Preferred name: Dryocosmus kuriphilus
Authority: Yasumatsu
Taxonomic position: Animalia: Arthropoda: Hexapoda: Insecta: Hymenoptera: Cynipidae
Common names in English: Asian chestnut gall wasp, chestnut gall wasp, oriental chestnut gall wasp
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Notes on taxonomy and nomenclature EPPO Categorization: A2 list
EU Categorization: Emergency measures (formerly), PZ Quarantine pest (Annex III)
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EPPO Code: DRYCKU

D. kuriphilus develops on Castanea crenata (Japanese chestnut), Castanea dentata (American chestnut), Castanea mollissima (Chinese chestnut) and Castanea sativa (European chestnut) and their hybrids. It also infests 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.

Host list: Castanea crenata, Castanea dentata, Castanea mollissima, Castanea sativa, Castanea seguinii

D. kuriphilus is native to China where it is recorded from several provinces but without details on its population levels (Murakami et al., 1980; Zhang et al., 2009). In the 1940s, it was reported in Japan (Murakami et al., 1980) and after several other introduction events between 1941 and 1999 (Japan, South Korea, USA, Nepal), it was first reported in Europe in 2002 (Brussino et al., 2002) where Castanea-based forests cover around 2.5 million hectares distributed across 17 countries (Conedera et al., 2016). Following its introduction, D. kuriphilus over 15 years colonized most of the European area where Castanea sativa are grown.

EPPO Region: Albania, Austria, Belgium, Bosnia and Herzegovina, Croatia, Czech Republic, France (mainland, Corse), Germany, Greece (mainland), Hungary, Italy (mainland, Sardegna, Sicilia), Netherlands, Portugal (mainland, Madeira), Romania, Russia (Southern Russia), Slovakia, Slovenia, Spain (mainland), Switzerland, Türkiye, United Kingdom
Asia: China (Anhui, Fujian, Gansu, Guangdong, Guangxi, Hebei, Hunan, Jiangsu, Liaoning, Shaanxi, Shandong, Sichuan, Zhejiang), Japan (Honshu, Kyushu), Korea, Republic, Nepal, Taiwan
North America: Canada (Ontario), United States of America (Alabama, Connecticut, Delaware, Georgia, Kentucky, Maryland, Massachusetts, Michigan, New Jersey, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia)

D. kuriphilus is univoltine (one generation per year) and reproduces by thelytokous parthenogenesis (virgin females produce only daughters). Males are unknown. The thelytokous reproduction of D. kuriphilus is not induced by Wolbachia infection (Hou et al., 2020; Zhu et al., 2007). Since most of the Cynipini species reproduce by cyclical parthenogenesis with a strict alternation between one arrhenotokous generation and one thelytokous generation, the univoltine thelytokous life cycle observed for D. kuriphilus and some other oak gall wasps is considered to be derived from the loss of the bisexual generation (Stone et al., 2002). The biology of D. kuriphilus is highly synchronized with chestnut phenology (Bernardo et al., 2013). The adult females are short-lived (2-10 days) (Yasumatsu, 1951; Bernardo et al., 2013). They emerge in early summer (end of May to July depending on latitude) and immediately lay eggs inside chestnut buds that will develop the following spring. Each female generally lay up to 300 eggs (Graziosi & Rieske, 2014; Nohara, 1956; Tokuhisa, 1981) with up to 30 eggs found in one bud (Otake, 1980; 1989; Kim et al., 2005; Gil-Tapetado et al., 2021). Described as proovigenic (emerging with a full complement of eggs and the ability to oviposit immediately after emergence), D. kuriphilus may be resorping eggs in the absence of suitable hosts, suggesting potential facultative synovigeny (i.e. the number of mature eggs within ovaries increases rapidly after adult emergence) (Graziosi & Rieske, 2014). Eggs hatch in 30-40 days and first instar larvae remain within chestnut buds where they overwinter. At bud burst in spring, larval feeding induces the formation of green- or rose-coloured galls, which are 5-20 mm in diameter on C. crenata in Japan (Otake, 1980; 1989) or 8-15 mm in diameter on C. sativa or C. sativa x C. crenata in Europe (Breisch & Streito, 2004). Each larva develops within an ovoid shaped chamber (Warmund, 2013). Depending on the climate (altitude, latitude), pupation takes around two months and occurs within the galls from mid-May to mid-July.

