Identification of Torymus sinensis and T. beneficus (Hymenoptera: Torymidae), introduced and indigenous parasitoids of the chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae), using the ribosomal ITS2 region

The parasitoid wasps Torymus sinensis and T. beneficus (Hymenoptera: Torymidae) are introduced and indigenous natural enemies, respectively, of the chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae), an invasive pest of chestnuts in Japan. T. beneficus has two emergence types in spring, here tentatively designated as the early-spring strain and late-spring strain. It is very difficult to distinguish these two Torymus species accurately according to their morphological and ecological characteristics. Although the sequences of internal transcribed spacer 2 of the nuclear ribosomal DNA (ITS2) of these parasitoids are very similar, we have developed a pair of primers for amplifying a part of ITS2 that distinguishes species and emergence type. By performing high-resolution electrophoresis of PCR products amplified by this specific primer pair, we have succeeded in accurately identifying T. beneficus (early-spring strain) and some T. sinensis parasitoids. However, this technique was discovered to be inapplicable to identify other T. sinensis and T. beneficus (late-spring strain) parasitoids with the same ITS2 genotype. In spite of this problem, the ITS2 marker appears to be more powerful than any other molecular markers so far reported, since it is applicable to detection of T. beneficus, T. sinensis, and their hybrids.     

Seasonal flight activity and genetic relatedness of Torymus species in Italy

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Rearing,release and settlement prospect in Italy of <Emphasis Type="Italic">Torymus sinensis</Emphasis>, the biological control agent of the chestnut gall wasp <Emphasis Type="Italic">Dryocosmus kuriphilus</Emphasis>

Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae), is one of the most serious pests attacking chestnut trees. Recently it was incidentally introduced into Northwest Italy and it is now spreading throughout Europe. This pest was successfully controlled in Japan by introducing a parasitoid, Torymus sinensis Kamijo (Hymenoptera: Torymidae), from China’s mainland. Following this successful experience, the parasitoid was introduced into Italy from Japan. One year of preliminary studies led to a successful method of rearing imported galls with a synchronization between the parasitoid’s emergence and the presence of the target galls in the field. In two consecutive years, a total of 2,117 individuals were released in several sites covering most of the infested area. There are encouraging data about the settlement of the parasitoid and its synchrony with the host’s cycle: from about 64,000 host galls collected in the field over 200 T. sinensis were reared. The role of native parasitoids associated with the chestnut gall pest in its Italian distribution range is also discussed.     

Wolbachia prevalence patterns: horizontal transmission,recombination, and multiple infections in chestnut gall wasp-parasitoid communities

The Oriental chestnut gall wasp, Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae), is a global invasive pest that causes serious damage to almost all chestnut species belonging to the Castanea genus (Fagaceae). Dryocosmus zhuili Liu et Zhu is a recently described sibling species of D. kuriphilus, which induces galls on Castanea henryi (Skan) Rehd. et Wils. There are many indigenous parasitoid species in China which play an important role in the natural regulation of their population dynamics. Wolbachia is a maternally inherited α-proteobacterium widely found in arthropods. This study screened for the presence of Wolbachia in the two chestnut gall wasps and in six parasitoid species from 12 populations, to investigate the prevalence patterns of Wolbachia in the chestnut gall wasp-parasitoid communities. We found that D. zhuili and four parasitoid species were infected with Wolbachia; among them, all individuals of the two populations of Megastigmus sp. had multiple Wolbachia infections. By using multilocus sequence types to characterize bacterial strains, three new sequence types were identified. The Wolbachia strains infecting D. zhuili (ST-507), Torymus sinensis Kamijo (ST-508), and Sycophila variegata (Curtis) (ST-508) belonged to supergroup A, whereas the Wolbachia strain infecting Megastigmus nipponicus Kamijo (ST-503) belonged to supergroup B. Our results also suggested that horizontal transmission of Wolbachia occurs between chestnut gall wasps and their parasitoids. Moreover, multiple Wolbachia infections of Megastigmus sp. may be due to gene recombination and horizontal transmission.     

Finding of Torymus koreanus (Hymenoptera: Torymidae) attacking Dryocosmus kuriphilus (Hymenoptera: Cynipidae) in Japan

Torymus koreanus (Hymenoptera: Torymidae) was reared from galls of Dryocosmus kuriphilus (Hymenoptera: Cynipidae) for the first time in Japan. We here report morphological features and partial mtDNA sequencing data of T. koreanus. Torymus koreanus has several common characteristics with species of the cyaneus group defined by Zavada (2003 ). According to the key to species groups defined by Graham and Gijswijt (1998 ), it does not belong to any species group because of the entire posterior margin of metasomal tergum five.     

