Thesis title: From ecology to pest control: development of heterospecific Sterile Insect Technique against the invasive species Drosophila suzukii
Invasive insect pests represent a major threat to global agriculture, causing significant economic losses each year due to their ability to rapidly expand and adapt to different environmental conditions. Drosophila suzukii is a particularly damaging species because of its unique ability to lay eggs in ripening fruit, leading to extensive damage and severe reductions in crop yield. Conventional pest control methods, mainly reliant on chemical insecticides, have raised considerable concerns related to environmental impact, non-target effects, and the evolution of resistance, necessitating the development of more sustainable and biologically based control strategies. In this context, the present PhD Thesis investigates a novel biological control approach called the heterospecific Sterile Insect Technique (h-SIT). This approach integrates the concept of reproductive interference with the classic Sterile Insect Technique (SIT). The h-SIT aims to suppress D. suzukii populations by releasing sterile males from a closely related, non-pest species to compete for mates and interfere with pest reproduction. The candidate species proposed for this control method is Drosophila melanogaster, a widely studied, cosmopolitan species that does not pose an agricultural threat but coexists in many environments with D. suzukii. The research focuses on three primary objectives: i) investigate the reproductive interactions between D. suzukii and D. melanogaster under laboratory conditions; ii) determine the optimal irradiation dose that effectively sterilizes D. melanogaster males while preserving their biological and mating performances; iii) evaluate the effects of irradiated D. melanogaster males on D. suzukii population dynamics in semi-field conditions.
The first part of this study investigated whether D. melanogaster could successfully court, mate with, and reduce the reproductive success of D. suzukii females. Through courtship tests, spermathecae analysis, and multiple-choice experiments, it was demonstrated that D. melanogaster males could mate and inseminate D. suzukii females. However, no viable offspring were produced, indicating the presence of post-zygotic isolation. Furthermore, the presence of D. melanogaster males led to a significant reduction of D. suzukii offspring, supporting the hypothesis that reproductive interference could be exploited for pest suppression. These findings provided the foundation for the following research phase, which focused on optimizing the sterilization of D. melanogaster males for h-SIT applications. To develop an effective h-SIT protocol, it was crucial to determine the appropriate irradiation dose that would sterilize D. melanogaster males while maintaining their biological qualities, including their ability to court and mate. In this phase, gamma irradiation was applied at different doses, and its effects on sterility, longevity, and mating performance were assessed. The results showed that an irradiation dose of 80 Gy effectively induced sterility in D. melanogaster males without significantly impairing their lifespan. Importantly, irradiated males retained their capacity to court and mate with D. suzukii females. The final stage of this research focused on evaluating the effects of irradiated D. melanogaster males on D. suzukii population dynamics under semi-field conditions. Experiments were conducted in controlled cage environments designed to mimic natural settings, testing periodic releases of sterile D. melanogaster males in D. suzukii populations. The results indicated a significant decline in D. suzukii population growth when irradiated D. melanogaster males were present. This research contributes to the field of pest control by demonstrating that h-SIT represents an innovative, sustainable alternative to conventional chemical control methods. The significant findings of this study highlight that reproductive interference is a viable strategy for pest management, as D. melanogaster males can successfully disrupt D. suzukii reproduction through mating interactions that do not result in viable offspring. Additionally, sterilization via irradiation does not compromise the competitive ability of D. melanogaster males, ensuring their effectiveness in h-SIT applications. Moreover, semi-field trials confirm that introducing irradiated D. melanogaster males significantly reduces D. suzukii population growth. These findings confirmed that h-SIT could effectively disrupt D. suzukii reproductive processes and limit their ability to establish and expand, demonstrating the potential for real-world implementation of this approach. While laboratory and semi-field trials provide strong evidence supporting the efficacy of h-SIT, further research is needed to refine and scale up this approach for practical agricultural applications. Future investigations should focus on optimizing release ratios to maximize suppression efficiency, assessing the long-term ecological impacts of h-SIT, and exploring the potential integration of h-SIT with existing pest management strategies. This doctoral research represents a pioneering step in applying h-SIT for invasive pest suppression and offers new perspectives for sustainable pest management. By integrating reproductive interference with SIT, h-SIT has the potential to become a cost-effective, environmentally friendly biological pest control, highlighting the importance of species interactions in designing innovative management strategies.