Symptoms

Galls are uni- or multilocular and contain from 1 to 25 larval chambers (Kato & Hijii, 1993; Bernardo et al., 2013). This multilocularity may be a strategy to protect larvae from parasitoids, with larvae in peripherical chambers being more vulnerable than those developing deeper within the structure (Reale et al., 2016). Galls are localized on shoots, leaf midribs or leaf stipules (Gehring et al., 2018). After the emergence of D. kuriphilus adults, galls dry, become wood-like and remain on the tree for several years. While galls are easily detected on plants or parts of plants, presence of eggs or young larvae inside buds cannot be detected by simple visual inspection. Gall size, in terms of number of chambers per gall, increases with time since invasion, as the abundance of D. kuriphilus increases in an area (Gil-Tapetado et al., 2021). Moreover, gall morphology (volume, mass) may be influenced by exposure to sun and precipitation (Gil-Tapetado et al., 2020a). 

Morphology

Eggs

Eggs are flattened, ellipsoid in shape, milky white in colour, somewhat transparent, and are about 0.15 mm long and 0.10 mm wide, with a long, thin stalk of about 0.4 mm in length at one end of the long axis (Nakamura et al., 1964).

Larva

D. kuriphilus has three larval instars: 

First larval instars appear in chestnut buds around 1 month after oviposition (July-August) and then develop very slowly until the next spring. They are 0.2-0.6 mm long, subglobular, with very small mandibles. This stage is hardly distinguishable from the egg (Viggiani & Nugnes, 2010).

Second larval instars are 0.8-1.5 mm long, hymenopteriform with mandibles with distally two teeth. This second larval stage appears in April-May and develops in less than one month (Viggiani & Nugnes, 2010). 

Last larval instars are on average 2.3 mm long, hymenopteriform with asymmetric mandibles with teeth. This stage, present in the field from late April to the end of May, is characterized by a wide variation in the morphology of mandibles and the respiratory system (Viggiani & Nugnes, 2010).

Pupa

The pupa of D. kuriphilus is 2.5 mm long, black or dark brown. In the field, pupae are present in galls from mid-May to mid-July (EPPO, 2005).

Adults

The adult female is 2.5 to 3 mm long and the body is brownish black; legs, scapus and pedicels of antennae, apex of clypeus and mandibles are yellow brown; head is finely sculptured; vertex is black; scutum, mesopleuron and gaster appear highly shiny and smooth; propodeum with 3 distinct longitudinal carinae; propodeum, pronotum (especially above) strongly sculptured; scutum with 2 uniformly impressed and pitted grooves (notaulices) that converge posteriorly; radial cell of forewing open; antennae 14-segmented with apical segments not expanded into a clava. Adults of D. kuriphilus are morphologically close to D. cerriphilus, a European oak gall wasp known to induce galls on Quercus cerris. However, D. cerriphilus has a vertex with large yellowish-red markings, a 15-segmented antennae and a propodeum without median longitudinal carina (Yasumatsu, 1951; EPPO, 2005). 

Detection and inspection methods

The induction of galls starts at bud burst in spring. Attacked buds remain therefore the infestation is asymptomatic by external plant inspection from oviposition (June-July) until bud burst. Stereoscopic observations may however reveal brown scars on attacked buds, as well as eggs or young larvae within buds. This technique is however time consuming (Reale et al., 2016). Molecular techniques (PCR) using several markers can be used to rapidly detect the presence of D. kuriphilus within buds even in absence of external symptoms (Sartor et al., 2012).  

