Use of entomopathogens, natural and semiochemical limiters for the biocontrol of recently introduced phytophagous species

In the era of globalization and climate change, the accidental introduction of invasive alien species represents one of the main threats to food production, ecosystem quality, and the local agro-economy. In this context, the present PhD thesis focuses on enhancing knowledge and developing techniques to promote sustainable and environmentally friendly control of two recently introduced alien insect species in Italy: Popillia japonica Newman and Drosophila suzukii Matsumura. The two case studies illustrate different invasion stages, for which are required distinct sustainable management strategies: P. japonica (Chapter 2 and 3) needs site-specific control strategies due to its quarantine status, while D. suzukii (Chapter 4, 5 and 6) requires large-scale and long-term solutions. In the introduction (Chapter 1), the main factors driving the introduction of alien insect pests, the damage they cause, and the strategies currently adopted to manage them are discussed. Then, the issues associated with the two studied insect pests are described. Regarding P. japonica, the effectiveness of mulching as a cultural technique to prevent beetle oviposition in potted plants intended for trade was tested (Chapter 2). Through controlled infestation tests, the most common mulching materials used in nursery chains were evaluated. Gravel was the most effective mulch and we observed that the effectiveness of the mulching depends on the specific weight of the materials. From a systems approach perspective, further studies were conducted to investigate the effectiveness of mulching and its integration with the use of entomopathogenic nematodes for the control of P. japonica larvae (Chapter 3). It was demonstrated that using thicker layers of mulch improves oviposition prevention, and that entomopathogenic nematodes are highly effective in reducing larval infestations in potted plants, although no significant interaction between mulching and nematodes was observed. In the second part of the thesis dedicated to D. suzukii, we assessed the current state of the biological control of D. suzukii in northeastern Italy (Chapter 4). This 3-year survey which analyzed infested fruit samples collected both from the ground and from plants, allowed the detection of native and exotic parasitoid species and their associations with host plant species at different altitudinal levels. Since the significant presence of the exotic parasitoid associated with D. suzukii, Leptopilina japonica Novković & Kimura, we investigated the ecological factors driving its parasitism activity in the three most common wild host plant species (Chapter 5). The results showed that L. japonica exhibits a density-dependent response to D. suzukii, effectively following its temporal and spatial dynamics. Higher parasitism rates were recorded in agroecosystems with high forest cover and landscape complexity, underscoring the importance of adopting a landscape-scale approach to promote the biological control of D. suzukii. Currently, the D. suzukii’s integrated management is based on repeated insecticide applications; therefore, it is crucial to understand the implications of their use on biological control. The non-target effects of commonly used insecticides against D. suzukii on Ganaspis brasiliensis Ihering were evaluated through laboratory and field bioassays (Chapter 6). Current chemical applications have a strong negative impact on the parasitoid, highlighting the need to adopt selective molecules, such as cyantraniliprole, to minimize the side effects on biological control. The sustainable management of invasive alien insects in the initial phase of invasion requires identifying cultural practices to limit pest proliferation and damage, while in advanced stages, efforts should focus on achieving biological balance through biological control. In this context, regarding P. japonica, the results demonstrate that cultural control through mulching and biological control using entomopathogenic nematodes represent sustainable and effective approaches for ensuring the production of P. japonica-free plants. The evidence collected during these experiments can provide valuable data for authorities, contributing to the improvement of nursery plant production guidelines in areas infested by P. japonica. On the other hand, the results obtained from the research on the biological control of D. suzukii provided insights into the composition of the parasitoid community and the ecological factors influencing parasitism. Additionally, the results concerning the side effects of insecticides on the newly introduced parasitoid G. brasiliensis offer practical information for implementing integrated pest management strategies against D. suzukii without compromising its biological control.