Gall Wasp Dryocosmus Kuriphilus Yasumatsu Research Articles

Anatomical Changes during Chestnut (Castanea mollissima BL.) Gall Development Stages Induced by the Gall Wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae).

This study delved into the larval development and the morphological and anatomical transformations that occur in the galls of chestnut trees (Castanea mollissima BL.) and are induced by the chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (GWDK) across various stages: initial, growth, differentiation, maturity, and lignification. Chestnut galls in the five development stages were collected. Gall structural characteristics were observed with an anatomical stereomicroscope, and anatomical changes in galls were analyzed with staining and scanning electron microscope techniques. The chestnut gall wasp laid its eggs on young leaves and buds. Chestnut gall wasp parasitism caused plant tissues to form a gall chamber, with parenchyma, protective, and epidermal layers. The development of the gall structure caused by the infestation of the GWDK gall led to the weakening of the reactive oxygen species (ROS) elimination ability of the host. The accumulation of ROS led to cell wall peroxidation, resulting in structural damage and diminished host resistance, and the parenchyma layer exhibited significant nutrient supply and thickening. The thickness of the protective and epidermal layers varied notably across different growth stages. The oviposition of the chestnut gall wasp induced modifications in the original plant tissues, with gall formation being most favorable in young tissues, correlating with the maturity level of the host plant tissues. Variances in the internal structures of the galls primarily stemmed from nutrient supplementation, while those in the external structure were attributed to defensive characteristics. This research contributes a foundational understanding of gall development induced by the chestnut gall wasp in Chinese chestnut, offering valuable insights into the intricate interplay between insect infestation and plant physiology.

Torymus sinensis and its close relatives in Europe: a multilocus phylogeny, detailed morphological analysis, and identification key

The introduction of the biological control agent Torymus sinensis Kamijo (Hymenoptera, Chalcidoidea, Torymidae) to control the populations of the chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera, Cynipidae) is considered one of the successful programs in biological control. The species was involved in interspecific hybridisation in Japan and the specimens imported into Europe were derived from this hybrid lineage, showing signs of introgression. The discovery of mitochondrial haplotypes or possible Enolase haplotypes from T. beneficus or of specimens with shorter ovipositor does not necessarily imply that T. beneficus is present in Europe, only that the European specimens are of hybrid origin. Of the native European Torymus species associated with D. kuriphilus, the molecular and morphometric results indicate Torymus notatus (Walker) as the closest species to T. sinensis. The two are part of the same species-group (cyaneus group), are nested together in the multivariate ratio analysis and are the closest genetically based on all three nuclear markers: Enolase (1.5% divergence), Wingless (2%) and ITS2 (13%). However, on the mitochondrial marker COI the closest species is Torymus rubi (Schrank) at 9.9% divergence. As such, T. notatus is the most likely candidate for accidental interspecific hybridisation if this is to happen in Europe. We provide an illustrated identification key for the European species of Torymus associated with D. kuriphilus, an important but lacking tool for biological control programs.

Brown Rot Caused by Gnomoniopsis smithogilvyi (syn. Gnomoniopsis castaneae) at the Level of the Chestnut Tree (Castanea sativa Mill.)

The European chestnut tree (Castanea sativa Mill.) has great economic importance, mostly due to the recognized nutritional value of its fruit. Thus, the development and improvement of the techniques of the production, preservation, and control of the diseases/pests of chestnut trees is a topic of great interest to producers, companies, researchers, and consumers to ensure the quality of this exceptional fruit. Recently, an emerging rot in chestnuts caused by the fungus Gnomoniopsis smithogilvyi (syn. Gnomoniopsis castaneae Tamietti) (Gnomoniaceae, Diaporthales) was reported both in Australia and Europe. Since then, the number of records of this pathogen in several countries of the world (Europe and Asia) where Castanea spp. is cultivated has been increasing. This disease, called “brown rot”, has been causing significant production losses, raising serious concerns for producers and the chestnut industry. This review describes the world distribution and life cycle of the causal agent of brown rot. The life cycle of G. smithogilvyi can involve primary infection, caused by ascospores, and secondary infection, related to the asexual phase of the fungus (conidia). Then, the analytical methods used to detect G. smithogilvyi are described. Furthermore, the incidences of the disease caused by G. smithogilvyi are presented, ranging from 5 to 94%, with high infection rates causing significant economic losses. The damages caused by G. smithogilvyi are discussed. In fact, it can act as an endophyte or as a pathogenic fungus, causing fruit rot, canker in several plant tissues, and necrosis in leaves, as well as in galls caused by the gall wasp Dryocosmus kuriphilus Yasumatsu. Possible pre- and post-harvest methods to mitigate the damage caused by moulds, and in particular G. smithogilvyi, are presented, including biocontrol agents and chemicals. Finally, some challenges and future prospects for a number of uncertainties related to the epidemiology, geographic distribution, spread, detection, and management of this disease are discussed.

