Use of Heterorhabditis bacteriophora for the biological control of Popillia japonica larvae and evaluation of its impact on non-target soil biota

Popillia japonica (Coleoptera: Scarabaeidae) is a scarab beetle native to Japan and accidentally introduced to Italy from North America in 2014. The high reproductive rate, the pathway of introduction, and the dispersal capability, as well as the polyphagous diet, led the EU to include this pest in the priority list. Popillia japonica spends most of its life cycle as a grub in the soil. At this stage, the grubs damage turf grasses, hay meadows, soccer pitches, and golf courses and are susceptible to biological control agents present in the soil, such as entomopathogenic nematodes (EPNs). As part of the Horizon 2020 Project ‘IPM-POPILLIA’, a large number of natural EPN strains have been collected from the soil in the outbreak area in Northern Italy along the Ticino River. The most virulent indigenous strain of Heterorhabditis bacteriophora (POP 16) was used in field trials to assess its effectiveness in containing the P. japonica larval population and its persistence in soil. Since September 2021, these trials have been carried out in the Piedmont region in three meadows with different irrigation management (non-irrigated, semi-irrigated, and irrigated fields). Two months after the treatments, a significant reduction of P. japonica larvae was observed in plots treated with EPNs (>90%) compared to untreated ones. Regarding persistence, H. bacteriophora was isolated again after the winter season and at lower concentrations one and two years after treatments, depending on irrigation management. Furthermore, in the irrigated meadow, the effects of EPNs on non-target soil biota were also evaluated, comparing soil arthropod assemblages with a Before-After Control-Impact experimental design. Compared to the control, no evident negative effect on non-target edaphic arthropods was disclosed on soil treated with EPNs. However, some taxa showed an increase in abundance, suggesting that some changes may occur in the long term. These results evidenced that the use of the selected EPN indigenous strain may be an excellent candidate for the integrated management of P. japonica, with no impact on non-target species, and at the same time highlighting the importance of evaluating the effects on soil biota to preserve the key- functioning of the ecosystem.