Asian Chestnut Gall Wasp Dryocosmus Research Articles

Assessing the susceptibility levels of chestnut cultivars/genotypes to Asian chestnut gall wasp (Dryocosmus kuriphilus Yasumatsu)

The Asian chestnut gall wasp Dryocosmus kuriphilus is a major insect pest affecting chestnut trees worldwide. Due to the galls that it creates on shoots and leaves, this pest prevents shoot development and blooming and decreases the fruit yield. In Türkiye, this pest was first seen in Gacık village of Yalova province in the year 2014. Today, it is seen in orchards and forests in different provinces of Marmara, Aegean, and Black Sea regions. The medium- and long-term control of this pest can be improved by making use of resistant or less-susceptible cultivars and hybrid clones. The susceptibility levels of our cultivars to this pest are not known. For this purpose, by using three infestation indicators, the susceptibility to Asian chestnut gall wasp (ACGW) of 3 hybrids (Castanea sativa x Castanea crenate) and 20 cultivars/genotypes (C. sativa), which came to the forefront via selection studies, were assessed for five years (2017-2021). The susceptibility to ACGW varied between the tested genotypes/cultivars and, since no gall development was observed in ‘YD-1’, ‘Tülü’, and ‘Ertan’ cultivars of C. sativa species, they were determined to be completely resistant to ACGW. Furthermore, the numbers of ACGW eggs on the buds of cultivars were recorded and it was found that ACGW laid eggs on the buds of all the cultivars. In microscopic analyses, it was revealed that the resistance originated from the prevention of larva development within the bud. The finding that resistant cultivars belong to C. sativa species is thought to be important for future genetic breeding programs.

Torymus sinensis against the invasive chestnut gall wasp: Evaluating the physiological host range and hybridization risks of a classical biological control agent

BackgroundA common strategy to limit the negative impact of biological invasions is biological control through the release of specialized alien natural enemies. However, biological control plans are not without risks, which include parasitism of native hosts and hybridization with related native species, particularly those that are potential natural enemies of the invasive species. Here, we evaluate these potential risks resulting from the introduction of the parasitoid wasp Torymus sinensis (Hymenoptera: Torymidae) in Europe to control the invasive Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae). ResultsWe found that, under laboratory conditions, the physiological host range of T. sinensis includes several native non-target species of Cynipidae, with oviposition observed on the galls of 8 of the 11 species tested. However, physiological host range of T. sinensis appears to be limited under field conditions, as we observed only one parasitized gall of Andricus curvator in the field. Regarding hybridization, inter-species mating between T. sinensis and its phylogenetically closest native Torymus species was not observed in the laboratory. Moreover, discordance between nuclear (ITS2) and mitochondrial (coxI) data does not support the presence of genetic introgression, suggesting that hybridization between T. sinensis and native Torymus species does not occur. In addition, we cite and discuss the unexpected presence of one individual of the related alien species Torymus beneficus in Spain. ConclusionOur results suggest that T. sinensis may negatively impact several non-target species, highlighting the need for careful monitoring of the extent of such undesired behaviour and its effects on the native fauna.

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.

Assessment of Chestnut Gall Toughness: Implications for a Biocontrol Agent.

(1) Torymus sinensis, the biocontrol agent of the Asian chestnut gall wasp Dryocosmus kuriphilus, is univoltine, but in NW Italy a small percentage of individuals exhibits a prolonged diapause, mainly as late instar larva. (2) In 2020, the diapause was investigated to evaluate its trend over the years. Due to the low survival rate of diapausing T. sinensis adults, the seasonal variation in the galls' toughness was evaluated, thus assuming that dry galls over time can negatively affect emergence. The window of vulnerability of the gall wasp galls was also evaluated in controlled conditions. (3) The results showed that the average number of second year T. sinensis emerging per 100 cells was 0.41 ± 0.05, and dead adults accounted for 4.1 ± 0.23 per 100 cells. Gall toughness resulted in lower values for galls collected in May and June, and then gradually increased over time. In general, no difference was detected in the wall toughness of galls formed during the previous year when compared to current-year dry galls. Oviposition was recorded on all the tested galls collected in May and June, and no difference in the number of oviposition events was detected. Conversely, no oviposition was observed in July. Comparing the number of oviposition events by T. sinensis and the gall toughness, a negative correlation was found (R2 = -0.99). (4) The present findings contribute descriptive information on this gall's structural traits, and the influence on gall wasp management is also discussed.

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.