No loss of genetic divergence in Torymus sinensis following its release for Dryocosmus kuriphilus control in Europe

Abstract Arrival, establishment, and further dispersal of non‐native natural enemies are considered essential for a successful biological control programme, while among the factors that may determine the success of such a programme, genetic diversity of the introduced population plays an important role in the establishment of a non‐native species. The Chinese parasitoid wasp Torymus sinensis Kamijo (Hymenoptera: Torymidae) was initially released in Europe in Italy to control biologically the Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae), and reduce the damage induced on sweet chestnut (Castanea sativa Miller). In the following years, T. sinensis was then released in numerous other European countries as a biological control agent of D. kuriphilus. Its presence has also been reported beyond the countries of release due to rapid natural dispersal. To assess the post‐release genetic diversity of D. kuriphilus, we screened T. sinensis populations from six European countries and tested the possibility of these populations suffering from frequently observed genetic effects that could threaten its successful establishment in Europe. Our results exhibit that T. sinensis populations have suffered neither from the Allee effect nor from genetic bottleneck after their release and establishment in Europe, something that increases the possibility to effectively control D. kuriphilus in Europe.

Damage ratio of the Asian chestnut gall wasp, Dryocosmus kuriphilus Yasumatsu, 1951 (Hemiptera: Cynipidae) in Samsun Province of Türkiye: First Report

The Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera, Cynipidae) is a critical pest that threatens chestnut cultivation. The chestnut gall wasp causes yield losses by deforming the buds of the infected trees. This pest, which was first detected in Italy in 2002 in Europe, was seen in Gacık village of Yalova province in 2014 in Türkiye. In the following years, it spread to Bursa, Istanbul, Sakarya, Kocaeli, Balıkesir, Bilecik, Düzce, Giresun, Bartın, Zonguldak, Sinop and İzmir provinces. In this study, the orchard of chestnut gene resources belonging to the Ali Nihat Gökyiğit research station in Samsun province was examined for the presence of the Asian chestnut gall wasp. As a result of the study, the presence of the Asian chestnut gall wasp was determined in the cultivars Marigoule' and 'Macit 55'. As a result of the counts made in 50 cm parts of the branches, it was determined that the presence of chestnut gall wasp is in a higher population in the 'Marigoule' cultivar than in the other cultivars. However, the invasion in Samsun is at the early stage.

Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae) in Minho (Northern Portugal): Bioecology, Native Parasitoid Communities and Biological Control with Torymus sinensis Kamijo (Hymenoptera: Torymidae)

The Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu, native to China, was first detected in Europe in Italy in 2002. In Portugal it was declared in 2014, and it has since affected the Portuguese chestnut production. The introduction of its natural parasitoid Torymus sinensis Kamijo started through inoculative releases according to the National Action Plan for the Control of Dryocosmus kuriphilus, established by the Direção Geral de Alimentação e Veterinária (DGAV), in 2015. This research was carried out during 2018 and 2019, in five chestnut orchards in the Minho region (Northern Portugal). Between January and March 2018, at each location, three monthly harvests of 100 buds were carried out to determine the infestation level. Between April and June 2018, 240 galls were harvested every two weeks, with the purpose of studying the bioecology of D. kuriphilus. In each sampling site, 160 galls were placed in emergence boxes to follow adults of the D. kuriphilus population, and 80 galls were dissected with a binocular magnifying glass, to count and record the evolution of the larval stages of D. kuriphilus and of the parasitoids. The identification of T. sinensis and native parasitoids was carried out. The parasitism rates and Simpson diversity indices were determined. The percentage of buds infested by D. kuriphilus was high in all studied places, varying between 67% and 80%, regardless of the number of T. sinensis releases made and of the chestnut cultivars present in each orchard. The highest rate of parasitism by T. sinensis was 0.5044% and it was recorded in an orchard where there were two releases of T. sinensis in 2016 and 2017 (Arcos de Valdevez—Rio Cabrão—P1). Considering that the exotic parasitoid has already been released in the region since 2016, its establishment in all the chestnut orchards studied was confirmed, although with very low parasitism rates. The natural parasitism associated with the native parasitoid species of D. kuriphilus was confirmed in this work, recording values between 34% and 51% of the parasitized chestnut galls, with 11 species of native parasitoids having been identified: Eupelmus azureos, Eupelmus uruzonus, Eurytoma brunnivientris, Eurytoma pistaciae, Megastigmus dorsalis, Mesopolobus sp., Ormyrus pomaceus, Sycophila biguttata, Sycophila iracemae, Sycophila variegatta and Torymus flavipes.

