We report the first detailed field survey of sickleweed (Falcaria vulgaris L.) in the United States. Sickleweed is native to Europe, the Mediterranean, Asia Minor, and Iran and was first reported in the United States in 1922. It is listed by the Nebraska Invasive Species Council as a Category II invasive plant species. In recent years, abundance and distribution of sickleweed has increased dramatically in and around the Fort Pierre National Grassland (FPNG), South Dakota. Management of such a rapidly expanding population is hampered by a general lack of baseline information on the biology and ecology of sickleweed. We used an environmental gradient approach to describe the abundance and distribution of sickleweed on the FPNG and found that sickleweed colonization and expansion may be largely driven by small and large scale disturbances that create gaps in ground cover (bare soil and total vegetative cover plus litter), reduced diversity (H'), and altered competitive relationships with western wheatgrass. Minimizing disturbances that create gaps in litter and vegetative cover, decrease diversity, increase bare soil, and alter the dominance of western wheatgrass may reduce the colonization and expansion of sickleweed.

Leafy spurge (Euphorbia esula L.) is an aggressive exotic species that has been successfully suppressed in a variety of situations using classical biological control (flea beetles; Aphthona spp.). This 9-yr study investigated patterns of vegetation responses following significant reductions in leafy spurge cover and density by flea beetles in southeastern Montana. We hypothesized that the vegetation following leafy spurge suppression would be dominated by species and plant functional groups able to persist through heavy infestations. Flea beetles were first released in 1998, and by 2006 leafy spurge foliar cover was reduced 80% to 90% compared to 1998 values on both release and nonrelease plots. Although total cover of the resident vegetation, excluding leafy spurge, increased 72% to 88%, relative cover of the functional groups (native forbs, native sedges, native grasses, and non-native species) was similar among years and between release and nonrelease plots. Mean diversity and mean species richness values did not differ among years or between release and nonrelease plots (P < 0.05), but mean diversity on both release and nonrelease plots was significantly less than noninfested plots, although richness was similar (P < 0.05). Indicator species analysis revealed that non-native Poa spp. replaced leafy spurge as the dominant species on release and nonrelease plots. Conversely, noninfested plots contained a variety of native species with high indicator values. Although total abundance of the resident vegetation in 2006 was significantly greater than 1998, plant species composition and relative cover showed little change for the duration of the study. Failure of the native vegetation to recover to a community that approached nearby noninfested conditions may be attributed to a variety of interacting scenarios, some of which may be ameliorated by treating infestations as soon as possible to avoid long-term residual effects.

Flea beetles (Aphthona spp.) are biological control agents introduced from Eurasia to reduce the cover and density of leafy spurge (Euphorbia esula L.). The potential for the introduced beetles to use alternate hosts for feeding and development in North America is slight; however, it is possible. Species at highest risk are native species closely related, ecologically and taxonomically, to leafy spurge. A native spurge, Euphorbia brachycera Engelm. is consubgeneric and sympatric with leafy spurge throughout the northern Great Plains of the United States, and was not included in prerelease host-specificity testing for Aphthona nigriscutis or Aphthona lacertosa. The objective of this study was to evaluate the actual and potential ecological overlap among leafy spurge, flea beetles, and E. brachycera. Wide-ranging and intensive field surveys indicate that E. brachycera is found well within the range of leafy spurge and flea beetles. E. brachycera occurs infrequently, in low densities, in areas with a high percentage of bare ground, and with a root system dissimilar to leafy spurge. Flea beetles released directly into populations of E. brachycera failed to persist beyond a single field season and plants showed no evidence of feeding by beetles. Our results suggest that the potential for flea beetles to host-shift is low due to differences in growth habit and root morphology between E. brachycera and leafy spurge.