Ganaspis kimorum, parasitoid wasp of spotted-wing drosophila larvae

Life cycle of spotted-wing drosophila and its parasitoid, Ganaspis brasiliensis. 

Spotted-wing drosophila (SWD) is a major pest of soft fruit world-wide. Parasitoid wasps are effective natural enemies of SWD. Surveys are currently underway to document these species and understand their biology. Adult male  SWD have dark spots near the ends of their wings, while females do not. Females also have a serrated ovipositor that cuts through fruit skins (like raspberries). Adult flies locate suitable fruit and lay eggs (each with two breathing tubes) within the fruit. The egg hatches into a white, first instar larva (maggot). An unparasitized maggot develops into a pupa (next life stage of the fly) within a tan puparium that protects the fly pupa before it can turn into an adult. 

SWD maggots within fruit may be detected by an adult female Ganaspis kimorum wasp, who will use a long ovipositor to lay one of her eggs in the SWD maggot. SWD possesses a potent immune system that can detect and encapsulate foreign invaders (including eggs of parasitoid wasps). The foreign body is then ejected from the body cavity at the next molt. Research into this ability is ongoing in the USA. However, Ganaspis kimorum has co-evolved with their host (SWD), and deactivate the host immune system using viruses harbored within their genome. The wasp egg hatches and the wasp larva knocks out the host (SWD maggot’s) immune system, developing and growing within the SWD maggot, eventually killing it. The SWD puparium now protects the developing wasp, rather than the developing SWD. Eventually the adult wasp emerges, male and female wasps mate, and the life cycle is repeated.

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Learn more about Ganaspis kimorum

(Hymenoptera: Figitidae)

Unlike many other frugivorous Drosophila species that typically oviposits into overripe or rotting fruit, the spotted-wing drosophila, Drosophila suzukii can lay eggs in intact and ripening fruits, puncturing the fruit’s skin with its unique saw-like ovipositor. Thus, D. suzukii often first attacks fruits before other Drosophila species, and this, in part, also results in its higher pest status. Furthermore, D. suzukii can attack and develop in many wild plant species that may provide reservoirs for fly populations to reinvade crops after they have been treated. Therefore, biological control through introduction or release of effective parasitoids into surrounding crop habitats may create an ‘ecological buffer zone’ to help reduce source fly populations moving into commercial crops (Wang et al. 2020b).

Although numerous parasitoid species can attack Drosophila larvae or pupae, extensive surveys in the fly’s invaded regions in North America or Europe have shown only low levels of parasitism by naturally occurring pupal parasitoids, and little or no parasitism by indigenous larval parasitoids (Lee et al. 2019, Wang et al. 2021a). Those common pupal parasitoids are not specific to D. suzukii while parasitoids commonly found attacking other Drosophila larvae in the rotting fruits rarely develop from D. suzukii due to the fly’s immune resistance (Wang et al. 2020b, Daane et al. 2021). In contrast, surveys in the fly’s native range in East Asia have discovered several larval parasitoid species readily attacking D. suzukii (Daane et al. 2016, Girod et al. 2018b, Giorgini et al. 2019). Among them, Ganaspis kimorum is one of the most dominant and widely distributed larval parasitoids of D. suzukii infesting fresh fruits in East Asia. Parasitism rates of D. suzukii by G. kimorum are highly variable according to geography and habitats, with the highest parasitism (when combined with parasitism by the closely related Ganaspis lupini) being 47.8% in wild Rubus in Yunnan, China and (Giorgini et al. 2019) and 75.6% in wild cherries in Nara, Japan (Girod et al.2018b).

