Biological control of Drosophila suzukii using natural enemies from Asia
The invasive spotted wing drosophila, Drosophila suzukii, is particularly difficult to manage in Europe. It has a very wide host range including fruits from many ornamental and wild plants and a short generation time, which allows the development of several generations per year. Consequently, crops are constantly re-invaded from the neighbourhood. Therefore, an area-wide management strategy is desirable. Classical biological control, i.e. the introduction of natural enemies from the region of origin for permanent establishment in the region of introduction, may provide a long term solution because these natural enemies would attack the pest in all habitats, thereby lowering populations at landscape level. The best candidates for the classical biological control of D. suzukii are larval parasitoids because they are usually considered as major mortality factors in Drosophila species and are, so far, totally absent from the natural enemy complex of D. suzukii in Europe. Furthermore, larval parasoids are considered more specific than other natural enemies of Drosophilidae, attacking only one or a few species of Drosophilidae.
Surveys were carried out in China and Japan to study the larval parasitoid complex of the fly and assess the importance of parasitism in its region of origin. At least eight parasitoid species were collected. Total parasitism rates in Asia varied from 0% to 80%. In all investigated regions, the parasitoid complex was largely dominated by two hymenopterans of the family Figitidae, Ganaspis sp. and Leptopilina japonica. Several strains of these parasitoids were selected for further laboratory tests in quarantine conditions in Switzerland and France. Their biology and ecology were investigated and compared with European parasitoid species. Studies are still on-going and particularly focus on the specificity of these parasitoids and on the mechanisms underlying specificity. Indeed, host specificity of potential classical biological control agents has to be determined prior to releases to avoid unintended non-target impacts on native species.
So far, the undescribed species Ganaspis sp. showed the highest specificity. However, important variations between strains were observed. Some strains are totally specific to D. suzukii in fruits, failing to attack and develop in other Drosophila spp. and even in D. suzukii in artificial diets. In contrast, other strains parasitised D. suzukii and the non-target D. melanogaster in diets enriched with fruits. These results are promising for the biological control of D. suzukii in Europe and suggest that Ganaspis sp. is the first candidate for introduction. This parasitoid may help lowering D. suzukii populations at a landscape level and, thus, has the potential to limit the need for other management methods. However, more studies are needed on its taxonomic status and the existence of biotypes or cryptic species before field releases can be envisaged in Europe.
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