Host Gall Size and Temperature Influence Voltinism in an Exotic Parasitoid

Insect phenology is highly temperature-dependent. Higher temperatures can lead to earlier emergence and lengthening of the active period, which enable many insect groups to complete more generations. Studies on the effects of climate change on insect populations are providing concerning evidence supporting this relationship. These kind of shifts in phenology and voltinism also occur in agricultural and forest insect pests and their natural enemies, with potential implications for biological control. The consequences derived from changing temperature regimes on tritrophic interactions remain poorly studied, particularly in gall-inducing insects and their parasitoids. Here we detail the occurrence of bivoltinism in the exotic parasitoid Torymus sinensis, previously categorized as univoltine, a widely introduced species to fight against the invasive Asian chestnut gall wasp Dryocosmus kuriphilus wherever this pest spread. This plasticity in voltinism has been observed in the southernmost European distribution of D. kuriphilus, and appears to be mediated by both temperature and gall traits, namely size or the number of gall chambers. Bivoltinism was most common at annual mean temperatures around 13.5°C and in galls with more chambers. Through this work, we intend to unravel the factors behind this phenomenon and discern the possible consequences on host-parasitoid interactions.

Impact of the Asian gall wasp Dryocosmus kuriphilus on the radial growth of the European chestnut Castanea sativa

Abstract The invasion of the Asian chestnut gall wasp Dryocosmus kuriphilus (ACGW) in Europe has caused serious biological and economic impacts on chestnut stands that have been partially solved with the introduction of the biological control agent Torymus sinensis. However, information concerning tree‐ring growth during the ACGW epidemic has been lacking so far. Using dendrochronology techniques, we analysed the impact of the ACGW on tree‐ring growth and the capacity of the chestnut tree to recover when biocontrol was achieved in seven sites covering the area of first detection and subsequent spread of the ACGW in Europe. In each site, a non‐target control species (i.e. trees not attacked by the ACGW) has been included as a reference in the analysis. Results show a reduction in the tree‐ring increment by 60% on average during the ACGW epidemic. Such effects were higher in the magnitude and longer in the duration when compared to other stresses such as insect defoliation or extreme summer droughts. Synthesis and applications. Marked reductions in radial growth were evident in the years of ACGW attack regardless of the age of the chestnut coppices. Since most of the trees only recovered to an almost normal growth rate after 3–4 years of biocontrol by T. sinensis, an immediate release of the antagonist at the first ACGW appearance is highly recommended. The consequences and perspectives of ACGW's attacks on the quality of the timber produced are also 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.

Environmental thermal levels affect the phenological relationships between the chestnut gall wasp and its parasitoids

AbstractStudies of thermal level‐related asynchrony in a host–parasitoid relationship are necessary to understand the effects of climate change on new host–parasitoid interactions. In the Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae) and its Chalcidoidea parasitoids, phenological synchrony is assumed to be weather‐dependent in a new area of expansion. To evaluate the effects of environmental thermal regimes on the host, a phenology model for different cynipid stages (larvae, pupae, adults, and adult emergence) and a host–parasitoid phenological estimator are developed in three chestnut fields during two successive growth seasons and subsequently validated in areas with chestnut fields at two different altitudes. Comparisons of the timings of the juvenile and adult stages with those of the parasitoid complex demonstrate that the shortest period of occurrence for cynipids within galls has negative effects on the host–parasitoid relationships at higher temperature levels, thereby increasing phenological asynchrony for some parasitoids species. Reducing the development time of pupae and adults decreases the likelihood of success for some parasitoid species at higher temperature levels. We also record the extension of the gall wasp development time (approximately 15 days) at higher altitudes (linked to a lower mean temperature of approximately 1.5 °C). These results highlight how parasitization on the new hosts is dependent on the host phenology and, in the present study, is limited by the short duration of the presence of the host in galls, which could explain the considerable differences in cynipid gall wasp parasitization recorded at different altimeters.

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.

Studies on the Gall Characteristics of Dryocosmus kuriphilus in Chestnut Genotypes in Yalova and Bursa Provinces of Turkey

The Asian chestnut gall wasp Dryocosmus kuriphilus is a global pest of chestnut (Castanea spp.). It has been spreading in Turkey’s forests and orchards since 2014. This pest imposes a big threat to the Turkish chestnut industry, which is among the top producers in the world. Its gall morphology has been related to pest pressure and host cultivar, thus eventually modulating plant damage with heavy impact on growth and fruit production. We compared gall characters (position on plant organ, ratios, dimensions, volumes, number of larval chambers) in wild Castanea sativa, two local cultivars and a Euro Japanese hybrid. Overall, leaf galls were more common (55.36%), followed by the stem (19.6%) and leaf stipule galls (15.29%). The mean number of chamber and volume value of gall types were 1.52-5.93 and 0.43-2.15 cm3, respectively. The highest values were observed in ‘stem gall’. The more gall formation was observed in the wild chestnut trees and ‘Marigoule’ than the other local varieties.

