Factors influencing the performance of Ganaspis brasiliensis G1 in Northern Italy as part of a biological control project against Drosophila suzukii

Drosophila suzukii, the spotted-wing drosophila (SWD), is a small fruit fly native to Southeast Asia. Its peculiar serrated ovipositor allows the female to lay eggs inside ripening undamaged fruit, particularly soft berries and stone fruits, causing huge damage. The life cycle is rapid, with multiple generations in a single growing season. Infestations lead to compromised fruit quality, reduced marketability, and economic losses for fruit growers. Managing SWD currently requires a combination of cultural, biological, and chemical control strategies to minimize its impact on fruit production. However, in invaded areas, classical biological control with specialized parasitoids coevolved in Asia can be the most effective and sustainable strategy, ensuring stable and long-lasting control. As a fact, by introducing native parasitoid wasps into affected areas, there is the potential to reduce the population of SWD reaching the natural ecological equilibrium between host and parasitoid and hence mitigate the damage caused by this invasive pest. Therefore, in Italy in 2021 a national biological control project was approved, involving the release of the most specialized and effective natural enemy of SWD, the larval parasitoid Ganaspis brasiliensis, Japanese strain G1, in many regions. After 3 years of field releases of G. brasiliensis, we are now committed to understanding its establishment capacity in the new environment and the ongoing multitrophic relationships with the host and other local and exotic biocontrol agents to better predict its spread, its SWD control capability, and, as a consequence, enhance the project outcomes. For these purposes, we performed field experiments exposing blueberries infested with D. suzukii larvae that had just been parasitized by G. brasiliensis in different natural environments. The aim was to evaluate the abiotic and biotic factors that influenced G. brasiliensis development. The data revealed that secondary infestations carried out by local drosophilids and secondary parasitization carried out by larval parasitoids, both autochthonous and exotic, resulted into a significant reduction of G. brasiliensis emerging adults. Moreover, emersion data demonstrated that Leptopilina sp. individuals were significantly faster to reach adulthood compared to G. brasiliensis. These data suggest that interspecific competition play an important role into regulate G. brasiliensis population in natural conditions in our release sites. We discuss the results obtained in relation to the potential implications, both positive and negative, regarding the achievement of the objectives set by the Italian project of classical biological control against SWD. Additionally, the collected data are valuable for optimizing the current methods of monitoring parasitoids and their effects in the field.