The invasive spotted-wing drosophila (SWD) is a devastating pest in berry, stone fruit and grape crops. A natural antagonist from the fruit fly-s area of origin in East Asia is now due to be released in Switzerland for the first time by Agroscope and CABI. The experimental releases in the Cantons of Jura and Ticino aim to clarify whether this parasitic wasp can become established in Switzerland to regulate the SWD population and reduce production losses.
Originally from East Asia, the spotted-wing drosophila (Drosophila suzukii) has been recognised in North America and Europe as an invasive pest since 2008. In Switzerland, where it has no efficient natural enemies, its presence was first detected in 2011. As the only fruit fly species in Switzerland, their females lay eggs in undamaged ripening fruit, rendering it unfit for sale. In this way, Drosophila suzukii causes major economic damage in Swiss agriculture, particularly in berry, stone fruit and grape crops.
Achieving a natural counterbalance
This week, researchers from Agroscope and the Centre for Agriculture and Bioscience International (CABI) in Delémont, Canton of Jura, will be releasing between 800 and 1000 adult parasitic wasps (Ganaspis brasiliensis) in the immediate vicinity of SWD-infested fruit at selected locations in the Cantons of Jura and Ticino. The wasps should parasitise the larvae of the pest, which will then die. After the release, the researchers will intensively monitor the area and determine whether the parasitic wasp can become established in Switzerland. In this way, a natural counterbalance to SWD could be achieved in the medium-to-long term so that it cannot continue to reproduce unhindered. The aim is to bring down SWD population numbers naturally, thereby reducing damage to agricultural production without additional use of plant-protection products. This antagonist would be particularly useful for high-stem fruit trees that are difficult to protect.
First controlled release of an exotic beneficial insect
What is different about this release? unlike with previous released parasitic wasps targeting the invasive brown marmorated stink bug in Zurich or the Comstock mealy bug in Valais, Ganaspis brasiliensis is not yet present in Switzerland. Hence, it is the first exotic beneficial insect to be released for biological pest control in Switzerland since the entry into force of the Swiss Release Ordinance in 2008. The same has already been done in the last three years in Italy and the USA, and is also being done this week in France. Initial findings from these countries give reason for optimism.
Longstanding battle against Drosophila suzukii
Between 2015 and 2020, Agroscope led the -Drosophila suzukii Task Force- in collaboration with the Research Institute for Organic Agriculture (FiBL) and other partners from research, extension, practice and enforcement. The aim of the task force was to develop sustainable pest-control strategies for farmers to protect berries, fruits and vines from the pest. Dominique Mazzi, a scientist at Agroscope and previous Head of the Task Force, observed that -Since Drosophila suzukii also infests fruit growing in the wild outside of agricultural production, large-scale, long-term measures are required to contain the unhindered reproduction of the invader-. Conventional biological control, in which natural antagonists in the region of origin of the invasive pest are sought and then introduced as beneficials, can be part of this solution.
Quest in East Asia successful
The quest for such a beneficial insect began in 2015, when various research groups, including scientists from CABI, undertook initial studies in Asia. The researchers found Ganaspis brasiliensis, a natural antagonist of the spotted-wing drosophila. Since then, this parasitic wasp has been investigated in Switzerland under controlled laboratory conditions. Lukas Seehausen, a scientist at CABI in Delémont specialising in invasive species and biological control, explains: -Before a release of this type, years of studies on biosafety are required whose main focus is to analyse the risk of negative effects on domestic species. Our research shows that Ganaspis brasiliensis is specialised in spotted-wing drosophila and is highly unlikely to parasitise native fruit fly larvae". The Swiss Federal Office for the Environment has examined and approved the application for an experimental release permit submitted by Agroscope.