Biological control of the chestnut gall wasp with T. sinensis: A mathematical model

The Asian chestnut gall wasp Dryocosmus kuriphilus, native of China, has become a pest when it appeared in Japan, Korea, and the United States. In Europe it was first found in Italy, in 2002. In 1982 the host-specific parasitoid Torymus sinensis was introduced in Japan, in an attempt to achieve a biological control of the pest. After an apparent initial success, the two species seem to have locked in predator–prey cycles of decadal length. We have developed a spatially explicit mathematical model that describes the seasonal time evolution of the adult insect populations, and the competition for finding egg deposition sites. In a spatially homogeneous situation the model reduces to an iterated map for the egg density of the two species. While, for realistic parameters, the map would support the hypothesis of biological control, the full model, in the same parameter range, does not give such a clear-cut answer. In particular, according to the spatially explicit model, the introduction of T. sinensis would spark a traveling wave of the parasitoid population that destroys the pest on its passage. Then, depending on the value of the diffusion coefficients of the two species, the pest may later be able to re-colonize the empty area left behind the wave. When this occurs the two populations do not seem to attain a state of spatial homogeneity, but produce an ever-changing pattern of traveling waves.

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.

Natural occurrence of Fusarium proliferatum on chestnut in Italy and its potential entomopathogenicity against the Asian chestnut gall wasp Dryocosmus kuriphilus

The Asian chestnut gall wasp Dryocosmus kuriphilus (DK) is one of the most important pests of chestnut trees worldwide, and sustainable control measures are urgently required for its control. In this study, the recurring mycoflora associated with DK galls in some Italian chestnut orchards was identified, and two Fusarium proliferatum strains (I3 and I4) were tested for their entomopathogenicity toward DK and their pathogenicity to chestnut in order to evaluate their potential as biocontrol agents. The two F. proliferatum strains, both isolated from the inside of DK galls and often covering dead bodies of larvae, pupae, and adults, caused a natural insect infection of about 4 % and resulted to be non-pathogenic to chestnut plants, while they induced 33 and 97 % of larvae mortality when applied in laboratory experiments to intact DK fresh or sectioned galls, respectively. Both strains infected maize seedlings, used as non-target hosts, and produced fumonisins in vitro. The efficacy in controlling DK in the galls, the absence of pathogenicity to chestnut, and the lower mycotoxin production indicated strain I3 as a promising biocontrol agent of the considered pest.

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.

Histological investigation on gall development induced by a worldwide invasive pest, Dryocosmus kuriphilus, on Castanea sativa

The Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae) is one of the most serious pests of chestnut trees worldwide. Wasps lay eggs into chestnut buds from mid Jane to late July, depending on latitude, and galls develop the following spring on new vegetation, reducing photosynthesis and nut production. We observed that modification of tissues surrounding D. kuriphilus eggs, which differentiate to form the larval chamber, started approximately 1 month after oviposition, shortly after eggs hatch. The larval chambers continued to increase slightly in size throughout the autumn months until January. After that, a period of stagnation, which corresponds to the plant's dormancy, occurred, followed by rapid growth from March to May, during the period from bud swelling to bud break. Galls continued to grow during the leaf expansion after bud break and stopped when plant organs achieved their final size. Our results have implications for the management of the pest, providing a better understanding of the critical time periods for the effective control.

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.

Hybridization between introduced Torymus sinensis (Hymenoptera: Torymidae) and indigenous T. beneficus (late-spring strain), parasitoids of the Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae)

Hybridization between introduced biological control agents and native species is a nontarget effect of biological control. However, the genetic impacts related to introduced insects, especially from the viewpoint of post-release evaluation, remain largely unexplored. Torymus sinensis (Hymenoptera: Torymidae) is a parasitoid wasp introduced from China into Japan to control the invasive Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae). After T. sinensis was first released in 1982, the damage caused by D. kuriphilus fell greatly. Previous studies showed that hybridization between T. sinensis and indigenous Torymus beneficus (the early-spring strain) was rare in the field, but individuals morphologically intermediate between T. sinensis and T. beneficus then appeared in the field. Here, we examined the frequency of hybridization between T. sinensis and the late-spring strain of T. beneficus by using the internal transcribed spacer 1 (ITS1) region of nuclear rDNA. Increasing numbers of F 1 hybrids were detected every year from 1993 to 1996, reaching 22%. It seems that T. sinensis can hybridize more easily with the late-spring strain of T. beneficus. Morphological indices of the F 1 hybrid type ranged between indices of the late-spring strain of T. beneficus and T. sinensis, with considerable overlap. Simultaneous with the occurrence of the F 1 hybrids, the indigenous late-spring strain of T. beneficus was rapidly displaced by T. sinensis.