Overwintering biology of Drosophila suzukii females in the Trentino region in northern Italy

Abstract: After being first recorded in 2009, Drosophila suzukii has rapidly became the key pest of the sweet cherry and soft fruit in Trentino (northern Italy). In the context of the experimental and research activities conducted at the Edmund Mach Foundation (FEM) in subsequent years, considerable efforts have been devoted to elucidating the biology of this new and damaging insect, with a particular focus on its overwintering behaviour. About 75,000 adult females, caught in area-wide monitoring activities from 2012 to 2016, were dissected under a stereo-microscope in order to classify their reproductive condition, according to a pattern of five recognisable ovarian maturation stages: no ovaries, unripe ovaries, ripening eggs in ovarioles, mature eggs in ovarioles, mature eggs in the abdomen. The results obtained from this work revealed that during the winter (December/February) females do not have mature fertile eggs available and ready to be laid. They pass through this period in different ovarian maturation stages, but clearly showing a reproductive diapause. Oogenesis normally resumes when temperatures and the photoperiod start to increase, and the first overwintered females with mature eggs appeared very early in the season (March). As new susceptible fruits were not yet known to be available in our region at that time, we speculated that these eggs may be laid in other substrates (e.g. compost), reabsorbed by the females or stored in their ovaries until the appearance of new fruit. In the framework of the area-wide monitoring implemented in 2016, several D. suzukii eggs laid in ivy (Hedera helix L.) fruits were detected. Adult females and males emerged from these infested berries when stored in laboratory conditions, confirming that the pest is able to exploit this very common and widespread host in a period when other fruits are not yet present. However, the smaller size of these adults in comparison with individuals emerging from early infested cherry fruit, as well as the high larval mortality recorded in infested berries collected in some of the sampled sites, indicate that this host plant is probably not highly appreciated. New generation adults were also obtained under laboratory conditions from domestic compost infested with overwintering D.suzukii adults collected live in an open field situation in February 2016.