D. kuriphilus can be transported over long distances in chestnut plants for planting and cut branches (EPPO, 2003). When present in the bud tissue, the pest cannot be detected by visual examination and the introduction of infested plant material is very likely to occur (EFSA, 2010).

Further diffusion occurs by natural spread. D. kuriphilus is thelytokous and each female can lay up to 300 eggs. Therefore, a single female can found a new population. D. kuriphilus follows a stratified dispersal comprising two components: local or short-distance dispersal and long-distance dispersal. Short-distance dispersal mainly includes the continuous dispersal of individuals at low spatial scale within the invasion front due to the natural random movement of adults as well as dispersal caused by natural (e.g. wind) or artificial (e.g. direct human transportation) driving forces. Long-distance dispersal is the result of discrete events that lead to the establishment of new infestation foci separated from the closest infested area by a non-infested zone. Long-distance dispersal events are mainly caused by artificial dispersal due to the transportation of biological material to new areas. According to recent studies, the mean speed of dispersal of the population front (short-distance dispersal) is around 7 km per year, with the mean distance of long-distance dispersal events being 76 km (Gil-Tapetado et al., 2020b; Gilioli et al., 2013). This distance is significantly shorter than the other values reported in the literature (Graziosi & Santi, 2008; Payne, 1981; Rieske, 2007), suggesting that although long-distance dispersal events represent a small proportion of the fraction of offspring dispersing locally, they drove the rate of colonization of D. kuriphilus in chestnut forest areas.

D. kuriphilus outbreaks severely alter branch architecture of chestnut trees, with a leaf area reduction of up to 70%, a decrease of dormant buds and a decrease of flower, fruit and wood production (Battisti et al., 2014; Gehring et al., 2018a; Ugolini et al., 2014).

Economic impact

High infestation rates by D. kuriphilus are reported to cause severe decrease of chestnut production. This pest is reported to cause 15-30% of yield reduction annually in China (Zhang et al., 2009) and 50-75% of yield reduction in the infested areas of the USA (Payne et al., 1983). In Italy, Sartor et al. (2015) showed that infestation rates above 0.6 galls per bud induce high yield losses, and Battisti et al. (2014) reported yield losses up to 80% when the mean number of galls exceeded six galls per twig. Although most of the chestnut cultivars are sensitive to D. kuriphilus, controlled infestations of D. kuriphilus on 64 cultivars resulted in variable impacts depending on the cultivars, with 14 cultivars classified as very susceptible (i.e. with more than 0.6 galls per bud), such as ‘Marsol’, ‘Marigoule’ or ‘Torcione Nero’, and 7 being resistant (i.e. no gall development). Among these, two are C. sativa cultivars, 4 are C. crenata x C. sativa hybrids and one is a C. crenata cultivar (Sartor et al., 2015). 

Evaluation of the economic impact of D. kuriphilus focused on chestnut production but, since this pest is affecting leaf area, branch architecture, production of flowers and fruits, its impact may be wider, in particular on natural ecosystems. For example, in the Southern Alps, D. kuriphilus is reported to induce significant changes in honey composition starting from an infestation level of 30%, with nearly all the chestnut component being lost when infestation levels exceed 40% of attacked buds (Gehring et al., 2018b). 

Control

Only a few management options have been identified for D. kuriphilus (Bosio et al., 2010; EFSA, 2010; Zhang et al., 2009). Even if conventional chemical control may be effective in controlling D. kuriphilus adults in chestnut orchards (Zhang et al., 2009), this method is expensive, hard to implement for large trees or in forests, and there are risks of side effects on the environment as well as on human health (toxic residues in honey for example) (Bosio et al., 2010). Pruning or hot water treatments seem not to be effective enough to be widely used (Maltoni et al., 2012; Warmund, 2014). Interestingly, mixed forests seem to be more resistant to D. kuriphilus since infestations of the pest decreased with the decrease of the relative proportion of chestnut (Fernandez-Conradi et al., 2018).

