A natural antagonist of Drosophila suzukii, a parasitic wasp from its native range, is harmless to the native non-target species Drosophila melanogaster. This has been proven in trials in secured field cages in Switzerland.
Native to Eastern Asia, the spotted-wing drosophila (Drosophila suzukii) has spread to other parts of Asia as well as to America, Europe and Africa. It attacks all soft-skinned fruits such as berries, cherries, plums and grapes, as well as the fruits of many wild plants. Unlike other Drosophila species, which lay their eggs in rotting, overripe fruits, females of D. suzukii lay their eggs in undamaged, ripening fruit. The invasive species causes significant damage to fruit crops.
Biological control with a parasitic wasp
A natural antagonist could be used to control D. suzukii biologically. In the native range of D. suzukii, researchers came across the parasitic wasp G1 Ganaspis cf. brasiliensis, which very specifically only attacks larvae of Drosophila species in fresh fruits. As D. suzukii is the only Drosophila species that attacks fresh fruits in the invaded range, the risk to non-target species of being attacked by the parasitic wasp is low.
Since laboratory studies with the parasitic wasps were successful, permits for their release are now being applied for in various countries. In Switzerland, the Federal Office for the Environment authorised their release in cages in June 2021. The aim was to study whether the parasitic wasp is still specific to spotted-wing drosophila under semi-field conditions, or whether it also parasitises the native fly D. melanogaster, a closely related species.
Native fly rarely parasitised
The experiments were conducted in summer 2021 in two regions of Switzerland, south and north of the Alps, in Cadenazzo (canton of Ticino) and Delémont (canton of Jura) in closed and repeatedly secured field cages. The released parasitic wasps had a choice between D. suzukii larvae in fresh fruits and D. melanogaster larvae in rotting fruits. An average of 15% of the D. suzukii larvae were attacked, while only a single parasitic wasp (0.02%) developed from the D. melanogaster larvae.
The results corroborate findings from laboratory experiments that the parasitic wasp G. brasiliensis seldom attacks non-target species that naturally reproduce on decomposing fruits. Hence, no serious negative effects on non-target species are to be expected from the biological control of D. suzukii with the parasitic wasp G. brasiliensis.
Release experiments planned
Release experiments with the parasitic wasp G. brasiliensis are now needed to test the efficacy of the biological control of D. suzukii under field conditions. A first release took place in 2021 in Italy. A release has been approved in the US, and a release application was submitted in Switzerland in February 2022.
- Field cages were used to investigate whether the parasitic wasp Ganaspis brasiliensis attacks only spotted-wing drosophila (Drosophila suzukii), or whether it also poses a risk to the native D. melanogaster.
- On average, 15% of the D. suzukii larvae were parasitised, while only a single parasitic wasp developed from the D. melanogaster larvae (0.02%).
- The results corroborate the findings from laboratory experiments and indicate a low risk to native Drosophila species from the release of the parasitic wasps.