Investigating the overwintering strategy of Ganaspis kimorum, a biological control agent of Drosophila suzukii

• Ganaspis kimorum survived overwintering in the introduction area. • G. kimorum goes through diapause as early larval instar (L1). • Survival rate varied across sites (0.4–17.6%). • Extreme low and high temperatures negatively affected overwintering survival. • A lower thermal threshold of 9 °C and 329 ± 6 post-diapause Degree Days were estimated. The larval parasitoid Ganaspis kimorum Buffington (Hymenoptera: Figitidae) is a classical biological control agent of Drosophila suzukii (Matsumura) (Diptera: Drosophilidae). This study investigated the overwintering strategy of G. kimorum under field conditions in Switzerland and Northern Italy. Field experiments conducted from 2021 to 2024 involved exposing G. kimorum to naturally variable temperatures in seven sites with different climates. Results indicate that diapause induction may be mediated by temperature, with exposures after late September successfully triggering diapause. Dissections revealed that diapause occurs in the early larval instar (L1) residing within early pupal stages of D. suzukii. In four out of seven sites, G. kimorum was able to survive winter conditions and emerged as adult. Survival rates varied significantly across sites (0.4–17.6 %). The analysis revealed significant negative associations between insect survival and the count of days with temperatures below 0°C and above 30°C. Analysing the variation of Degree Days needed until adult emergence confirmed that G. kimorum arrests its development as a first-instar larva and suggests a lower thermal threshold of 9°C, with 329 ± 6 post-diapause Degree Days needed to develop to the adult stage. The study provides valuable insights into the biology of the parasitoid that may be helpful for optimizing field releases and enhancing the effectiveness of G. kimorum as a biological control agent of D. suzukii.