Spatial and temporal dynamics of wild boars Sus scrofa hunted in Alpine environment

This study evaluated the trend and spatial distribution of wild boar population harvested in the Alpine hunting district C.A. CN1 (Piedmont, Italy) from 1996 to 2018, and its relation with hunting effort. Protected areas were found to shape the distribution of the harvested wild boars, which decreased in number as the distance from those zones increased. The hunting bag data presented large yearly fluctuation, with a trend in line with the hunting effort until 2007 when the maximum capacity of the population to cope with the hunting pressure was reached. The variation of reproductive parameters (percentage of piglets in the hunted population and piglets to sexually matured female ratio) showed a decreasing trend in both time series. Conversely, hunting effort increased over the years, with significant trend changes in 2000 and 2015, probably associated with the increased preference for hunting activity on wild boars, and the parallel reduction of the extension of hunting areas. Predation, hunting activity, and environment could have modulated the wild boar population dynamics in the study area. Decrease in chestnut Castanea sativa production, due to the gall wasp Dryocosmus kuriphilus Yasumatsu, were reported during the period of study. This might be the main factor determining the downtrend of piglets in 2003. In addition, predation by wolves Canis lupus, whose population has sharply increased in the southwestern Alps in the last decades, might have contributed to the decline since 2010. This work outlines the importance of a proper management of protected areas, which influence the density and distribution of wild boars. In this context, hunting bags analysis is of pivotal importance to monitor population dynamics and develop proper wildlife policies.

Recruitment of native parasitoids by an introduced gall wasp Dryocosmus kuriphilus Yasumatsu, 1951 (Hymenoptera: Cynipidae) in Britain and France

The invasive chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae) is a major pest species of Castanea spp. worldwide. It only arrived in Britain in 2015 and the parasitoid community associated with it was studied in 2019. Eight native species of chalcidoid, previously known to be associated with oak gall wasps, were identified. These results are compared to a similar study (2016–2019) in France where four parasitoids were reared. However, this community included the deliberately introduced Torymus sinensis Kamijo, a specific biological control agent of D. kuriphilus. The presence of T. sinensis and the differences between the two communities is discussed.

The Asian Chestnut Gall Wasp in Italy: Surveys on its Native and Exotic Parasitoids as Well as on Chestnut Cultivar Susceptibility

In the context of biological control against the Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae) in Italy, the aim of this study was to survey its native parasitoids as well as the exotic Torymus sinensis Kamijo which had been introduced and released as a biological control agent. Furthermore, both parasitism and pest susceptibility of local chestnut trees were investigated. Surveys were carried out in 2014 in the Municipality of Sambuca Pistoiese (Pistoia), in a chestnut forest which included three cultivars: Nerattino, Carpinese, and Pastinese. On a total of 35 trees, selected among the three cultivars, 24 current-year shoots were randomly collected and examined in the laboratory. After the damage by the cynipid had been assessed, leaf samples were used for molecular analyses. Galls were also collected on the 35 selected trees both for dissection and to rear parasitoid specimens in the laboratory. The three cultivars were confirmed by genetic tests. The Carpinese cultivar resulted to be the most damaged, with highest percentage of attacked shoots (84.38%). Moreover it was also the cultivar with the lower parasitism rate (about 16%). From reared galls seven parasitoid species emerged, all belonging to the superfamily of Chalcidoidea. Exception made for the non-native T. sinensis, the other six were native species associated to oak gall wasps. Native parasitoids were more scarcely present, while the exotic T. sinensis amounted for the 84.27% of all emerged parasitoids. Our results confirm how chestnut susceptibility to D. kuriphilus varies depending on the cultivar. Overall, the Nerattino cultivar resulted as the best among the three examined: it was subject to a less intense attack than the other cultivars, and the resulting smaller galls were more effectively exploited by parasitoids.