The most effective methods for reducing D. kuriphilus populations are the use of resistant varieties of Castanea species and biological control using natural enemies. Following the introduction of the pest in Japan in 1941, the first attempts to manage D. kuriphilus focused on the development of resistant varieties, leading to an increase of the area of C. crenata in Japan (Shimura, 1972). However, damage caused by D. kuriphilus increased on resistant varieties in the 1970s (Moriya et al., 2003). Despite 40 years of selection of C. crenata in Japan and the wide use of resistant varieties, the mode of inheritance of resistance was not established (EFSA, 2010). In Europe, some resistant varieties were found to be completely effective in preventing damages by D. kuriphilus, such as ‘Bouche de Bétizac’ (C. sativa x C. crenata), ‘Idea’ (C. mollissima x C. crenata), ‘Muraie’ (C. sativa) or ‘Vignols’ (C. crenata x C. sativa) (Botta et al., 2009; Dini et al., 2012; Sartor et al., 2015).

In addition to the use of resistant varieties, and since increasing damage was reported on these varieties in Japan, the use of biological control using natural enemies was considered by researchers. In all the invaded countries, native parasitoids were reported to switch from native oak cynipids to invasive D. kuriphilus. Around 40 species of parasitoids were thus recorded worldwide (Aebi et al., 2007; Cooper & Rieske, 2007; Jara-Chiquito et al., 2020; Kos et al., 2020; Matosevic & Melika, 2013; Murakami et al., 1994; Muru et al., 2020; Quacchia et al., 2013). All these species are polyphagous and multivoltine (i.e. several generations each year). Among these species, Bootanomyia dorsalis, Torymus flavipes and Eupelmus urozonus are the most abundant. Nevertheless, the effectiveness of these native parasitoids to control D. kuriphilus remains low due to phenological asynchrony (Aebi et al., 2007; Bonsignore et al., 2019; Budroni et al., 2018; Panzavolta et al., 2018). Their use as biological control agents may thus be difficult. Moreover, increasing the level of parasitism by native parasitoids may lead to unintentional effects on their primary hosts (mostly cynipids) since the second generation of these parasitoids, more numerous due to additional progeny from the ‘new’ host D. kuriphilus, can parasitize only the asexual generation of their primary hosts.

To date, the most effective method to control D. kuriphilus is the use of Torymus sinensis, a parasitoid originating from the same area of origin as the pest. In the 1970s, field expeditions in China led to the discovery of this parasitoid that was the only species with high host-specificity and a life cycle synchronised with that of D. kuriphilus (Moriya et al., 2003). Releases of this parasitoid in Japan starting in the late 1970s have very successfully reduced D. kuriphilus infestation levels (Moriya et al., 2003; Murakami et al., 1977). This parasitoid was also introduced in the USA where it reduced pest populations (Rieske, 2007). Following the introduction of the pest in 2002 in Italy and its spread all over Europe, several countries (Croatia, France, Hungary, Italy, Portugal, Slovenia, Spain) have implemented classical biological control programs more or less recently (Avtzis et al., 2019). In Italy and France, where T. sinensis was first released in 2005 and 2010 respectively, results showed a drastic reduction of D. kuriphilus populations (Borowiec et al., 2018; Ferracini et al., 2019). Moreover, post-introduction dynamics of T. sinensis were found to follow a two-phase process: firstly exponential growth of T. sinensis populations without significant decrease in D. kuriphilus populations, and secondly a general decrease in both T. sinensis and D. kuriphilus populations starting 5 years after the first releases (Borowiec et al., 2018). The use of T. sinensis to control invasive D. kuriphilus is considered as one of the most successful cases of classical biological control against a forest pest.

