Resistance evolution in parasitoid biocontrol: understanding the rule, managing the exceptions

In contrast to chemical pest control, biological control (biocontrol) is generally considered evolutionarily stable, with pests rarely evolving resistance to agents such as parasitoid wasps. In 1997, Holt & Hochberg outlined five principles to explain this pattern. Here, we review post-1997 case studies where resistance has contributed to the breakdown of parasitoid-based biocontrol. We evaluate how these examples align with or challenge Holt & Hochberg's framework and propose updates to reflect new findings. While resistance remains rare, reported cases suggest that breakdowns are more likely when pests possess greater standing genetic variation than their parasitoid enemies. We argue that long-term stability depends not just on host constraints but also on the potential for parasitoid virulence to evolve. Finally, we offer practical recommendations for biocontrol practitioners and regulators to minimise the risk of resistance evolution in parasitoid-based systems.