High genetic diversity in the landscape suggests frequent seedling recruitment by Euphorbia virgata Waldst. & Kit. (leafy spurge) in the northern USA

When weeds can reproduce both sexually and vegetatively, limits on weed density and population growth can vary with the relative success of different reproductive modes. Such considerations are particularly important for sustained management with biological control (biocontrol), which relies on selectively targeting demographic processes that limit weed populations. We conducted a survey of 100 Euphorbia virgata Waldst. & Kit. (leafy spurge) populations across three U.S. states to quantify weed genetic diversity, density, and associated root-feeding biocontrol agent (Aphthona species complex) abundance. Aphthona spp. reduce survivorship and clonal recruitment, and their impacts may indirectly influence the value of seed-based recruitment to weed population increase and persistence. We intended to provide insight into: 1) the frequency of seedling recruitment among populations, which is strongly related to population genetic diversity in clonal plants such as E. virgata; and 2) the degree to which E. virgata density is related to the abundance of Aphthona spp. agents and inferred clonality within local weed populations. We found high genetic diversity across all sites, suggesting frequent recruitment from seed. Aphthona complex biocontrol agents were ubiquitous, but associations between agent abundance and E. virgata density were variable. Sites with the lowest E. virgata density had low Aphthona abundance, but otherwise agent abundance was not strongly associated with plant density. Increased population-level importance of seed production may be changing the priority of demographic targets for biocontrol, e.g., elevating the importance of seed-reducing agents, after decades of Aphthona pressure on clonal reproduction in E. virgata invasions. Further empirical evaluation is needed to ensure management tools, such as biocontrol, are sufficient for long-term sustainable control.

Automated summary

This survey examines the relationship between seed recruitment frequency and weed density, as well as the association between weed density and associated biocontrol agents. The results indicate high genetic diversity and abundance of biocontrol agents, but the associations are not strong. The increasing importance of seed production suggests the need for further research to ensure the efficacy of long-term sustainable management tools.