T. sinensis is univoltine but can undergo an extended diapause within a 2-year cycle (Ferracini et al., 2015). Moreover, only a small proportion of the population (up to 17% of the adults) emerged only a few months after female oviposition (June to August) (Borowiec et al., 2018). Risk assessment concerning the release of exotic T. sinensis in Europe highlighted two types of unintentional effects that should be investigated: the attack of non-target species (i.e. oak cynipids) and the hybridization with native Torymus species (Gibbs et al., 2011). Because of the asynchrony between T. sinensis and native oak cynipids, the attack of non-target species should be limited. However, T. sinensis was collected from 15 oak cynipids species in Italy, representing 1% of the total number of parasitoids collected (Ferracini et al., 2017). First concerns about hybridization between T. sinensis and other Torymus species appeared during the translocations of T. sinensis from China to other Asian countries. Indeed, there are three Torymus species parasitizing D. kuriphilus, all belonging to the subgenus Syntomaspis: T. beneficus in Japan, T. koreanus in Korea and T. sinensis in China (Yasumatsu & Kamijo 1979). Integrative taxonomy showed the high similarity between these three entities, and particularly between T. sinensis and one ‘ecotype’ of T. beneficus (‘T. beneficus late’) (Yara, 2004). In Japan, hybridizations between T. sinensis and T. beneficus were reported (Yara, 2004). Based on around 800 T. sinensis specimens collected in France and Italy, a recent study showed for the first time that the European stock of T. sinensis has some rare molecular signatures of historical hybridization with T. beneficus that took place in Japan (Viciriuc et al. in press). To date, hybridizations between T. sinensis and other European Torymus species have never been reported. Among these species, the morphologically and phylogenetically closest to T. sinensis is T. notatus (Pogolotti et al., 2019), indicating that a specific survey should be carried out to investigate more precisely the risk of potential hybridization between these two species. 

Phytosanitary risk

D. kuriphilus is considered the most serious pest of chestnut worldwide. Following the first report of D. kuriphilus in Europe in 2002, a risk assessment for this pest was produced by the European Food Safety Authority (EFSA, 2010). In its conclusions, experts concluded that the risk of establishment and spread of D. kuriphilus in Europe was high, chestnut being widely grown in Europe for timber, fruit, landscape conservation and as ornamentals. D. kuriphilus is now reported in all the main areas at risk, i.e. areas of the EPPO region which have the highest degree day accumulations and the largest areas of chestnut production. 

When D. kuriphilus is regulated as a quarantine pest, plants for planting (except seeds) and cut branches originating in countries where the pest occurs should be produced in pest free areas. Plants for planting should be transported in appropriate conditions (not transported through infested areas, transported outside the flight period, or transported closed to prevent infestation) (EPPO, 2017). EFSA (2010) also suggested the production of plants in pest free places of production surrounded by a buffer zone. The technical feasibility of insect screening was considered to be very low by EFSA (2010) due to the small size of the insect. No management options are available to reduce the likelihood of spread following introduction to Castanea forests/woodland (EFSA, 2010). 

Once introduced, sustainable management with new planting using resistant varieties and the use of biological control agents have shown to be effective in controlling the pest (see Control section). 

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This datasheet was extensively revised in 2021 by N. Borowiec from the National Research Institute for Agriculture, Food and Environment (INRAE, France). His valuable contribution is gratefully acknowledged.

EPPO (2024) Dryocosmus kuriphilus. EPPO datasheets on pests recommended for regulation. https://gd.eppo.int (accessed 2024-04-20)

This datasheet was first published in the EPPO Bulletin in 2005 and revised in 2021. It is now maintained in an electronic format in the EPPO Global Database. The sections on 'Identity', ‘Hosts’, and 'Geographical distribution' are automatically updated from the database. For other sections, the date of last revision is indicated on the right.

EPPO (2005) Dryocosmus kuriphilus. Datasheets on quarantine pests. EPPO Bulletin 35(3), 422-424. https://doi.org/10.1111/j.1365-2338.2005.00849.x