Evolutionary Stability of High Virulence in Vector-Borne Forest Pathogens: Evidence from Pine Wilt Disease and Management Thresholds

Pathogen virulence is a central trait shaping disease dynamics and management outcomes. Classical evolutionary theory predicts that virulence should be constrained by trade-offs between transmission and host survival, often favoring intermediate optima. However, many vector-borne forest diseases exhibit persistently high virulence despite severe host mortality, challenging this expectation. Increasing evidence suggests that ecological context, particularly transmission mode, vector abundance, and spatial structure, can fundamentally reshape virulence-transmission trade-offs, biasing selection toward elevated virulence rather than attenuation. Pine wilt disease, caused by the pine wood nematode Bursaphelenchus xylophilus and transmitted by Monochamus beetles, exemplifies this paradox. Infection leads to rapid host death, yet highly virulent nematode lineages persist and spread across forest landscapes. In this system, host death directly facilitates vector reproduction by creating suitable breeding substrates, potentially reversing the classical trade-off in which host mortality constrains transmission. Why such systems remain evolutionarily trapped in high-virulence states, despite widespread host depletion, remains unresolved. Using pine wilt disease as a motivating and representative vector-borne forest pathosystem, we develop an individual-based evolutionary framework to investigate how transmission conditions and management interventions shape long-term virulence evolution. By integrating stochastic population dynamics with selection-gradient analysis, we identify the mechanisms sustaining high virulence and determine quantitative management thresholds capable of reversing selection pressures and driving durable virulence suppression.