within the galls. In Japan, North America and Italy a parasitoid wasp (Torymus sinensis) has been released as a biological control agent, however, environmental risks associated with this method are still being assessed and this parasitoid is not available or known to be present in the UK.

Biology

The adult female wasps deposit clusters of up to 30 eggs in summer (June to July) on the developing buds of sweet chestnut trees. The eggs hatch after 30 to 40 days and the larvae grow very slowly through autumn and winter, overwintering in the buds. The galls develop from mid-April as the larvae feed within them for 20-30 days before pupating. New adults emerge from the end of May until the end of July. All adult wasps found to date have been female and do not need to mate in order to lay eggs (they are parthenogenetic).


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