Variation in the impact of biological control on leafy spurge is associated with agent abundance

Classical biological control (biocontrol) is a valuable tool for leafy spurge (Euphorbia virgata) management, but what drives variation in biocontrol efficacy remains unclear. Few large-scale data are available to assess long-term control impacts after agent establishment, though such evaluations are necessary to identify gaps in biocontrol management and ensure the sustainability of biocontrol programs. We surveyed leafy spurge and biocontrol agent communities for four years in 60 sites across Idaho, Montana, and North Dakota, and evaluated how leafy spurge density and cover varies and correlates with agent presence and abundance across ecological gradients. Initial findings suggest biocontrol agents are ubiquitous. The agents present vary across space, but agent community composition varies less across time, suggesting some ecological partitioning. Associations between agent impacts and leafy spurge populations are nuanced, though the highest leafy spurge densities often occur where agent abundance is low. Variability in the abundance of Aphthona spp. flea beetles, the dominant agent complex used in leafy spurge management, appears to increase with elevation and is associated with more problematic leafy spurge infestations. Leafy spurge infests millions of acres in the central and western US and remains a priority weed. These survey data provide multiple avenues for quantifying biological control efficacy across a wide region, and a baseline for considering how agent communities may establish and change over time, and change the invasion dynamics of their target weed. Such information can inform how management may need to evolve to maintain a successful weed biological control program.