Dryocosmus kuriphilus
(Oriental chestnut gall wasp)

D. kuriphilus is considered the most serious pest of chestnut worldwide. It is very likely to be able to establish in many European and Mediterranean countries particularly in the centre and south where cultivated or wild chestnuts grow. Spread of the pest from the infested area in south Piemonte, Italy is very likely by females flying and movement of infested young chestnut plants and grafts (Brussino et al., 2002). Transfer of the pest from infested areas in Asia and America to Europe and the mediterranean may occur, on a limited scale, by introduction of infested grafting materials with eggs or first instar larvae inside the buds.

D. kuriphilus attacks Castanea crenata (Japanese chestnut), Castanea dentata (American chestnut), Castanea mollissima (Chinese chestnut) and Castanea sativa (European chestnut) and their hybrids. It infests also Castanea seguinii in China, but is not known to attack the wild North American species Castanea pumila and Castanea alnifolia, which are very often grown adjacent to infested chestnuts.

D. kuriphilus is a univoltine species, reproducing parthenogenetically. Early instar larvae overwinter inside chestnut buds. At the time of bud burst in spring, the adults emerge to induce the formation of 5-20 mm diameter green or rose-coloured galls, which develop in mid-April on new shoots. The larvae feed 20-30 days within the galls before pupating. Depending on locality (altitude, exposure) and chestnut cultivar, pupation occurs from mid-May until mid-July and adults emerge from galls from the end of May until the end of July. They are all females (males of this species have never been collected). The females lay 3-5 eggs per cluster inside buds. Each female can lay over 100 eggs. Some buds contain 20-30 eggs. The female lifetime is short (about 10 days, some of which is spent boring a tunnel to emerge from the gall). The eggs hatch in 30-40 days. Larval growth then proceeds very slowly through the autumn and winter.

Associations

Magro et al. (2010) reported a species of Gnomoniopsis associated with the necrosis of chestnut galls caused by D. kuriphilus in Italy.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Infestations in small chestnut orchards may be reduced by pruning and destroying the infested shoots, but commercial growers cannot rely on this strategy because of the cost. Persistent insecticides may prove effective against females and young larvae, but side-effects on the environment could be serious. At present, there are no efficient plant protection products for control of this pest. After the Second World War, Japanese breeders selected chestnut cultivars with some resistance to the insect, but the pest developed a new strain overcoming that resistance. There are now new Japanese and Korean chestnut cultivars resistant to the pest (Anagnostakis, 1999).

In its native range in China, D. kuriphilus can be effectively controlled by natural enemies, particularly hymenopteran parasitoids. Several new chalcid parasitoids reared from galls of D. kuriphilus have recently been described from China, Korea and Japan, e.g. Torymus sinensis, Torymus beneficus, Megastigmus maculipennis, Megastigmus nipponicus (Chalcidoidea: Torymidae), Ormyrus flavitibialis (Ormyridae) and others (Yasumatsu and Kamijo, 1979). Some of these parasitoids appeared to be very effective. T. sinensis has already been introduced as a biological control agent in Japan and Korea and appears to be very effective in mass-release programmes (Moriya et al., 2002).

Some transpalaearctic species of parasitoids, e.g. Torymus geranii, Ormyrus pomaceus, Eurytoma brunniventris and others, which are also effective parasitoids of D. kuriphilus in Japan (Yasumatsu and Kamijo, 1979) are very common and widespread in oak cynipid galls in Europe and will probably easily move onto D. kuriphilus. O. pomaceus has already been reared from galls of D. kuriphilus by authors in Piemonte, Italy. A host shift of native Western Palaearctic parasitoid species onto D. kuriphilus can be expected. This already happened in Georgia, USA, where native parasitoid species, Torymus tubicola and T. advenus were reared from galls of the introduced D. kuriphilus. However, native parasitoids in Japan and USA (and very likely in Europe) will probably not provide good control of this introduced gall wasp, since they are not specific or well synchronized with the life cycle of the pest.

Phytosanitary Measures

D. kuriphilus was added in 2003 to the EPPO A2 action list, and EPPO member countries are thus recommended to regulate it as a quarantine pest. Introduction of D. kuriphilus from Asia and America is effectively prevented by the fact that import of all plants of Castanea (except fruits and seeds) from non-European countries is in fact prohibited by most European and Mediterranean countries on account of other pests. Within Europe and the mediterranean, plants for planting of Castanea (young plants or shoots for grafting) from infested areas should be produced in a place of production kept free from D. kuriphilus by appropriate systemic insecticide treatment.