Optimal clutch size of the chestnut gall-wasp,Dryocosmus kuriphilus yasumatsu (Hymenoptera: Cynipidae)

Optimal clutch size of the chestnut gall-wasp, Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae), was examined in galls on wild and resistant chestnut trees in 1988 and 1989. The rate of escape success of newly-emerged adults from galls was an average of 60%, irrespective of cell numbers per gall. Dry mass per cell of a gall (as an index of nutritive condition) decreased with increasing cell number per gall, but was proportional to the mean number of mature eggs of new adults per gall. The number of cells per gall that occurred most frequently did not agree with that attained by the maximum survival rate from young larva to adult emergence of the gall-wasp. This discrepancy was examined from the viewpoint of three factors: 1) quality of offspring, 2) defensive response of the host plant causing mortality of the gall-wasp before cell formation, and 3) fitness per gall vs. fitness per egg. It is concluded that the third factor is most likely to be the one best in explaining the discrepancy.     

A native and an introduced parasitoid utilize an exotic gall-maker host

Dryocosmus kuriphilus (Hymenoptera: Cynipidae) is non-native to North America and induces formation of galls on petioles and leaves of all chestnut (Castanea spp., Fagales: Fagaceae). We investigated the interactions between the gall wasp D. kuriphilus, a native parasitoid, Ormyrus labotus (Hymenoptera: Ormyridae), and a non-native parasitoid, Torymus sinensis (Hymenoptera: Torymidae). Galls were collected monthly from May to August and in January from four locations in the United States consisting of orchard-grown hybrid chestnuts (Hiram, OH and Meadowview, VA), suburban-grown ornamental Chinese chestnuts (C. mollissima) (Broadview Heights, OH), or forest-grown American chestnuts (C. dentata) (Bowling Green, KY). Parasitoids were removed from galls and T. sinensis and O. labotus were identified using PCR-markers. The relative abundance of each parasitoid was compared in relation to collection date, habitat, presence of alternative hosts, and gall characteristics. T. sinensis was collected from each location and date, and was dominant in the orchard and suburban locations. However, relatively more O. labotus were collected within the forest, which had significant oak component and alternative cynipid hosts. O. labotus was only collected in spring and early summer, indicating the use of different summer and winter hosts. Observations suggest that in addition to parasitizing D. kuriphilus, O. labotus hyperparasitizes T. sinensis. T. sinensis has a longer ovipositor than O. labotus, and parasitized larger galls. This study improves our understanding of interactions between an invasive gall wasp, an introduced parasitoid, and native parasitoids, and illustrates novel relationships that may form as exotic species expand their geographic range.     

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

1. 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.
2. 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.
3. 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.
4. 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.

     

<Emphasis Type="Italic">Torymus sinensis</Emphasis>: a viable management option for the biological control of <Emphasis Type="Italic">Dryocosmus kuriphilus</Emphasis> in Europe?

The chestnut gall wasp Dryocosmus kuriphilus is a global pest of chestnut (Castanea spp). Established as a pest in the mid-twentieth century in Japan, Korea and North America, this species was first reported in Europe in 2002. Following the successful release of a biological control agent Torymus sinensis in Japan, this parasitoid species has been released in Italy since 2005. Here we discuss the potential of T. sinensis as a viable management option for the biological control of D. kuriphilus in central Europe. We suggest that more consideration should be given to determining, (i) the conditions under which T. sinensis may attack alternative native gall wasp hosts and (ii) the likelihood of hybridization of this species with native Torymus. Both issues are central to predicting unassisted range expansion by released T. sinensis, and to assess the environmental risks associated with a more widespread release of this species in Europe.     

Rapid on-site identification of the biocontrol agent of the Asian chestnut gall wasp

In classical biocontrol programmes, a rapid and correct identification of the introduced antagonist is a key issue during both the release and establishment monitoring phases. It is often difficult to distinguish morphologically cryptic species or immature stages. An accurate diagnosis can now be provided by molecular diagnostic methods. Among the conventional and real-time PCR-based methods, loop-mediated isothermal amplification (LAMP) is a particularly suitable technique as it allows a rapid amplification of target DNA directly in the field. During the programme implemented in Italy against the Asian chestnut gall wasp (ACGW) Dryocosmus kuriphilus, we developed a real-time LAMP assay, combined with a simple DNA extraction method, for rapid in-field identification of larvae, pupae, and adults of the biocontrol agent, the parasitoid Torymus sinensis. Validation of the assay comprised adults as well as preimaginal stages of parasitoids obtained from ACGW galls collected from different localities. Results confirmed the effectiveness of the LAMP assay to rapidly and specifically identify the target parasitoid in the field. This assay will be a valuable tool for quick on-site checking of the parasitism rate.     