Torymus sinensis Kamijo, a biocontrol agent against the invasive chestnut gall wasp Dryocosmus kuriphilus Yasumatsu in Spain: its natural dispersal from France and the first data on establishment after experimental releases

Aim of study: The globally invasive gall wasp, Dryocosmus kuriphilus Yasumatsu, 1951 (Cynipidae: Cynipini), reached Spain seven years ago and is already regarded as an important pest of chestnuts (Castanea spp.) in this country as well as worldwide. In this paper, we present comprehensive data on the establishment in Spain of Torymus sinensis Kamijo, 1982 (Chalcidoidea: Torymidae), an effective non-native natural enemy of this pest, as a result of both natural dispersal and settlement after controlled releases since 2015.Area of study: Sites of the Spanish autonomous communities of Galicia, Asturias, Basque Country, Catalonia, Andalusia and Madrid where D. kuriphilus is present.Material and methods: To study the natural dispersal of T. sinensis from France, we selected two sampling sites in Catalonia, six in the Basque Country and two in Navarra known for their heavy Asian chestnut gall wasp (ACGW) infestation; to study T. sinensis establishment after authorized controlled releases by the concerned authorities, the field samplings were done mainly in Galicia (35 sites) and Andalusia (8 sites). Additionally an experimental release study was made in Madrid.Main Results: Our results showed that T. sinensis has spread throughout Spain by natural dispersal across the French border and now occurs in Catalonia (two sites), the Basque Country (three sites) and Navarra (one site) but not in the neighbouring region of Cantabria. The percentage of parasitism by T. sinensis on D. kuriphilus is higher in the Basque Country sites, which are close to the French border, thus indicating that its establishment in these localities is not recent. After controlled releases, T. sinensis has been successfully established in five release sites in Andalusia (Valle del Genal and Sierra Blanca, Málaga Province) and one release site in Madrid. However, in the region of Galicia (NW Spain), where the number of authorized releases has been higher, the establishment of T. sinensis still appears to be very low.Research highlights: Established populations of T. sinensis may exert a positive buffer against D. kuriphilus-driven chestnut infestation in Spain, similar to what is observed in other invaded European countries.Additional Keywords: Controlled releases, Torymidae, invasive species, Cynipidae, natural spread, biological control.Abbreviations used: Asian chestnut gall wasp (ACGW); Torymus sinensis parasitism rate (TsPR); mean of ACGW larval chambers per gall (MACGW).

Seasonal flight activity and genetic relatedness of Torymus species in Italy

Abstract Torymus sinensis Kamijo was released into Europe to control the Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu, proving to be an effective biocontrol agent. In Japan, hybrids between this exotic parasitoid and the native congeneric Torymus beneficus Yasumatsu and Kamijo have been recorded, whereas no case of hybridization with natives has been reported for Europe. The seasonal phenology of the Torymus species was investigated in north‐west Italy and 172 997 chestnut galls (Castanea spp.), 7866 oak galls (Quercus spp.), 29 wild rose galls (Rosa spp.) and 44 bramble galls (Rubus spp.) were collected over a 5‐year period. The seasonal flight activity was recorded for eight native Torymus species [Torymus affinis (Fonscolombe), Torymus auratus (Müller), Torymus bedeguaris (L.), Torymus calcaratus Nees, Torymus cyaneus Walker, Torymus flavipes (Walker), Torymus geranii (Walker) and Torymus notatus (Walker)] and five of them were found to overlap with T. sinensis flight, namely T. affinis, T. auratus, T. flavipes, T. geranii and T. notatus. Among the Torymus native species, T. notatus revealed to be morphologically and phylogenetically closely related to T. sinensis, thus suggesting hybridization potential.

