Field cage study highlights safety of classic biological control agent against devastating invasive fruit fly

CABI scientists have led new research which highlights the safety of a classical biological control agent against the devastating invasive fruit fly Drosophila suzukii which attacks over 150 wild and cultivated fruits, including cherries, blueberries and strawberries, as well as the fruits of ornamental plants.

Drosophila suzukii, or commonly called Spotted Wing Drosophila, is a frugivorous insect native to Eastern Asia that was accidentally introduced to the Americas and Europe in the 2000s, where it rapidly spread. Unlike sympatric Drosophila species in invaded areas, D. suzukii females are able to lay eggs inside unwounded ripening fruits due to their specialized egg-laying organ that is equipped with saw teeth, providing it with a unique niche virtually free from competition.

The resulting high abundance of D. suzukii is leading to extensive damage, making it a major problem for fruit growers, especially in the soft fruit industry.

Field cage releases of the parasitoid G1 Ganaspis cf. brasiliensis carried out in two regions of Switzerland in August 2021 supports findings from previously conducted laboratory-based experiments and the low risk for non-target effects on native Drosophila spp.

The study, carried out with colleagues from the Repubblica e Cantone Ticino, Agroscope, and the Institute of Agricultural Sciences (IAS) of ETH Zurich, and – all in Switzerland, revealed that larvae of the target species D. suzukii feeding in fresh fruits was readily parasitized and of 957 emerging parasitoids, only one was from larvae of the non-target species D. melanogaster feeding on decomposing fruits.

Lead researcher Dr Lukas Seehausen, based at CABI in Switzerland, said, “Released parasitoids had the choice to parasitize either D. suzukii larvae in fresh fruits, blueberries or elderberries, or the non-target native species D. melanogaster in decomposing fruits, which is their natural habitat.

“The results were unequivocal in that parasitism of D. suzukii larvae feeding in fresh fruits was on average 15%, whereas only one parasitoid emerged from D. melanogaster feeding on decomposing fruits, which is a mere 0.02% parasitism.

“The results achieved under semi-field conditions supports findings from previous laboratory experiments that the parasitoid G1 G. cf. brasiliensis is highly specific to D. suzukii larvae feeding in fresh fruits and parasitism of the closely related D. melanogaster naturally feeding on decomposing fruits is very rare.

“Because in its invaded range, D. suzukii is the only Drosophila species that can attack and develop in undamaged fresh fruits, we conclude that possible non-target impacts are a low and acceptable risk for the control of the destructive invasive spotted wing drosophila.”

In their conclusion, the scientists note that with the first releases of G. cf. brasiliensis in Italy in 2021, a recent acceptance of the application for releases of the same parasitoid in the US, and the submission of an application in Switzerland in February 2022, the research starts to be implemented into practice.