Host specialization is ancestral in Torymus (Hymenoptera,Chalcidoidea) cynipid gall parasitoids

The host range of parasitoids varies greatly among species: Some only parasitize one to a few hosts (specialists), while others parasitize multiple species or a variety of host types (generalists). The direction of most host range shifts in parasitoid groups, that is from generalist to specialist or, alternatively, from specialist to generalist, is unknown. To explore the origin of host range shifts, we studied a clade within the genus Torymus (Hymenoptera, Chalcidoidea) that includes both generalist and specialist parasitoids of Cynipidae (Hymenoptera). We analysed the phylogenetic relationships of the species of Torymus on the basis of two gene fragments (cox1 and ITS2) of 246 specimens and performed an ancestral state reconstruction of the specialist/generalist trait. Our results revealed the following: (a) The ancestral state of this group of Torymus is specialist, with the generalist state evolving through a loss of specialization. (b) The species Torymus cyaneus and Torymus flavipes both have a strong genetic structure, suggesting the existence of different biological identities. (c) There has been a host plant shift in the lineage(s) leading to Torymus rubi and Torymus bedeguaris from galls on Quercus to those on Rosaceae. (d) The alien species Torymus sinensis and the native European species Torymus notatus are phylogenetically closely related. (e) Speciation within Torymus was likely associated with the diversification of their cynipid hosts, which itself was driven by the dramatic changes in climate and vegetation that occurred during the Miocene.     

A plant pathogen causes extensive mortality in an invasive insect herbivore

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Invasion by the chestnut gall wasp in Italy causes significant yield loss in Castanea sativa nut production

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Asian chestnut gall wasp in Tuscany: gall characteristics,egg distribution and chestnut cultivar susceptibility

• 1 Preliminary investigations were carried out on Dryocosmus kuriphilus Yasumatsu on Castanea sativa Miller in Tuscany to assess variations in gall characteristics in coppice and high forest at two crown heights (height < 2 or 2–6 m), influence of bud size and bud position on oviposition rates and susceptibility of three cultivars.
• 2 Gall size may depend on various factors, including wasp population density. In the present study area, small galls (with one or two cells) were the most numerous in 2008, whereas larger galls (with more than three cells) prevailed in 2009.
• 3 Dryocosmus kuriphilus oviposition occurrence was influenced by both bud size and bud position. Buds with eggs tended to be larger in size compared with bud without eggs, suggesting that D. kuriphilus females prefer to lay eggs in larger buds (approximately 6 mm³) compared with smaller buds (approximately 3 mm³). The mean number of eggs per bud tended to decrease from the apical bud toward the basal bud.
• 4 Three C. sativa cultivars, Carpinese, Fusca and Cesurone, were examined. Fusca grafts had significantly more galls compared with Carpinese and Cesurone, whereas Cesurone grafts had more larvae per bud compared with Carpinese and Fusca. Overall, the Carpinese cultivar may be less susceptible to D. kuriphilus galling compared with the Fusca and Cesurone cultivars.

     

Success of classical biocontrol agent Torymus sinensis within its expanding range in Europe

Torymus sinensis Kamijo (Hymenoptera, Torymidae), a classical biocontrol agent of chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera, Cynipidae), was released in Croatia, Slovenia and Hungary in 2015. Following the introduction, the research of parasitism rates and population genetic indices on 40 different sites was performed to monitor and evaluate the success of establishment. The observed parasitism rates were unexpectedly high and negatively correlated with the distance from westernmost locality, while population genetic indices showed that populations of T. sinensis did not suffer from bottleneck‐induced founder effect phenomenon. Moreover, lack of genetic differentiation demonstrates that all populations share similar genetic structure, which could be shaped only by high levels of gene flow. We conclude that T. sinensis established viable and genetically diverse populations and successfully spread naturally from Italy across Slovenia to Croatia and Hungary.     

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

Studies 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.     