Ganaspis kimorum was previously described under the name Ganaspis brasiliensis (Ihering)  by Buffington and Forshage (2016), and so G. kimorum referred to as the “G1” strain, or lineage, of G. brasiliensis in publications prior to 2024. Initially, there appeared to exist geographically distant lineages of G. brasiliensis, with the populations collected from D. suzukii in China, Japan and South Korea, consisting of two lineages (named G1 and G3) (Wang et al. 2020b). G1 was the dominant lineage (> 65%) in both the Chinese and South Korean collections and these two lineages were sympatric and co-existed on the same host plants inhabited by D. suzukii (Daane et al. 2016, Giorgini et al. 2019). The G1 lineageappeared to be strictly specific to D. suzukii while G3 could also attack other closely related species such as D. melanogaster and D. simulans (Girod et al. 2018a, Biondi et al. 2021, Daane et al. 2021). These two lineages were then shown to have different host-searching behaviors, very divergent genomes, and incompatible crossing, suggesting that they represent cryptic species (Seehausen et al. 2020; Hopper et al. 2024; Sosa-Calvo et al. 2024).  Subsequently, Sosa-Calvo et al. (2024) redescribed the G1 and G3 lineages as distinct species – Ganaspis kimorum Buffington and Ganaspis lupini Buffington, respectively.   

Ganaspis kimorum was been approved for field release in the USA and Europe and classical biological control releases in several states in the USA began in 2021 (Gariepy et al. 2024). Unintentionally introduced populations of G. kimorum were found in the Pacific Northwest beginning in 2019 and already appear to be well-established in this region (Abram et al. 2020; Beers et al. 2022; Abram et al. 2021).

The biology of G. kimorum (under its previous name, G1-G. brasiliensis) has been studied in quarantine laboratories in California (Wang et al. 2018, Hougardy et al. 2019, Wang et al. 2019, Wang et al. 2020a, Biondi et al. 2021, Daane et al. 2021) and Switzerland (Girod et al. 2018a, Seehausen et al. 2020). Under suitable conditions (22°C with honey water), adult females of G. kimorum emerged with about 40 mature eggs and mature egg load reached the peak (~80 mature eggs) after 2–3 days (Wang et al. 2018). The parasitoid preferred to attack young host larvae, although host age did not significantly affect the parasitoid offspring’s survival, developmental time, sex ratio and body size of female wasps. Adult G. kimorum females survived about 18 days and produced about 100 offspring per female. Estimated net reproduction rate of the parasitoid was 40 (the average number of female offspring produced per female’s lifetime) (Wang et al. 2018). The parasitoid’s development time decreased with increasing temperature from 17.2 to 27.5°C; at constant 29.3°C no parasitoids completed their development. Temperatures below 17.2 °C triggered a facultative diapause. This cold temperature response varied among populations of different origin: South Korean populations of G. kimorum entered diapause at 17.2°C, whereas only a proportion of their Chinese counterparts entered diapause at the same temperature (Hougardy et al. 2019). A climatic model predicts that the Asian strains of G. kimorum would likely establish in the western, south-eastern, and east coastal states in North America and most southern European countries (Wang et al. 2021b). 

To aid in the release, rearing methods for G. kimorum have been developed (Wang et al. unpublished data). Also, recommended methods for standardized sampling and identification of introduced or released larval parasitoids have been developed, which will facilitate multi-research team sampling efforts in the coming years to characterize the biological control impact of this parasitoid against D. suzukii in different regions (Abram et al. 2022a).

It is worth noting that another closely related Asian larval parasitoid Leptopilina japonica has also established in the same areas as  G. kimorum in the Pacific Northwest as well as in several states in the Eastern USA, possibly through accidental introduction (Abram et al. 2020; Abram et al. 2021; Beers et al. 2022; Gariepy et al. 2024). Many aspects of the biology of L. japonica were similar to that of G. brasiliensis, except that the former species developed faster and have a wider host range than the latter species (i.e. L. japonica has a broader host range than G. kimorum) (Wang et al. 2018, Hougardy et al. 2019, Wang et al. 2020a, Biondi et al. 2021, Daane et al. 2021). The fast development of L. japonica may confer a competitive advantage over G. kimorum when they both attack the same host (Wang et al. 2019). However, G. kimorum discriminated strongly against hosts previously parasitized by L. japonica, that may allow their co-existence as found in their native range in East Asia (Daane et al. 2016, Giorgini et al. 2019) and invaded regions in the Pacific Northwest (Abram et al. 2020; Abram et al. 2022b), where they attack spotted-wing drosophila on a wide range of crop and non-crop host plants. The establishment of these two larval parasitoids will likely enhance the overall impact of larval parasitoids on D. suzukii. Further studies are underway to better understand the field ecology of these parasitoids.