Early population dynamics in classical biological control: establishment of the exotic parasitoid Torymus sinensis and control of its target pest, the chestnut gall wasp Dryocosmus kuriphilus, in France

AbstractIn response to the introduction and spread of the chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae) in metropolitan France, including Corsica, the exotic parasitoid Torymus sinensis Kamijo (Hymenoptera: Torymidae) was deliberately introduced in more than 50 locations covering most of the areas of chestnut production. To test the impact of propagule pressure (i.e., the number and size of introductions) on establishment success and post‐release dynamics, the introductions were done according to three experimental modalities: (1) a single introduction of 100 females, (2) two sequential introductions of 50 females, or (3) a single introduction of 1 000 females. Post‐release surveys demonstrated a very high local establishment of T. sinensis, regardless of the introduction modality. Post‐introduction dynamics were found to follow a two‐phase process. First, early dynamics were characterized by an exponential growth of T. sinensis populations without a significant decrease in D. kuriphilus populations. Later, middle‐term dynamics indicated a global decrease in both D. kuriphilus and T. sinensis populations. Quite surprisingly, propagule pressure had a limited effect on post‐introduction dynamics, with a lasting demographic advantage for the largest introductions, but no influence of the number of introductions. Finally, site connectivity had a positive effect on T. sinensis population size, suggesting successful colonization and establishment at a wide spatial scale beyond the introduction sites. In addition, we observed that some T. sinensis exhibited a very short developmental time compared to the majority of the emerged individuals, which may be interpreted as a diversified bet hedging strategy against environmental stochasticity. These findings confirm the strong potential of T. sinensis for successful establishment and sustainable control of the chestnut gall wasp in Europe.

Occurrence of oriental chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera, Cynipidae) in Slovakia - short communication

During 2014–2016, damage by the oriental chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera, Cynipidae) was found on sweet chestnut trees at 4 localities in Slovakia. Dryocosmus kuriphilus is a specific plant pest on Castanea spp. In Europe it is considered as the most harmful insect pest on Castanea sativa. It is the first report of the oriental chestnut gall wasp occurrence on chestnut trees in Slovakia.

Gnomoniopsis castaneae associated with Dryocosmus kuriphilus galls in chestnut stands in Sardinia (Italy)

Abstract: Invasive fungal pathogens and pests of sweet chestnut (Castanea sativa Mill.) forests drastically reduce their productivity. The recently described Gnomoniopsis castaneae is one of the main agents involved in the epidemic of brown rot of chestnut nuts worldwide. In 2014, during an investigation aimed at evaluating the health status of chestnut forests in Sardinia, a high incidence of necrotic galls induced by the Asian gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera, Cynipidae) was observed. Several fungal isolates were consistently isolated from necrotic gall tissues. Based on their morphological characters and analyses of both the ITS and EF1-α-coding gene sequences, all isolates were identified as Gnomoniopsis castaneae.

Spread of the introduced biocontrol agent Torymus sinensis in north-eastern Italy: dispersal through active flight or assisted by wind?

In successful classical biological control, natural enemies can provide enduring pest suppression if they reproduce and disperse without continued human management. To explain the efficient control exerted on the Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae) by the parasitoid Torymus sinensis Kamijo (Hymenoptera: Torymidae), it is required that the latter has the capacity to track the range expansion of its host. In the present study, we hypothesized that the distribution of infested patches and the prevailing wind directions would interactively influence and accelerate the spread of the parasitoid. It emerged that the spread capability of T. sinensis is high and likely affected by a combination of short- and long-distance flights (stratified dispersal). In particular, the ability to disperse long distances represents a successful trait of the parasitoid because the natural dispersal of its host is high and frequently aided by unintentional human-mediated transportation.

A plant pathogen causes extensive mortality in an invasive insect herbivore

Abstract We investigate a plant–microbe–insect interaction where a plant pathogenic microorganism is utilizing an invasive insect herbivore as an alternate resource, and test the hypothesis that the microorganism is affecting the population dynamics of the herbivore on its woody host plant. We investigated the presence of necrotic lesions formed by an unknown microbe on galls induced by the Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae), a global pest on Castanea hosts. We hypothesize that colonization by this plant‐associated microbe impacts survival and invasiveness of the gallmaker and, using Koch's postulates, we evaluate its effects on the gallmaker and on the gallmaker's primary natural enemy. Morphologically identical colonies were isolated from galls and insects, and DNA sequences matched with the fungal genus Colletotrichum, a causal agent of plant‐pathogenic anthracnose. Inoculations generated comparable symptoms. After the formation of superficial lesions, the pathogen rapidly penetrates galls, colonizing insect tissue in gall chambers and causing extensive larval mortality. Minimal effects were evident on the associated parasitoid Torymus sinensis Kamijo (Hymenoptera: Torymidae). We report a plant pathogen utilizing a gall‐inducing arthropod at the same time as sparing its parasite and expand reports of Colletotrichum impacting insects. We shed light on a complex of interactions influencing an important invader.