Population dynamics of the chestnut gall wasp,Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae)

Summary During the course of a 10-year investigation on the population dynamics of Dryocosmus kuriphilus, a rapid decrease in the number of individuals was noticed in the adult stage. To detect the role of predation by spiders in causing this high adult mortality, spiders collected from the survey station were tested for their reaction to Drycosmus antiserum. About 2050% of the spiders collected during the emergence period of D. kuriphilus reacted positively, showing that the species of Thomisidae, Argiopidae, Agelenidae and Salticidae were the important predators. The predation ratio for the total adult wasp population was estimated at 8.1% in 1968, 20.2% in 1969 and much higher in 1970, increasing with decreasing population density of the wasps. However, the predation by spiders could not account for all of the adult mortality of D. kuriphilus.     

Population dynamics of the chestnut gall-wasp,Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae)

Summary As a part of serial study on population dynamics of the chestnut gall-wasp,Dryocosmus kuriphilus, analyses of the distribution of eggs, gall-cells and emergent holes were made from the statistical point of view. Many of distributions of the eggs per bud could be described by the truncated Poisson, but some cases showed slight overdispersion than expected by chance. Because of no linear increase of with increasing (expected mean for complete sample), however, the truncated negative binomial seemed to be not so available for whole series. Distributions of the gall-cells and the emergent holes were, on the other hand, well described by the truncated Poisson distribution when the observed frequency was calculated for respective trees. Negative-binomial tendency found in distributions from some stations consisted of a certain number of trees would be resulted from mixture of Poisson populations with different means. Random or slightly concentrated oviposition and random mortality within galls was thus supposed for future study unless the proof favouring other distribution models would appear.     

Comparison of the suitability of Liriomyza chinensis and L. trifolii (Diptera: Agromyzidae) as hosts for Neochrysocharis okazakii (Hymenoptera: Eulophidae)

Effective parasitoid production or mass-rearing is one of the key components in practicing biological control with parasitoids. Selection of host species for rearing parasitoids is a major approach to improve the production efficiency. The stone leek leafminer Liriomyza chinensis (Kato) is a destructive pest of onion crops in many Asian countries, which is difficult to control with chemicals. The present study examined whether the production of Neochrysocharis okazakii Kamijo, a promising native biological agent of L. chinensis, could be effective if kidney bean-Liriomyza trifolii (Burgess) system was used as an alternative rearing procedure. The suitability of L. chinensis and L. trifolii as hosts of N. okazakii was investigated in the laboratory. Neochrysocharis okazakii completed its development on both two host species, and the total development time was similar on these hosts. Longevity of N. okazakii females provided with L. chinensis was longer than those on L. trifolii. There were no significant differences in fecundity and offspring sex ratio when females had been provided with these two host species. The adult parasitoids emerged from L. chinensis were significantly larger than those from L. trifolii. However, the intrinsic rate of increase was higher, and mean generation time was lower, for L. trifolii than L. chinensis. Thus, mean net reproductive rate (Ro) was higher when L. trifolii was used as host. Although the two host species were equally suitable as host for N. okazakii, our findings suggested that L. trifolii and kidney bean is an ideal system for N. okazakii production.     

Biological Control of Anastrepha spp. (Diptera: Tephritidae) in Mango Orchards through Augmentative Releases of Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae)

Diachasmimorpha longicaudata (Ashmead) parasitoids were released by air on a weekly basis over 1600 ha of commercial mango orchards, backyard orchards, and patches of native vegetation, at a density of ca. 940 parasitoids/ha. Releases were made during 2 consecutive years, beginning at flower onset and lasting until the end of the production cycle. Two areas, 7 km apart, were compared. In one area parasitoids were released, whereas the other area was used as a control. During the 2nd year treatments were reversed. Fruit was sampled in commercial mango orchards and in backyard orchards to assess levels of parasitism in fruit fly larvae. Highly significant differences in percentage parasitism were found in release and control zones in backyard orchards. Furthermore, trapping results indicated that D. longicaudata releases were associated with ca. 2.7-fold suppression of Anastrepha spp. populations in backyard orchards. Results suggest that suppression might be affected by environmental conditions and by the parasitoid:fly ratio achieved. Anastrepha obliqua McQuart populations were suppressed more effectively by use of parasitoids than those of Anastrepha ludens Loew, perhaps due to the type of host fruits used by each species. Augmentative parasitoid releases in marginal areas surrounding commercial orchards (backyard orchards, wild vegetation) can substantially suppress fly populations. Through this approach, the number of flies that later move into commercial orchards can be significantly reduced. Such a strategy, when combined with sound orchard management schemes, can allow growers to produce clean fruit without the need to resort to the widespread use of insecticides.