Flies on the dance floor: social group shapes mating success and rivalry in Drosophila suzukii

The social environment, characterised by interactions between individuals and their surrounding group, has a significant influence on behavioural and life-history traits in many animal species. This study investigates the impact of the social environment on the mating behaviour of the invasive pest Drosophila suzukii. We conducted experiments to examine how variations in density of individuals and sex ratio - two main characteristics of the social environment - affect mating behaviours. First, we observed that both male and female flies were attracted by fruits and aggregate around them, suggesting that fruits may serve as social hubs. We then tested the influence of male density and ratio of males to females on mating behaviour. Results showed a clear mate-finding Allee effect at low densities, with a lower mating probability and an increase of the latency before mating, when male density decreases. The general pattern of results highlights male density as the key determinant of reproductive success, while female density and sex ratio had no significant influence on mating outcomes. Conversely, high density environments facilitated mating probability and increased male-male competition. Thus, physical interactions among males, primarily in the form of aggressive attempts to dislodge copulating pairs, were prevalent at high males’ densities. These results demonstrate that D. suzukii uses social cues to modulate reproductive strategies, a flexibility likely contributing to its invasion success.