Revisiting Canada’s oldest invasive plant biological control system : exploring ecological associations influencing physical and metabolic characteristics of the invasive Hypericum perforatum.

Hypericum perforatum is an invasive plant introduced to North America in the 1700s. H. perforatum thrives in open, sunny areas, and produces toxic metabolites, hypericins and hyperforin, that cause health problems in herbivores. The plant’s toxicity has resulted in considerable expenses to ranchers. In response, Canada developed its first biological control program in the 1950s, introducing seven specialized insect herbivores from H. perforatum’s native range, of which five established successfully. Several decades after the successful control of H. perforatum across Canada’s grasslands, there are rising questions regarding the effectiveness of the insects in British Columbia as new reports claim observing persistent plant populations. To identify factors causing possible variation in biocontrol effectiveness we quantified the phenology, abundance and morphology of 25 plants and the secondary metabolic profiles at the flowering stage of 5 individuals per site, across 17 populations exposed to different geo-climatic conditions weekly from April to December 2024. We also measured the abundance, phenology, identity and herbivory of the released insects on the plants and across all sites to identify variations in plant responses to biological pressures. H. perforatum shows phenological differences associated with growing season length and biocontrol pressures. Increased herbivory associated with earlier plant senescence after a threshold of damage. Plants in populations facing east and west show greater toxic metabolite concentrations than other populations and biological control abundance is associated with lower hyperforin concentration. Hypericin and hyperforin concentrations were weakly associated across sampled plants and their orthogonal loading in PC analysis suggests they vary independently in response to different environmental drivers. Our results flag candidate variables that may contribute to variation in biocontrol effectiveness, particularly abundance and herbivory, highlighting the need to investigate insects’ preferences to ensure future effective control.