Novel insight in the life cycle of Torymus sinensis, biocontrol agent of the chestnut gall wasp

Torymus sinensis Kamijo (Hymenoptera: Torymidae) is a biological control agent of the chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae). It is reported in the literature as univoltine, but in NW Italy it exhibits a prolonged diapause mainly as late instar larva. Diapause is extended for 12 months, and adults emerge in April as usual, showing a two-year life cycle. Second year emergence individuals are able to mate, and the presence of mature eggs was confirmed in females which parasitised fresh chestnut galls, showing the same parasitism behaviour as first year emergence individuals. Both sexes of second year emergence individuals proved to be smaller than the univoltine ones according to ovipositor sheath length, pronotum width, and hind tibia length. Proving evidence of the extended diapause plays an important role for the establishment of T. sinensis especially in the first years after its release. Future studies are needed to clarify the factors which trigger off this response.

Potential fecundity of a highly invasive gall maker, Dryocosmus kuriphilus (Hymenoptera: Cynipidae).

Fecundity is a key factor in modulating population growth rate, and is of particular significance when considering the invasiveness of introduced species. In insects, fecundity is affected by body size, age, and nutrition. We investigated the potential fecundity of the invasive Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae), an introduced parthenogenetic gall former of Asian origin and a global pest of chestnut (Castanea spp.), to better understand its invasiveness. We compared ovarian, egg, and body metrics of adult wasps of different age. We evaluated insect weight, body length, mesosomal and metasomal lengths and widths, hind femur length, number of eggs, and size of eggs in wasps from four age cohorts. Adult weight and metasomal width were positively correlated with number of eggs. Egg load decreased with wasp age, and egg size initially increased before decreasing. Our findings suggest that adult D. kuriphilus, previously reported as proovigenic, may be resorping eggs in the absence of suitable hosts, and reallocating nutritive resources for body maintenance and egg quality to increase fitness, implicating a plasticity in its reproductive strategy. D. kuriphilus may be able to vary its potential fecundity in response to nutrition and host availability, thus increasing its invasiveness.

Invasion by the chestnut gall wasp in Italy causes significant yield loss inCastanea sativanut production

The Asian chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera Cynipidae) is an invasive species in chestnut forests and orchards in many parts of the world. 2 Nuts produced by the European chestnut (Castanea sativa Miller) are important in human food and culture, and as a component in food webs in forest ecosystems. 3 Severe infestations are reported to reduce nut yield, although precise data are lacking because of large natural year-to-year variability in yield. 4 The recent colonization of chestnut orchards in north-eastern Italy, where nut yield has been continuously and precisely recorded for several years, offered an opportunity to calculate the impact of gall wasp infestation level on yield. 5 The nut yield of C.sativa chestnut trees was negatively related to the gall wasp infestation level, with losses as high as 80% being reported when the number of current-year galls was above six galls per 50-cm twig. 6 Yield losses can be explained by direct and indirect factors related to gall formation, and a fuller understanding of the mechanisms involved could identify possible mitigation measures.

Influence of the introduced parasitoid Torymus sinensis (Hymenoptera: Torymidae) on T. koreanus and T. beneficus as indigenous parasitoids of the chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae) on chestnut trees in Nagano Prefecture, Japan

We describe here the parasitoid wasps Torymus sinensis Kamijo and T. beneficus Yasumatsu & Kamijo (early-spring and late-spring strains), which are introduced and indigenous natural enemies of the chestnut gall wasp Dryocosmus kuriphilus Yasumatsu, an invasive chestnut pest in Japan. We recently discovered specimens of T. koreanus Kamijo, endemic in Korea, among Torymus parasitoids collected from D. kuriphilus galls in a Japanese chestnut orchard. In this study we compare the composition of Torymus parasitoids emerging from D. kuriphilus galls before and after the release of T. sinensis. Before the release of T. sinensis, early-spring and late-spring strains of T. beneficus predominated (58.3 and 20.8% of specimens collected). However, a few years after the release, both T. beneficus strains had been almost completely displaced by T. sinensis. In contrast to the rapid decrease in T. beneficus, T. koreanus did not decrease drastically before and even after the release of T. sinensis (approximately 10–20% of specimens collected). These results suggest that not a few T. koreanus were present in the Japanese chestnut orchard investigated at least several years after the release of T. sinensis, although both the T. beneficus strains were rapidly displaced by T. sinensis during this period.