The international trade of potted plants represents a pathway for the unintentional spread of harmful organisms, leading to new infestations in pest-free areas. The Japanese beetle Popillia japonica, which is considered one of the most threatening insect pests in Europe, can evade detection at the larval stage by hiding within the substrates of potted plants. To reduce the risk of P. japonica dispersal via potted plants, we tested the efficacy of entomopathogenic nematodes (EPNs) and fungi (EPF) as biological control agents (BCAs). Field trials with potted cherry trees were conducted in an Italian nursery in northern Italy, using preventive treatments applied before the beetle’s flight period and curative treatments after larval establishment. Two Heterorhabditis bacteriophora (a native population, labelled as POP16, and a commercial product) significantly reduced larval survival, achieving mortality rates up to 97% after 60 days. By contrast, EPF treatments with Metarhizium robertsii and M. brunneum showed persistence in the substrate, without efficacy. EPNs proved to be much more effective when applied shortly before the adult beetle flight, rather than as a curative treatment targeting larvae already present in the pots. These results suggest that, at present, the use of H. bacteriophora applied between 15 and 30 days before the commercialisation of the potted plants could represent the best trade-off between management effort and effectiveness in integrated pest management (IPM) programmes to reduce the unintentional spread of P. japonica through the nursery trade.